Mystery Machine

[Erick]

Alright guys this just doesn’t happen …. got a good one working.

I’ll give you the symptoms and you tell me what’s up??

It’s a Stihl MS660 starts fine revs right up to 13,000 but if you hold it at WOT for say 10 seconds it goes lean…. Will come down to idle lean, and if you hit the throttle on deceleration it will bog out lean, but if you let it idle for a few seconds it will recover and rev right back up to 13,000 and four stroke for another 10 seconds until it screams lean again.

Hit quitting time so I had to give up on it for the day, but the damn thing is haunting me. So lets see what the collective AS mind can come up with here.

Ask away and I’ll give you the results of the tests so far… maybe you guys can come up with something I’ve missed or trigger my little pea brain into gear on this one?????? :dunno: :bang:


Fire away!!!!…. I really hope I come out looking ignorant on this one.

 

[Sprintcar]

Intake boot.

 

[Saw Dr.]

Starving for fuel. Change fuel line and filter with new OEM stuff.

 

[MuleyJ]

Having the same problem with a husky 350, but it is one with the plastic intake clamp. I have ordered a new one hoping that there will be no more probs after metal clamp is installed. This only happens when the saw is warmed up real well though.

 

[outdoorlivin247]

Maybe the guy working on it?...Okay, I say tank vent?...

 

[Erick]

Maybe the guy working on it?...Okay, I say tank vent?...

Well he's a tard for sure, but the saw is still a mystery.

 

[AUSSIE1]

Tank vent blocked ?

 

[Jacob J.]

Tank vent blocked ?

That's my guess, tank vent or the nylon one-way check valve in the WJ carb is not working right or there's some corrosion going on in the high-speed circuit.

 

[Sprintcar]

Any or all the above + more to follow.

 

[Erick]

All right all good suggestions so far and all on the money for the symptoms.

First up lets say tank vent..... test's out fine... not only passes a vacuum test but you can take the thing completely out where you can see the fuel through the hole...... no change.

Second lets say carb.... torn down... all looks good... replaced with known good new carb.... no change.

And C.... Saw passes pressure and vacuum tests with flying colors even went so far as to let it sit with 10psi vacuum for over 30 min. never leaked even so much as a half pound....

 

[stihlboy]

sounds like when we built up a modular v8, the fuel supply was there initally but it would out run the pump.

was solved with a bigger fuel rail.

its initially getting the fuel but its supply runs out

you have a restriction somewhere

check your lines very closely for holes or a clogged filter

 

[rupedoggy]

Check carburetor. Little screen partially plugged. Needle and seat passage part plugged or the lever set too low. Pulse line cracked, pin holed or just barely on the barb. Junk in the high speed circut or under the high speed welch plug. Pump diaphram to hard and needs to be changed (can't pump enough to keep up). Of course the high speed needle could be set too far in. One last thing, believe it or not, a new spark plug could fix it. Mike

 

[Erick]

sounds like when we built up a modular v8, the fuel supply was there initally but it would out run the pump.

was solved with a bigger fuel rail.

its initially getting the fuel but its supply runs out

you have a restriction somewhere

check your lines very closely for holes or a clogged filter

Ahhhh yes, the fuel line... tests out fine, but was replaced for the sake of argument, new filter too.... it's all good.

 

[ckelp]

i had the same problem with my 024 even after i rebuild the carb.
i did a shot in the dark took the carb apart again soaked it on some super hot water and dish again soap rinsed it off with water and blew it off with air it has't done it sense

but thats just my two cents

 

[Mastermind]

check your lines very closely for holes or a clogged filter

I'm in the "you got a pin hole in the fuel line" camp...and that's where I'm staying...

 

[outdoorlivin247]

Spark plug loose?...Sounds stupid, but it happens...

 

[Erick]

Spark plug loose?...Sounds stupid, but it happens...

Not a chance.

 

[outdoorlivin247]

Not a chance.

You're going to feel really dumb tomorrow when you check it and it is loose...LOL

 

[Erick]

You're going to feel really dumb tomorrow when you check it and it is loose...LOL

Wouldn't pass pressure or vacuum on the crank case with a loose plug.... not to mention I checked it.

A loose plug will just lose compression not necessarily affect the fuel mixture, same with a leaky compression release.

 

[AUSSIE1]

Partial impulse line blockage?