carb not getting enough fuel at high rpms

meanoldrunk · Apr 23, 2013

hi. ive been reading this forum and another one since january and found a lot of useful info on both sites. now for my problem that im having and hoping someone here has a solution, i have a 66/88

cc something or another that i got from ebay. it runs perfect except... when im getting on it at wide open throttle according to the gps on my phone i only hit 37mph. at that point the bike dies from lack of fuel. the clear filter is dry. if i back off the filter gets gas and everything is good as long as i dont go above 3/4 throttle. at the point it starts to do die i can really feel it starting to reach its full potential and then it craps out. the bike is a k2 zed with a 36 tooth sprocket, speed carb, and a expanded chamber exhaust. i know its capable of doing more not that i want to more , i just want to know what i can do. i read something earlier about vibrations that might be the problem but it doesnt really shake much. and if i unhook gaslines etc. fuel pours out. any ideas? thanks for reading

meanoldrunk · Apr 23, 2013

sorry didnt mean to post twice trying to figure it out how do delete it

bigbutterbean · Apr 23, 2013

It sounds like the fuel filter is positioned in a way that is not allowing fuel to flow through completely, working fine when less than full throttle but the carb is sucking too hard at full throttle. re-position your fuel filter for better flow and you should be good to go. The two best positions are right below where the fuel line attaches to the gas tank, or right above where it attaches to the carb. a fuel filter sitting sideways will cause fuel flow issues, I recently had a fuel flow issue caused by the same.

bairdco · Apr 23, 2013

i used to have that problem til i got rid of the stock petcock on the tank.

i use a bunch of differen ones, my favorite are from old tecumseh lawnmowers, but sick bike parts sells a good inline one.

there's been a lot of debate about this, whether changing the petcock works or not, but there's a lot of people besides me who've tried it and the problem went away.

Risingsunn · Apr 24, 2013

meanoldrunk said:
at the point it starts to do die i can really feel it starting to reach its full potential and then it craps out.

I think that the petcock is not the only issue that is going on here. He also indicates that when the bowl empties it gains power. Generally to get that power all the time you have to adjust the position of your float by slightly bending the metal forks that are right above it, but for the life of me, I don't remember if it's up or down. I will have to look into that and see if I can find the post I remember reading...

crassius · Apr 24, 2013

not sure what it is, but these engines take VERY little gas - 50 or 70 miles on a half gallon tank is a pretty slow stream - seems hard to dry out the carb bowl

bairdco · Apr 24, 2013

Risingsunn said:
I think that the petcock is not the only issue that is going on here. He also indicates that when the bowl empties it gains power.

as the float bowl empties, it'll lean the motor out, causing that sudden surge of power.

biknut · Apr 24, 2013

Another good thing to check is to make sure the vent in the gas cap is adequate for top speed fuel flow.

Risingsunn · Apr 24, 2013

I found one of the posts I was looking for:

http://motorbicycling.com/showpost.php?p=403288&postcount=5

"Sounds to me like you need to do a float adjustment to lower the fuel level in the bowl. If you're getting a surge of power when you ride with the fuel off, and it is reproducible, then a float adjustment should be able to help keep your carb constantly in that condition. The float adjustment alone might lean out your carb enough to not need the wire or smaller jet after all. Give it a try...."

Toadmund · Apr 24, 2013

biknut said:
Another good thing to check is to make sure the vent in the gas cap is adequate for top speed fuel flow.

Check that one first.
This is obviously a fuel flow problem.

Is there any bubbles in your line at WOT?

meanoldrunk said:
i only hit 37mph. at that point the bike dies from lack of fuel. the clear filter is dry. if i back off the filter gets gas and everything is good as long as i dont go above 3/4 throttle.

Hang on, reading comprehension check, your filter goes dry?
Check your petcock for rust, flow rate or other debris, or even a kink in your line, sounds like a fuel valve (petcock) problem. (or what biknut sez)

rustycase · Apr 24, 2013

All good advice above!

... yet it seems to me 37mph is exceptional for a motorized bicycle. Really Great!!!

IMO, be very cautious making changes, if it's running that kinda speed!

rc

meanoldrunk · Apr 24, 2013

just got home form work and am gonna tinker with it. just wanted to say thanks for the reply's. i dont know if it has airbubbles in the fuel line as i am using black gasoline tubing from autozone. the fuel filter does sit sideways and always has a large air bubble in it. distance between the tank and carb is about 9 inches. so im starting to lean to float and /or sticky needle valve. could also be the gas cap not venting well either. not sure what to look for on that though. once again thanks for the replys. if all else fails i'll order another carb and see how it goes.

wan37 · Apr 24, 2013

Just losen the gas cap but don't take it off and ride it you'll notice a different if that is it.

meanoldrunk · Apr 24, 2013

ok got half the problem fixed, kinda. i bought a fram gas filter that is bigger and repositioned it so that it is facing downward to the carb, also bent the forks slightly to achieve my best guesstimate of what 21mm should look like(by the way a nickel is 21mm, i didnt happen to have one or a metric ruler). rode a bit filter never showed any signs of air bubbles or drying out. so that worked. but it still did the same thing at above 3\4 throttle. acting like it wants to die, then i found this video and this is what its doing. well thanks again im off to do some research on 4 stroking.

http://www.youtube.com/watch?v=0wzph3hL6EE

2door · Apr 24, 2013

If your engine is "four stroking" that is a result of an over rich condition. Not fuel starvation. You might want to start playing with jet sizes to lean out the mix.
Just FYI, the needle only controls fuel flow up to about 3/4 throttle. After that jet orifice size determines how much fuel goes to the engine. After 3/4 throttle the needle is pulled up out of the jet and has no bearing on fuel flow.

Tom

rustycase · Apr 24, 2013

Let's try this, Tom...

MOD,
When your motor is warmed up, take off on a high speed run, in an appropriate level ground location, and turn off your petcock just as you take off.

As you begin to run out of fuel, does the engine speed up? or does it just stop running or spit back out the carb a few times...

If the engine speeds up and begins running really well before it dies out for lack of fuel, you have been running it too rich.

If it spits back through the carb, or just stops running, it has been running too lean.

...........

You can also try engaging the choke, partly, when on a hi-speed run...
Does it speed up? or just bog down completely???

If speed improves, your jet is too lean.
If it just bogs down, you might be running too rich.

How's that for simple testing?

rc

Aw crud... of course I have forgotten to mention intake leaks and such! lol
:-||

CrzdCtryRebel · Apr 27, 2013

I had the same problem the fuel tank wasnt able to breath the stock cap wasnt venting

biknut · Apr 27, 2013

CrzdCtryRebel said:
I had the same problem the fuel tank wasnt able to breath the stock cap wasnt venting

This is a very common problem.

2door · Apr 28, 2013

I did a little tutorial on gas caps in this thread> http://motorbicycling.com/showthread.php?t=30967

If you suspect your tank isn't venting this might explain why.

Tom

meanoldrunk · Apr 28, 2013

im pretty ceartin its not the gas cap. i loosened it and went for a spin, sameproblemas before. then i took the rubber gasket off and still the same problem.so ok i want to goto the local motor cycle shop to see if they have 5mm jets.along the way my air cleaner falls off no big deal, musta wiggled loose. continuing on my way with air cleaner in my pocket, it starts running like real crap, sputtering conking out. my choke was loose. now i tightened pretty well so that was odd. looking around for a piece of something to hold the choke open i notice that the front mounts were a tad loose. so i give the motor a little wiggle to see how loose it was. it would move a bit and a bit is a little over too much. on closer inspection the one rear mount bolt was really backed out kinda far, looked at the otherside and it was gone. oddly enough i have never noticed any type of vibration out this bike. so pedaled home, thanking my self for leaving the shifters on the handle bars. went to homedepot they didnt have what i need so i'll try fastenall and lowes tomorrow if not i'll order online somewhere. basically the topspeed issue is on hold for now
this bike is like my kids it can be frustrating at times but SO rewarding the other 90% of the time

carb not getting enough fuel at high rpms

New Member

New Member

Active Member

a guy who makes cool bikes

Member

Well-Known Member

a guy who makes cool bikes

Well-Known Member

Member

New Member

Gutter Rider

New Member

Member

New Member

Moderator

Gutter Rider

New Member

Well-Known Member

Moderator

New Member