I was going to post this in the UTV section, but since my issue involves the 660 motor (Raptor), I thought I might be able to get more response on the Yamaha section.

I have a 2007 660 Rhino. I put an aftermarket billet exhaust tip on the exhaust. The only difference between stock and the billet tip is increase from approx. 1? to 1.5? opening, and the billet tip is straight.

Ever since changing the exhaust tip, the Rhino backfires when I let off the gas.

The motor and carburetion is bone stock. I have a BRP CDI but I had that with no issue before the tip change.

Can anyone provide info on what the issue might be?

Thanks a million!!!

Rob

 

Backfire on letoff is a sure sign of lean idle circuit.
If youre using ethenol blended gas, try pure gas. If that doesnt do it, turn the mixture screw,1/4 turn richer .

 

I thought it meant a rich mixture.... Unburned fuel escapes and detonates in the pipe...

 

Yes, backfire from the exhaust means unburnt fuel is collecting and igniting in the muffler or header pipe. A lean backfire is created when the fuel mixture burns too slowly due to the lean condition and is still burning in the combustion chamber when the intake valve opens, igniting the incoming air/fuel charge. This results in a backfire out the carb. If it is backfiring out the muffler only on deacceleration, the most likely cause is too much fuel being sucked through the idle circuit under the high manifold vacum created when egine rpm is high and the throttle is closed. Try turning your mixture screw or raise your idle speed.

 

Heres Mikuni's explanation of the backfire. And yes, its due to gas exploding in the muffler...but its caused by lean.

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.

 

Thats is Mikuni's explanation of an exhaust pop or crackle, I thought we were talking backfire here. A loud boom. On most carbs when the throttle is closed, the increase in vacuum sucks additional fuel out of the pilot circuit, since the carb slide is closed down, the resulting air fuel mixture becomes to rich for complete combustion to take place in the cumbustion chamber, the unburnt fuel collects in the exhaust then ignites.

 

Its very easy to troubleshoot, and figure out which is correct.
Just before throttle let off, pull the choke on, while still on the throttle...now slam the throttle shut. It it was lean, the pop probably went away, but if it were caused by rich...it should get much worse. Try it....itll be better.

 

backfiring out the muffler upon deceleration is caused by a lean condition. i just went through this on my warrior, when i removed the airbox lid. i turned the fuel screw out 1/2 turn and it was fixed. i was told that it was caused by being too rich but i knew this was impossible because i did not change anything except going up one size on the main jet because i was already running lean before the lid removal. i did the choke test hondabuster just explained and it conformed that i was running lean on the pilot when i let off the throttle.

 

Lots of info!

Can someone please provide me step by step details on how to gain access to the idle screw, where it is, what is looks like, which way is lean/rich and how much I should turn it? You can e-mail it to me if you would like.

Thanks again for all of your help tonight!

Rob
rob4092@***.net

 

does that 660 in the rhino have 2 carbs? im not too farmiliar with the 660 motor, just seen a few on the trails. i dont know if you need to adjust both. usually the idle fuel screw is on the bottom side of the carb on the motor side, its a flathead brass screw. i can only turn the one on my warrior with a stubby screwdriver or a drillbit, so access is tight on some machines. turning it left (loosening) richens the mixture. hope that helps.