Technical Tips > Engines - Carbies - Exhausts - Dissys

Exhaust Popping?

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Matt G:
Hi guys, just curious to understand a phenomenon that happened recently. My ‘70 was a little harder to start than normal (it had been a while between starts...) and when I did get some life, it settled down to an pretty normal idle, however it was popping (almost more like a backfire) out of my right side exhaust pipe. Frequency was probably every second or two. I parked it back in the garage for a later check/review.
Today I pulled all the plugs and the right side (cyl 2/4/6/8) were pretty even, but all were wet/oily. The left side (cyl 1/3/5/7) were a little carboned up ( similar to the right) but mostly dry.  I gave them all a good clean and loaded them back in. I figured that it was running too rich, so I checked my mixture screws: FL @ 1-1/8 turn from seat), FR @ 1-1/8, RL @ 1-1/2, RR @1-1/8. I screwed in the RL in to 1-1/8 turn, figuring that it may have worked it way out a little, but didn’t expect it to change the heavier fuel load on my Right hand side.  I check all of my spark leads and they looked perfect.
Started up okay, however again a little harder than usual. When started, no exhaust pops/backfires were noticed.
Any ideas?

sirfixalot:
Sticking valve?

bfit:
 Two saying come to mind when it comes to  motor vehicles

If it ain't broke, don't fix it

 Guessing is cheap. Guessing wrong is expensive.
Bfit 

Matt G:
I think an intermittent valve seat might be close to the problem (after deep thought an many beers). The bit that has me confused it why most of my right side cylinder bank had similar spark plug characteristics.  If it was one valve (logically my exhaust valve in this case) then wouldn’t only one cylinder show different characteristics?  It’s highly unlikely that it’s all four cylinder 2/4/6/8.
I overheated my ignition box (MSD 6A) back when we had the 46deg day, and she stopped until the flat bed delivered her home (and cash was exchanged 🤬), but  I can’t imagine how it would create a cylinder or bank specific issue with firing.

Assuming it’s a valve seat or sticky stem seal, is there an easy way to test? I have a braced roller rocker train, but I could test clearances if valve/rocker adjustment is the key problem.  I can’t get a cylinder pressure tested in behind my extractors to test variable pressure. Can I buy a screw-in pressure tester? Even better if anyone knows of an electronic logging version...!!!
Any suggestions on how to test for valve stickiness?
If you pm me I can send a video, pics, etc.

Scott:
Check exhaust runner temps with an IR temp gun?  Might narrow it down to which cylinder is playing up.

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