Technical backfire under load

Ok I have begun to have a backfire issue when I extend acceleration in first and second. (3 speed on the tree). It only happens after the engine has heated up to 170. Before that it runs great. After choke open it runs really strong. This is a 1962 327 with a Holley 600 CFM, Pertronix ignition. I have checked timing (4 degrees above). Wiring on distributor is correct. It has an electric choke and seems to be adjusted correct and is open all the way after engine warms up. I have checked the float and it is correct.

I saw that the radiator hose was touching the fuel line so I realigned thinking it might be creating a vapor lock but no help there. I understand a lean condition can cause issues like this but The car ran great a couple of weeks ago. The only thing I have done under the hood is change the radiator. Before that I was having an issue with the engine dying when I turn left but I found there was no ground from the engine and I installed one and that corrected the dying problem.

Any ideas?

 

Fouled plugs or bad plug wires , cap & rotor in other words , a weak spark ..

 

I checked the plugs and they look fine. No fouling there. I guess it could be wires but the car runs fine unless I push it beyond a normal take off from dead stop or extend second gear. Maybe opening the back two on the Holley.

Also remember while the car is warming up it runs really strong. Even when I open up the back two on the Holley it pulls really strong and engine sounds great.

 

Are both advance mechanisms in the distributor working correctly?

You didn't by chance kink or twist the fuel line when you realigned it? My experience with a fuel line restriction on a 327 was that the carbs ran out of gas on hard acceleration and the bowls filled up when I let off and shifted and then ran out of gas again when I topped out ins second. That was 4 WCFBs on a Corvette intake with an engine with an Engle cam 12.5 pistons and 2.02 heads. No popping back just running out of gas until I let off.

I'd be checking for the little things though. Loose wire or connection inside the distributor. Check the screws that hold things down to make sure that they are tight. Check any ground wire inside the distributor. Make sure that the top of the coil is clean and doesn't have anything that might cause a spark jump issue. Check inside the cap for carbon tracking. Check the contact for the rotor inside the cap. We always hunt for the big things but forget to look at the little things.

 

Backfire,,,,,,through the carb ?
Tommy

 

yes backfire through carb. Only after the engine has warmed up. Again, before that the engine runs great. For maybe 10 or 15 miles. Great acceleration and engine pulls hard. Then when the temp gets up about 170 if I push the engine I get the backfire. One other thing I put a fuel press gauge on and the pump is maintaining 6.5 psi.

 

Car runs fine when cold (choke shut)... the richer mixture may be hiding a lean-out in the power or accelerator circuits. Choke opens, mixture goes lean, and the lean-out becomes evident.

Does the miss occur when you initially mash the throttle (accelerator pump), or a little later, when the engine spools up (secondaries)?

To check, frig the choke shut (or partially shut) and see if it still does it.

Cheers,
Harv

 

Solid or hydraulic lifters?

 

hydraulic

 

I agree with Harv wire the choke a little shut and try it!

 

Hmmm, then it won't make any sense to suggest you might have an intake valve or two with lash set too tight.
Resetting valves cured the same symptoms you're having on a solid lifter motor I had awhile back.

 

Lobe wear on your camshaft?

 

Exactly what I'm thinking. I hope not, but ...

 

Find someone who has an ignition scope and knows how to use it. You can eliminate any ignition problem in less than 15 minutes with a scope.

 

Spark plug wires performance fail under heat and load. Car could idle beautifully, rev out in neutral like it's fine, and crap out under load...especially when the engine is warm/hot.

 

Warm engine, intake manifold or carb gasket leaking vacuum .

 

Check very closely for an intake manifold leak.

 

Do a compression check

 

Jack up the radiator cap & put a new car under it ..

 

20210125_125532 by genebrownlow posted Feb 4, 2021 at 5:41 PM

I would hate to do that....

 

Beautiful 57!
My wife´s 53 Suburban had a backfire through the carb under acceleration, it also has a 327. It had a flat lobe on #1 exhaust valve.I´could still kick myself for using a cheapo enginetech cam.Finally the worn lobe wouldn´t open the valve enough to vent the gases through the exhaust, so it popped back into the intake and through the carb. I cross my fingers you ´ve got a different issue...

 

Does it do it when its warm but sitting still and reved up..............or only under load and reved up?

If you can replicate it sitting still, I would wait till its good and dark and rev it and see if you can see any spark issues when its dark. Could just be a bad ground connection. When it does backfire, is it continuous or kinda random?

 

I too vote for a flat cam lobe.

 

A lobe can be easily detected if your needed to adjust one often or by looking a valve movement with the covers off. A push button cord really helps and no ignition needs to be on.

 

So, a little more info please.

Has it always done this?
Is this car new to you or do you know the history?
What has changed before this started? Any recent maintenance?

 

OK found it! I always seem to make it more complicated than it really is.. ACTUALLY was thinking it had to be something related to one of the 5 or 6 things I had done in the last few weeks because it just started recently.

However, the troubleshooting trophy will be shared by..... 2OLD2FAST (even though he wanted me to just keep the radiator cap and install a new car!) and Black Panther..... Thanks guys for reminding me to look for the simple things first.

The wires had just been replaced right before I bought the car but must have been on the cheap side. Change the plug wires and it runs great again.

I did like all the suggestions and I am sure if I had given more or better information you guys would have zoned in on the problem like 2OLD2FAST and Black Panther. Thanks for all you input. This is a great website because of you guys!

 

Timing


Sent from my iPhone using The H.A.M.B. mobile app

 

no sir.... plug wires.....

 

You got it buddy! It is always best to start with the easy stuff first. People talking about cam lobes on this thread confuse me..if it was a cam lobe it would be running crappy always...the temperature wouldn't matter. If the lobe is flat or going south it will always be. A problem that occurs at temperature instead of a cold engine, will most likely be electrical...second option would be to look at the fuel system. That's been my experience..

 

Interesting comment. I get it, start with the easy stuff first. However a cam lobe that is just starting to disappear is going to behave differently than one that is flat or nearly flat. An exhaust lobe that is completely worn flat or nearly flat will behave like a broken rocker arm and backfire with a regular rhythm. An exhaust lobe that is worn just enough will only misbehave under acceleration and can be temperature sensitive as well. I diagnosed and replaced dozens of camshafts in late 70s and early 80s Chevy smallblocks and saw a few that behaved that way.