Bonneville America Forums Forums Technical Forums General Tech Idle Mixture # of turns out.

Ok, I'm here!
Here is the Reader's Digest version and the general operation.

The difference between in/out-lean/rich is entirely dependent upon whether the pilot adjustment is in the front (engine side) or the back (air cleaner side) of the carbs.

In the front (like ours are) the pilots turn in to lean, out to richen. They are adjusting the GAS flow for the pilot circuits.

In the back (closer to the air filters), in is richer, out is leaner. This adjusts the AIR flow for the pilot circuits.

Edited cause I got it backwards too, oops....

Now I'M confused...I thought ours WERE on the engine side???? I know they are DEF on the engine side of the bowls!

Sorry Rob, I wrote it bassackwards, I corrected it, you are correct, I am a spaz. Still recovering.....

I concur with Pat. From my experience I have found that increased airflow via less restricted intake and/or less restricted exhaust requires that the screws be turned out to compensate at low throttle.

I recently installed a Freak kit. Before the Freak I had ran the following setup: Staintune exaust, snork out, 45 pilots, 138 mains, 3 turns average ( I fiddle with the air mix screws a lot. I tried 2.75 to 3.25)

After adding the Freak I now run 45 pilots (same as before), 150 mains, but I found that I needed to go to 3.5 turns to compensate for the increased air flow. I was even running 3.75 turns for a while, but that seemed a bit extreme. I might try installing 48 pilots so I can bring the air/mix screws back to around 3 turns.

This tells me that turning the mix screws out (counter-clockwize) will introduce more fuel and richen the mix.

Just my 2 cents.

Cody

I stand corrected then!

I guess my original assessment was the right one! WooHoo! I am 50% right!!

Whew what would we do without Pat?
1 thing is, there would be a bit more brew in the world so we would really have to start drinkin'.
Thanx Pat for the clarification.
That darn tech vault.... It has it moments too huh?

I edited that post in the Tech Vault to reflect this discussion.

4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.


Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Well thanks Pat for stepping in and bringing this whole ugly mess to a close. I was just about ready to rip my carbs off and start all over again.

I agree. It will be nice to see this post start rolling back into the abyss now.
Thanx Grump for that explaination.

Last...
Back to the very first question of this post. (well sorta)
Would being at 1 to 1 1/2 turns out justify going down a pilot jet? Or is 3/4 turns the minimum per the carb tuning thread?

A quick note...
When I removed the 3rd baffel from my exhaust, it got louder, removing the last baffel really deepened the sound. (REALLY DEEPENED). This makes my pops sound a bit more mean than before. This is why the backfire/pops sound a bit more excessive. That last baffel is holding back the BOOM (so to speak).
That took me awhile to get..