Hey all, I've done a search, and can't seem to find out if this is a problem for me or not.  I recently upgraded to K&N individual pods, Jardine full system, and DIY rejet (40 pilots/150 mains/2 washers/3 turns out).  The bike runs fantastic, except for a slightly annoying popping from the exhaust under closed throttle deceleration (coasting).  It seems to happen more when the bike is warmed up.

I can't figure out if this is a sign of richness or leanness.  Also, since the bike runs fine under both partial and wide open throttle, i'm hesitant to start 'fixin' things...can anyone comment?

Check for leaks at the exhaust port/header junction. Also, when you chop the throttle while running the engine goes into a high vacuum mode and pulls in fuel with little air. This cause fuel to burn in the exhaust. If air enters into the exhaust you get a popping. Just about any engine with a big exhaust will pop like that. You might try the ign. adv. mod for the GS too. Love the bike, the forum and the Remington 700 action.

C.......

I have the same setup as you except I have the V&H full system.  Mine pops too on decel when the throttle is closed.  I read somewhere that means it's too rich, but my bike runs fine and I'm hesitant to screw with something that's running fine.  I just live with it.

Jake
Oh, I also read somewhere on here when doing research on my exhaust that popping on decel is common for a full system.

All my GSs with race pipes have always popped on decelleration, and I was never been able to tune it completely out of 'lita's street bike. :dunno:

Me thinks that with a closed throttle the cylinders aren't building up enough compression to get a good burn, so lots of unburnt mix is going out the pipe and igniting there.  To get rid of it you would have to lean out the idle mix to next to nil, which of course would require redoing the rest of the jetting, and probably screwing up the idle to mid transition.

I never heard the popping with the stocker, but that is probably due to it being so quiet.  since the D&D pipe I hear it, but I don't feel any difference. So, I've just left it alone.

i've grown to sort of enjoy it.  

jake

The idle mixture becomes too lean to burn with an open pipe.  Uncombusted fuel collects in the pipe and ignites.  Richening the idle can help, but must compromise between popping and throttle response.

At least, that's the info I gleaned last time I did a web search. :dunno:

Which doesn't mean the mixture is lean at idle.  Lean when decelerating with a closed throttle.  Which is why the popping is hard to tune out without buggering up the idle.

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.

Ignition:

If exhaust system popping  is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes

Many thanks to Mikuni Home Page

and many thanks to sparkplug for the detailed response.  there is no loss of power, and the bike runs fantastic on throttle...so i think i'll just have to live with it.  to tell the truth, it doesn't bother me that much, but i'm beginning to feel bad for all the cagers with their windows down as i slow up to red lights...hehe.