Sherco 290 Running Lean

[brinky]

My 290 is running lean at high power settings. After an audit of the jets I note that the Main Jet is a 120. The manual calls fot a 126 Main Jet. I am new to trials and wonder if this is a trials thing to stop loading up or to help with unusual attitude issues. I am inclined to return to the manuafacturers recommended jet size.
Any guidance appreciated.

[Neo]

Hi Brinky,

I assume your talking about the standard Dellorto?

Best place to start is with the standard Sherco specs and work from there.

Take the carb apart and give it a good compressed air clean up. And make sure those two vent tubes (one either side) are completely clear. Also, check the float height is correct.


Best of balance.

Neo

[brinky]

Dear Neo,
I agree, float level correct, vents/passages all clear. I shall return the jetting to OEM. Nothing worse than a 2T running lean at high power setting.
Regards,
Brinky.

[phoenix75]

hey there Brinky,

Usually the Dealer sets the Jets as they are so they run a bit rich, but the 120 main jet in it I would send to someone who knows how to get the Carby setup properly with the air fuel mixture screw.

For when I got a new bike the dealer told me to go down in size with the jetting for this reason ( rich mixture) for the run in period and then the smaller jetting for Comp, but with me changing the jets it was pushing to much fuel through and it was loading up on big hills. I got someone who knows there stuff to listen to the engine and to do the mixture and that hill that the bike was loading on I was shooting up the next time round

[brinky]

Thanks Frank. Good info.

[phoenix75]

how did it go ?

Did you get someone to play with the air Fuel mix ?

[brinky]

Removed the carby today and used an Oxy tip cleaning tool which measured 1.25mm in diameter to increase the size of the 1.20mm diameter main jet. This I felt was a reasonable jet size as I am 1000ft above mean sea level and the manufacturer recommends a 1.26 jet at sea level. I adjusted the idle mixture and the bike is running well at all throttle setting and not loading up with a full synthetic oil mixed with non Pb fuel at 2.3%.
Thanks for the advice.
My next problem is excessive carbon in the head. I fell off last week and the throttle stuck on. I pushed the kill switch but the engine continued to rev. I pulled the plug lead off and the engine continued to rev. I eventually yanked the rear guard off and shoved a piece of rag into the aircleaner to stop the engine. I have see this glow plugging on motocross bikes when hot with hot carbon in the head.
Your thoughts.

[Stork955]

Gday, pull the head off and decoke it (dont scratch the head with anything sharp - oven cleaner will help). While you are there, decoke the exhaust system as well and repack the muffler. This is standard 2-stroke servicing that virtually no one performs. After this you may find that you need to change your jetting back to how it was before.

Also, a clarification for mixture setting -mixture screws only affect idle and very slow speed running. Main jets only affect full throttle. Slide cutaway affects the transition from idle to open throttle and the needle pretty much does the rest. If you are having full throttle issues adjusting the idle mixture screw wont change anything there.

HTH,

Cheers,

Stork

[Neo]

Also, a clarification for mixture setting -mixture screws only affect idle and very slow speed running. Main jets only affect full throttle. Slide cutaway affects the transition from idle to open throttle and the needle pretty much does the rest.

Something like this..

Best of balance.

Neo

[brinky]

Dear Stork,
Great Diagram.
What is the I.M.S just below the Throttle Valve Cut Away envelop.
Regards,
Brinky.