Author Topic: 394xp dies after throttle release  (Read 2552 times)

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Offline Spruce_Goose

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394xp dies after throttle release
« on: November 29, 2015, 01:01:31 pm »
Hi all,

I have a 394xp i bought used. Runs great in general, but it oftentimes cuts out after a cut when the throttle is released. I have played a bit with the carb, but it seems like it goes straight from this dying action to racing at idle (in other words I don't feel it is a simple L or T adjustment unless I'm just not hitting the mark).
The saw did sit for awhile before my ownership I think, and also sat a bit under my ownership. My shot in the dark response is to do a carb rebuild.

Other details: If i let go of the throttle suddenly (not IN the cut but after) it is more likely to die than if I let off slowly and keep gassing it a bit until eased into idle. It does sometimes die in idle but usually it is after the cut. It also seems to happen more once warmed up. Power in the cut seems fine as far as I can tell (never had bogging or anything, seems to really rip).

Ideas?  ??? Appreciate any input.


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Offline mdavlee .

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Re: 394xp dies after throttle release
« Reply #1 on: November 29, 2015, 02:38:56 pm »
Does the chain creep at idle? If not then you're probably lean on the L. Where are the carb screw now in relation toighy seated?

Offline sharkey

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Re: 394xp dies after throttle release
« Reply #2 on: November 29, 2015, 03:14:49 pm »
If you turn up the idle and it races you may have an air leak, 394 has to be what 10-15 years old so the seals are getting up there in age. 

Places to look for leaks are the intake adapter, crank seals, cylinder gasket and case gasket.  Borrow a brake bleeder that will do pressure and vacuum. Test the case and then look for the bubbles.  5psi.  Hold the intake adapter up to a strong light and look for cracks.

Offline Spruce_Goose

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Re: 394xp dies after throttle release
« Reply #3 on: November 30, 2015, 08:44:01 pm »
Thanks I'll try to fiddle a bit more with the carb, but If that doesn't do it I may have to delve into the world of testing for leaks (never done that before).

I'm still tempted to rebuild carb. Perhaps my issues are not symptomatic of bum carb parts though...

Offline Spruce_Goose

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Re: 394xp dies after throttle release
« Reply #4 on: December 06, 2015, 09:56:19 pm »
So I did a vac test and it passed. But in the process I uncovered some piston damage. The compression was getting readings close to 150 psi (maybe 145).

Not sure if this is what was causing the issue I previously described, nor am I sure when this damage occurred (before my ownership or during). Saw is used for milling. I didn't really notice a decrease in performance since I bought it however.

So I figure regardless, I'll probably need a new piston. Noteworthy also is decent carbon build-up on piston top and exhaust port.

I would REALLY like to avoid a new jug seeing as that's the pricey part, and the saw is already so old. Plan is to get a new piston assembly. I'm leaning OEM since I'm willing to pay more for a part that will last longer and/or perform better if that is true of OEM. I'm open to differing opinions on that though.

I'll probably replace base gasket, do a carb rebuild (maybe) and make sure it is tuned right to avoid future problems. Clean the cylinder best I can. Oh and replace fuel filter as its pretty dirty. Haven't checked fuel line for leak yet but appears like it wasn't running too lean based on carbon build-up?

Any other precautions I should take? Such as making sure case is clean? I obviously don't want to put a new piston in there and have it go to sh** again, but short of just double checking there are no leaks and making sure fuel filter/line is good and carb is tuned, not sure what to do. Could it have been running too rich and the carbon build up caused the damage?


Offline Spruce_Goose

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Re: 394xp dies after throttle release
« Reply #5 on: January 06, 2016, 06:52:52 pm »

I cleaned the cylinder up, replaced piston, new fuel line and filter, carb kit, and its back together. Pressure and vacuum test passed. Carb passed pressure test too. Compression pre-break in is between 145-150 psi.

I haven't dialed in the tuning yet. It still died a couple times when I let off throttle during tuning. Hopefully its just a tune issue, but if not, it seems like something with carb internals is wrong that the rebuild didn't fix? Check valve held against vacuum too. Replaced welch plug (no nail polish). I feel like i've eliminated virtually every other problem...

unless ignition could cause this?

Offline 67L36Driver

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Re: 394xp dies after throttle release
« Reply #6 on: January 08, 2016, 07:31:28 am »
Low side set too lean.  Richen it up 1/4-1/2 turn and reset idle speed and see if that helps.


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