ET250 Bog and Plug fouling

DoktorC

DoktorC

Member
Joined
May 11, 2003
Messages
431
Age
50
Location
Parry Sound
I've got an ET250 with a fresh top end, new coil, VM36 swap and it has a flat spot/bog through the bottom and mid. Once the revs come up look out and if you keep the revs up and the sled moving it works well but if you slow it just has no power. Along with this it seems that if I let it sit and cool when I restart it, it fires up and will idle but it seems to load up once I apply throttle (won't move). So, I lift the back to "clean it out" and it just bogs, backfires once and kills the plug. The plug doesn't seem to be oil/fuel fouled but its dead...torching it does not bring the plug back to life. Change the plug and it runs and goes again.

The whole sled is just out of my normal area of expertise...it has points, the goofy splined clutch, mixed gas, and only 1 cylinder lol.

VM36 specs - Main 250, Pilot 30, Tube 159 P4
 
Do you have a filter on the carb? If a pod, is it a motorcycle one or a snow one? Guys have had trouble with the MC ones on sleds.

I suspect it's loading up, others with more experience will know better. I suspect the main is a little big, maybe a 230 or 220 might be better.
 
have you checked the timing and points gap? Has the condenser been replaced? Are you running an air filter? What spark plug are you running? What condition is the external coil in? (any cracks/breaks in the wire etc?) What's the compression like?

VM 36 is probably a little big, I'm running a 34. The backfire makes me think the points/timing might be the problem, or a part of a problem. On my 250 I had similar issues, the solution wasn't necessarily repairing one thing but addressing the combination of small problems that were adding up to poor running. Have you checked the decompression valve for leaks? Leaking valve would cause a lean condition, but is worth checking. The de-comp valve isn't necessary anyways and is a source of problem, mine is plugged with a bolt.
 
I'm not running a filter yet. I think the 250 is likely too rich...It was so long ago I thought it was actually a 220 until I checked my notes to write the post lol.

Matty - good info! I'm certain it's a combination of small problems. I have NOT checked the points but will do that ASAP. Everything was good when the engine was rebuilt BUT it has sat for a couple of years since so the points could definitely be an issue. Same deal with the decomp I've never even looked at it...I'll pull it and plug it just for safety sake.

The coil, wire, cap is new when it was rebuilt. I'm running a BR8HS. I haven't checked compression but the motor is fresh so I didn't bother...sounds more like tuning than blown up.
 
It sounds like a rich idle mixture, and mid range. Try the air screw at 2 1/2 out. If that does not lean the idle mix enough, try a 25 pilot.

You can also drop the needle, move the clip to the top.

It really sounds like your clutches are shifting way to fast. A little more tension on the secondary spring will slow down the shift, and hold it in low gear to get it going.

Also the drive clutch could have a totally sacked out spring, try a new spring, or a stiffer spring.
 
Last edited:
matty k said:
let us know how it goes.
Click to expand...

I'm getting there. It was warm this weekend so I threw a 210 main in it and it improved greatly. Then I dropped the needle 1 position and it got better again. I think the 30 pilot may be lean because it has no power below 1/4 throttle. If I load the motor by being off trail it gets worse. I'm going to try a 35 pilot.

Once it's rolling now if I pin it it goes good. This is all a case of I just should have followed the TY jetting recommendactions from the get go lol.
 
That 30 Pilot is definitely to small, I'm running a vm34 and Uni Pod filter on mine and I had a 30 in mine, I had to have the mixture screw all the way in to idle. Now I have a 40 in it one turn out idles well. I am running a 220 main jet at 20-30 degrees and it works well.
 
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