Quick question about carburator pressure test (Stihl ms260)

[MartDalb]

Hi all


Trying to test a Stihl MS260 carburator, need a quick tip on doing it.


Do I need to somehow stop air from coming out of the "impulse pipe" on the carburator?
I am putting pressure from fuel end, but air is going out of the "impulse pipe", is that surpossed to be like that?


See picture 1


Thanks in advance!

 

[SteveSr]

Your carb has a DEFINITE problem. To get air to come out the impulse port you have to blow up/out the fuel pump diaphragm. So how much air pressure did you apply?

The carb should hold 7-10 PSI at the fuel inlet indefinitely. Anything else and you have a leak to find and fix.

 

[Ozhoo]

Make sure that you have the fuel pump diaphragm and gasket in the correct order... diaphragm goes against the carb body.

 

[MartDalb]

Thanks for the replies!

Yeah i figured something was up when it just let air out of the damn impulse pipe. (7-10 PSI)

This issue had to come from the other side of the carb, so I have changed the needle/spring etc, tested 3 different ones!

I finally have pressure and no leakage, (7 PSI) for 5+min now and I have it under water just in case. Looks tight now.


This was VERY weird, 3 needles, different springs, several diaphrams/gaskets. Switching around to find something that works.
Granted, I do have too many spare parts from old machines, I should just throw out, obviously saving old stuff isn't always smart


Thanks a ton for the help!

 

[MartDalb]

This thing has been driving me nuts too.

I tested EVERYTHING on this damn saw, it would not run well. (Got if from an old friend who didn't know where it had been used, he gave up on fixing it, Stihl MS260 from 04)
Pressure tested, changed all hoses. manifold, checked for leaks etc etc etc.

I will run the saw tommorow and see if this carb adjustment fixes everything, if so, this saw should run a few years after this major service

 

[president]

This thing has been driving me nuts too.

I tested EVERYTHING on this damn saw, it would not run well. (Got if from an old friend who didn't know where it had been used, he gave up on fixing it, Stihl MS260 from 04)
Pressure tested, changed all hoses. manifold, checked for leaks etc etc etc.

I will run the saw tommorow and see if this carb adjustment fixes everything, if so, this saw should run a few years after this major service

it should ,but if it doesn;t ,subject it to Viking eviseration,&toss it into the sea! (lol)

 

[MartDalb]

it should ,but if it doesn;t ,subject it to Viking eviseration,&toss it into the sea! (lol)

Hell no, it's either gonna work or it will be a spare part saw for my other 260's!

Had to work on the oil pump too, now it's fitted with adjustable pump and new worm gear, it wasn't oiling when I got it, first clue was unoriginal worm gear that was all worn out......

 

[DND 9000]

Don` forget the carburetor has to be wet (full with fuel) inside to do this test right.

 

[MartDalb]

Ok i need some help here.

I tested the carb, as per Stihls own service manual. It held pressure.

But... The Damn Thing is still acting up.


I took another carburator off a working ms260, tested and it runs but it dies at low speed and only way to keep it running is turning the iddle screw way up, leaving the chain to run in iddle.

Other issue is hitting the chain break kills the saw as well. Unless again its turned way up in iddle.

Ive tested All leaks, changed All hoses, impulse, manifold, cylinder and piston are smooth.


Im out of ideas here... Any help appriciated!!

 

[MartDalb]

Now granted I have No idea what this saw has been through, I have noticed several indications of mistreatment.

But still... All things have been checked now.

Worn ignition coil? Should not make it die at low iddle, should it?

Searching for ideas....

 

[MartDalb]

@HarleyT

 

[MartDalb]

Maybe I should just split it apart, bit by bit. Part of me wants to see the cylinder piston in the open.

Im thinking someone worked on this saw and did something stupid along the way, then got rid of it.

Also noticed liquid gasket between the muffler and exhaust, when i first took that off. Never seen that before either. Not that i think it matters much.

What would the indication of a piston turned the wrong way be?

Its weird the piston is so neat and clean while the rest of the saw is clearly used and a used.

 

[SteveSr]

I tested the carb, as per Stihls own service manual. It held pressure.

Pressure test is only ONE requirement for a working carb. It could also have clogged internal passages or other issues. BTW, what is the carb model and what are the L&H screw settings? The carbs in the E.U. are considerably different than the U.S.

I took another carburator off a working ms260, tested and it runs but it dies at low speed and only way to keep it running is turning the iddle screw way up, leaving the chain to run in iddle.

This is pretty worthless as it doesn't compare performance between each carb. What happens when you put this saw's original carb on the other (assumed to be working) 260?

Other issue is hitting the chain break kills the saw as well. Unless again its turned way up in iddle

Clutch is binding. Pull the clutch drum and figure out why. Pay close attention to the idler bearing and closely inspect the end of the crankshaft for any galling. If the crank is buggered the saw is toast.

If this is the PRO version with the clutch driven oiler pay attention when installing the clutch drum to make sure that the notch in the drum lines up with the drive arm on thew worm gear. The worm should also turn easily with a small screwdriver. If it doesn't you have another problem.

cylinder and piston are smooth.

Pull the muffler cover and check the spark screen for blockage. Remove it and burn it with a torch if necessary.

Not that we don't believe you but I have seen posters swear up and down that the cylinder & piston were fine until they posted a photo of a roached top end. Take a photo of the piston through the exhaust port and post it.

 

[SteveSr]

Maybe I should just split it apart, bit by bit. Part of me wants to see the cylinder piston in the open.

Hold off on this until you can do a pressure and vacuum test on the crankcase. Once it's apart you're hosed.

Im thinking someone worked on this saw and did something stupid along the way, then got rid of it.

Wouldn't be the first time. BTW , what does a compression test show especially in comparison to your "good" 260?

 

[MartDalb]

It's late for me here, ill post picture tommorow, trust me, piston looks smooth, top and buttom, muffler is clean too.

The clutch was actually binding, no idea why? I put it all on again, and clutch doesn't bind now

Ill retest the saw again tommorow, hoping the binding part had something to do with it dieing in low idle mode. (The saw runs good full throttle, but wont idle, tested various carb settings without luck)

This saw has been through some rough time for sure, there's a little piece of the cylinder broken off on the top near the spark plug (I am guessing someone slipped while changing spark plug?) and in the buttom of the saw, the metal pin holding into the plastic handle is also broken off (prolly been dropped on the ground?). Someone has been mistreating this poor thing.

I'm by no means a pro saw guy, but I have repaired alot of machines, never ever seen one act like this, and never had this many issues with everything being checked and tested with "working" parts etc.

 

[MartDalb]

Here we go.

Pass your judgement.

I did find it Odd, the screws holding the cylinder to the case were extremely easy to unscrew. Littery by regular hand held screw driver All four screws came right out. And no sign of lock tight on the screws either?
The gasket was broken ofc.. Gotta replace that anyhow.

Am I back to Searching for issues within the carburator?

 

[MartDalb]

Perhaps I should have made a new threat but....

Ill continue to explain this saw project, for those interested in that sorta thing

Having taken the saw mostly apart, I started to backtrack from when I first got this machine.

The first thing I noticed was that the carburator had no gaskets on it, at all, so that was obviously something, someone, somewhere had been working with.

Next up, the "O" ring/washer between the Carb and the inlet/manifold was missing, the one you put on the bolts before the carburator. Again sign of someone not doing his job right?

Today, I took off the cylinder, and noticed the 4 screws holding it, was very easy to unscrew.
I have never ever had cylinder screws come out just by a regular hand held screwdriver, and the gasket was ripped, but actually looking almost brand new?



Now granted, I could have done a poor job testing pressure here, and I will surely redo it once I got a new gasket for the cylinder.

Could the issue actually come from the cylinder not sitting tightly screwed in?
I have no idea how my pressure test didn't show this, but perhaps it held the pressure, but didn't when the saw was actually running?

This saw has seen use, and also mistreatment/poor handling, but it's not THAT old (2004).
Also as pictures show, not damaged/worn heavily in piston/cylinder

I have seen several Stihl 026 saws, way older, that had no signs of needing oil seal replacement or any other leakage.

I think i'm getting close to figuring this thing out! I will repost when I get the spare parts.

 

[1hander]

Wow sounds like quite a few things contributing to this one.. A loose base gasket would certainly affect how the pulse line or port operates the carb ..

Stretched out clutch springs or a dragging drum needle bearing will cause it not to able to idle because of the friction..