2 Cycle Engine Questions

Old 05-26-03, 08:32 PM
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25cc Sears Weedeater fuel mixture

I recently bought a fleamarket weed eater for 5 bucks. I left the engine internals alone for now, I am reluctant to open the engine for fear of having a spring pop out somewhere that I cannot get back in. But since this is a learning engine for me I will be doing that next weekend.

The eater is a Sears 25cc. I don't have any other documentation.

It will fire and run but only if I pour a small bit of gasoline directly into the piston. It will then run for only as long as the gas in the piston remains, about 2-4 seconds.

Never having used 2-stroke engines before I don't know what the fuel mixture was so I guessed at 30:1 based on what I could find on the internet for related engines.

Is this a sign that the mixture is off? What mixture should I use? Is the carburetor not pulling fuel? Do they use a diaphragm to pull fuel? I don't even know how to properly mix 2-cycle fuel.

Any debugging advice?

Old 05-26-03, 09:24 PM
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Hello Oscar. Welcome to our Do-It-Yourself Web Site and our Small Engine forum.

Based upon the problem description, the carb is gummed up with stale fuel gums and varnishes. The carb will need to be cleaned out and rebuilt with new carb kit rebuilding parts.

Most likely the fuel tank will need to be flushed out and the fuel filter inside the tank replaced.

Quick test for fuel flow from the tank is to remove the fuel hose at the carb. Fuel should flow out freely for at least one minute. If it does, the flow is good. If not, there is a restriction.

Most likely place but not the only place is the filter and or the fuel line. If the fuel will not flow out freely or do so for a minute or so, remove the fuel tanks fill cap.

If the flow resumes flowing freely, the vent hole in the cap is restricted or plugged up. Cap replacement is the best solution.

If there is no change in fuel flow out of the tank with the cap removed, the restriction is in the fuel line or filter.

Should all test be satisfactory, the carb is gummed up and in need of cleaning and rebuilding.

Most likely I haven't covered all of the potential possibilities. The other resident small engine service and repair professionals may offer additional suggestions, advice & help.

Check back on your question several times over the next few days for additional replies.

Small engine repair parts are available at all local small engine repair shops and or lawn mower repair shops. They are listed in the phone book directory in every town.

Read the already asked questions and the replies offered in them on this same type of problem for additional help & information. Doing so will provide additional possible causes and the solutions to correct them.

If you need further assistance, use the REPLY button. Using the reply button moves the topic back up to the top of the daily topic list automatically.

Regards & Good Luck.
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Old 05-26-03, 11:53 PM
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Hello Oscar!

I agree...you have a fuel delivery problem. If the mentioned tests go well, purchase a carb kit and clean and rebuild the carb. Let us know how it goes!
Old 05-27-03, 10:26 PM
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Thanks for your help

Hello All,

I took off the carburetor and removed the slime that was coating everything. I didn't have any ether/carb cleaning spray so I could only mess around with some gas that was handy.

It worked well enough, the motor still would not turn over but this time fiddling with the Low/High/Idle got it to run without cutting out. I think the slime was the main culprit though.

What does that L/H refer to? I know that Idle affects the air flow through the center of the carb but I don't know how adjusting the Low/High screws makes a difference except that too little low and the motor will not stay running. Too much high and the motor chokes off.

I am amazed at how tiny everything is. The through holes are near the diameter of a needle and I could not follow where they went into the carb. I have never taken a close look at a carburetor before.

I could not get access to the piston and cam. Neither the rope pull racheting sprocket (or whatever it is called) would come off nor the clutch on the other side of the block. I don't know how to get them off.

For being a flea market weed eater left out in the rain and smashed against a wall or something (the body is broken and the gas cap was and still is missing when I bought it) it runs pretty well now.

Thank you for all of your recommendations, I hope to have actual worthwhile and sophisticated questions as I get further into this and seek out more engines before casting my own.

Thanks again,

Old 05-28-03, 12:21 AM
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You adjust the low speed screw when the engine is at idle, and you adjust the high speed screw when the engine is wide open. Adjust the each until the engine runs smoothly.
Old 05-29-03, 06:11 PM
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LH Control


I figured that is what it was but I guess 1-turn increments are too much, it makes it either run like it is going explode or stall. I finaly have it.



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