Just put a full FMF Q2 exhaust on my son's 300EX. FMF says to increase main jet from 122 to 128-130. Nothing else. I used 130 and it popped. I just lowered the needle clip one and it still pops a little. Is this OK or should I raise the clip again? I'm also going to watch the plug..............

 

A little is ok but none is better. I would take out the 38 pilot jet and install a 40. You'd be good to go then.

When I had my 300ex it was a 40 pilot, needle up all the way (or down one,can't remember) and a 130 main. No lid and a uni foam filter.

 

Thanks but I thought the pilot was for idle only which is fine. I'm running a Uni and open box as well. I'll give it a try before I go all the way up on the needle (one more).

 

TractorPacker is right, you probably need to richen your idle circuit. You said you notice the popping on deceleration, which would mean that your thumb is iff the throttle and your carb slide is closed all the way, initiating the idle circuit. So you can either raise the needle or go a bit richer on your pilot jet. I'd start with the pilot jet before moving the needle.

 

Turn in the air fuel mixture screw about a quater turn..

 

Yes, that's what I am doing. Running at about half throttle and letting off the throttle completely. I'll try the bigger pilot jet and fuel mixture screw first and if that does not do it, I'll raise the needle to the last setting.

Thanks

 

Changing the needle position wont effect a thing, its all idle circuit, when you let off the throttle.
Have you checked and sealed all the exhaust leaks?
Heres a guide from mikuni, about why an exhaust will pop on decell.

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.



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.

 

Thanks all for your help! All good info. I'll report back when done.

 

Just finished replacing the slow jet with a #40. The quad is perfect now. I also found that the old slow jet was clogged so between the two issues, all is well. Thanks for the great and correct advise!