Got the buddys 68 200 I6 back together and vacum plumbed right. Retimed it too but still have a strange problem.

At idle the engine has a random slight missfire or backfire. The carb is one of those new "professional" rebuilds that he ordered. The distributor is new and even swapped it out with another to see if that was the fault but it ran the same. All plugs are firing correctly, everything seems cool but somehow it gets this random pop?

 

Mine does it and I am used to it.

 

I know allot of them do, mine however does not and its so quiet you can almost not hear it so I know it can be remedied.

I just wonder what the heck it could be?

 

Hmm mine doesn't do anything like that at idle.

Have you replaced your fuel filter? Is it dirty? Backfiring can be caused by low fuel pressure.

 

Does it have an aftermarket muffler? My grandfathers stock exhaust doesn't do it, but my no restriction exhaust does.

 

Pretty sure his filters are clean, I will have him buy a new one just to check.

Now his exhaust system isnt factory........low backpressure may indeed be a reason although I dont see how.

 

Fuel filter isn't it. I just replaced mine last week.


I don' know how it would, but it could be possible. I have a straight through style muffler on mine and that is the only exhaust mod. I don't know when mine started the popping noise though.

 

buddy of mine's 68 with a 289 does the same (at least it fits your description).
since it's a V8 and it's got straight through pipes without crossover, it was easy figuring it was coming off the left pipe (driver side).
When I pull #7 cylinder on that one it stops. drove me freaking insane as I changed plugs and cables, seperated them thinking it was crossfiring. I can't get down to it

just as you describe it the occasional pop ---- pop-pop -------------pop ...
Another thing I figured is that even though idle mixture is correct it makes this typical load poppiong on deccelleration that indicates to lean (at least in my experience), but I couldn't find any leaks

 

Maybe your valves were leaking in that cylinder. Did you do a compression test on that cylinder to see what was up?

Now this is wild and you'd think it impossible but I have had it happen......the plug may have not been grounding on that cylinder too. Might try hitting it with some brake parts cleaner.

 

I don't know how to fix it but if it makes you feel any better my 289 does the same thing.