QuestionThanks Eric, i`m replying to your reply re. clean petrol tank. No, just the diaphragm and gaskets. The trimmer is 20 years old.The compression seems ok. There`s no spark arrester on it and the exhaust port is quite clear. It seems that when it starts and stops it has used up the petrol in the carby and petrol is not continuing to be fed from the petrol tank. I have adjusted the " L and H screws" several times as per the manual. Terry
AnswerAre you familar with how those little carbs work? You can find manuals on the Walbro and Zama websites depending on which carb you have.
Being the trimmer is pretty old, I would get a compression reading. In my younger years I did not have a compression gauge and spent many hours trying to repair a "carb" problem when it turned out the compression was too low.
However, you can be pretty sure the carb/fuel system is working by checking a few things. First, the carb needs a "pulse" from the engine crankcase in order for the carb fuel pump to pump fuel from the fuel tank. Check to make sure the cylinder jug bolts are tight as well as any engine case assembly screws/bolts. Crankcase leaks can often be found visually as dirt will accumulate around the leaky area. We use a crankcase pressure tester to check for leaks.
I have a lot of time on similar trimmer models as yours and they don't tend to leak but it can happen.
The second thing to check is the carburetor to engine block insulator and gaskets. You will notice a hole in the carburetor outer housing...this is for the "pulse". This hole needs to get the "pulse" from the engine. Make sure the carb insulator block and gaskets allow the "pulse" to enter the carb. FYI, the carb insulator hole and carb hole rarely line up directly. The "pulse" is usually routed to the carb...the main point it to make sure the hole/pulse is not completely blocked...I have had folks make their own gaskets that sealed off the port.
Finally, if you put a metering diaphragm in the carb did you check the inlet needle arm height? Are the gaskets/diaphragms in the correct location? Incorrect gasket/diaphram position can cause the carb fuel pump and/or metering diaphram not to function. Refer to the online carb service manual for your carb.
Finally, if you want to see if you have an actual fuel issue, you can use a primer bottle with fuel/oil mix and keep the engine running by manually priming fuel through the carb. If you can keep the engine running for 3-5 minutes the compression is probably alright. If the engine will not idle while you are priming then I would suspect poor compression...just be sure you don't flood the engine, i.e., don't provide too much fuel and cause the engine to stall.
Let me know if this helps or if you need help finding the carb service manuals.
Eric
