Yamaha rt100 won't rev!

[Tbstauffer]

Hello there, I'm not sure how much this forum focuses on old dirt bikes but I have an issue. I just recently picked up a 1990 Yamaha RT-100 two stroke, it was rebuilt with a Wiseco piston and Boyesen performance reed valves. It also has a brand new carburetor. It has an issue where it won't rev up, no matter how the mixture screws are adjusted. It will rev to about 2000-2500 rpm and then bog down and cut out, but will stay running fine- meaning it doesn't die. When I remove the air filter and screw out the pilot jet most of the way it revs up almost all the way and runs good but still not perfect. I have checked the exhaust and don't believe it's clogged because the thing ran great before the rebuild (that's what the guy I bought it from said at least).. Spark plug was not fouled, just wet. I'm thinking it's a jetting issue in the cheap carburetor, possibly needs a smaller one- but need some tips!! I don't think it's a spark problem because messing with the air intake changes the performance. Please give thoughts, Thanks, Tom.

 

[MB165]

most likely, the carb needs to be jetted, main jet is too rich what number is in it?

 

[Tbstauffer]

most likely, the carb needs to be jetted, main jet is too rich what number is in it?

Haven't pulled the jets out yet, just picked it up last night. Figured it would be the stock jet size because the carburetor is stock remanufactured (Chinese of course). Was thinking on ordering a jet kit and going the next size down. Why would it do this with a stock style carb?


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[kc57]

My first question is, what happened to the original carb ? The originals are usually better than the replacements and not much goes wrong that cleaning wont fix. Second have you checked the clip on the needle ? There are usually five positions . The middle being a good place to start. Moving the clip up leans it out. moving the clip down makes it richer. Who knows how the new carb is jetted ? a lot of the Chinese jets are not marked,

If you are going to order jets. Look up the specs for stock jetting. If the stock main jet is a 150, I would order a 140,150 and a 160. And I would do the same with the pilot jet. Lots of good info on how to tune mikuni's online. Good Luck

 

[older then dirt]

I agree with KC57. I'm a yamaha guy from way back. Have been messing with enduros since I was a kid. Before you order anything, Try adjusting the main needle as said. Unscrew the cap on top of the carb and pull the slide out. The needle is in the center of the slide. You will see the C clip that is at the top of the needle. Like what was mentioned, there are 5 slots (some have 4) Those control how deep or shallow the needle goes into the jet. Try different slots and see if you can tune out the problem that way before spending the money on new jets.
This past spring I had a Kawasaki 250 that was bogging badly and would not rev. By adjusting that needle depth, it turned out to be a great daily driver. I have a RT3 360 I'm restoring right now. When I got that bike, It needed some adjustment there as well. With the addition of the fiberglass reeds also change the whole intake dynamic as they flow better at low RPM. The reed change alone would require some carb retuning. If the new carb is set up in stock configuration, it will not run correctly with the power reeds. And if you can get your hands on the stock factory carb, rebuild that and park the China carb on a shelf.

 

[Tbstauffer]

Thanks!! I bought the bike with the new carb, so there isn't an original to work with. Last night I moved the clip to the middle slot.. it was on the top slot making the needle delve fully down. I adjusted it to the middle slot and the thing ran the same. Think I just need to put it on the bottom slot and see if it changes or should I look into a smaller jet?


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[MB165]

china carb, you should certainly open it up and check the jets. they may be properly marked and drilled wrong, or vice versa, or not marked at all. stock sizes are main= 140, pilot = 17.5, the needle jet and jet needle only come up as part numbers, even if they are off you should still be able to dial it in very good with jets alone.....the jet needle only has a small effect on tuning at transition from partial to almost full throttle.

 

[Tbstauffer]

china carb, you should certainly open it up and check the jets. they may be properly marked and drilled wrong, or vice versa, or not marked at all. stock sizes are main= 140, pilot = 17.5, the needle jet and jet needle only come up as part numbers, even if they are off you should still be able to dial it in very good with jets alone.....the jet needle only has a small effect on tuning at transition from partial to almost full throttle.

That's really helpful, tonight I will crack it open and check. Thanks for the responses!!