Leaderboard

You are right in stating the exhaust is holding fuel either a rich vapor collection or actual raw liquid. That muffler is extremely hot and will ignite either. When it's cold put your finger inside the tailpipe and wipe the surface to see if it's black soot. Indicating a rich fuel mix. If it doesn't backfire while running it is telling you it's collecting and holding a little bit inside that muffler and when the engine stops breathing/running the excess in the muffler ignites from heat inside of there. I've seen mufflers blow up an split out on cars. I know you said you had the carburetor off a couple times but a couple things need checked again. Mikuni's CV series choke is an enrichment needle that sends extra fuel for starting from cold. Not like most American carbs that actually cut off the air intake to make the carb pull more fuel in. Set the correct slack in that choke cable and check that the spring is on that plunger needle at the end of the cable where it screws into the carb. It must close correctly when you move the choke lever to OFF. Next, I know the float setting is critical on these carbs so make sure it is set to the service manual float height. My 1998 500 Quadrunner is set at 17mm or 0.67" +/- .04" that's about 11/16ths of an inch. This is with the carb upside down measuring from the rim of the float bowl up to the highest point of the float. Use a machinist ruler or calipers. Bend the tang on the float that rides above the needle to adjust up or down. Be sure the needle has a good clean point with no damage and no obstruction in the seat, like a fine crumb of rust or something that might prevent a seal. Now go to the top of the carb and remove the cover and gently lift and inspect that metering rod diaphragm for fine splits or cracks. Also someone may have re-jeted at some point which could make it run rich so check those jets. The factory jets on my 98' are; Main #130 and Slow #55. The mixture adjustment screw is on the carb bottom on the engine side of the carb on mine. It's set about 2-1/2 Turns out and very hard to get at with the carb mounted. You need a #1 flat blade screwdriver about 1-1/2 to 2" length. If the engine fully heat up it's very hot work turning that needle screw. These are the things I would check if mine were running rich. When a float and needle aren't seating good you can get so much fuel it can hydraulic-lock from a full cylinder of gas when the engine is off. I am not a fan of the problematic vacuum petcock safety design. I replaced mine with a common On/Off/Reserve petcock and plugged the vacuum port at the carb. Works fine. Good luck out there

15/40 of your choice just be sure to run after change at idle then CHK again to make sure it's full