520 jetting - backfiring

[ml36]

i rejetted my 01 520 mxc
ocemn needle 3rd clip
165 main
.75 turns on air screw
i also removed the crank breather tube from the carb.

the problem is that it seems to be backfiring way too much when off the throttle. normally, from a half throttle acceleration to closed throttle, it will pop 3-4 times as soon as the throttle is closed. after a high rev full throttle acceleration, its even worse. i have tried moving the needle and changing the air screw with no results. have the stock mxc exhaust w/ sa. i know its not a big deal, but if i can fix it, it will make the muffler packing last longer. my buddy rejetted his 400 and it doesn't backfire at all.

later

 

[dstktmusa]

Backfiring on decell can be a tricky thing to diagnose. It can be caused by an exhaust system air leak i.e. faulty exhaust gasket at the cylinderhead or any of the exhaust joints (RFS' are very prone to leak at the mid-pipe) it can also be caused by a lean air/fuel mixture, usually at an idle circuit. (it sounds to me that with a stock 48 pilot and 3/4 turns out, that would be pretty lean on a bike that runs lean in this area out of the box). It can also be caused by an over rich idle circuit (unburnt fuel in exhaust pipe). You stated that you removed the breather from the carb, you did plug the hole in the carb right?
If you are running the 48 pilot, I would richen that up to a 50 and it would certainly help.

 

[drehwurm]

Servus ml36,

If the bike also starts harder than before I would take a look at the valve clearance. Backfiring CAN be a sign of too tight clearances.

3/4 turn on the fuel screw seems also pretty lean to me - 1 1/4 seems more reasonable.

I'm running very similar jetting to yours, but had a lot of success with an even leaner main jet - try a 160 or 162. This will wake up your top end quite a bit running an e-series needle.

Michael

 

[SFO]

What pilot and pilot air jet are you running?
The effectiveness of the pilot screw is derivitive of these.

 

[ml36]

fellas, i took apart my exhaust and sealed all the joints with red rtv. fixed the problem. i hadn't started the bike since last being washed. i was suprised by how much rust was in the head pipe. there was even some light oxidation up in the exhaust port from getting wet. i dont' use a pressure washer either. pretty crazy.

later

 

[flea]

Ml36
where did you do the RTV? I have done both mid pipe areas with no changes. did you do the union to the cylinder head?

 

[dstktmusa]

The "joint" at the cylinder head is just a slip fit, so I would also apply a small bead of sealant around this area too:D

 

[ml36]

i sealed all three joints, head pipe, mid pipe, and muffler. i thought it went away, but the trails i rode on after fixing it were really tight and i didn't rev the motor much. last weekend i rode on some faster trails and the backfiring was back. i don't know, who cares. it doesn't hurt anything, so i give up. actually i think it sounds cool.