550 sx pp motor backfires?!

jonathan hinson · May 14, 2014

So i got a 91 550sx stock carb recently has been rebuilt with mikuni rebuild kit.. Stock choke. Back fired when starting the ski and would cut off..Got it to run on water with allot of adjusting but never started consistently. So i swapped the carb with a 86 550 i had that i knew was good. This time with a primmer installed..Same end result it just hiccups and shuts off. Im running single cooling with a Coffman half pipe coffman waterbox and a more open K an N filter. Im wondering if the crank seals are the issue.. I have turned adjustment screws checked compression (115psi and 120psi) an the carb is clean im lost. The problem must be the engine right? Iv tried 2 carbs now.. Any help would be much appreciated

kswiatk · May 15, 2014

Sounds like an electrical issue. Air leaks don't cause backfires i believe. First place I would check is the flywheel, you may have sheared the flywheel key and your timing is off. Most likely the issue. After that you may have a problem with the CDI. I will check my manual and see what else it could be later.

jonathan hinson · May 15, 2014

Il check that tonight i was going to swap motors but il double check my flywheel tonight thanks il let you know what i find

kswiatk said:
Sounds like an electrical issue. Air leaks don't cause backfires i believe. First place I would check is the flywheel, you may have sheared the flywheel key and your timing is off. Most likely the issue. After that you may have a problem with the CDI. I will check my manual and see what else it could be later.

flgliderpilot · May 15, 2014

Most common cause of backfire is exhaust leak. Next most common cause is fuel buildup.. either poor spark, poor spark in one cylinder, etc. Fuel builds up in exhaust and ignites.

jonathan hinson · May 15, 2014

I would buy the fuel build up statement but after it backfires an cuts off i have to choke or prime it to restart the ski.. If it were loading up with fuel i would be flooding it right? Since i have to throw more fuel to it to get it to restart?.?. Maybe something spark related though.

flgliderpilot said:
Most common cause of backfire is exhaust leak. Next most common cause is fuel buildup.. either poor spark, poor spark in one cylinder, etc. Fuel builds up in exhaust and ignites.[/QUOT

Fatman · May 15, 2014

Like stated earlier, the first place I'd look is at the woodruff key.

jonathan hinson · May 17, 2014

Fatman said:
Like stated earlier, the first place I'd look is at the woodruff key.

Well checked out the flywheel key.. All was well.. Nothing abnormal no wear on the key or anything.. Someone previously had re set the timing because the screws were blew out so i rotated the stator counterclockwise which advanced the timing an now it runs much smother still hiccups just a little but not a full blown back fire so im getting somewhere haha

kswiatk · May 17, 2014

I checked my manual.
These were the issues listed as possible:
incorrect timing
poor fuel quality
Faulty needle/seat
Faulty Spark plug
Damaged CDI
too rich or too lean

I read somewhere that misfirings can cause backfiring if severe enough. Your compression is really low for that engine, that may be causing the misfires which may cause backfires.