Engine sputtering at higher speeds

greasy tom · Apr 29, 2010

Heop! http://motorbicycling.com/images/smilies/auflg.gif
I am having trouble with my 80cc bike running poorly at higher speeds. It starts fine and idles fine. It has tons of torque at low RPM but "breaks up" at higher rpm. It pulls great right up to 10 or 15 mph then starts sputtering and continues to run but quits accelerating. It keeps running but runs as if it is starving for fuel....which I don't think is the case. The tank is full and the filters (2) are clean. New spark plug wire, original plug; properly gapped. Oddly, when going downhill at WOT it quits firing altogether at about 15 mph and will start firing again in a few seconds if I let off the throttle.
Any thoughts?

Brent436 · Apr 29, 2010

Im going to guess that the issue is probably something with the carb. i would check float height and check what setting your c-clip is on the carb needle. a lot of people recommend the c-clip be on the second notch from the top of the needle. i know ive had similar issues with my motor and they were carburator problems. hope this helps.

-Brent

Venice Motor Bikes · Apr 29, 2010

Thy usually aren't starving for fuel, they're too rich! Try moving the C clip inside the carb up a notch.

greasy tom · Apr 30, 2010

Aha! I had swapped carbs with another to see if the problem changed but it stayed the same. I did, however, keep the same slide attached to my throttle cable when I swapped so it could be the needle position. After I finish my coffee I'll go out and move the needle position up a notch.
Thanks!

drhofferber · May 1, 2010

sure it stayed the same....the biggest problem we have with these engines running properly is the intake...in my opinion...either port out your intake or the best move is buy a small intake from pirate or jim at manic machanics....i buy the scrap ones and grind a 40% chamfer on the intake surface...it has a o ring so its going to seal great...you might have to adjust the clutch cable holder, or get rid of it and go a different route...Dennis

diceman2004 · May 1, 2010

why ya tellin him to port the motor and buy new parts ?
you sound like the garages i won,t go to . ( if you put on a new starter , your brakes should work a lot better , cuz the white wire is usually the wrong gauge and the lack of grease usually causes the muffler to burn out prematurely ,,, but if you going to do the starter . ide recommend doing the water pump and put a fresh belt on while everything is apart , or you,ll just end up needing it in 6 months anyways . at least this way we can save you all the labour charges if you get it done now )

Tom ( pictures are worth a thousand words )

the problem worsens going down hill , So the more your carb gets tilted forward .
sound pretty obvious its a fuel supply issue , too little OR too much .
i,m gonna make an assumption and say your motor is sitting at a pretty good angle .
try changing the needle setting and float first like the other guys said ,

gavmiester · May 2, 2010

turn the fuel off and if it starts to improve just before it dies you have to much fuel alternatively try putting the choke on gradually if this helps its running to lean.

Bikeguy Joe · May 2, 2010

Sound like you are running out of fuel in the float bowl.
Also, what does the plug look like?
Is the jet screwed into the carb body all the way?

crazywhiteboy072291 · May 12, 2010

it can also be your muffler to test just take your pipe off and run and see what happens
if runs right get a pipe that is not a pipe with a catalytic converter

Bikeguy Joe · May 12, 2010

So, did you ever get it sorted out?

greasy tom · May 12, 2010

I had to sideline the bike to help with more pressing matters so I haven't had a chance to work on it lately. I should have an opportunity to spend some time tinkering with it soon & will post any results.

saolom · Jul 13, 2010

hi I'm getting the same exact problem with two of my 80cc gt4.

riding at high speeds 45km's I get the feeling starving for fuel 3-4seconds then normal again and then it happens again.

The C clip in the carb is on the second lowest position. Yesterday it was riding fine with great results but today this happens?

I dadded new fuel in today , ratio I think was 50mls -1.2ltr? is this to much guys?

Did you find out wats causing this?

bairdco · Jul 13, 2010

another thing it could be is fuel flow. at higher speeds and higher rpms, you've got a lot more vibration, which could cause fuel foaming in the line.

my bike started doing the same thing at top speed, and i reached down and held my fuel line, and it went away.

i fixed mine by putting a drop of epoxy glue on the line, and sticking it to the frame.

it came off after 2 months or so, and boom, same problem. glued it back on, and it's all good.

bairdco · Jul 13, 2010

crazywhiteboy072291 said:
it can also be your muffler to test just take your pipe off and run and see what happens
if runs right get a pipe that is not a pipe with a catalytic converter

this is very bad advice.

greasy tom · Jul 13, 2010

greasy tom said:
I had to sideline the bike to help with more pressing matters so I haven't had a chance to work on it lately. I should have an opportunity to spend some time tinkering with it soon & will post any results.

I never did find the problem but have been using this MB for slower cruising around the neighborhood & running errands. I did, however, try different settings on the carb needle, replaced intake gasket, checked for air leaks, checked for muffler clogging, checked gas cap vent, spark plug, and plug wire. Nothing helped but now I am wondering if it is foaming in the fuel line. When I rev it up to high rpm, the fuel in the line immediately foams. I'll try to remedy this & let you know what happens.

saolom · Jul 14, 2010

Took my bike out for a test ride and it started up fine and ran great, then when the engine starts to get warm/hot the problem starts over again!? No foaming in the lines. ??

evogav · Jul 14, 2010

saolom said:
hi I'm getting the same exact problem with two of my 80cc gt4.

riding at high speeds 45km's I get the feeling starving for fuel 3-4seconds then normal again and then it happens again.

The C clip in the carb is on the second lowest position. Yesterday it was riding fine with great results but today this happens?

I dadded new fuel in today , ratio I think was 50mls -1.2ltr? is this to much guys?

Did you find out wats causing this?

I too have that problem with my GT4. Runs sweet all the time apart from when i take her up to 30mph. Initially it will do the speed fine but all of a sudden it feels as though the choke is on and theres less power for about 5 seconds - knocking my speed back down to 25mph ish - then it'll build up speed and do the same thing again.

Gonna zip tie the fuel line to the frame to see if this helps a little as mine too vibrates like **** and foams the fuel in the line and filter - no doubt its probably foaming in the float bowl too...

Initially i thought i would lose compression a little when it heats up - i did tighten my head bolts and spark plug and i thought i'd cured it but the second day it all came back again.

klb6154 · Jul 22, 2010

sounds like the floats bowl is drying out if the needle clip didnt change anything this could be caused by the carb being tilted

gtv-gr · Aug 24, 2010

I got THE EXACT SAME PROBLEM on a till now flawless engine, removed the spark plug it's running lean , i know it's something wrong with the ****ty carbyretor might also be an air leak, i use my bike daily going to work here in downtown Athens Greece the thing is a GOD in heavy traffic

ferball · Aug 25, 2010

Foam would indicate air in the lines...Where is the air coming from? Vibes should not put air in the line, unless the air was already there. Right? I am using the logic from days of working on oil burners... So if your gas is foaming at high revs I would think there is a problem with the fuel line, it is sucking air from somewhere at high revs (greater vacuum) the petcock itself might have issues, or where the line seats on your fittings, or even a pinhole in the line some where. The other side of the bad fuel line logic, is that you looked at everything else, and all that remains is the line itself...

Engine sputtering at higher speeds

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