by KE4AVB » Wed Aug 09, 2017 7:45 am
With fuel should only venting if there is vacuum on the tank. Since you have pressure then it is probably okay. With the tank pressuring some I suspect you have a flooding problem or metering problem. BTW the primer has one way check valves (two of them) so reverse venting isn't possible.
Try pressurizing the fuel in line of the carburetor and see if it will hold 5-7 PSI. If it doesn't then you have a leaky needle/seat or stiffen metering diaphragm. Also a stiffen metering will not allow the fuel to flow if it can't open the metering valve. It does how the fuel is working if the metering valve doesn't open fuel can't get pushed to the metering chamber. As for checking metering side normally it is not possible due to setup. Very few I have found that I can do vacuum check on. And with main nozzle check valve you definitely can't pressurize can't pressurize it.
When it comes to the metering diaphragm first make sure metering lever height is correctly and the gaskets are the correct order. It is gasket to carburetor body then the metering diaphragm. Most carburetor issues are usually related to the metering diaphragm or leaky metering needles once a carburetor has been cleaned here. I have only seen a handful of bad seats and a few bad main nozzles over the years.
Personally when it comes to any cube carburetor issues I generally clean them from it start in my UC which saves a lot fooling around trying different things. But in your case with internal filter missing I would definitely clean it with both mixture screws removed. Trash in the metering ports will give you fits as mixtures can vary as partial plugging occurs then clears.
If the carburetor proves to be fine then you should pressure and vacuum check the crankcase as cylinders do come loose and crankcase seals do go bad. Most times they show up when you operating the trimmer at various angles. You must have a good vacuum from the crankcase via the impulse port in order for the metering diaphragm to open the metering needle via the lever; hence, to allow the fuel to flow.
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