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Vacuum advance/Acceleration problem

12K views 40 replies 10 participants last post by  RoddinRob 
#1 ·
Hi Gang,

I'm still working through the kinks on my 67 fastback. It is a re-built 289 (302 block) with what I believe is an Autolite 4300 4v carb & C4 transmission that the PO had installed 20 years ago.

One thing I noticed when I would drive it is if I tried to goose the throttle, it would 'fall off' then kick-in... A significant hesitation. I wanted to verify the timing. It was way advanced - maybe around 12 degrees or more. It took me over a week to free-up the distributor to be able to adjust it. I put it back to 6 degrees BTC & it ran worse. I opened up the distributor & it looked pretty nasty in there, so I took apart the internals of the distributor down to the springs/weights & cleaned it all up/put it back together. I tested the vacuum advance unit & it functions/holds. I don't have anything more than a basic timing light & a DVM, so my diagnostic tools are limited. After putting the distributor back together, I could hardly give it gas at all without the engine wanting to shake out of the mounts. When I would disconnect/cap the vacuum line off the distributor to re-check the timing, it actually ran better. SO - I played with the throttle & adjusted the vacuum advance unit's screw until I figured I had it smoothed-out. When I took it for a run, it seemed to perform alright... the hesitation on acceleration is still (periodically) there like before... but now when I am at steady speed for a few minutes, it starts to run rough, like it's missing or something. If I let off the gas, or give it slight acceleration, the roughness goes away. Doesn't matter what speed I'm at.

I've read quite a few posts on the whole vacuum advance topic... Some of them are quite involved. I'm wondering if there is an easy way to adjust/set it up when you don't have all kinds of diagnostic tools handy. I suppose I could just advance the timing a bit & plug the line that goes to the distributor & run it like that for now, but I don't want to leave it that way. If I have to spend some $$ I'd like to make sure I'm putting it in the right place... do I get a tach, do I replace the whole distributor/rebuild? It still needs to be set-up...

Any suggestions welcome...

Thanks,

R.R.
 
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#2 ·
Rob, the 4300 is notorious for this sort of thing. Stumbles, hesitation, and general annoyance (along with an occasional engine fire) are the hallmarks of this vintage carburetor. It was supposed to supercede the 4100, and get better emissions. In some regards, this is true, because vehicles with the 4300 often got parked instead of driven.

In order to rule out timing as the source of your trouble, I suggest you follow these steps:

1) Make sure your timing ring has not slipped, by checking top dead center. Pull plug #1, disconnect your ignition, and using a dowel inserted into the spark plug hole (which will not mar the piston or fall into the cylinder), turn the engine over until the piston is at the top of its travel. Verify that your timing mark matches the mark on the harmonic damper.

2) Disconnect the vacuum advance, and plug the hose or port. Reconnect your ignition. Start the car, and using a timing light, set your ignition advance to its recommended value (according to the shop manual). Probably 6-8 degrees BTDC. Test your mechanical advance, by increasing RPM with the throttle. You should see it stop moving around 2800-3000 RPMs, and be in the mid 30's for timing. Do NOT have the vac advance attached for this part. Your engine will not be idle smoothly and perfectly at this point, because your vac advance is not hooked up, but it should smooth out and run pretty good when you give it throttle.

3) Reconnect the vacuum advance. Adjust your idle bleeds until you get the smoothest idle possible. Adjust your idle speed to 650-800 RPMs (with the car in Drive and someone standing on the brakes for you, presumably).

Go test drive your car. Do some full-throttle acceleration runs, and check for any ping. If you hear it rattling, back off one degree on the timing, and then go check it again. Ideally, with your heads, you want to run the most timing you can get away with, for best performance. Too much will make your car sensitive to the type of gas you use, and cause trouble going up hills, into the wind, etc. If you are tuning to the 'ragged edge of performance', find where it pings, and then back off a couple of degrees for safety.

If you are still experiencing stumbles and hesitation at that point, I think it's safe to say that your 4300 is up to the typical shenanigans. Some people claim that most of its bad habits can be cured through a rebuild, along with some ritual chanting and chicken bones. What I have observed personally is that these carburetors are not so good compared to most other designs, and even if you spend the effort to make it the best it can be, it will still be mediocre.

If you want to drive your car, box up the 4300, get a Summit M-series 500 CFM carburetor, and enjoy the hell out of it.
 
#3 ·
The vacuum advance is not a performance part, its basically an emissions device. Its function is to increase fuel economy and reduce engine temperature at light loads. Leaner A/F mixtures require more timing advance to burn efficiently. Timing and vacuum advance are 2 separate features. Vacuum advance drops out under WOT.

First you need to get your mechanical timing curve and carburetor A/F mixture adjustment sorted out with the vacuum advance disconnected. Then work on your vacuum advance.

Use the link below to mark your harmonic balance in increments to attain 34* and +/- 2*.
Timing Mark Calculator

Find the rpm point at which your mechanical timing stops advancing, adjust the distributor for 34* at that point. Note at what RPM this happens. Then set your preferred idle speed and note the initial timing and RPM for future reference. Your engine may like a few more degrees of advance beyond this so feel free to experiment with your initial timing. For every degree of initial timing change, it will change your total the same amount.

Then reconnect your vacuum advance to manifold vacuum set so it adds around 10*.
 
#6 ·
All good advice here, but I want to stress the importance of this:

Then reconnect your vacuum advance to manifold vacuum set so it adds around 10*.
(Actually adds 15 degrees or more, but that's not my point.)

Some guys incorrectly connect the distributor vacuum to a carburetor port, at or above the throttle plates. In most street applications that is wrong.

Mechanical timing should be in the 6-10 degree BTDC range at idle RPM, and advance smoothly with increasing RPM. The reason is simple physics- it takes the same time increment to build pressure, but the power cycle is happening in less time.

Vacuum advance should increase advance at low engine load. Maximum vacuum advance at idle and cruising, zero added advance at wide open throttle. The reason for this is chemistry: the carburetor provides a lean mixture at low load situations, and a rich mixture at high load situations. Lean mixtures take more time to burn.
 
#4 ·
I hate to bring this up too, but messing with the vacuum advance's screw is generally the very last thing you ever mess with. It's more of an ultra-fine-tuning sort of thing. If you've turned it a bunch, it's likely that you have effectively disabled your vacuum advance, or made it so sensitive that it wants to give too much advance at inappropriate times. On stock distributors, its job is to determine how much vacuum has to be present before it starts to advance (or stops advancing) - not 'how many degrees of vacuum advance'. You'd only change it if you had a car with a cam that gave you too much lope.

Bottom line here: at low throttle (either idle or cruise) the engine's mix has very little air or fuel to burn. It lights slowly. Vac advance just makes sure that the spark happens sooner, so your engine runs smoother and more efficiently. It has little to no effect on how your engine behaves when you open the throttle though, because it should 'go away' when there's no vacuum in the manifold anyway.

It's a very important thing to a street driven car that spends most of its life at low throttle, cruising around and idling, but your car should still be driveable without it. It would just run rougher when you are at low (or no) throttle. Redstang's right! Get your mechanical advance sorted first, make sure that's spot on, and then worry about fixing the adjustments you made to your vac can's sensitivity.
 
#5 ·
I hate to bring this up too, but messing with the vacuum advance's screw is generally the very last thing you ever mess with. It's more of an ultra-fine-tuning sort of thing. If you've turned it a bunch, it's likely that you have effectively disabled your vacuum advance, or made it so sensitive that it wants to give too much advance at inappropriate times. On stock distributors, its job is to determine how much vacuum has to be present before it starts to advance (or stops advancing) - not 'how many degrees of vacuum advance'. You'd only change it if you had a car with a cam that gave you too much lope.

He's right, I bought an old mustang where the vacuum advance had been adjusted and every time I took a left turn the car would stall out, once I got the mustang professionally re-tuned they explained to me why the vacuum advance adjustment gave me that problem and many others. I suggest that you don't mess with that unless you know exactly what you are doing.
 
#7 ·
Adding to what Yadkin has to say - even if it's rich, the small amount of fuel and air that get let into a cylinder with the throttle plates closed just cannot light off as fast. If you take a tiny amount of gas and compress it into a given amount of space, pressure will be pretty low when you set off the spark, allowing for a very lazy explosion.


Do the same thing with a full cylinder, shoving all the fuel and air molecules into a tiny space and making them get very close to each other before igniting = BIG BANG.


Since the engine mostly just cares about reaching peak pressure at the right time, lighting off the mix early when there's not much in the cylinder is imperative. Vacuum advance makes that happen. It's stupid simple, really: High vacuum in the manifold = not much in the cylinders = light it earlier, in an adjustable way that pays attention to manifold pressure.
 
#9 ·
I'm going to say leave the distributer alone as long as you have a very smooth idle and medium RPM engine. if the engine is smooth at both low and medium ranges its very unlikely that the ignition system is the problem

Look at the carb. theres are a ton of internet resources on rebuilding the 4300 and fine tuning it. The delay on dropping the pedal to the floor is what we used to call signal lag. Some small adjustment to the accelerator pump rod may alleviate this issue. Its not uncommon for those diaphrams to spilt also. Basically when you push down on the pdeal the carb should squirt extra fuel into the throat of the carb, if you are getting lag the extra fuel is either not happening happening at the wrong time or just not enough, the sudden surge is when the vacuum created actually begins to pull more air and with it more fuel due to the "venturi effect" but that little hesitation before the air start to pull more fuel as it rushes over the ports is annoying as heck, hence the need for an accelerator pump or diaphragm.

We used to have similar problem on the old big block GM cars with the spread bore carbs.
 
#10 ·
Hi Guys,

I'm just reading through everything now. Man there is a lot of good information here. I was only playing with the vacuum advance unit because I thought that's what was giving me the trouble since it seemed to run better without it. I'm not a big engine guy... my troubleshooting skills lie more in the automation world. (That said, I've only backed-out the vacuum advance unit 1.5 turns, so no issue if I have to put it back.) At first I was thinking this isn't making sense, but after re-reading it I think I'm getting it. I like the procedure outlined for setting up the timing first & how to ensure it is actually right. I never would have thought the balancer could slip. That would have me screwed from step one & everything after. I need to pull plug one & do the dowel thing anyway since I don't know what heads are on my car. I could very well have a 302 on my hands even though I was told it was a 289, the block codes say 302 from 1980. Markings on the heads are inconclusive. For this problem it doesn't matter since timing/firing order I understand are the same for both engines, I just want to know.

I also have to say I like Grim's subtle jokes. :)

There's a few things I wanted to hit on to make sure I'm on the right path here & to make sure nothing you guys said is affected by the additional information I'm providing as a result of reading your posts...

1- If I understand right; I'll know when the mechanical advance (weights/springs in the distributor base) are maxed-out when the timing mark stops moving as I accelerate, correct? I don't necessarily need a Tach to do this, just the timing light.
2-My vaccuum advance unit is ported to the base of the carb, not the manifold. There were a couple references to this in the thread & I didn't previously state how it was. Not sure that I need to move it or not. Sounds like there are arguments for/against.
3-I run 'premium' gas in the car - always - for the extra octane. I don't add anything to it other than stabilizer/Seafoam when I put it away for winter. Right now it's got a full tank, a little less than half of that is last summer's gas with stabilizer/seafoam, the rest is fresh.
4- I'm assuming my Carb is an Autolite 4300 based on pictures I've seen trying to look-up/identify it online. The distinguishing feature being the teeter-totter arm for the fuel pump towards the top/front of the carb./driver's side.
5- I DO see fuel squirting in if I open the primaries when the engine is off & pump the accelerator.

If anything I've added changes anything or puts a different spin on it, by all means let me know before I go to far down the wrong path. You guys have given me lots to get started. It's gonna take me a bit to get to it, but I will & definitely post any updates.

Thank you all so much!

R.R.

:)
 
#12 ·
2-My vaccuum advance unit is ported to the base of the carb, not the manifold. There were a couple references to this in the thread & I didn't previously state how it was. Not sure that I need to move it or not. Sounds like there are arguments for/against.
There are two basic vacuum sources. The first is manifold vacuum and it is any port below the throttle plates. All the manufacturers made numerous changes over the years (due to emission requirements) so I have no idea what the connection on your carb is ported to. It's simple to figure out by connecting it to your distributor and looking at how it changes the ignition timing at idle.

My advice is predicated on pre-emission controlled carbs and distributors, and street cars with compression ratios 8 or above. For Fords this was the Autolite 1100 (one barrel carb, 2100 (2 barrel) and 4100 (4 barrel). Your 4300 (think version 3.0) was some later version that I'm not familiar with, because most of the changes were emissions related and and not performance oriented.

Same thing with distributors, there were changes made for emissions. My advice is predicated on the simple vacuum advance distributor with a port pointing towards the front of the engine. Latter versions had retard functions with one or more ports on the side.

So the "debate" over what vacuum source to use is largely due to emission changes over the early years of those systems. Big changes came after 1972, and none were performance oriented. Various configurations were used to retard spark during certain situations. Admittedly I haven't spent any time considering all that because from what I've read, what I know based on my knowledge of physics and chemistry, and based on my experience running a 1972 car through mid 1980's emissions tests (required back then in CT where I used to live), its very easy to tune a 1972 or earlier car to run well and with low emissions.

No doubt someone will disagree as they always do. But I have yet seen one of these guys explain their reasoning in scientific terms.
 
#13 ·
Thanks for the kind words, R. R.! And Yadkin, +1 for diving into the giant pile of ... discussion ... about ported vs. manifold vacuum.

I'm just going to support Yadkin's points by adding a few more:

1) If your car needs more timing to run most efficiently when manifold vacuum is high (closed throttle) then doesn't it make sense that *idle* needs as much or more advance as your car does at cruise, with the throttle plates slightly open? Ported vacuum shuts off vacuum advance completely, as soon as the throttle blades close, making the car idle slower because of incomplete combustion. It spits more unburnt fuel into your exhaust, because the mix didn't have time to burn in the cylinder.

2) If your car has a big cam with lots of overlap, and manifold vacuum surges wildly because of lope, you have a valid reason for using ported vacuum. With such a situation, the manifold vacuum spikes and drops, causing the timing to also spike and drop, sometimes making it impossible to tune idle bleeds or keep the engine running. Using ported vacuum can help stabilize idle, when the engine is likely to die because of surge. A mild cam will never experience this situation.

3) Nobody's really suggesting you disconnect the vac advance, but I wanted to also mention this. While it is true that vacuum advance mostly just affects economy, it really improves a lot of other things. It allows crisper throttle response without risk of detonation or ping, and it changes the feel of light-throttle driving quite dramatically. Having driven cars with no vacuum advance for years, I think the best way to explain it is this: A car without vacuum advance feels like it's an ill-tempered beast, always complaining and sometimes even stumbling or stalling at inopportune moments - until you step on the gas. Then it's all joy. Adding the vac advance has all the joy, but adds manners too, even when puttering around in traffic, or on a Sunday drive.


If you set the mechanical advance right, then you've set the perfect timing for Wide Open Throttle. Stepping on the gas brings you back to that baseline, taking away all the vac advance, no matter what's going on. Adding the vac advance just means you're putting in more timing to adjust for the slow-burning mostly-vacuum mix you get when you're not WOT. That need doesn't change when your throttle plates close completely - so my vote's for Manifold Vacuum, not Ported.
 
#14 ·
... Typical... Pooched at step one...

I tried to get a measurement off the balancer, but can't get an accurate one. It's 'about' 6 inches... I saw a part number on it, so I thought - that's great - I'll just write it down & search it out, then I'll know for sure/exactly what it is... WRONG AGAIN! I suppose I could try to beg wifey's forgiveness & take her seamstress measuring tape that is flexible & wrap it around, get a circumference & do the math - hopefully I can clean the grit off her tape...

Does anybody perchance have reference material that can tell me what the balancer is from this number?

C8ZE 6316 B1 B7

Also, I found a timing light that has a built-in tach & will allow you to input the advance degrees you want when checking the mechanical advance so that you're still aligning to the initial timing mark. This would mean I don't need to tape/mark in the 34* range, I just need to make sure that the TDC mark is correct. It's about 140$. Might be worth the investment...

R.R.
 
#17 ·
Although I think you are onto something (timing related) there is something that jumped out at me.....fuel (about 50%) in the tank from last summer and seafoam.....


Ok, Seafoam is not really good for most everything/anything IMHO....I can go into detail but it contains a variety of products that although will help with fuel turning bad it can/will create issues as well...including destruction of gaskets, etc. in the carb and impact (negatively) gasoline burning characteristics...….


Fuel begins turning bad at about 6 months.....earlier if you are "lucky" to have the California type blended fuels. IMHO, although the timing may very well need to be addressed.....I would 1st siphon/remove all the fuel from the car, fill with fresh......now you know you have a "clean" base that you are working from......you might also consider adding about 1/2 oz per gallon of acetone.....this will "convert" and "clean" most any of the residual, clean the carb a bit (without damaging anything) and burn about a 1/4 tank of gas through the engine before addressing the timing side...this will clean any small valve related residues as well...….
 
#18 ·
Hi All,

I had a look at that video... 14 minutes long... I didn't catch anything about how to set the length of the bolt for TDC. Maybe I drifted off... :smile: I still don't know if I have a 289 or 302 for sure yet, so that would definitely be a factor in that bolt length I would expect?

Whether I mark the balancer myself, or buy pre-fab tapes, I still need to know the correct diameter, right?

The Seafoam went in this spring. Stabilizer went in last fall. The car has had this problem since I got it two years ago. It was doing it last summer when I had fresh gas in it. I know there are arguments for/against Seafoam, but I figured it couldn't hurt to try it to clear out the system. Stabilizer - I don't have any choice - winter is a solid 6 months long here, so the car is stored. From all accounts, better full tank than empty for storage. I don't drive it much (yet) since I can't trust it enough yet to go very far - dependably, so if I'm lucky, I put maybe 2 tanks of gas a summer through it...

I'll wipe down the balancer the best I can & get wifey's tape out... :laugh2:

R.R.
 
#19 ·
You could get a timing light that has the electronics in it that sets the advance itself, like the INNOVA 3555. That way you just mark correct TDC and forget about damper diameter and markings.
 
#20 ·
Rob, a bolt in the cylinder won't show you the stroke. A dowel small enough to fit in the spark plug hole works better.. You put it in, and let it down to the lowest point, then make a mark on the dowel. Then turn it till it can't go 'up' any more, and make another mark. A 289 has a 2.87" stroke. 302 has 3 inches.

The 'piston stop' doesn't measure the whole stroke - it just stops the piston at the same place on each side of the stroke, and the spot in between tells you where TDC is. I'm not fond of them. The all-metal ones can mar your piston, and the ones with a rubber stop can fall off inside the cylinder.

Seafoam is not very good for anything. It is 'snake oil'. It usually doesn't have much effect except to make a lot of smoke, but I've seen it fool very experienced mechanics into trying it out. If your engine ran a tiny bit better after you used it, you'd say that the Seafoam worked! If it failed in some way or ran worse, you'd likely just figure the engine was too worn out for the Seafoam to do its job. Most people I know that have used it justified it by saying "Well, it couldn't hurt!" but that's like giving $100 to a street corner huckster that says they won't spend it on booze and drugs.


The real truth is that Seafoam is irrelevant. If your engine runs bad, you fix it. If it runs fine, you leave it alone. Seafoam doesn't enter into that equation.


Stabilizer, on the other hand, forms a barrier over the fuel, like an oil slick. It keeps oxygen from getting to all the eagerly reactive chemicals in the gasoline, and helps it last longer.
 
#21 ·
The length/depth of the TDC bolt is set so that the distance between the #1 piston contact points is not very large when marking the dampener in a CW and CCW rotation. My first attempt at setting the bolt depth resulted in a distance of about 3" on the damper. I considered that too big. I backed the bolt out a turn and tried again. This time the distance was about 1". Good enough. Marked the mid point and now had TDC on the damper known. I took a punch and made a stamp on the face of the damper for reference. Next, I cleaned the damper and installed the timing tape. It's been a year and the tape is still good. If it comes off, I have the stamp mark to install a new one in the future.
 
#23 ·
Update...

So I got my timing light with the tach & advance feature on it. I got a chance yesterday finally to take it out to verify/set the timing. Here's how things went down:

I got the engine to operating temperature. I disconnected/plugged the vac. advance line, set the base timing to 6*btc & was idling about 650rpm at that point. I ran the throttle up & it looks like the mechanical advance stops at around 2750rpm. I set the timing for 34* & locked it down. The idle was up a bit - I adjusted it to about 800rpm. There is a seemingly weird thing with that though. As I adjust the idle screw I'd have to give it a little throttle & let it go for the change to take. I was finding that even not touching the idle screw, each time I'd give the throttle a bump, it wouldn't always return to the same rpm. I would be up/down as much as 200 rpm. Is this an indicator of carb trouble? To me, it should always return to the same rpm.

In any case, I adjusted the vac. advance back to 'where I found it & re-connected the hose - no change in timing. I ran the adjustment in/out reconnecting the hose after each adjustment & still no change in timing... Very weird. Not sure what to make of that.

I ran the throttle up a few times, no ping/hesitation or anything, so I took it for a ride. Performance is a lot better in that it is smooth all the way through acceleration. The other thing that was happening where holding a speed would eventually feel some roughness also went away. I am still however, experiencing the odd hesitation or falling off when goosing the throttle. I'm thinking this, combined with the varying idle speed is telling me that there are carb issues...?

The other thing I did last week when trying to prove TDC on the balancer was to use a pencil in #1. It was the best thing I had around to use in lieu of a dowel. I'd roll the crank & mark the pencil at TDC - which did line up to 0* on the balancer. Roll the crank some more - almost lost the pencil in the hole. The plugs go in on an angle, so not sure how accurate that depth measurement is at that point... It's over 3" - which can't be right. SO - I proved TDC, but still don't know if I have a 289 or 302 on my hands.

I welcome any thoughts on my results...

R.R.

:smile:
 
#24 ·
The reason you have no advance when you connect the hose is that its probably hooked to ported vacuum, try connecting it to manifold vacuum. After you set your timing to 34* did you note what your timing is at idle? The reason the rpm may be moving around is that you need to reset you idle a/f mixture screws on the carb.
 
#25 · (Edited)
After you set your timing to 34* did you note what your timing is at idle?


You beat me to it.


Rob, you stated you set up initial at 6* BTC but there is a big chance there was a little change on initial timing when you locked the dizzy at 34*. Once you know the actual # you can calculate mechanical. If you used the right port on the dizzy and there was no change on idling the canister is probably done. Question on the carb, do you have a good solid throttle return spring?These wear out over time and you have to fiddle with the thing to bring back down the rpms.



J
 
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#26 ·
Hi Guys,

I did not note what the timing was at the end of it all. My timing marks are hard to see. I had to use a sharpie & dot the TDC & marked a line at the 6* mark - otherwise very tough to read the marks. I'll try to note that the next time out though.

I'll also try to move the vacuum line as well. I know the diaphragm on the advance is good, because it moves if I suck on the hose (checked when I had he distributor cap off before).

The throttle seems to return pretty well/strong. Where do I adjust the A/F? is that on the base of the carb? Will resetting that also help with the hesitation on hard aceleration?

Thanks!

R.R.
 
#27 ·
Rob, i don't know much on the Autolite 4300 other than it must have screws to play with the air to fuel ratio, but i'll advice you to buy a Vacuum gauge to help you figure the ratio out. Very inexpensive tool and it will make wonders to your engine. The gauge can pretty much give you a diagnostic of your engine. There are plenty of articles online on how to read inches of mercury on the gauge. Find a nice intake manifold port and plug it in there, crank up the car let it warm up and start playing with screws.


found this site by doing a quick search https://www.onallcylinders.com/2016/03/31/vac-visual-quick-guide-vacuum-gauge-readings/




J
 
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#28 ·
Right on. Thanks for the link - it looks good. I'll find out about those screws before I start tinkering. I agree a vac gauge is worth the investment.

The other thing I was thinking is I can use the 'advance' feature on the timing light to figure out where my timing is at now (d'uh!) so I don't have to struggle with trying to read the marks...

:smile:

R.R.
 
#29 ·
A pencil works just as well as a dowel. Kudos for being creative. You just don't want to use something metal that will scratch the piston or score the cylinder walls.

Again it's very important that your distributor is plumbed to full manifold vacuum. That is high at idle and cruise and should add 15 to 18 degrees at idle. Plugged up, your initial timing at idle should be in the 6 to 10 range. THEN add in vacuum and total advance at idle should be 21 to 28.

After you set the timing then adjust the two carburetor idle mixture screws. Turn one in clockwise until it gently bottoms, then back out 1.5 turns. Then do the other. The idle at this point should be smooth and steady. Then turn both clockwise in 1/8 turn increments until the idle gets rough. Then turn back out 1/8 to 1/4 turn and you're done.

If your idle does not return to a consistent RPM then first check the dashpot. This is a little damper that acts against the return spring in order to keep the engine from stalling when you release the throttle. I've always had problems with them and have routinely removed them, or adjusted out of range so they don't act on the mechanism. Next is the return spring, but it sucks to have to have a big strong spring on there.

Lastly, on these old carbs the steel throttle shaft rides on the pot metal casting and will wear an oblong hole in it, especially on the linkage side. Test by removing the spring and moving the shaft back and forth. If the casting is worn it can cause the shaft to stick. It also creates a spot where unfiltered and un-metered air can enter the combustion chamber and that can screw up your idle, mess up your air-fuel mixture and cause wear in the cylinders. There are creative solutions to fix that casting but it is far easier just to buy a new carb.
 
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#30 ·
Hi Yadkin,

Thanks for clearing up about the A/F screws. I'll move the advance hose & see what that does for the advance. There's no dashpot on my carb, so someone already got to it. Looks like I've got some more work to do. All good! :grin: I'm making progress!

R.R.
 
#31 ·
... Update...

I reset the screws at the base of the carb. I also verified my idle timing. With the vac. advance hose disconnected/plugged, I'm at 16*. Unfortunately, my vac. hose wasn't long enough to get it to the manifold vac. port, so I'll have to try that one once I find my hose stretcher. I did connect the hose back to the dizzy & sucked on it & saw the timing come up & the idle pick up, so I know it'll work once I get it ported.

Also, I do get a bit of 'side-side' wobble on the carb. shaft if I wiggle it. Not a lot of slop, but enough to notice. It kinda 'clicks'. That could be why the idle doesn't return to the same RPM every time, right?

When I take it for a run, it's a lot more responsive & haven't really felt the hesitation on acceleration, so it IS getting better!

I'll post again when I have more results.

:grin:

R.R.
 
#33 ·
16 initial does sound a bit steep - but the real important thing would be your *total* Mech. advance, because at WOT that's when your engine's making enough 'bang' to wreck things if detonation starts. A rowdy cam and stock heads might want 40 degrees or so, but most stockers would want your total timing (no vac, but fully advanced from RPMs) to be around 34-36. With good heads like the GT40P you might even be down around 29 degrees BTDC for total advance.


If you're just a little bit retarded (and by that, I mean your IGNITION TIMING, Rob! *grin*), it'll hurt power, but it won't damage the engine beyond possibly sooting things up and making your exhaust hot/bad mileage. Too far advanced will kill power too - along with your rings, valves, pistons, rods, and bearings.
 
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