Engine that likes to backfire out the drivers side ONLY !

Retro Jim

Retro Jim

Senior Member
I have a 1971 Ford 302 . I have replaced the points , rotor , condenser , cap , plugs , wires , new fuel filter, new header gaskets , have checked the timing many times , change the carb to a different one , have double checked to make sure the firing order is correct , made sure the distributor is top dead center 4 times ! I even tried 3 different coils !
OK here is the problem . While the engine starts fine and has plenty of pick up , the drivers side header is cold and the passengers side is hot like it should be . It backfires out the drivers side ONLY and sounds like a gun shot . I put my hands in front of the header exhaust and both have good flow and exhaust pressure out the headers collectors . It doesn't do it all the time but ever so many cycles of the engine . The spark plugs look fine on both sides for fuel mixture . When it idles or runs steady at 1500 to 2000 rpms it's OK but still backfires through the drivers side only every so often , but not all the time like it should when the timing is bad or have a bad valve or lifter problem .
So I pulled the valve cover to check the valve springs , push rods , rockers and all are fine . Everything is super clean inside !
When you start the engine I can keep reving the engine and it's fine with open headers until I keep it either at idle or a steady rpm . Then it will backfire out the driver side ONLY but only now and then and not at a continuous cycle or all the time .
This one has me puzzled to no end and I just can't figure out what the problem is . Please not I DID NOT REBUILT THIS ENGINE ! I know someone else has done engine work on it because of the new gaskets and it hold 60-80 lbs of oil pressure all the time so I guess someone used a HV oil pump .
I am open to any and all suggestions ! I NEED HELP !!!!! This is a first for me !

Jim
 
for the engine to back fire, out the exhaust, you need 2 things raw fuel and a ignition source, the changing rpm will hide the backfire as it changes fuel/ignition combo.

try pulling one plug wire at a time and see if the back fire goes away?

look for a burr or weld bb that could glow red inside the headers this could be the ignition source.

the cold header on one that backfire is very intresting???? this means lack of combustion fuel no being burned in the clyinder but burned in the header, why??

spray some carb (gas is better but watch for fire) around the intake gaskets there may be a fine hole (gasket did not seat correctly common on fords) in the intake gasket changing your mixture.

tell us the results
 
Boy, that is weird! It almost sounds like a valve timing issue. As far as the firing order goes, did you check it for your specific engine? As I recall there are different firing orders on early and later 302s.
 
The cold header is a big clue. Too much gas will cool them off right now and give the back fire thing.

If you have access to a therma reader ($70 at most HVAC sourses) get one and check your temps on each pipe on the drivers side and the passenger side. This will tell you if you have a vac/carb issue or it is isolated to one or two cylinders.

Vacumn leaks will give a false reading to the carb and tell the power valve to dump fuel. A valve(s) could be hanging open, do a compression test. But first verify the cam you have, HO cams use a different firing order then the standard 302. Try switching the firing order to the 351 order and see what happens.
 
It's a 1971 -- 302 so it isn't a HO /351w cam . If the firing order was wrong then it would run like hell ! Idles good and runs strong if you just keep revving the throttle but keep it at a stead RPM and bang it will go . It was doing it on both side once but I replace the header gaskets with new thick ones and the drivers side has no more back firing . Now only the drivers side does it .
I have gone through 2 different carbs , 2 dist.caps & rotors , 2 sets of points and condenser , 2 sets of plug wires . The engine is on a test stand so there is no wiring to worry about . It's a straight wire from the distributor to an on/off switch then to the battery . I have tried 2 different resistors at the distributor and even without one and doesn't make a difference . I have tried 3 different coils as well .
The difference in the temps on the headers really bothers me allot . The passenger side gets got pretty fast like a header does but the driver side stays warm . I can run the engine for a minute or 2 and the passenger side is hot and will burn you but the drivers side is warm and you can actually touch it ! It's hot but not like it should be . If I put my hand over the passengers side at the collector it's hot exhaust like it should be . When I put my hand over the drivers side collector the exhaust is barely warm . I can hold my hand there and not worry about it getting hot or burned at all . It's like the #5-6-7-8 cylinders are running but not making any heat from it firing .
I have been working and building Ford and Chev (sorry but they pay money just like a Ford engine ) engines since the late 60's and this is the very first engine I have had this problem with . It really has me puzzled . the main thing to remember is , I got the engine together ! I had NO part in building it at all . A customer brought the engine minus the carb , the balancer was missing but he had one in a box of parts that would work and the distributor was pulled out . I did find TDC with my finger over the #1 cylinder that way . Been doing it for years and always worked out fine for me . I installed the balancer to TDC . I dropped the distributor to #1 on the distributor cap . I changed the oil and filter , did a complete tune up and install a good 2 bbl carb I had . Now when the engine start it runs fine . Rev the engine and it's fine but leave it at any level RPM and It will backfire on the passenger side only and at no certain time . That's what has me puzzled . If it backfired all the time that's an easy fix / find of the problem . BUT NO it won't do it all the time like it had a bad miss-fire !
I am going to do a compression test on that side in a little while and will post what I find later tonight .
Thanks for the help and suggestions so far . I am really concerned about the cool air on the drivers side exhaust . That's not right and it's like it is burning no fuel to make heat but still running .
Of to the shop !

Jim
 
But......a rebuilt 71 from an unknown source.....verify the cam and try the 351 firing order. Nothing to loose switching a couple of wires on the distributor.
 
Use some carb cleaner and spray with engine running around carb and intake.
I have a feeling you will find a vacuum leak on the intake to head gasket union.
I had this on a 77 302 and found a leak on the passenger side intake gasket, where I had installed a new intake, cam, and carb kit, and peeled the gasket surface. that caused a small leak that was VERY hard to find.

GOOD LUCK
 
It's a 28 oz engine . It's a 1971 , 302 . Even if I did have the 50 oz it would make it back fire on the drivers side only .

I am going to try the carb cleaner on the intake tomorrow evening or Thursday .
I will let you know what happens .

Jim
 
Could also be a valve hanging or closing too slow (binding), but the cooler exhaust doesn't really seem to fit that scenario. The exhaust temp issue is where I keep thinking valve timing and/or firing order as Ford blue blood stated.
 
Make sure the breaker plate in the dist. isn't worn out and creating a huge dwell variation. You mentioned looking at the valve springs, double check that you don't have a broken one. My last suggestion is the cam itself. I have purchased in the last few years, new, brand name cams that were ground with a "bump" in the back side of the base circle, the engine idled fine, but with a little throttle the lifters would pump up and open a valve slightly at the wrong time. This used to be a common problem with a reground cam, but it seems to be showing up on new as well. You can check this with a dial indicator.
 
You say the engine runs great, but if one bank of cylinders is not getting hot there is certainly no ignition/compression going on in a couple of cylinders at least. It sounds to me like you have a couple of plug leads crossed. Easily done and overlooked. To be getting backfire out the exhaust you are obviously getting fuel into the cylinder but it is still burning in the exhaust.
You mention that you have checked all the basics, but my experience is it is always something simple.
Regards
 
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