91 Marathon New carb and it won't run.

[barndog324]

I have 91 2stroke Marathon that doesn't run well at low speeds. It wants you to push the peddle to about half throttle and then it launches, which makes it very difficult to tow a car with. I bought brand new carb and put on it and it initially ran fine for about 3 minutes before it started bogging down and eventually would no longer run. So I ended up putting the old carb back on and the engines runs, but just like before. When I put the old carb back on we found that a gasket has fallen off, therefore we thought maybe it was a big enough vacuum leak to cause the issue, so we put the new carb back on. Same scenario, ran fine for a brief time and then wouldn't start at all. Any suggestions would be appreciated. Also the new carb has a 6 inch long clear tube which attaches to a nipple that has a very small hole in it. The old carb has the same nipple but nothing on it. When I remove this clear tube from the new carb it starts dripping gas from the small hole, however nothing comes out of the old carb at this point. I can't find any info anywhere on what this tubing is supposed to be connected to. Any ideas? Thanks!

[cart around]

The tubing doesn't connect to anything. It is a vent for the float bowl.

Both carbs should be spring loaded open. Your throttle cable pulls them closed until you step on the pedal. I would check the spring on the first carb to look for smooth operation.

The new carb should be taken apart, and cleaned. Also check that the float valve isn't catching on anything right at the hinge.

[barndog324]

Thank you for your response. The main reason I replaced the carb is because the return spring under the throttle arm was broken. I had to connect a spring from the governor arm to the throttle arm in order to be run at anything but wide open. Does the tube on the float vent get removed? Otherwise I don't see how the vent could get any air.

[cart around]

Mine is a hollow tube that just vents overboard like an overflow.

Is yours solid, or plugged on the end?

[barndog324]

Mine is hollow as well, however the vent hole is in the side. That's why I figured the tubing would be blocking any airflow.

[cart around]

I have heard of that before.

You need to remove that hose.

[BrewCityMusic]

I'm guessing You didn't dismantle and thoroughly clean the new carb prior to installing it, did You? The Chinese carbs that all the aftermarket guys are selling are about 50/50 as to whether they'll run right without a good teardown first, they tend to have leftover manufacturing gunk, etc in them and it's generally just a matter of time before it loosens up and plugs a jet, etc.


Also, that "vent" is your overflow, and you should only get gas out of it if the float is set incorrectly or stuck (see first statement above RE cleaning carb, LOL) - that short hose is just to direct the overflow (if it happens) away from the hot engine and should NOT be removed. I'd pull the new carb and pull and clean the jets, float valve, all passages, etc, set the float correctly and try it again.

[cart around]

It's drilled on the side like this. That tube is going to prevent any venting, or overflow.

[barndog324]

Yes, that's exactly what the vent looks like. In fact that picture looks exactly like my carb.

[kyerik]

Yes- as others have said- remove the tubing because it blocks the air vent holes in the sides of the metal tube. I bought one of the $18 new carbs of the internet- it flooded immediately. I then removed the plastic tube and it worked great.