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Discussion Starter · #1 ·
Hey guys I bought a 20mm from tboltusa for my 110cc and I’m having a problem with the bike idling then going to wide open throttle almost instantly and it bogs and some times it bogs for a second then goes and other time it’s bogs and stalls. If I play with the choke I can make it run perfectly . My fuel mixture screw is almost all the way in currently with the slide needle as far in as it can go I’ve tried turning the fuel mixture screw out to give it more fuel and it doesn’t like that it bogs the same if not more so I thought maybe I should’ve just the needle but that doesn’t do anything, so does that mean I need to change the main jet? If so do I go bigger or smaller than my current main jet size? I’m open to retrying stuff I already tried if you guys have any advice. Thanks guys.
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The 20mm ShengWay carb is relatively easy to tune right out of the box. It comes with the #89 main and a #35 pilot jet, both of which you can buy the extra jets from TrailBikes or any TB sales rep. As a baseline setting, I use the #90 or #92 main jet, the #38 or #40 pilot jet, set the jet needle e-clip in the 3rd or 4th notch from the top, and the air screw at 1-1/2 to 1-3/4 turns OUT from seated. Make the necessary air screw adjustments IN to richen it a 1/4-turn at a time and see what the bike does. With some of the carbs, I've run it to within a 1/2-turn of being all the way IN to get rid of 98% of the off-idle hesitation. You might have to raise or lower the clip on the needle, depending upon your altitude, I'm at sea level.
 

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Discussion Starter · #3 ·
The 20mm ShengWay carb is relatively easy to tune right out of the box. It comes with the #89 main and a #35 pilot jet, both of which you can buy the extra jets from TrailBikes or any TB sales rep. As a baseline setting, I use the #90 or #92 main jet, the #38 or #40 pilot jet, set the jet needle e-clip in the 3rd or 4th notch from the top, and the air screw at 1-1/2 to 1-3/4 turns OUT from seated. Make the necessary air screw adjustments IN to richen it a 1/4-turn at a time and see what the bike does. With some of the carbs, I've run it to within a 1/2-turn of being all the way IN to get rid of 98% of the off-idle hesitation. You might have to raise or lower the clip on the needle, depending upon your altitude, I'm at sea level.
Ok thanks, what engine are you using? Also does the bigger the number mean the smaller the jet?
 

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I've used the carb on everything from a 75cc engine to a 131cc engine with the same performance results. Once it has been fine-tuned, it pulls strong through all the gears.

A #92 main jet is bigger than a #89, likewise, the #40 pilot jet is bigger than the #35.
 

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Discussion Starter · #5 ·
I've used the carb on everything from a 75cc engine to a 131cc engine with the same performance results. Once it has been fine-tuned, it pulls strong through all the gears.

A #92 main jet is bigger than a #89, likewise, the #40 pilot jet is bigger than the #35.
Ok thanks, just for laughs I adjusted the needle all the way up so it would be the most fuel and it’s still Boggs from idle to wide-open throttle and wants to stall if you hold it at wide open throttle, it seems like no matter what I try with the stock jets it reacts the same, the jets you recommend, you recommend for a 110cc?
 

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Yes, you have to go up to a #38 or #40 pilot jet to eliminate as much of the off-idle hesitation as you can. You may not get rid of it all together, but these carbs do not have an accelerator pump.

Besides, you don't ride the bike from idle to wide-open throttle every time you twist the throttle, do you?
 

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Discussion Starter · #7 ·
Yes, you have to go up to a #38 or #40 pilot jet to eliminate as much of the off-idle hesitation as you can. You may not get rid of it all together, but these carbs do not have an accelerator pump.

Besides, you don't ride the bike from idle to wide-open throttle every time you twist the throttle, do you?
No lol but I would like to know that it can lol, so all I need is a #38-#40 pilot jet?
 

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Discussion Starter · #10 ·
No, that's not all you'll need. You'll have to fine-tune the carb with the air screw, and possibly the jet needle clip as well.
Hey I tried the #38 pilot jet and first time starting it it didn’t wanna start and after that start it would go up to a high RPM and just sit there even though I backed off the idle screw all the way and almost turned the fuel mixture screw all the way in almost seated and the needle is as far in as it can go, I checked for a vacuum leak, any ideas?
 

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Try setting the carb up with the baseline settings I posted above, make sure the throtle slide is closing all the way down in the carb throat, and follow the instructions for adjusting the air screw.
 

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Discussion Starter · #12 ·
Hey I tried the #38 pilot jet and first time starting it it didn’t wanna start and after that start it would go up to a high RPM and just sit there even though I backed off the idle screw all the way and almost turned the fuel mixture screw all the way in almost seated and the needle is as far in as it can go, I checked for a vacuum leak, any ideas?
I tried 38 and 40 jets with every combination of adjustments you can think of and it’s actually bogging more so I tried a smaller jet and it doesn’t like that either so I guess I’m gonna go back to stock
 

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sounds like you have a lean condition due to a leak at the the intake manifold. try spraying starting fluid/ carb choke cleaner etc... at the carb intake connections while the engine is running. you'll know if you have a leak if the idle goes up or down while spraying at the connections
 

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Discussion Starter · #15 ·
sounds like you have a lean condition due to a leak at the the intake manifold. try spraying starting fluid/ carb choke cleaner etc... at the carb intake connections while the engine is running. you'll know if you have a leak if the idle goes up or down while spraying at the connections
I sprayed starting fluid on the intake where it meets the head and the idle increased and it seems like wherever I spray it the idle increases so I changed the gasket and the O-ring on the carburetor and it’s still doing it it idles fine but if you even just rest your hand on the throttle it dieI sprayed starting fluid on the intake where it meets the head and the idle increased and it seems like wherever I spray it the idle increases so I changed the gasket and the O-ring on the carburetor and it’s still doing it it idles fine but if you even just rest your hand on the throttle it Bogs and stalls
 

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Discussion Starter · #16 · (Edited)
sounds like you have a lean condition due to a leak at the the intake manifold. try spraying starting fluid/ carb choke cleaner etc... at the carb intake connections while the engine is running. you'll know if you have a leak if the idle goes up or down while spraying at the connections
Also when you do turn the throttle and then let go of it the rpm increases almost like a three second throttle response delay if that makes sense, The harder you turn the throttle and let it go the more the RPMs increase when you let go. I have a #38 jet in the carburetor cause that’s what I was told would work but if I adjust the air fuel mixture it won’t idle at the right speed either.
 

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Try applying a light coating of wheel bearing grease to the gaskets and o-rings of the carb and intake manifold, to see if you can get a better seal. A hanging idle is a sign of a lean mixture, possibly due to a recurring air leak somewhere
 
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