Rebuilding a Stihl 034, should I try it?

[dabills]

Well I bought a cheap carb on Amazon but when it came it said zama on one side and stihl on the other? Anyways I put it in last night, fired it up and voila, it idles now. Super stoked gonna do some cutting today. Thanks all for the help!

Edit: oh yeah the thing only cost $13 lol

 

[Mad Professor]

Glad it is fixed.

Don't forget to tune the new carb. Sharp chain, clean filter, in the cut.

Which brings us back to what was problem in carb?

Many years ago, I worked in a motorcycle shop, 4-,6- cyl bikes with as many carbs. Some carbs cleaned several times, by several mechs...... When it was "fixed", then you had to sync the carbs w/vac gauges.

 

[dabills]

Carb, I dunno. Filter clean, chains sharp

 

[SteveSr]

I thought of one carb defect that could probably cause these symptoms...

The main nozzle assembly has a built-in check valve to prevent an internal "air-leak" in the carb that will prevent the "L" idle circuit from functioning. At idle the valve should be closed preventing preventingthe internal air leak. As engine speed increases eventually there will be enough vacuum in the venturi to open the check valve and begin pulling fuel through the "H" circuit. This check valve is made of a 1/8" rubber or fiberglass disc and "floats" in tside the main nozzle assembly..

If the check valve is stuck closed the engine will idle fine all day but will never be able to transition to the main "H" circuit for high speed operation. Conversely, if the valve is stuck open (or missing!) the engine will run at speed but not at idle which sounds like the OPs current issue.

So how would the check valve disc go missing? Blowing compressed air through the "H" circuit would be the best method, especially if it was a rubber disc type. If the OP did this it is probably in the next county somewhere!

The attached documentation provides the theory of operation and how to test this system. Basically cover the "H" feed hole in the metering chamber with a piece of electrical tape, remove the "H" screw and use a piece of fuel hose to alternately blow/suck air through the main nozzle. You should be able to blow air through it (valve open) but not suck air back through it (valve closed). You should also hear the click-click of the valve as it moves back and forth.

If the valve is still in there but stuck you might be able to dislodge it (gently) with the bent end of a paperclip. You can also try running some good carb cleaner (Berryman's B-12) through it. If that doesn't work it is probably time for a new carb. The normal symptom for a sticking check valve is that the saw runs fine one minute and not the next as the valve gets stuck in either position. Either it won't rev up or it won't idle.

 

[Ax-man]

Good explanation and I have done basically the same thing with mixed results.

Any tips if the H screw has a limiter cap on it to test for that pesky check valve not working and we don't want to remove the limiter cap ??? What if the H screw is absent like that 025 I just did ???

I tried to see if the main nozzle check was working in that carb by testing the H speed circuit in the metering chamber using a piece of fuel line and blowing into the hole and seeing if the valve was stuck one way or the other . I could hear the valve clicking and was able to blow air one but not the other. The main nozzle checked out but the darn thing still wouldn't run right . I might also add that this carb wouldn't always let the inlet needle seat test good off the saw but tested bad while mounted on the saw. I even after changed the needle out from a carb that was good but the same problem kept showing up. I finally just threw in the towel and got a new carb which solved my problem but I had to try and see if I could get that crummy carb to work right .

 

[cuinrearview]

It's in these instances I ask myself WWHTD(what would @HarleyT do)?

 

[Nathan Graff]

It's in these instances I ask myself WWHTD(what would @HarleyT do)?

Send you a parts list and a price...

 

[SteveSr]

Good explanation and I have done basically the same thing with mixed results.

Any tips if the H screw has a limiter cap on it to test for that pesky check valve not working and we don't want to remove the limiter cap ??? What if the H screw is absent like that 025 I just did ???

Access for carb cleaner and check valve testing is through the "H" circuit feed hole in the metering chamber.

I tried to see if the main nozzle check was working in that carb by testing the H speed circuit in the metering chamber using a piece of fuel line and blowing into the hole and seeing if the valve was stuck one way or the other . I could hear the valve clicking and was able to blow air one but not the other. The main nozzle checked out but the darn thing still wouldn't run right . I might also add that this carb wouldn't always let the inlet needle seat test good off the saw but tested bad while mounted on the saw. I even after changed the needle out from a carb that was good but the same problem kept showing up. I finally just threw in the towel and got a new carb which solved my problem but I had to try and see if I could get that crummy carb to work right .

Sounds like this carb had something stuck in the needle seat area. Try carb cleaner and the cardboard shaft of a Q-tip. Get out your flashlight and magnifying eyes! Hook up your Mity-Vac and test to hold 10 PSI.

 

[dabills]

I didn't use air compressor on carb fyi. I think she was gunked up.

 

[dabills]

Cut some wood today. Best saw I've ever run.

 

[SteveSr]

I didn't use air compressor on carb fyi. I think she was gunked up.

Now you know how to test it and perhaps repair it for a spare.

 

[drf255]

Well I bought a cheap carb on Amazon but when it came it said zama on one side and stihl on the other? Anyways I put it in last night, fired it up and voila, it idles now. Super stoked gonna do some cutting today. Thanks all for the help!

Edit: oh yeah the thing only cost $13 lol

That carb is built on the same assembly line as the OEM zama’s. Sometimes you get lucky. Most are, but not all have the Stihl or Zama logo on them. All saw “C3-“ on the side of the casting.

Many of the best runners I’ve built have that carb on them.

And yes, the check valve stops the metering area from pulling air in. If it’s defective, the metering diaphragm won’t get pulled up via fuel siphoning because air I’ll not allow that vacuum to build.