I have a 33cc gas scooter and it starts but when I pull the throttle it cuts off. what could be causing this to happen?

i've already got a new carb,fresh gas with correct oil mixture, i replaced the piston and rings, yet i still find small amounts of oil on the spark plug, is this normal?  it starts and runs but cuts off when i try to ride it.       ????

Picture of I have a 33cc gas scooter and it starts but when I pull the throttle it cuts off. what could be causing this to happen?
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dsénécal5 years ago
by the looks of your picture you need a air breather with a chock and sponge.
lemonie7 years ago

What about the clutch?
Does it run throttled on the stand with t'rear wheel off t'ground?

L
japanda1 (author)  lemonie7 years ago
no, as soon as i pull the throttle it cuts off, but if i slowly pull the throttle the wheel will slowly began to spin. i am not able to get it up to full throttle befote it cuts off. im not shure about the clutch

Rebuild regardless, I think it's a fuel flow problem.

L
japanda1 (author)  lemonie7 years ago
where might this fuel flow problem be occurring? what could be the cause of such an issue?
japanda1 (author)  japanda17 years ago
i just got a carb that i ordered the other day and bolted it on. the motor is still displaying the same symptoms that i described.

There's all the line from the tank and the throttle-linkage to check. Then there's air-flow through the filter and reed-valve.

Assuming it was rebuilt perfectly correctly.

L
japanda1 (author)  lemonie7 years ago
i've already replaced the carb with a brand new one, yet the motor still cuts off etc. also,where is the reed valve located on the carb, and how could i check and service it if necessary?

If it's 2-stroke the reed-valve is on the block were the carb' bolts on.

L
japanda1 (author)  lemonie7 years ago
after a final inspection, I noticed that 1 of the 2 mounting screws on the carb was still loose after i had already secured it. it turns out that the threading in the intake manifold has been stripped pretty badly. i think the loose screw is causing an air leak between the carb & intake manifold. i placed an order for a 33cc manifold on e-bay i'll see if it does the trick and let you guys know.
There you go, have a good look and you find something. Do you think they could be re-tapped?

L
japanda1 (author)  lemonie7 years ago
I doubt it, the threadings are really destroyed.

Could you cut new threads?

L
japanda1 (author)  lemonie7 years ago
OK i'm going to test out those posibilities. thanks
NachoMahma7 years ago
. Loss of compression will result in less power, but should not keep the engine from reving up (at least a little bit) under no load. (Usually, with a 2-stroke engine, combustion chamber problems tend be be too much compression due to the buildup of carbon deposits.) It still sounds like a carb problem to me.
. Find a pump oiler (or a mister you can afford to throw away when finished) and put some gas/oil mix in it. Crack the throttle and squirt a _little_ bit of mix into the carb throat (don't want to flood it). If the speed picks up, you're not getting gas to the high-speed circuit.
. You might be able to look down the throat and see if gas is flowing from the venturi, but it will be very difficult to do with such a small engine (there's not much gas flow even when it's working right).
. Hey! I don't see a choke! Where'd it go?
japanda1 (author)  NachoMahma7 years ago
this scooter never had a choke when it was given to me about a year ago;i have an aftermarket air filter that i usually strap on when i ride it. how would not having a choke affect the operation of the motor? also, i just got a carb that i ordered the other day and bolted it on. the motor is still displaying the same symptoms that i described.
. The choke enrichens the A/F mixture (primarily when the carb and engine are cold) so that enough gas will vaporize to burn.
.
.
.  Not sure what to think about a brand new carb displaying the same symptoms.
.  Possibilities I can think of:
  • Gas tank cap is not vented (pulling slight vacuum on tank)
  • Fuel line from tank to carb is partially blocked
  • No choke (try my experiment from a prev msg)
  • Reed valve(s) not fully opening
  • Exhaust valves not fully opening
  • Muffler plugged up
japanda1 (author)  NachoMahma7 years ago
after a final inspection, I noticed that 1 of the 2 mounting screws on the carb was still loose after i had already secured it. it turns out that the threading in the intake manifold has been stripped pretty badly. i think the loose screw is causing an air leak between the carb & intake manifold. i placed an order for a 33cc manifold on e-bay i'll see if it does the trick and let you guys know.
. I may owe you $0.35 after all. ;)
. A very little bit of RTV silicone will help seal the gasket (and hold it in place if re-assembly is tricky).
.  If it has reed valves, this would be a good time to inspect them.
japanda1 (author)  NachoMahma7 years ago
hey,no worries! thanks for the tips, i'll keep you updated on my progress.
japanda1 (author)  NachoMahma7 years ago
i'm going to test out those posibilities. thanks
japanda1 (author) 7 years ago
yes the carb is adjusted properly, and there should'nt be any blockage because I rebuilt it a couple days ago. as for the clutch,i'm not shure how to tell what condition it's in. when it's on stands,and i run the motor and give it some throttle, the wheel starts to spin a little and the motor just cuts off...........
> I rebuilt it a couple days ago
.  Let's be blunt: if you're that good of a mechanic, why are you asking questions on a general DIY web site? I've got $0.10 (US) that says you let a piece of trash get into a passage or misaligned a gasket. $0.25 says that whatever the problem is, it's because of a botched rebuild. Carburetors are precision instruments.
japanda1 (author)  NachoMahma7 years ago
actually the initial purpose for rebuilding the motor was because the problems that I listed had already occurred. I guess that eliminates your "botched rebuild" theory -$0.35 (US)... Also, I don't consider myself a "good mechanic" at all; in fact i've never worked on any motor before. Everything that I know is the result of research that I have done online............thanks for the tips though
> that eliminates your "botched rebuild" theory
.  Nope. You could have even made it worse. Very easy to do. I've had to rebuild more than one carb the second time because of dirt I didn't manage to clean out or a lopsided gasket. Rebuilding a carb is not an easy job for even an experienced mechanic. If the carb had been rebuilt properly, the engine would be running. ;)
.  I'm still betting it's a blocked passage or gasket. Keep us informed of what you find.
japanda1 (author)  NachoMahma7 years ago
Yep. the motor does run and idle fine; before i rebuilt the carb the motor wouln't start at all. it could not be a blocked passage ither because i blew all of them out with high pressure air when the carb was torn apart. a gasket problem is unlikley because i made shure that each was replaced back to it's original location. i'm confedent in the work i did on the carb,and i'm shure the issues that i'm facing have nothing to do with it. i do things right the first time around. ;)
i'll let you guys know what happens though
.  Been there. Done that. Got the t-shirt. Carbs are very finicky.
.
.  I'm still not convinced that the rebuild went well, but, assuming it did, the only other option I can see is that the high-speed A/F mixture is way off. But you should have set that "in the ballpark" during the rebuild.
.  If the carb has a "power pump" (or "acceleration pump"), it may not be functioning properly (but I don't remember seeing the pump on small engines I've worked on).
.
.  BTW, a little oil on the spark plug is not unusual, especially in an engine that has a lot of wear. Spark plug images via Google Images.
japanda1 (author)  NachoMahma7 years ago
i'm quite certain that this motor does not have a "power pump", nor an "accelerator pump". the air-fuel mixture has been set to it's default position(1 & 1/2 turns out). i am convinced that the rebuild was a success based on the fact that the motor didn't even start before the rebuild,and now it starts right up and runs no prob. I was thinking that it may be that the cylinder head has gone bad somehow, causing a loss of compression; what are your thoughts on that?
airsofter17 years ago
Is the carb adjusted properly?
NachoMahma7 years ago
.  It sounds like the idle circuit is OK, so I'd look at the main jet circuit, especially any blockage between the bowl and venturi.