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This "popping" has me stumped. Any help? Been an all summer thing.

2K views 11 replies 7 participants last post by  Ivy66GT 
#1 ·
So I posted on here about a month ago about my 1968 Mustang with a rebuilt 302 in it. I've rebuilt the carb twice. New distributor, new pertronix ignitor (my problem was still there with the points) new cap and rotor. Already did a compression test. Did the spraying around the intake for possible vacuum leaks. New fuel pump. New fuel filter. Vacuum lines are new and running to the right places. Timing is set, checked with a timing light. So here's the problem. Once it's started and warmed up, it runs perfect. It will sit and idle there all day long. Put the minute I rev it a little bit, it pops through the carb with a puff of white smoke, not the exhaust. Not a backfire, a pop. I took the valve covers off both sides and everything is working like it's suppose to be.

Now here's a kicker, I put my vacuum line off of my distributor (I don't need it with the new pertronix) and put it in the overflow hole on my carb just to get it out of my way one night, and I heard the rpm's change. So I said to heck with it and drove it. And it didn't do the popping near as much. But as soon as you take that vacuum line out of that overflow hole, you can hear the change in rpms. So the other night I tried driving it 10 miles to the nearest gas station. While I'm accelerating it makes this popping noise hit and miss. But once I got it up to 55-65 and kept it there, it ran like a dream. But if I tried to push the throttle down or get into it, it starts popping again. But as long as I keep it at a constant speed it stops.

Does anyone have any ideas?!?! I've been chasing this problem all summer. And I just put an entire new front end under it, and the trip to the gas station was the most I've really been able to drive it. Hopefully someone has had this happen (sorry to say that lol) or has an idea of what is causing this. Any help would be appreciated as always. Thanks guys!
 
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#2 ·
Did you trouble shoot the carburetor?
 
#5 ·
Motor was rebuilt. New valve springs. I've even taken the valve covers off, everything is working fine while trying to start it, and while running. No busted springs, no bent push rods, and I even took a hammer and a small block of woods and hit the valve springs. No luck :(
 
#6 ·
Expalin what an 'overflow hole' might be? I have no clue what you mean.

Also, a Pertronix does not change your timing in any way. It simply fires the plugs; any advance mechanism originally on the engine still has to be used.

Your whole problem still sounds to me like terribly screwed up timing. If its not ignition timing, then your valves could be out of time from an incorrect timing chain installation. Anything coming back out of the carb is from burning charge or exhaust coming out through an open intake valve.
 
#7 · (Edited)
Popping up the carburetor on acceleration, is usually, a lean condition caused by not enough accelerator pump shot. And as Ivy66GT pointed out, it can be ignition also, as when you accelerate the voltage required to fire the plugs increases dramatically, so you need to check the point gaps, spark plug wires for resistance (no more than 1K ohm per inch of length), etc. ALso double check your firing order on the spark plug wires, I have seen many times someone gets 7 and 8 switched. Post your results. Good Luck.
 
#8 ·
Rex, he got rid of his points for a Petronix so no points gap issue. All of the running lean would be valuable to check out for popping back through the carb. I'm also guessing the popping has to do with timing. Like as has already been mentioned, the Petronix just fires more reliably than points. You'll still need your vacuum advance line from the carb to the distributor advance. My initial timing at idle is just about the same with the vacuum disconnected from the distributor and plugged and with the vacuum line connected. I do know that vacuum advance and centrifugal advance work together over the range of RPMs and I'm guessing your popping comes because of a timing problem..maybe because of no vaccum advance. That's just a shade tree guess that I would take a look at with my own car with that popping issue.
 
#9 · (Edited)
Almost all old Fords use ported vacuum for the vacuum advance, which means they only have vacuum advance when the throttle is opened up some, they do not have vacuum advance at idle, the only exceptions to this is when they came up with the dual diaphragm distributor for emissions, and they use a combination of retarding the timing and advancing it with the emission controls on the vehicle. The I6 with the load-a-matic setup might be a little different as it has no mechanical advance. I think I meant for him to check his spark plug gaps (.032-.035"), and not the point gap. Just rebuilding a carb, does not mean it is working properly, due to design of the venturi's, the air bleeds as well as most of the fuel passages go through them, so if you had this problem before rebuilding the carb, you very well could still have a problem with the carb. Good Luck.
 
#10 ·
When you talk about the "overflow on the carb", do you mean the spark ported vacuum outlet by any chance? On Holley carbs this is a fitting on the choke side of carb, just above the idle mixture screw on that side. If this fitting is not blocked it won't cause a vacuum leak at idle, but you will have a leak when driving. Also if your new distributor has a vacuum advance, it should be connected to this port. You need to set your timing at idle without the vacuum advance connected (and the carb port blocked), and it should be around 9deg at idle.
 
#12 ·
I am not sure how you would make that hose connection without removing the air cleaner, but, if so, that means there is a huge vacuum leak once the throttle is off idle. Extremely lean mixtures from a vacuum leak can cause backfires just like timing too far advanced.
 
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