i have a 250ex with an hmf system, kn filter and i had a honda atv dealer rejet the carb when i bought the new exhaust. When I decelerate the exhaust pops. It is fine when all other times. I have never had a problem with the quad. Always starts, very reliable. I run Mobil 1 synthetic motorcycle oil and it is changed regularly. I never brought it in for the first 20 hr. service. I just changed oil, gear oil, cleaned air filter/re-oiled it myself.

What is this popping noise from? Was it jetted incorrectly? I know that the manual says something about valves being adjusted or something at the first 20 hours. Would this make it pop loudly? Is the engine running too rich or too lean or something? I am going to look at my spark plug today to see if i can figure anything out. Any help is appreciated.

 

Its caused by a free flowing exhaust and a lean idle circuit.
Youll be able to confirm this by pulling on the choke, just before decellerateing...the popping will stop.
Some aftermarket exhaust systems, are so free flowing , that its hard to adjust the carb so it will idle correctly AND not pop.
Heres the explanation from the mikuni carb website, and although you have a kiehin carb, and they talk about harleys, the theory is the same.
You could try going 1/4 turn richer on the mixture screw,if you dont find any leaks. Also try to use only pure gas, dont use the ethenal blended stuff, if you have a choice. The blended gas is about 10% leaner than pure gas, and is just leaner enough to cause exhaust pops.

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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.

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.

 

It's tough to get the 250ex carb dialed in correctly after piping it. I piped my wifes with an FMF and went up one size on the main, to a 110 I think. It was also popping on deceleration which is the pilot jet (idle circuit as hondabuster said). Adjust the air/fuel mixture screw and it should correct it.

 

thanks, i'll try that. the idle screw and the air/fuel mixture screw are two different things correct?

 

Yes, and on a four stroke more times than not the air/fuel mix screw is a fuel screw meaning that as you screw it in you allow less fuel into the mixture (leaner) and as you turn it out you allow more fuel into the mixture (fatter).

 

so should i just richen it up a little and see if it helps?

 

yes