Trimmer fuel tank pressure

[Bill_C]

I'm fixing FIL's old trimmer. It's a Redmax with a Walbro carb. I rebuilt the carb and it starts and idles OK but loses revs when I give it full throttle. It will rev up to about 3/4 throttle.

I notice that the fuel tank appears to be intentionally pressurized. I suspect this may be my trouble, the rubber plug (where the 2 fuel lines enter the tank) is leaking and gas is venting out, making the tank lose pressure. I've found vague mentions of this type of fuel system design on the 'net, saying the tank needs 1 or 2 psi of pressure to help force fuel up into the carb for full throttle response.

Sound right? thanks...

 

[Fordlords]

Yes, it's very common on reed valve type 2 stroke engines and still used on almost all even 4 stroke trimmers to this day. It's exhaust gas flow being used as a poor man's fuel pump to keep fuel flowing in to the carb and prevent starvation when the engine is tilted at various angles. It works, but is not always the best method as sometimes that type of aeration can make the fuel foam up putting air bubbles in the fuel lines, causing erratic engine operation.

-Fordlords-

 

[Nasty135]

Alot of two stroke engines once they get some time on them start to carbon up the exhaust spark arrestor (screen/muffler) and cause poor performance...
cleaning it will usually restore it to proper running performance

Assuming the mixture screws are adjust good

 

[J_J]

Bill,

If the engine does have a screen in the exhaust tube, remove it if using it around the house. The screen works as a spark arrester, and are hard to clean, or, buy a new screen for the exhaust.

 

[Eagleview]

I rebuilt the carb and it starts and idles OK but loses revs when I give it full throttle. It will rev up to about 3/4 throttle.

There are usually two mixture screws , one is adjusted while the throttle is wide open . When it is on song it will rev to it's limit . Wear ear protection while doing this .

 

[Bill_C]

This carb does not have mixture screws (Walbro WYJ 117). High speed mixture is "adjusted" by bending the needle lever.

I found the check valve (beneath the purge bulb) was torn. I've replaced it but the carb still runs very rich, it 4-strokes at full speed. I now suspect the needle seat is worn--even bending the needle lever to an extreme still has the carb too rich.

Time to buy a new carb since the needle seat is not replaceable. Thanks for the advice!