2600Views31Replies

Author Options:

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? Answered

Picture of

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.       ????

31 Replies

user
dsénécal (author)2012-07-27

by the looks of your picture you need a air breather with a chock and sponge.

Select as Best AnswerUndo Best Answer

user
lemonie (author)2010-11-23


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

L

Select as Best AnswerUndo Best Answer

user
japanda1 (author)lemonie2010-11-23

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

Select as Best AnswerUndo Best Answer

user
lemonie (author)japanda12010-11-24


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

L

Select as Best AnswerUndo Best Answer

user
japanda1 (author)lemonie2010-11-24

where might this fuel flow problem be occurring? what could be the cause of such an issue?

Select as Best AnswerUndo Best Answer

user
japanda1 (author)japanda12010-11-24

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.

Select as Best AnswerUndo Best Answer

user
lemonie (author)japanda12010-11-25


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

Select as Best AnswerUndo Best Answer

user
japanda1 (author)lemonie2010-11-25

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?

Select as Best AnswerUndo Best Answer

user
lemonie (author)japanda12010-11-26


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

L

Select as Best AnswerUndo Best Answer

user
japanda1 (author)lemonie2010-11-28

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.

Select as Best AnswerUndo Best Answer

user
lemonie (author)japanda12010-11-29

There you go, have a good look and you find something. Do you think they could be re-tapped?

L

Select as Best AnswerUndo Best Answer

user
japanda1 (author)lemonie2010-11-30

I doubt it, the threadings are really destroyed.

Select as Best AnswerUndo Best Answer

user
lemonie (author)japanda12010-12-01


Could you cut new threads?

L

Select as Best AnswerUndo Best Answer

user
japanda1 (author)lemonie2010-11-27

OK i'm going to test out those posibilities. thanks

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)2010-11-24

. 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).

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)NachoMahma2010-11-24

. Hey! I don't see a choke! Where'd it go?

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-24

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.

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)japanda12010-11-25
. 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

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-28

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.

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)japanda12010-11-29

. 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.

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-29

hey,no worries! thanks for the tips, i'll keep you updated on my progress.

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-27

i'm going to test out those posibilities. thanks

Select as Best AnswerUndo Best Answer

user
japanda1 (author)2010-11-23

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...........

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)japanda12010-11-23

> 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.

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-23

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

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)japanda12010-11-23

> 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.

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-23

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

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)japanda12010-11-24

.  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.

Select as Best AnswerUndo Best Answer

user
japanda1 (author)NachoMahma2010-11-24

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?

Select as Best AnswerUndo Best Answer

user
airsofter1 (author)2010-11-23

Is the carb adjusted properly?

Select as Best AnswerUndo Best Answer

user
NachoMahma (author)2010-11-23

.  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.

Select as Best AnswerUndo Best Answer