Ford Mustang Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
503 Posts
Discussion Starter #1
Hi all,

66 coupe, 289 bored .030 over, mild Edelbrock cam with Edelbrock lifters, intake and 500 cfm carb. Has probably 2k miles on the rebuild if that.

The car sat for the better part of a year in my garage while I did work to it, installing an electric fuel pump, subframe connectors, disc brakes, etc. The basic problem goes like this: I'll fire it up, it idles high with the choke on, but as it warms up and the choke opens it will sputter and die. More often than not it backfires thru the carb spraying some fuel around the bay. If I give it gas to try and keep it alive it sort of works but still dies and sometimes backfires out the exhaust (loudly).

I've been playing with the timing and a/f adjustment screws on the front of the carb, but to no avail. Today I changed all the spark plugs and they were carbon fouled like it's running too rich. Same problems as before but it ran a little better. I sprayed some carb cleaner around the intake/carb area to see if I had any vacuum leaks and sure enough when I sprayed between the intake and valve cover on the pass. side it revved up (repeated this a few times, same result). So I'm going to pull the intake and replace the gaskets.

I know I have a few problems, but need some direction. I think the vacuum leak is affecting my timing, etc. and making it backfire thru the carb. Improperly adjusted a/f screws making it too rich and backfire out the exhaust?
 

·
Registered
Joined
·
2,355 Posts
Vacuum leak will cause all kinds of massive problems. That particular intake has a pretty skinny seating area for the gaskets, and the first place I'd look for a leak is actually at the base of the carb. It is possible that your manifold to head gasket on that side is sucked in too, of course. You can't tune around the leak. And yes, it can cause backfires, rough idle, hard miss - all kinds of weird things.

Once you get that fixed and sorted, then just grab your timing light, set it back to 10-12 degrees BTDC (or whatever your engine likes). Gently reset your A/F screws (Do not screw them in hard! They frequently get damaged by guys who don't know better) and set them - usually 1 1/2 turns out is right, but I don't know about your particular model of carb.

Once things are running, let it warm up and get off the choke, and then you can start tuning. Don't forget to disconnect and plug your vacuum hose while you set your timing.

Good luck!
 

·
Registered
Joined
·
503 Posts
Discussion Starter #3
Vacuum leak will cause all kinds of massive problems. That particular intake has a pretty skinny seating area for the gaskets, and the first place I'd look for a leak is actually at the base of the carb. It is possible that your manifold to head gasket on that side is sucked in too, of course. You can't tune around the leak. And yes, it can cause backfires, rough idle, hard miss - all kinds of weird things.

Once you get that fixed and sorted, then just grab your timing light, set it back to 10-12 degrees BTDC (or whatever your engine likes). Gently reset your A/F screws (Do not screw them in hard! They frequently get damaged by guys who don't know better) and set them - usually 1 1/2 turns out is right, but I don't know about your particular model of carb.

Once things are running, let it warm up and get off the choke, and then you can start tuning. Don't forget to disconnect and plug your vacuum hose while you set your timing.

Good luck!
Thanks for the help and guidance, I'm really hoping the vacuum leak is the main source of my problems. I actually replaced the paper carb gasket with a 5/16" composite one because the fuel was getting too hot in the bowls and causing hot start issues. I think I'm going to leave it bolted to the intake when I take it off, redo the intake gaskets, and then see if I have any more leaks when I get it bolted back on.

I think I'll check fuel pressure too. I installed a Holley "Mighty Mite" electric pump by the tank, it's only supposed to put out 5-7 psi but who knows.
 

·
Registered
Joined
·
2,355 Posts
It may be that your composite gasket is fine, but a 'thick' gasket is actually a really bad idea, because it offers more surface area for air to press in on, and tends to leak more easily. A half-inch carb spacer made out of phenolic, with two gaskets (above and below it) will usually work better in every regard - and frequently will add a few horsepower too, by effectively lengthening your intake runners. For street engines, a four-hole spacer probably works best. For a more race-oriented application, an open spacer is a better choice.

Of course, that's assuming hood clearance permits the spacer in the first place.

If you've checked, and the leak really seems to be your manifold gasket instead of the carb, then I think you're smart to pull the intake and not have to buy a new carb gasket though, as you suggest.

The fuel pressure thing really won't give you any grief unless you're overwhelming the float valves in your bowls - in which case, you often see fuel pouring out the vents, as your engine coughs and chokes. Sounds like you're on the right track to fixing this thing though! One step at a time, Jr! You've got this. ;)
 

·
Registered
Joined
·
22 Posts
Timing issue, points @ condenser, old gas, cam chain timing. I would pull number one plug, find TDC on the dampener, put the plug back in the spark plug wire rest it on the engine block, turn key on, twist the dizzy, you should see a spark at the plug. No spark, points and or condenser, or no current to coil. When the new cam was installed was valve timing checked, some cams grinds may required off set key ways. This could be as simple as old gas, or having spark plug wires crossed. I hope this helps....
 

·
Registered
Joined
·
2,355 Posts
If it *ever* ran right, with a timing set this new, unless you are hearing horrible grindy noises from the front or something crazy, I can't imagine you having valve timing issues.

Knowing that there is for sure a vacuum leak, just fix that. THEN if there are any other problems you can address them.
 

·
Registered
Joined
·
503 Posts
Discussion Starter #8
I think I found my vacuum leak :|

Apparently when I installed my intake (a few years ago at this point) the passenger side gasket must have slipped down some before I bolted it down. So while the gasket itself was the right size, it was sitting so low that it wasn't sealing the tops of the intake ports on at least 3 of the 4 on that side. Luckily I used some RTV on the coolant ports otherwise I probably would have had some getting into the oil. This probably explains why I had to keep messing with timing and the carb tuning to try to get it to run right. And why it was backfiring out the carb?

My plan now is to clean everything up, attach the new gaskets to the head first with some permatex high-tack and then install the manifold. Then I'll recheck for vacuum leaks and see what happens.

Thanks for all the help so far!
@dicer I know I have spark because with it running I get consistant flashes with the timing light and I have the magnetic pickup on the #1 plug wire. I even set the engine to TDC with #1 piston up top and rechecked that I had the distributor lined up with #1 on the cap, and rechecked firing order. Valve timing was checked before the engine even went back in, and I even used a degree wheel on the cam to make sure it matched the cam card.
 

Attachments

·
Registered
Joined
·
1,532 Posts
I good hint for reinstalling the intake, if you aren't aware of it, which I've seen posted on this forum: cut the heads off 2-4 long bolts the same thread as the intake bolts and screw them into the corners of the block before dropping the intake down. They will guide the intake to the right position so there is no danger of it moving after settling down before you get the other bolts in. I do realize that wasn't the case for your first install (it was the gasket that moved, not the intake). The High-Tack is the way to go.
 

·
Registered
Joined
·
22 Posts
Yes, it looks like the problem, good trouble shooting for sure. I did read you used a degree wheel for your new cam set-up, excellent. The extra steps insure a proper install. Nice tip on the intake Charles, will do it, if I ever need too. Keep us posted, t.mcginley.jr .....
 
1 - 10 of 10 Posts
Top