GT-5 bogs down at higher RPM

PatTheThird

New Member
Apr 24, 2011
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Southern Maryland
I've had this gt-5 for a few months now. It starts imediatelly and idles great. For some reason once i reach an RPM range the motor backs off like its loosing fuel\air. I think there is a leak in my air intake somewhere. I have a carb ring from Sick Bike Parts and the intake itself is air tight. I am using an exaust gasket as the gasket for the air intake. I had a CNS carb on it until i broke the mount by overtightening. So now it has a NT. I have tried the needle in all of the settings. Ever since i put the expansion chamber from SBP the problem is exagerated. if i rev the motor with the clutch in it modulates up to around 6500 (guess) and then bogs down to 2000 or so and then back to 6500. i have changed the CDI, plug, wire and magneto so the electric is straight.the motor was broken in good and has good compression. Oh yeah, i had to buy the offset intake to clear the frame. I had to cut the intake and reweld it in a slightly different angle. The carb isnt sitting flat but it is close to flat. I have tried positioning the bike so the carb is flat with no change.

PLEASE advise :)
:-||
 

Allen_Wrench

Resident Mad Scientist
Feb 6, 2010
2,786
3
36
Indianapolis
I remember Bairco mentioning something about petcocks, and getting enough fuel when the engine needs it most. If it runs fine otherwise, start looking in this direction. See how big a hole you got running through your petcock when the lever's open. If it's teeny tiny, you may want/need a better petcock. And look at your fuel line, fuel filter, etc. You want to be able to get plenty of fuel into that float bowl quickly when you need to run wide open.

And if that was the problem, don't thank me. I read it here on the forum, so I owe these people too.
 
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Bob Mac 18704

New Member
Jan 24, 2010
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penna.
Mine did the same thing when I put the SBP expansion chamber on. I went to a #72 jet and it works perfect. Others mileage may vary.dance1
 

PatTheThird

New Member
Apr 24, 2011
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Southern Maryland
Really?! you guys had the same issue exactly? Thats very relieving to know its such an easy deal. I could look it up but.. what do you do, take off the float bowl and thread it out?
 

Bob Mac 18704

New Member
Jan 24, 2010
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penna.
Get the jet from SBP. Mine is a speed carb. You mite get a few size jets. # 72-74- 68. both sides of the spectrum. You could also buy the drill bits and drill your own.
 

DuctTapedGoat

Active Member
Dec 20, 2010
1,179
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34
Nampa Idaho
I ended up settling on the 74 - but it all depends on how your bike runs. I'd say get a 72, 74 and a 76. Your stock one is going to be something like a 70ish. On mine it is on the NT. Word to the wise : It's tough to thread them cause they're new and have sharp pointy threads, so don't be scared and just torque em down. Though - don't cross thread it, but with a thick pitch like they've got it's pretty obvious.
 

PatTheThird

New Member
Apr 24, 2011
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Southern Maryland
So, it turns out the float needed to be adjusted. I bent the arms on the float needle and the carb started geysing fuel but its not surging anymore! Unfortunatelly not before taking my main jet to a local mechanic and having it drilled out to .76. So now its way to juicy. Oh well, now im ordering a new .70 to get back to baseline. I am seriously considering the Dax carb.
At one point my bike hit the perfect fuel air mix and rocketed foreward from where i thought top speed was close to. I was going way way faster than was comfortable.
LOL! theres nothing like realizing your engine has been poorly tuned for 6 tanks of gas and the true potential is twice what you thought it was.
 

Bob Mac 18704

New Member
Jan 24, 2010
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penna.
76 is way to small of a jet.76 would be .020. A # 72 would be more like it .025 The standard is #70- .028. Your move to .020 is .008 way to large for these little motors. You want to lean it out just a little. I would suggest that when you order your #70 to get a #72 and try it first. I think you'll be happy with the results. Just my opinion
 

roger41

New Member
May 25, 2011
2
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winnipeg
these are all good ideas to try but i have the same or close to the same problem. here goes ,,, i just got my gt5 and put it together over the weekend and did take my first ride in the rain but not far. it did seem to run ok then , i have ran it close to a full tank at 16 -18 to 1 ratio and so far it seem like i have to play with the choke to get any mid range power and if i play with it right i can get to top end and it runs good ,, i will slow down and will have to start all over to get to speed, it idles great and with out any load it will rev up fine. if i dont play with the choke it dies out like it has no fuel....i am at a loss with this and not to happy... oh yeh every thing is stock or came with the kit
 
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PatTheThird

New Member
Apr 24, 2011
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Southern Maryland
Roger i cant give you any advise on your prob. I havent ever had to give my bike any choke even on start. i had massive issues with the stock POS red cover carb that I bet you have. The CNS V2 needed to have its fues/air screw run all the way in to run half way decent. After i installed the SBP pipe i had to unscrew 3 turns to bring it back into range. I bought the thatsdax carb. I hope it was good money well spent.
 

phoenix-on-wheels

New Member
Jun 23, 2011
4
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0
Charleston South Carolina
Can some one please explain to me why when i reach 20mph on my Schwinn motorized bike with a 2 stroke one cylinder engine, it has way too much excessive vibration that feels and sounds like the engine is going to blow up and fall apart starting at about 20mph and wont reach its potential top end speed of 35mph and also bogs down and acts like its being starved for air & fuel, but when i back off from full throttle at that speed the bike slows down no vibrabtion , engine running smooth, but when i reach 20mph it does the same thing! Im frustrated and trying to find a solution to this problem with my engine! Can anyone please explain to me what i need to do to correct this problem?
 

phoenix-on-wheels

New Member
Jun 23, 2011
4
0
0
Charleston South Carolina
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any updates on the thatsdax carb?
Can some one please explain to me why when i reach 20mph on my Schwinn motorized bike with a 2 stroke one cylinder engine, it has way too much excessive vibration that feels and sounds like the engine is going to blow up and fall apart starting at about 20mph and wont reach its potential top end speed of 35mph and also bogs down and acts like its being starved for air & fuel, but when i back off from full throttle at that speed the bike slows down no vibrabtion , engine running smooth, but when i reach 20mph it does the same thing! Im frustrated and trying to find a solution to this problem with my engine! Can anyone please explain to me what i need to do to correct this problem?