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Just got a kit for my String trimmer. (kit made in Taiwan) Upon inspection I find that:

The gas tank Vent Valve in the kit allows only very little draw, & if you add a lil pressure the rubber inside pops away from seated position, while the cap remains on.

Is my original one serviceable ? It does draw air much easier. & it will not allow out going air at all.

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You test a trimmer fuel tank with a pressure / vacuum meter.
It should hold 10 psi of pressure and have no resistance to vacuum
Pull the tubes off the carb clamp the supply line off and fit the gauge to the return line.
Do the vacuum first then the pressure.
While the tank is pressurised remove the clamp off the supply tube and you should get a very strong flow of fuel out of the hose if the filter is clear.

While it is pressurised spray some soapy water around the tank grommet & fuel cap you are looking for bubbles, or rather the lack of bubbles.

Pull the pressure gauge off the tank and using some new tube do the same to the return line from the carb.
Again it should hold 10psi and have no vacuum
Release the pressure & hook up to the return line again then pump the primer.
It should be able to pump up to 10 psi ( will take a lot of presses ).
Swap your gauge to the intake side of the carb.
Again it should take 10psi, don't go any higher cause the fuel pump is only good for 7 psi max.
when you have 10 psi, pump the primer and the pressure should drop then hold each time you press the bulb

This is how to do it
blowing & sucking will tell you next to nothing as your lungs will have difficulty telling the difference between 10psi ( holding pressure ) & 15 psi ( pop of pressure)

Bert- Thank You Sir, will do. I was testing like that, but not to 10 psi, I only went to 5 psi & all checked as you've stated.

The replacement part is crap, it won't hold any pressure, & quite restricted on vacuum test even @ only 5 psi. . .. My OEM one may be just fine. I'll know soon..


One surprise was when I put my trimmer up on my bench, I popped off the tank vent valve & got a good shot of gas from that line.

I should have opened the gas cap & de-pressurized the tank first. Doh..

You ain't alone on that one and some of us should know a lot better :ashamed: so have no excuses.
Most of the repair manuals state 7 psi or there abouts for pump capacity so they should not go over 10,
Just for the heck of it I pumped up a disposable Lawn Hornet tank , it took about 35 psi then blew the grommet out, I was quite impressed about that,

Most cube carbs have a pop off pressure on the inlet valve of around 15 psi but you should never test it as forcing it to pop off & leak can damage a perfectly good metering diaphragm.

There is a lot of stuff online that is rubbish.
If it looks too cheap to be good then usually it is too cheap to be good.
Echo stuff is expensive but rarely needs to be replaced because it is really good quality.
Even better there seems to be a good supply for anchient parts sort of opposite to most of what comes out of Japan.

Why are you putting a kit through your trimer in the first place ?
About the only thing I regularly replace is primer bulbs. & the occasional fuel line.

Bert- Thanks again..

The Kit I got was from Stens. So I thought it would be ok. In tank gas filter looks very good, just the tank vent did not pass testing. My OE one is fine.

Reason I'm doing a lil work on my trimmer is that it ran super last use. But I ran out of trimmer line, so I shut it off & put it on my porch in shade to get more line from shop. When I got it fired back up it seemed to not wanna take the throttle, idle = fine & hi speed = ok, just bogging in mid throttle.. Almost like a vapor lock ?

I only use 100% 93 octane gas & Echo mix. PS: Primer bulb is fine & since this I added a new spark plug, replacing the original plug.

It is only 4 years old & only used every other week for my home.

Not unheard of for a tank to pressurise enough to pop the float needle off its seat from the engine heat
Then raw fuel will run down & fill up the crankcase .
As these engines have exposed bearings the fuel can build up between the bearing & oil seal which can take quite a while to fully clean out.
A vent that is not working usually will cause the engine to die out after a few minutes from starvation
It will also struggle to pump fuel up hill so you turn the trimmer carb side up & the engine stalls.
Sounds more like the idle jet is partially blocked.
You sound like you know your way around engines so clamp it in a vice of some sort and slowly increase the speed.
Listen to see if the engine is stalling out lean or rich as it transfers from the idle to main jets .
If it seems to suddenly pick up speed then die, the idle jet is partially blocked and it is starving before the main jet can take over.
IF it just bogs down and 4 strokes a little then the idle jet is too rich.

Thanks Bert, my 2 stroke weed trimmer could fill the crankcase with oil/gas mix ?

I was gonna check my spark arrester next... But will test as you have mentioned..

I ran it & didn't shut it off & it was fine.

Only time it acted up prior was when I gave it a break to restring it, & get some water..

Yes they can.
Got that from one of my commercial customers who always opens the fuel cap before he puts the trimer back in his truck to release tank pressure.
He was finding that at the next house the trimer would not start which is a big problem cause you trim before you cut.
He went one step further and made a cover for the trimer when it is on it's rack to keep the sun off.

Also send the kit back to Stens ask for a refund as the kit is faulty.
I get 12 months on Stens parts down here but warranty seems a bit shorter in the USA

bertsmobile1 said:

Yes they can. Bert- Wow, never heard of a 2 stroke filling the crankcase before.

Got that from one of my commercial customers who always opens the fuel cap before he puts the trimer back in his truck to release tank pressure. I always crack open the gas cap after use to relieve pressure.

He was finding that at the next house the trimmer would not start which is a big problem cause you trim before you cut. Very true & good advice. Mine started immediately, just had that mid range bog, just the once so far.
He went one step further and made a cover for the trimer when it is on it's rack to keep the sun off. Good idea..

Also send the kit back to Stens ask for a refund as the kit is faulty. I got it online thru Walmart (lesson learned) but the in tank filter looks very good.
I get 12 months on Stens parts down here but warranty seems a bit shorter in the USA IDK what it is here ?

Thank You Sir.

Click to expand...

:thumbsup:

Have a good look at the carb set up.Nothing to stop fuel under pressure from being pushed out of the main jet and then strait into the cylinder.
Some of the older ones had check valves which were activated by pressure drops in the venturi
However most of them have been deleted over the years.

Very few would get enough pressure to open the metering needle , remember it should have a pop of pressure of around 15 psi and the tank should start to vent around 10 psi
However there are few too many shoulds in that statement and even if the needle is off the seat for a second or two, a lot of fuel can pass through the main jet.

Some of the older 2 strokes had drain plugs in the crank to drain off water, oil & fuel that accumulates in there.

I have looked at it. & I can see how it could fill the combustion chamber..

But I don't see how it could fill the crankcase ?

Piston rings only seal when they are moving ( or are under pressure )
The inlet port never sees the top of the piston.
It is opened by the under side of the piston skirt, not the piston crown.

Point taken, no I didn't have the carb or muffler off to see the piston travel.. 2 strokes I don't do anything deep on them, as yet anyhow.

At any rate the trimmer seems to be running fine again.. Plus I still have 1 more year of warranty..

Thanks for all Bert..

Piston goes up creating a partial vacuum in the crankcase till the intake port is uncovered by the BOTTOM of the piston.
Some have cut aways to get a tighter control over this.
Piston comes down blocks the intake off & compresses the crankcase gasses till the TOP of the piston opens the transfer ports to allow the fresh charge to enter the cylinder.
This is why you test the crank case in both compression cause it can blow the fuel air out past the seals
And vacuum cause it can suck fresh air in past the seals so the engine runs very lean

Certainly appreciate the schooling on 2 strokes... Not my forte, but I like learning new stuff.

Gave the trimmer a good workout today, runs fine again..

After it cooled I wanted to check the spark arrestor screen & exhaust port. So I took it apart.

Screen was clean, exhaust port only had very minimal soft carbon in it. Soft probably from the Seafoam. Cleaned carefully & thoroughly.

I did spray out the muffler with Brakleen thoroughly, just for good measure (has the Cat in it). After I rinsed it out w/water & blew it dry w/air comp.

Reassembled & fired it up..