Small Engines (Lawn Mowers, etc.)/carburetor choke problem


QUESTION: My knapsack trimmer won't start without the choke 'ON' and when it does, the engine revs up and finally dies off as I put the choke 'OFF'. The throttle is set at minimal. In the meantime,  the air filter is not fitted.

I have checked the carburettor and found nothing abnormal. What could be the fault? Could it be air leak somewhere?

Please share your expertise.


ANSWER: It is possible it is an air leak but I would suspect the carb is the problem.

Do you have a Walbro or Zama carburetor?  What is the carburetor model number?

Let me know.


---------- FOLLOW-UP ----------

The marchine is China made, quite a good design. The carb is a very simple unit. How do I check for air leaks?

You will need a pressure tester/pump with a gauge.  There are many available online.

You can find adapters that you can use to pump air into the engine through the spark plug hole. They are pretty handy for 2 cycle engine pressure testing.

Many engine manufactures sell special block-off plates and gaskets that seal the muffler port and intake port.  There are too many and they cost too much.

Often I just use an old rubber inner tube or rubber gasket material to seal off the ports.  Just remove the muffler and carb, cut the rubber to the basic shape, make some hole for the bolts and re-install the muffler and carb with the rubber gasket you made in between them...tighten and the rubber should seal.  Just pump 3-5 PSI into the engine and see if it holds pressure.


Small Engines (Lawn Mowers, etc.)

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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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