My build with a GT 90 Kit sputters until it's over ~15mph

Ryan843

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Hey everyone! I'm new to the forum, but not exactly new to building motorized bikes.

This is my 3rd build. I kept wanting something faster after building the last two, so this time I went with a GT7 kit with the GT90 conversion kit. Rebuilt the engine, got it set up and all.

The problem I'm having is the bike sputters so badly it won't run on its own until it reaches about 15mph, then it's like a switch flips and it runs great and gets up to 30mph with a 44 tooth sprocket. But under 15mph it's useless.

Tried different spark plugs, a different voltage regulator that worked on another bike, and tried with the choke on and off. Only things I can think of are that the carb that came with the GT90 kit is too big to pull in enough air at low RPMs or something is wrong with the coil on the engine that generates the electricity for the spark plug.

Any ideas? Thanks
 
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indian22

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Ryan if I follow what you have it's a case inducted, reed valve, third port jug with a window piston? If I'm correct in my previous assumption congrats on assembling & getting it running, however poorly. I've not built one, but I've read discussions on this setup and it seems Gas bikes shipped some with OYO and most with Kehin carbs and these are a bit more challenging to get running right than NT. Dellorto or Speed...more on the order of a Mikuni with more circuits and jets to adjust. I wouldn't be too quick to start jetting etc. just yet since it's running ok with revs. If you're still running heavy break in mix 16:1 I'd suggest riding three tanks of fuel through her first, and that shouldn't take too long as reed setups on the 66cc are fuel hungry beasts. I also suggest keeping speed down until after you jump to the 32:1 mix. Your engine will tell you when it's broken free and it should start run even on 16:1 mix. Jetting only makes real sense after engine break in.

I've 2 engines running third port, window piston, Dellorto carb, reed valve, but inducted through the jug and not case with x-chamber. I wasn't really impressed with either one till I put about 500 miles on each and a smile on my face!

Something I'd keep an eye on is the plug mainly if it's running wet right now at lower speeds of say 20 mph, not 30mph or more. If it's wet I'd suggest that's part of the low speed running problem. Since your running ok above 15 mph and the motor starts ok it's not the electrical system. Are you running a good expansion chamber and if so what model? If you aren't running one why not? If your carb is running rich with a kit stubby muffler then a good expansion chamber that's designed for low to mid range performance will convert that rich jetting to power on the bottom end.

One other thing. The reed valve petals are really stiff when new and require time to break in and flex, so low end performance will especially suck and be quite noticeably poorer than when induction air volume picks up with rpms and this added to the lack of an x-chamber (my apologies for the assumption, if you have one already) is likely drowning the combustion mix explosion at 16:1 break in as well....bunch of things going on here that riding time alone will help greatly.

30 mph and speed shouldn't be a concern at this time....once it's broken in and your on 32:1 mix & an x-chamber your motor should pull mid forties or more with a 36 tooth rear. Reading the plug after rides post break in will determine if final jetting adjustment is required & perhaps a plug range change as well....especially if it's still not fun to ride at under 15 mph!

Once again congratulations on getting it up and running I think you're a lot closer to a finish than you probably think.

Rick C.
 

Ryan843

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Thanks for taking the time for that long reply.

I did buy an expansion chamber, the tunable one from sickbikeparts, but I haven't installed it yet mostly because I want to know how much of a difference it makes compared to the stock muffler. You're correct, it is a ported jug (dunno what you mean by third port) with reed valves.

I think I understated how bad it runs under 15mph. until about 10mph, while pedaling furiously, the piston fires very randomly, only like 2-3 times per second. From 10-15mph it's still sputtering but it's close to being able to run on its own without pedaling, then it's good over 15. Surely letting it break in wouldn't fix something that significant. And I mostly ride around the city so I'm not even riding it right now.

I think it's running lean right now because I can get it to sorta run under 15 if I quickly crank the throttle open and closed and open and closed as fast as I can, so the metering rod is pulling gas through the jets
 
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indian22

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Really not how it works Ryan. Show some good photos of the engine & we can go from there...little things can really cause big problems and break in is huge, every change you've made and each one made going forward causes change as well. you can't expect the engine to break in sitting in the garage.

Rick C.
 

bairdco

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Sounds like your idle jet is too big. If your carb has one. Or if there's an air mixture screw, it needs tuning.

Note: main jet is for upper mid to full throttle. Changing that won't do anything for your problem. Neither will changing anything electrical (magneto, CDI, plug, etc, unless your plug gap is way off.)
Mixture won't cause that either. The only thing that will cause a low range bog is the idle jet, air mixture, and maybe needle setting in the carb. Float level might cause it, but I doubt it. It's a fuel/air related problem, so don't over think it.

Timing may cause it, but there's no way to really adjust that, unless your motor comes with some fancy timing adjuster that older engines don't have. I dunno, I haven't been keeping up on the Chinese motors
 
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Ryan843

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Here are some pics of the build.

bairdco, you happen to know where I could find an idle jet kit for a GT90 kit? The GT80 jet kit has main jets that are compatible with the GT90 but a reviewer said only the jet kits’ main jets are compatible with the Keihin clone carb that comes with the GT90.

We’ve already tried adjusting the float level and like you said, it didn’t change much
 

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indian22

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Ryan you have the engine I referred to: it's a case inducted reed valve engine and if it came from Gas Bikes it has a third port machined into the jug and a window piston...Kehin carb....so what I suggested is quite applicable although the idle circuit could be way off as well. Reed engines are night and day different than piston port timed motors; like come with the stock China girl kits.

Stiff reeds when new block almost all air/fuel flow coming into the motor at low rpm, the engine has to work hard to get started but then when rpms build up the motor acts like an air compressor and causes the reeds to open really open up and the engine then has ample air and fuel to run properly at speed once the reeds start to flex the motor should start and pull at low speed, though idle circuit may need adjustment it's true. Good news is the reeds typically don't take nearly as long to break in as the engine does.

Good luck it's a nice looking motor!

Rick C.
 

Ryan843

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The bike we built before this one had a reed valve and we never had any issues like this with it. I’m sure there’s a fair amount of break in with it and I hadn’t considered the stiffness of the reeds. If nothing else works I’ll try breaking it in some but it’s difficult to do when you can only run it between 15-20mph lol
 
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indian22

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Ryan that's one way to learn trial and error. I prefer the RSE reed valves because they have soft pedals like the major motorcycle manufacturers used, no noticeable obstruction from new (though they made better power later after break in) so it's relative. Did you check ring gap with feeler gauge when installing the piston, a tight piston makes a bike hard to start and sluggish as well.

Rick C.
 

bluck

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The piston came preinstalled in the cylinder body, so no I didn’t check that
I honestly would put the expansion chamber now. It will allow the engine to breath better it will lean it out which sounds like thats might be the problem with your carb(too rich). I only use the keihin carb on all my build now. Same carb for both 4 strokes and 2 strokes they run great. The GT80 and GT90 carb jet kit will work(pilot and main jet). The SBP pipe you got is a decent pipe but for a beast like this you could go with something a bit bigger. I honestly your pilot or air screw are out of tune. Try screwing in youe air screw all the way incand then turning it out 2 full turns see how it runs then try 1.5 turns then 1 turn. Keep the idle jet in the middle and adjust from there. Theres a good site that shows the process of tuning this carb which I follow on all my builds and works great. I'll post it later today (its 4am im on my phone and gonna sleep soon).
 
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Ryan843

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I’d love that video if you can find it!

Swapped the reeds out for some that have already been broken in from another bike and it made a noticeable improvement. It’ll idle now if you feather the throttle a bit.
 
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indian22

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I'm glad you followed that piece of advice, now put the x chamber on as suggested to lean it out and build low end torque you will feel immediately, especially since it's been hesitant to run at all to this point, then start to tune the idle and mid range adjustments, don't bother swapping jets till you get to the point when you are running 32:1 mix carb...this stuff is old news I've been building 2 strokes for almost 60 years...putting miles on the new motor is so important to getting it running right...ride and adjust one thing at a time. Now that it runs better take time to ride close to home...it's a pain to pedal far when something quits on you.

Bluck's right, this engine should perform really well with the 44 tooth, which is a really good city ratio. Also his advice on adjusting the Kehin carb sounds solid since he runs them on multiple bikes and for the same reason his advice on a good video for Kehin carb tune should be helpful.

It's a joy when a new motor runs well for the first time!

Rick C.
 
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indian22

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I honestly would put the expansion chamber now. It will allow the engine to breath better it will lean it out which sounds like that might be the problem with your carb(too rich). I only use the Kehin carb on all my build now. Same carb for both 4 strokes and 2 strokes they run great. The GT80 and GT90 carb jet kit will work(pilot and main jet). The SBP pipe you got is a decent pipe but for a beast like this you could go with something a bit bigger. I honestly your pilot or air screw are out of tune. Try screwing in you're air screw all the way incand then turning it out 2 full turns see how it runs then try 1.5 turns then 1 turn. Keep the idle jet in the middle and adjust from there. Theres a good site that shows the process of tuning this carb which I follow on all my builds and works great. I'll post it later today (its 4am im on my phone and gonna sleep soon).
Good advice Bluck as I've noticed you giving, and I've not commented on, in several of your previous posts. It's a good thing to see solid advice from forum members helping others. We don't always have the correct answers for specific problems encountered, but providing correct motor information may help down the road as others read these posts they pick up detailed information that is useful.

Rick C.
 
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Ryan843

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Yeah I really appreciate all the advice from you guys, you know your stuff. The next thing is adding the expansion chamber, just gotta buy some flex pipe because using the metal pipe that came with the SBP x chamber will get in the way of the pedals. I’m not really sure what to change on the carb other than the float level and the idle screw, I don’t see anything else to adjust
 
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indian22

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Yeah I really appreciate all the advice from you guys, you know your stuff. The next thing is adding the expansion chamber, just gotta buy some flex pipe because using the metal pipe that came with the SBP x chamber will get in the way of the pedals. I’m not really sure what to change on the carb other than the float level and the idle screw, I don’t see anything else to adjust
Ryan some more advice you didn't ask for. Instead of flex pipe buy a set of wide pedals to clear the pipe. Yours is a common problem with these bikes and the wide cranks do the trick. These are for the JAP square taper crank axle and normally are for 1/2" pedals, that is pretty common. Many vendors have them, but price varies. If you don't have a pedal lever puller you'll need one to get the old pedals off, they are inexpensive Changing the hard pipe for flex will alter any benefits the x chamber provides...plus the flex material ends up leaking in short order.

These bikes will nickle & dime you to death, but are worth it.

Rick C.
 
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Ryan843

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And we got another great suggestion! I like that idea, I think that’s what I’ll do. You aren’t kidding these projects nickel and dime you, but it’s worth it
 
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indian22

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And we got another great suggestion! I like that idea, I think that’s what I’ll do. You aren’t kidding these projects nickel and dime you, but it’s worth it
Yes Ryan in the end all our input into your projects, or anyone's builds, are just suggestions. Your build and your final decisions always hold sway and that's the way it should be. It's very easy for me to spend your money with my suggestions sometimes based on my personal preferences for bike fabrications, but in the end they are just suggestions on my part and after all these years I'm still learning too.

Rick C.
 
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Ryan843

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My dad’s put ~25 miles on the bike and says it’s starting to run better. I’m gonna go over there this weekend and ride it myself, probably gonna bring it back with me to ride to class to put more miles on it.

Still need to order some wide pedals to install the expansion chamber, I’ll do that soon
 
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indian22

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That's great Ryan sometimes these bikes come to life on the first or second tank of fuel others a bit more, but you'll know when it happens. When it does switch to 32:1 mix & that motor should really sing, especially with the x-pipe eventually installed.

Both you guys have fun with it.

Rick C.
 
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