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What the f..........is going on here?

traffic903

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My 660 problems are back. I thought I had them solved but no. I did rule out the coil, which is good. And, I have it narrowed down to the carb. So here is my problem.

Saw will start and idle perfectly. Will usually rev up fine and is tuned to 13000ish. However (especially once warm) then it will suddenly "flood" at high rpm. This drops the revs back to around 9-10000 and then if I let off the throttle it will mostly die.

I have taken off the filter and watched this as it occurs. It revs straight up to 13 then after about 2 to 3 seconds you can see fuel just pour into the venturi. I mean like a car wash! Why is this happening, and why does it mostly only happen once warm? I have set metering lever higher and lower for no difference. I changed the welch plug and added some sealer which helped for one tank, but now it is back to doing the same again. I have removed the needles and sprayed brake cleaner through the passages. Have changed metering lever spring. Renewed all gaskets.

I swapped my genuine 660 carb over to test and it runs perfectly. I have 3 chinese carbs. 2 do the EXACT same thing and the other is in the saw at the moment and is working so far, albeit with a slightly erratic idle.

What can make a carb basically flood at high rpm like this? Could it be something like some swarf moving around in a passage? A check valve? Alien? It just seems so weird that 2 different carbs would do the exact same thing in the same circumstances.

Am keen to hear some opinions if you could offer something please.
 

traffic903

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Check valve in the nozzle would be my guess.
But I dont claim to be a carb guru by any means
Are you able to say why you think that could be it? Can I do anything about that? I thought I read somewhere that the check valves aren't serviceable. But I'm probably wrong there. Carbs aren't my thing. Give me fuel injection any day!
 

Stihlms

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This is a pretty tough situation, if you have a oem carb, put that back on but make sure you get a rebuild kit for it with all new diaphragms and gaskets. Make sure you adjust the float needle and seat to the proper specification. And this goes without saying, make sure you clean out all the ports, passageways and screens in the carb body.

After it's all put back together start out with the high and low needles one and a half turns out on both. Then you can go from there.

I would also check the the impulse line, if there's any damage of slight cracks dry-rot Etc that could be an issue also.
 

Stihlms

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Some of the tools that I use to clean out carburetors are sewing needles because of how small they are. They work wonders
 

traffic903

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This is a pretty tough situation, if you have a oem carb, put that back on but make sure you get a rebuild kit for it with all new diaphragms and gaskets. Make sure you adjust the float needle and seat to the proper specification. And this goes without saying, make sure you clean out all the ports, passageways and screens in the carb body.

After it's all put back together start out with the high and low needles one and a half turns out on both. Then you can go from there.

I would also check the the impulse line, if there's any damage of slight cracks dry-rot Etc that could be an issue also.
I have an OEM carb on my genuine 660, but this is a china saw I built. I have done a rebuild on the carb and cleaned it as good as I can. The other one that does it also is brand new, did it out of the box.
 

traffic903

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I agree with you that you want to know whats wrong with the carb.
So expensive down under?
We get bent over on pretty much everything down here mate. And to make matters worse, Stihl Australia has a policy of discouraging their dealers from keeping traded or junk saws for parts. It isn't just the odd dealer here and there either. You are flat out struggling to find anyone that has used parts. They all just throw them straight out!
 

Stihlms

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I have an OEM carb on my genuine 660, but this is a china saw I built. I have done a rebuild on the carb and cleaned it as good as I can. The other one that does it also is brand new, did it out of the box.
Do you have the tools or anybody anybody you know have the tools to pressure vac the crankcase?
 

traffic903

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Is there a little brass plug on the outside indicating it's a governed carburetor? If so it's just doing its thing.
It's 2.30am here so I might have to leave that til later!! But it is just a typical China 660 carb. Copy of the Wabro WJ. Don't believe they are governed.
 

traffic903

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Do you have the tools or anybody anybody you know have the tools to pressure vac the crankcase?
Pressure and vac is all good. This is strictly a carb issue I believe. The pulse line is all good too. There is no air leak anywhere. This is just some crazy over fuelling going on!
 
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