The Engine that just won't behave !

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 !

Retro Jim

 

Have you tried both firing order?

 

Maybe it has a HO / 351 firing order cam in it? Or if it has a dual plane intake it could have one booster plugged in the carburetor.

 

the 351 W firing order...not a C or M the W

 

Yes and if the firing order was wrong then it would run like shit and backfire on a continuous basis .
It idles smooth and picks up fine but I get that damn backfire out the drivers side ONLY every now and then . If it would do it all the time that would be one thing but it doesn't so it doesn't have the 351w firing order .
It's a 1971 --- stock 302 .

Retro Jim

 

I have tried a few different carbs !

Retro Jim

 

Some of these motors had adjustable rockers, others have positive-stop rocker studs and are not adjustable. Disassemble one of the rocker assemblies to see which type you have. Find a good/old Chiltons or a service manual to help you determine which ones you have. If they are adjustable double check the adjustment. If they are non-adjustable then double check the valvetrain geometry and lifter pre-load to verify that the push rod length is correct (enough). If heads or deck was resurfaced you could have issues with pushrod length.

Do a compression check (or better yet a leak down test) to see if there are any differences. This may help you narrow down to which cylinder(s) are giving you problems.

Burnt valves and liifter issues can sometimes act sporatic like you're describing so don't count them out until you've positively ruled them out. You may have to pull the driverside head to make sure everything is OK.

Don't rule out a vacuum leak at the intake to head surface....exterior or interior. Also, if the carb is sitting on one of the fat hollow OEM spacers check both the top and bottom gaskets closely for leaks as I've had them suck the gasket on more than one motor.

 

Have someone else double-check your firing order. If it is off it will still start and run,but could very well only backfire when you give it gas.

BTW,how good is the timing chain? A 1971 302 with a 1971 timing chain maybe quite a bit sloppy by now.

 

Open headers...burnt valve? I have also had 3 condensors in a row fail, all new. Try another new condesor.

 

I have been through 2 sets of points , condenser , dist. caps , different plug wires , 3 different coils and 2 different carbs ! The engine is on a test stand so the distributor is hook straight up to an on/off switch so there is no wiring in-between to worry about ! I also have tried 2 different resistors for the distributor and even tried it with out a resistor.
Couldn't tell you about the chain since I wasn't the one who rebuilt that engine . The engine has signs that it was totally apart at some time not long ago since someone used silicon for some gaskets . There doesn't seem to be much play in the crankshaft when I do move it and check the rotor as it moves to see if there would be slop in the chain but even if it was a sloppy timing chain it wouldn't be back firing through the drivers side Only and not on a continual basics . That is what has me puzzled . It's almost like un burned fuel loads up in the headers and then decides to fire off . similar to when we use to be able to turn the key of driving down the road and start pumping the gas peddle then turn the ignition back on and get that BANG out the exhaust !
I am going to do a compression check and double check the TDC again . I will post the results later tonight .
Thanks again for the help .

Retro Jim

 

Make sure #7&8 plug wires are not touching each other at all. They can "cross fire" if they touch.

 

Exactly...this is a known problem on Fords. Make sure your wires are good....and not only should these 2 not be touching each other, make sure they are not routed in the wire loom next to each other.

 

unplug one wire at a time and see one is fouling.

 

Maybe an exhaust valve is sticking open occasionally?

 

Yeah....and alot of those had plastic teeth on the cam gear that would magically make the journey to the oil pan.

 

Have you checked your intake manifold and gaskets for a leak on that side?

 

how stock is it.is it a 2 or 4 barrel carb.maybe jam some mufflers on it and try it.might not have enough back pressure with open headers on a stock engine.

 

Seeing its on engine stand how easy does it get ,I,d pull the head and check valves and head gasket.

 

Loco Breath has it I think. Drizzle some A/T fluid into the carb at
a fast idle to clear up sticking valves. It works!

 

I've seen cam timing off cause similar issues. Too advanced and it will pop out of the exhaust, although usually both sides. Through the carb is cam timing is retarded. 1 tooth can be around 15*.

Just a thought.

 

It had mufflers on it ! How you been doing anyway . haven't heard from you . I will be at the HAMB Drags OCT 1 . You going again this year ?

Retro Jim

 

I did a compression test ad this is the results . Now we know the story !


Well I did a compression test on all 8 cylinder to see if that would help any . Well I thought the engine was rebuild not long ago and I am right .
Here is the cylinder results .
#1--150
#2--150
#3--150
#4--150
#5--140
#6--150
#7--130
#8--150

All spark plugs are clean with no carbon .

I would say it's didn't have a valve job or a terrible one done to the heads ! Maybe a bad guide , maybe a bad valve , bad valve or a bad seat IF they were replaced . I also think the intake gasket is leaking as well .

Jim
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hey jim,
planning on going.i`ll have to stop by soon and see you.i hope you get your issue figured out.are all four tubes cool or just one?

 

It's really weird . The passengers side headers get hot like normal but the drivers side stays cool and even after a min or 2 of running I can actually touch the drivers side header and not get burned right away like the other side . All 4 tubes are the same . I can also put my hand behind the exhaust and the passengers side is hot and the drivers side you can hold your hand there as long as you want to . I have never seen anything like this before but I didn't build the engine either . the backfiring comes from one cylinder and when I took the compression test I knew I would have a problem with that cylinder . Someone has rebuilt/freshened up this engine at some time but not long ago . the guy bought it and I am finishing the project for him . it's on a frame with a radiator and a main panel with gauges and everything . He just had the engine , balancer , coil for me to build from . He did pay much for the engine and it's a decent engine except for the problem .
So he bought everything I didn't have and now I am at thins point .

Retro Jim

 

Have you tried a different dist?

 

its running on only one bank of Cylinders?

A compressor? air pump.. maybe? do you know any history on where it came from?

 

No , I didn't try another distributor . Don't have any but dual points right now . I did check the distributor when it was out and everything felt OK with no slop play in it . I am thinking it's a bad exhaust valve or seat . I checked the springs for breaks or any problems and they all looked OK . I think the intake is leaking some but the drivers side head has a problem . All the cylinders are 150 or 140 but the one is 130 and that was the cylinder I thought had the problem .
I am still open to any ideas someone might have .

Jim

 

I would be surprised if one cylinder being low compression would make the whole header cold.

I have run across an odd one though, sort of sounds like what you have, there are two plug wires you can switch on a 302 that will let it start, run normally but won't make as much power under a load (only under a load) and will backfire occasionally.

I have a friend that used to do that to people's cars if they left the hood up, he told me about it and I thought he was full of it.

Then I accidentaly did that to myself working on the model A in my avatar (it has a SBF in it).

Took me a bit to find it, would run great on the stand, or while parked, except it would bark once in a while, then when I tried to drive it, it was gutless, after troubleshooting (fuel pressure, filter, carb, timing, air filter, and several others I don't remember at the moment) I remembered what said friend had told me and rechecked the firing order, sure enough two were swapped.

Swapped them back and all was good.

 

No the engine does run strong until I keep it at a steady RPM then the back firing will start . It idles good and sound good when giving it the gas but keep it at a steady RPM or idle and there is goes ! The passengers side bank header gets hot like normal and the exhaust is hot . The passengers side is cooler and the header is cooler . ALL 8 cylinders are firing but something is very wrong someplace . I have set the timing , adjusted the carb and even swap out another carb and still the same thing starts . The only thing running on the engine is a alt. and nothing else .
Yes it will run a compressor .

Retro Jim

 

flat cam or valve to tight