trimmer tank pressure

[38racing]

Can't recall if it was this forum or another that had a thread on this. I have a friend's craftsman trimmer with wt298B walbro. It quit running well and i suspected bad gas because I sold my friend some that might have come from another can found to have bad stuff. I can get it running but not consistent. I keep having to adjust needles. I got a good 15 minute run but then had string issue and didn't want to run right after. At this point I conclude likely plugged screen and open up carb. In this case there was no screen. (hope I didn't do that. I had it before but maybe just adjusted it). Anyway when taking off carb I pulled primer line from tank and tank to carb line off carb. The carb line will stop draining fuel if I cover the primer hose hole in the tank. It will start if I either uncover the hole or loosen the fuel cap. Does this indicate a fuel cap problem? I don't see the tank venting through the primer lines as bulb usually has gas in it so can't be a vent?

[KE4AVB]

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.

[KE4AVB]

Bump...

Just wondering if you got this resolved 38Racing?

[38racing]

I sprayed inside with carb cleaner and compressed air.Installed a screen, new pump diaphragm and gasket, new metering diaphragm. Found that fuel pickup line was pretty brittle and the filter was loose in the tank. And when I pulled line from tank I left a piece right in tank hole (found it when putting new line in). New line from primer to tank. Line from primer to carb seemed ok. Back together I took a bit of effort to get it running. Used carb cleaner into intake. Finally got it running both high and low. Not sure what rpm should be for trimmer but low is about 3600 and high will hit 8000. After running a bit I tried an immediate restart. Ok. Later tried a restart after sitting. Ok. So tomorrow will be the big test. And I'm doing it with the original fuel cap on tight.

[38racing]

friend has been laid up a bit so still has tried it.