Author Topic: 5100S Problem  (Read 1193 times)

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Offline wkpoor

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5100S Problem
« on: April 24, 2012, 05:52:48 pm »
First off I gotta say I'm probably working on the all time high hrs 5100. Used daily to cut firewood year round for 5 yrs.
Saw runs fine off idle but won't idle right. Is real irratic at idle and mostly just dies but will occasionally go lean before it quits. No air leaks that are easy to check for. Rebuilt carb, checked fuel line, impulse line, compression is 170+, muffler is clear, no scours on piston and ring and spark is great. I'm guessing its time to set up for a case vac check. Maybe he has plum wore out the crank bearings and seals.

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Offline Cut4fun .

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Re: 5100S Problem
« Reply #1 on: April 24, 2012, 06:58:41 pm »
Try checking tank vent, intake boot and seals.  Have you checked to see if cylinder bolts have come loose?  Had a racesaw do that to me once.
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Offline wkpoor

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Re: 5100S Problem
« Reply #2 on: April 24, 2012, 07:47:03 pm »
Ran saw with cap loose no change. Intake boot just fine. Cylinder bolts all tight and sprayed brake clean at base with no change. Talked with Brad and he thinks it has to be an idle circuit plugged in the carb.
We had a Stihl 034 couple yrs ago that we couldn't get to idle right and that ended up being crank seals. Don't want to get steeered down the wrong road though.

Offline Cut4fun .

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Re: 5100S Problem
« Reply #3 on: April 24, 2012, 08:21:57 pm »
Sounds like you need to pull a known good carb and test it on that saw.
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Offline sharkey

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Re: 5100S Problem
« Reply #4 on: April 26, 2012, 01:00:26 am »
If it really has alot of hours, pull the muffler and try to make the piston move in the bore like the skirts are worn.  Use a screwdriver and try to push the piston back out through the intake.  If you get some play that is the problem. 

Here is a short video of a different dolmar that has alot of hours on it with the skirt problem.  The cylinder should be ok, its just that the piston skirts wear away.
http://www.youtube.com/watch?v=YoUMR7SKkxQ&feature=plcp
   

Offline wkpoor

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Re: 5100S Problem
« Reply #5 on: April 26, 2012, 06:11:13 pm »
Just put a new piston in last year.
Downloaded the Zama manual yesterday. Learned alot reading it. Talked about erratic idle caused by a leaky main nozzle check valve. Turns out both Zama and Walbro have a main ejector check valve. Without it atmosphereic  pressure is available on both sides of the diaphram when the throttle plate is closed. Checked it out per the maunal and sure enough no check action. Compared to another carb and it did check. Haven't proven this on the saw yet but got a donor carb from a shop that has other issues but the nozzle checks fine. Going to try and remove it and install in Earls carb. Supposedly its a service part but I doubt it can be bought outside of Zama.
Earl didn't start having trouble with his saws till he went back to pump gas. I've never had a fuel related issue on any saw. I only run AvGas.

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Offline wkpoor

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Re: 5100S Problem
« Reply #7 on: April 26, 2012, 08:37:31 pm »
Here is the link to the Zama Tech Guide http://www.zamacarb.com/pdfs/TechGuide_2007.pdf where I got my info. I doubt very few people know the main nozzle has a check valve or what its function is. I'm guessing there must be rubber involved and therefore subject to ethanol deterioration.

Offline wkpoor

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Re: 5100S Problem
« Reply #8 on: April 29, 2012, 09:47:44 pm »
Well I haven't finished the saw but I put it all back together yesterday and tried to start it on a different carb. Do dice. Tried old carb with same results. Trouble shot to fuel pump not working on either carb. Installed new pump diaphragm and tested impulse line. All good. I'll know more in a few days when I get back on it but its looking like crank seals are shot. Noticed today pto side has quite abit of radial play. I hear these saws are famous for spun bearings.

Offline wkpoor

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Re: 5100S Problem
« Reply #9 on: May 01, 2012, 04:29:57 pm »
OK here is the deal. PTO bearing is spun. Did crank pressure and vac test. Won't make any vacuum and if you blow into the hose attached to the impulse port is free flows out the bearing seal. I'm curious to see how tight a new saw is and good running older one for comparison.

 

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