Backfiring popping thru exhaust at idle.

[Rusty McNeil]

Backfiring popping thru exhaust at idle.

Running Mikunis with a new motor, 1540cc, 13:1 compression with the .380 cams, dyna 3000 set at stock curve.

I have a handle on how to get the top end and midrange right.
Issue I'm having is that it is backfiring thru the exhaust at idle.

I've;
Synched the carbs.
Set the idle mixture screws by a couple of different methods, turning them until it stumbles and then back the other way 1 turn.
Then also did the bump throttle test to see if it hangs up and comes down slowly or if it dips below idle set point and then recovers back to idle set point. I have it to where it just quickly settles back to idle set point.
After all that I synched em again.
No matter what I do it still pops or backfires thru the exhaust, intermittently, like maybe every 10-15 seconds.
I've tried going a couple of turns either way with no improvement.


Any one have any ideas? I've already checked for vacuum leaks etc, carbs are freshly cleaned. I've always heard backfiring exhaust was a lean condition but usually when we talk about that it involves backfiring on decell, not at idle. Mine backfires only slightly on decell, nothing really annoying tho'

Running DJ 175 mains which need to go bigger since WOT is still lean at about 15:1 a/f ratio
Running Stg. 7 needles on 4th clip from blunt
Running stock PAJ 1 & 2
Fuel bowls are set at 1.115"
My part throttle cruise a/f is really rich at 12:1 or so and I think I can address that with either Bigger PAJ 2 and/or a float level adjustment?????

 

[gamorg02]

100% sure your exhaust gaskets are good, no exhaust leaks? that can cause it too eh?

 

[customizedcreationz]

Check exhaust for cracks also. My new systems I just installed had a 2 1/2 " crack in one of the pipes

Todd

 

[rusty]

Quote: RustyMcneal= backfires thru the exhaust, intermittently, like maybe every 10-15 seconds..... Mine does the same thing when it's idealing cold. When warmed up it stops & runs fine with no issues.......Rusty :biglaugh:

 

[tugla]

all your needle and seats good,no worn out ones?,more of a issue at slow cruise but could effect at idle maybe

 

[mr.maxx]

Quote: RustyMcneal= backfires thru the exhaust, intermittently, like maybe every 10-15 seconds..... Mine does the same thing when it's idealing cold. When warmed up it stops & runs fine with no issues.......Rusty :biglaugh:

mine too....only on the right side muffler...the supertrapps really exagerate it too.