Author Topic: Exhaust Popping?  (Read 221 times)

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Offline Matt G

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Exhaust Popping?
« on: March 08, 2020, 07:13:48 PM »
Hi guys, just curious to understand a phenomenon that happened recently. My Ď70 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 didnít 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?

Offline sirfixalot

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Re: Exhaust Popping?
« Reply #1 on: March 10, 2020, 11:31:28 AM »
Sticking valve?

Offline Bfit

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Re: Exhaust Popping?
« Reply #2 on: March 10, 2020, 04:29:06 PM »
 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 
Freedom's just another word for nothin' left to lose
George Orwell 1984

Offline Matt G

  • Four Barrel
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  • Location: The Hills District, NSW
  • Car: 1970 Coupe 350 manual.
Re: Exhaust Popping?
« Reply #3 on: March 10, 2020, 11:54:28 PM »
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 wouldnít only one cylinder show different characteristics?  Itís highly unlikely that itís 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 canít imagine how it would create a cylinder or bank specific issue with firing.

Assuming itís 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 canít 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.

Offline Scott

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Re: Exhaust Popping?
« Reply #4 on: March 12, 2020, 08:49:56 AM »
Check exhaust runner temps with an IR temp gun?  Might narrow it down to which cylinder is playing up.

Offline Bfit

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Re: Exhaust Popping?
« Reply #5 on: March 12, 2020, 11:11:19 AM »
yes,
you could try this
 with engine cold , you can use the finger pyrometer test . as exhaust starts to warms up.
DONT BURN YOURSELF if you use this method.  only viable for the first 10 to 15 seconds of engine running.
bfit
 
Freedom's just another word for nothin' left to lose
George Orwell 1984

Offline gtc

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Re: Exhaust Popping?
« Reply #6 on: March 12, 2020, 05:25:37 PM »
My 1970 454 with solid lifters and stud girdles had a similar problem until I got the valve lash adjusted back to the cam spec. That fixed it.
It's C3 chromie for me, see? Si!