Here is a good one you guys to figure out.

Fish said:
Bad cam?
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Not that one either. Good guess though as I replaced two 793880 camshafts last year for bad exhaust lobes but the backfiring was constant and not as random this one was.

As I said (typed) I used no additional replacement parts to make the repairs.

I only pull the rocker cover to do valve adjustment check and for the leak down test.

It sounds as if I had recorded and posted the engine running it would have even confused you guys even more but I have no means of doing it here.
 
You say you couldn’t idle below 2500 RPMs and it isn’t a mechanical or ignition problem. This leaves fuel. I’m guessing the idle mixture screw was turned out too far or missing and possibly found idle speed screw in too far. This would produce an extremely rich mixture, plus intermittent flooding, causing back firing and if the flooding occurred at the right time would kill the engine.
 
Rivets said:
You say you couldn’t idle below 2500 RPMs and it isn’t a mechanical or ignition problem. This leaves fuel. I’m guessing the idle mixture screw was turned out too far or missing and possibly found idle speed screw in too far. This would produce an extremely rich mixture, plus intermittent flooding, causing back firing and if the flooding occurred at the right time would kill the engine.
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You now on the right path but it was not a rich burn condition but was lean burn issue. Adjusting the idle mixture screw had no effect; hence, the modification. It was pre-ignition cause by burning way too lean; otherwords, the fuel load was igniting from heat and not spark [reverse dieseling effect]. Applying 3/4 choke at all speeds lead to the lean burn issue.
 
StarTech said:
You now on the right path but it was not a rich burn condition but was lean burn issue. Adjusting the idle mixture screw had no effect; hence, the modification. It was pre-ignition cause by burning way too lean; otherwords, the fuel load was igniting from heat and not spark [reverse dieseling effect]. Applying 3/4 choke at all speeds lead to the lean burn issue.
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Applying 3/4 choke will lead to a rich run not lean (if carb is operating proper). Where was the fuel igniting at prior to the cylinder causing it to go lean? Fuel/air path is short on these engines. Intake manifold? Plugged air filter pulling excess air from block vent hose?

Also was thinking gov issue.

slomo
 
I have been doing repairs for 40+yrs and know better than to use those torch tip cleaners except on my cutting torch tips. Matter of fact I haven't used carburetor cleaner since 2013 to clean these carburetors. I actually gave away my last dip can of cleaner back in 2014.

Yes it was pre-ignition. For it to be back firing it had to be at the intake valve head. Couldn't have been the air filter as it and the air intake manifold was removed, plus the vent was discoonected due the manifold being removed. The carb to cylinder was check for both cracks and warpage. I find the restrictor plate was missing, which I replaced.

It took enlarging the jet involved by 0.0381mm [.0015"] on the new Chinese crap carburetor. Could stand to go another 0.0127mm [.0005"] but there is no bit in that size available The other shop threw away a perfectly good Nikki carburetor because they were too lazy to clean it.
 
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Rivets said:
Now I’m thinking you punched the main jet couple thousands over.
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And no it wasn't the main jet. Remember these engines run off the idle mixture ports when off load and also run off it under light loads.

Btw going from 1.15mm [.04527"] to 1.18mm [.04646"] on the main didn't help anyway under the light deck load so put back the 1.15mm jet.
 
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