125M jetting advice needed

[86T3]

I have a 125m motor in my rokon and i need some help with the jetting. The exhaust is now 1 1/4"(double the stock size) and the intake tube is 1 1/2" (stock size) with an open uni filter. It has the stock 92 main in the carb. I rode it around a bit and ita fine at low rpms but cuts out when i give it full throttle. I didnt do a plug chop but i pulled it and its white at the tip. I only drove it for 30 seconds max just to try out the chain tensioner. So my question is where should i start with on the main and do i need to increase the slow jet also? This is my first excursion into jetting so be gentle on me.

[Badbmwbrad]

I had a similar problem in that my ATC 110 exhaust was free flowing w/o spark arrestor when I first got it. The air filter element was stock Honda but it had no oil and the foam had deteriorated (free flowing). The engine would operate OK at low speed but would cut out at higher RPM because it was fuel starved.

Your 125M motor with double-diameter exhaust pipe and Uni filter will develop higher air flow through the motor. Your pipe probably will not produce very much back pressure so engine low-range and mid-range torque will be less than it would be with a stock pipe and airbox.

The 110 and the 125M share the same Keihin PB20A carburetor but the 125M has a #92 main jet and #38 pilot jet whereas my 110 had #82 main jet and #35 pilot. In addition, the jet needle on the 125M is a different taper (narrower) ...and it was January here on the New England coast (sea level) with very cold (i.e. dense) air.

So where am I going with this? I re-jetted the carb by purchasing a Moose carb rebuild kit for a 125M to install on my 110. The stock size #82 main jet was increased to size #92 and the stock size #35 pilot jet was increased to #38. A 125M jet needle was installed and the clip set to 2nd groove from top of the new (narrower) jet needle (stock 110 jet needle clip is set third groove down from the top). This should give you some idea of how much larger pilot jet and main jet you need.

In my case, it pretty much fixed the problem with the engine cutting out at high RPM. I've subsequently welded a spark arrestor onto my exhaust pipe and installed a new, oil-wetted stock Honda air filter element. My sparkplug color is pretty dark (too rich). As the outside daytime temperatures become warmer, I can predict that my carburetor will be delivering a fuel:air mixture ratio that is too rich; especially during hot Summer months.

I'll stick with the size #38 pilot jet and keep the same jet needle but will probably raise the clip one notch and decrease the main jet size to #90 or #88.

[kormos_93]

If the plug is white you're running lean & need to get some bigger jets. Don't run it with your current jets or you will end up putting in a new piston. You'll need to replace both the main & slow jets. I think you can get 2 sizes larger at a time, maybe 3 (meaning they the number on the jet will increase/decrease in increments of 2). I'd try at least 4 sizes larger first & go from there, don't be scared to buy a bunch of larger jets. You can return the ones you don't use. Run the engine for about 30-45 seconds after you've got the new jets in & then check your sparkplug. Repeat the process if necessary. When the plug is nice & tan you've got the right size jets. You can raise or lower the needle after that is done & play with the fuel/air screw to adjust the throttle response. (You may need a different needle if you're unable to get a perfect response after adjusting the fuel/air screw, depends how far from stock the proper main & slow jets are)

[86T3]

Thanks for the advice fellas. I went to the local shop here and had to order some parts. Im getting a 98 and a 100 main but im kinda stuck with the pilot jet. They looked for about an hour and couldn t find anything. I went to a smaller specialty shop on the way home and the guy ylthere suggested i stick with the stock pilot jet since it idled and ran correctly to half throttle. Ill have to run it with the new mains and see if t works out. Parts should be in thursday, ill update you then.

[Badbmwbrad]

I ordered the ATC 125M carb rebuild kit from DRATV.com. It was the only way I could figure out how to get a size 38 pilot jet; a larger pilot jet than the ATC 110's stock size 35 pilot jet.

Your carburetor probably has the size 38 pilot jet. You might try to find a pilot jet larger than size 38 at www.dratv.com


Thanks for the advice fellas. I went to the local shop here and had to order some parts. Im getting a 98 and a 100 main but im kinda stuck with the pilot jet. They looked for about an hour and couldn t find anything. I went to a smaller specialty shop on the way home and the guy ylthere suggested i stick with the stock pilot jet since it idled and ran correctly to half throttle. Ill have to run it with the new mains and see if t works out. Parts should be in thursday, ill update you then.

[DasUberKraut]

I hope you figure it out. I have the same issue with my 84 125m's.

[86T3]

I ordered the ATC 125M carb rebuild kit from DRATV.com. It was the only way I could figure out how to get a size 38 pilot jet; a larger pilot jet than the ATC 110's stock size 35 pilot jet.

Your carburetor probably has the size 38 pilot jet. You might try to find a pilot jet larger than size 38 at www.dratv.com

DR ATV is awesome, i wish i had checked there earlier. They have the pilots and mains and they re half price of the mains i ordered. Also, they have some slick open element filters that i think will help me out when i finalize my intake. As a side note, they have a chinese carb thats under 50 bucks. If i had known about that i wouldnt have soaked this one for two moths trying to get the crap out of it.

Does this help you out uber or are you having jetting issues?

[DasUberKraut]

I have the same symptoms. But I won't be able to tinker for a month or two.