Author Topic: Micro Carburation issues...  (Read 413 times)

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Offline Drew_Uth

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Micro Carburation issues...
« on: November 22, 2016, 02:37:29 pm »
So I have 5  micro series saws I have been rehabbing to gift and or sell, plus two to keep.  Mostly 2.0 craftsman units, with two Daytons thrown in.  I have a common issue with each one- carbs have been rebuilt with new gaskets, cleaned, new fuel lines, etc.  While running them, when they idle for somewhere between 5-10 seconds, they load up and almost die.  If you keep cutting and blipping the throttle constantly they run fantastic, but let it set idle for a few seconds and then whack the throttle again and they blubber, smoke and then clean out. 

If I set the L any leaner, they have no throttle response and wont run right at all.  In one unit, I put a longer metering lever spring in and that seemed to help a bit.  I replaced the needles in each carb.  Is my thought that the diaphragm pressure is exceeding the metering spring valid?  Can you think of any other causes?  I really don't think it is a mixture screw adjustment because they all run great otherwise.  But it is baffleing that I have 4 runners and they all do the same thing!

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Offline Cut4fun .

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Re: Micro Carburation issues...
« Reply #1 on: November 22, 2016, 02:53:16 pm »
WAG Have you looked into  the reeds under the carbs?
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Offline fossil

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Re: Micro Carburation issues...
« Reply #2 on: November 22, 2016, 03:02:41 pm »
If you blew them out with high pressure air you may have damaged the nozzle check valve. That will give you idling issues.
Tim
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Online Efisher26

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Re: Micro Carburation issues...
« Reply #3 on: November 22, 2016, 04:36:12 pm »
I can't see this for all of them, but old clogged fuel filters? Idle may not pull enough gas through.


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Just a home owner that likes the older better made machines

Craftsman 3.7
Echo 290evl
Husqvarna 36

Offline Drew_Uth

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Re: Micro Carburation issues...
« Reply #4 on: November 23, 2016, 09:52:11 am »
New fuel filters

Have not looked super closely at the reeds...in my (dirtbike) experience, bad reeds prevent the engine from running at all, but maybe these are different because they have a fuel pump rather than relying on crankcase vacuum to draw fuel into the carb?

Regarding high pressure air, I did not.  But the item you mention, where in the carb is it located/what purpose does it serve? I can jet a motocross or enduro bike to perfection but I am only about at about 60% understanding of how these tiny carbs work.

Offline fossil

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Re: Micro Carburation issues...
« Reply #5 on: November 23, 2016, 01:28:36 pm »
I assume you have a WT carb on yours. The reed is located under the carb. You would have to remove the carb and the box it's mounted in to access the reed. Make sure it's flat and seated and that there is no crap or rust on the reed where it needs to seal against the plastic.

Here's some reading from Walbo.

WT manual
http://www.walbro.com/wp-content/uploads/2016/01/WTseries.pdf

General service manual. Good reading here on how they work and issues

https://app.box.com/s/vee4p32zuxpb2xnfvlm05wjaibsizekk

Tim
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Offline Cut4fun .

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Re: Micro Carburation issues...
« Reply #6 on: November 23, 2016, 02:02:07 pm »
You say if you set the L leaner. Where are you at on L from lightly seated?
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Offline Eccentric

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Re: Micro Carburation issues...
« Reply #7 on: November 24, 2016, 04:39:38 pm »
These saws have a LONG fuel line (something like 15") that snakes up and over inside the handle and then down to the carb.  If the pump section of the carb isn't 100% and/or the impulse signal is weak............they'll run like you describe.  You've rebuilt the carbs, so we can focus on the impulse signal.  A leaking reed (grit under the reed, 'tired' reed that won't lay against the carb box surface, etc) can give a weak signal.  Leaking crank seals will too. 

Every Micro I've brought back into service has benefited from crank seal replacement.  They're easy to get and cheap too.  530019059 from any Husky/Poulan parts source (including your local Husky dealer).  Should be around $3-$4 each.  Chicago Rawhide/SKF 4913 will also work, although they'll probably be more expensive than the OEM seals.  Seals on these saws are SUPER easy and quick to change too.  I harp on crank seals a lot..............but in my experience they're a "must replace" when rehabbing an old saw for work.  They're rubber items with a finite life.  They degrade over time just like fuel lines and carb diaphragms....
-Aaron

For older saws:
Tune the H side so that it 4-strokes (burbles) at WOT unloaded and just cleans up when under load.
When you lift cutting load, the saw should immediately revert to 4-stroking.  Fine tune the transition point for the wood you're cutting.
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Offline sharkey

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Re: Micro Carburation issues...
« Reply #8 on: November 27, 2016, 03:29:27 am »
Because both the H and L circuits of the carb feed from the same metering chamber, the barometric pressure of the metering chamber must be kept below atmospheric barometric pressure or the fuel stops flowing.  The high speed nozzle which is fed by the H circuit, has a check valve contained somewhere in its system to prevent atmospheric barometric pressure from seeping past its check valve and back into the metering chamber.  Some of these valves are screens that are wetted by fuel, and others are of a mechanical check valve style.  The mechanical check valve style nozzle has been more problematic than the screen type.  All of the screen type systems are serviceable, but the mechanical check valve types are not.  The reason is some of the components are not sold by the carb manufacturer.  I guess they want you to buy a new carb?  Navigate over to Walbro's website and look up the carb.  The parts listing will show which components are used.  Before I bought any carbs or parts though I would go with what Eccentric posted and replace the seals and all of the fuel line.                   
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Offline Drew_Uth

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Re: Micro Carburation issues...
« Reply #9 on: November 28, 2016, 08:13:57 am »
Thanks all for the excellent responses.  I will order some crank seals and swap them out, it sounds like a good place to start and at that price its not like I am concerned about throwing "good seals" away.  I will check the reed(s) as well.

They are all running Walbro carbs.  Interesting detail- they aren't all identical.  The first difference I noticed was that one or two had three screws for the diaphragm cover, while the other 2 had 4.  Then I noticed one that had a much bigger venturi than the others.  I have already given one saw to a BIL and lent one out, so I can't check those, but I have three carbs in my shop right now and two are of the smaller variety and one is larger. Naturally I put the larger one on the saw I plan on running the most  ;D

Cut4Fun- I will have to get back to you on the L setting.  I tune them to run the best, I rarely count turns; but I know that is the best way to have a discussion like this.

 

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