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coppicer

Cleaning carb on smoking Stihl

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1 minute ago, woodwizzard said:

You will probably need to be screwing that in a bit to get it to keep running.

Er, I've been unscrewing it further! Is that the wrong direction?

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Unscrewing the idle screw too far out will disable the idle circuit but you'd have to go a long way. If engine starts like this it will rev high - like the high idle position on choke switch. 

 

The pump in the carb is pretty straight forward, as you say if the diaphragm material is in good condition, correctly positioned against the carb, gasket against cover, then probably ok. You need the crankcase pulse signal to reach the carb but there's no separate impulse line on these, its combined in the rubber boot, worth checking for splits though.

 

A few other things, 

-did you check and set the meter arm height in the metering side of the carb ? Maybe fuel is getting to pump but going no further, coz the metering valve is closed. Research on here, been covered many times. If you strip the carb after the engine stops (no choke) pay special attention to where you find (or dont find) fuel, it can help with diagnosis.

-did the filter in the carb get changed ? Same logic as above.

-main jet one way valve not doing is job,  happens if an air line has been used to clean main jet. Check by bIowing each way through the jet with a piece of soft tube. 

-accerator piston and its oring worn out, needs removing and inspecting, happens on old or high mileage saws. Also been covered on here.

 

Edited by bmp01
Filter note added.
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2 hours ago, bmp01 said:

A few other things, 

-did you check and set the meter arm height in the metering side of the carb ? Maybe fuel is getting to pump but going no further, coz the metering valve is closed. Research on here, been covered many times. If you strip the carb after the engine stops (no choke) pay special attention to where you find (or dont find) fuel, it can help with diagnosis.

-did the filter in the carb get changed ? Same logic as above.

-main jet one way valve not doing is job,  happens if an air line has been used to clean main jet. Check by bIowing each way through the jet with a piece of soft tube. 

-accerator piston and its oring worn out, needs removing and inspecting, happens on old or high mileage saws. Also been covered on here.

Didn't check any of these! Thanks, will get onto it.

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I would set the metering arm if not done already and then hold the carb up to the light, open the choke and turn in the idle screw until you see a very small amount of light around the brass throttle valve plate. 

The carb should be pretty close to going then. The other suggestions are a bit more complex and would be a phase two if the saw still didn't function like this.

It is always worth doing the obvious and easy things first and then progressing to the more difficult if the easy stuff fails.

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4 hours ago, spudulike said:

I would set the metering arm if not done already and then hold the carb up to the light, open the choke and turn in the idle screw until you see a very small amount of light around the brass throttle valve plate.

I'm all for doing the easy things first (and exclusively, if possible). Anyway, now been waylaid by wife to begin assembly of IKEA item. Looks possibly more complex than the carb.

 

EDIT Furniture is supposed to have 20 screws, M4 x 10mm. There are only 19 in the bag...

Edited by coppicer
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Sorry about this @bmp01 and @spudulike, but with regard to your comments, I'm using the Stihl repair manual and it doesn't mention a metering arm anywhere (only a metering diaphragm). Is it the "air valve lever" referred to in the screenshot below?

 

image.png.4efc0db8aa0f784620e71954070bcbc2.png

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Metering arm lives under the metering diaphragm, requires the carb to be pulled apart...

IIRC its a Zama carb on these, the Zama website is quite helpful if you can tolerate some reading and pondering.  There's a full explanation of how the carb works on choke, part throttle and open throttle plus the pump and metering arm layout. ...

Head to http://www.zamacorp.com/ then look for SUPPORT, then TECHNICAL SUPPORT then DISSASEMBLY AND SERVICING (types C1U etc).

Down the page you'll see this:

 

Screenshot_2020-08-03-14-17-43-1.png

 

It used to be you could download this technical guide, not sure that's possible now. 

Anyway,  that's the metering arm stuff,  you need to decide if it's the type A or B, can't remember.

Whie you're in there, there's the filter to attend to. 

Pretty sure you can check the main jet one way valve in situe by accessing one or t'other drilling while closing high speed screw.....

 

Edited by bmp01
Further info added below image
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