Troubleshooting my 1970 Ford Mustang

I have a 1969 GT350 351W stock engine. Long story but engine runs fine at 1400rpm and above but at 900rpm it will blow back through primary’s every 4 or 5 seconds. I have done everything possible on the timing elements and valvetrain (carb, dizzy, lifters, pushrods, valve adj. timing chain, fuel pump, and much more),but cant stop the problem. Please help!!

Ok, “blow back through the primaries” means what? Pops into the header pipes near the engine?

If that is what is happening, the idle circuit is a bit too rich and the excess fuel is lighting off in the pipe. Mitght look for a leak in the header gasket as well.

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Or do you mean backfire through the carb primaries?

Yeah, need to clarify. I took it as backfiring out the carb primaries.

What is your ignition timing at idle?
Is this a stock carb or something more performance oriented?

Stock exhaust or headers??

This might be a case to contact the local Mustang club if there is one. Those people just live for the time when they can solve a problem or send you to someone who can.

Technically backkfire but no flame. Spits allot of gas when it does this. I’ve tried 4 carbs, 1 Holley 600 fresh out of the box. It’s a stock autolite now. It does have headers.

Or the ignition is out of time. Maybe because the timing pointer has moved because the rubber isolator ring on the balancer has slipped.

Or possibly an intake valve is burned. Compression tests OK?

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Timing is at 6btdc, its hard to get good idle because once car is warm it begins backfiring (coughing) through carb. Stock carb. Damper could be slipping but Ive moved dizzy all ways possible. Tried a MSD dizzy, same results, new stock dizzy, same results. I’ve pulled dizzy, verified TDC and re-installed dizzy at least 30 times. I know this is a logical timing issue but I’m out of ideas???

Hook a vacuum gauge to the intake manifold and see if it flips from a healthy vacuum to a pressure condition as the spitting back happens. If it does, I’m going to suggest there is something funky going on with the timing chain.

You’ve probably already addressed this but cross-firing is another possibility. Age of wires and how they are routed? They are better as a rats nest than ty-wrapped into a neat bundle…


Already checked and changed wires. Vacuum gauge shows low vaccum but again when you can’t idle car at normal idle it’s hard to check. I did put a plug in the rear intake port to disconnect all vaccum a cseries and it’s still the same results. Thanks everyone for all your help. Keep it coming!!!

intake bolts are tight? intake been off the car in the recent past? good gaskets?

Ok, stock intake manifold? Stock cam?

Yep all stock shelby equipment. Yes intake was off because I replaced lifters thinking one or more might be stuck. All bolts torked to specs and lots of rtv used on front and rear. Of course it’s possible for a leak there but I’ve checked numerous times. I’ve had front cover off 3 times checking timing marks for alignment. Very, very frustrated.

I realize I’m hearing hoofbeats and looking for zebras instead of horses, but I’m down to two thoughts, either you are over carbed, or an intake valve is hanging open on a compression stroke.

Is this a problem that just occurred out of the blue or after some change/repair was made? Just curious about the genesis of the problem.

Compression by cylinder? Worn timing chain?

Thanks mopar guy! I’ve had the same thought 1000 times. It has the stock ( rebuilt) carb now. I tried a new Holley 600 just to see and it did the same thing within 3 min of running. To the valve; I’ve done compression testing and leak down and I still come up with nothing. All results are with in spec. The valve still seems like the most logical. I have a new set of heads but frankly if I went through the effort to change them and I end up with the same result I might loose my mind!!

How old is the timing chain?

Comp came brand new.