# Walbro WT640-1 carb Question



## Spit (Nov 3, 2006)

I have a HomeLite trimmer with a Walbro carb on it. I have cleaned it and rebuilt it with a repair kit, I have also run fuel mixed with Chevron Techtron fuel injector cleaner and gotten all of the carbon out of the ehhaust, engine & muffler. But I still get some hunting when in idle. I went looking for air leaks with carb cleaner spray and it seems like if I spray on to either the top or the bottom of the throttle shaft the trimmer will stall. Has anyone else experienced anything like this? I think that there may be seals on the shaft..do they wear and leak? Is it more likely wear on the carb body or the throttle shaft itself? 

There is only a Idle speed adustment screw on this carb...and it sits right If I below the throttle so that when I spray I may be hitting that a little also. Do the Idle screws or the matching threads in the carb housing wear and leak air?

If I spray around the carb mounting area, base of the cylinder etc. no change but it hit the areas mentioned above it stalls.

Any ideas and advice welcome.

Thanks

Spit


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## geogrubb (Jul 28, 2006)

Spit said:


> I have a HomeLite trimmer with a Walbro carb on it. I have cleaned it and rebuilt it with a repair kit, I have also run fuel mixed with Chevron Techtron fuel injector cleaner and gotten all of the carbon out of the ehhaust, engine & muffler. But I still get some hunting when in idle. I went looking for air leaks with carb cleaner spray and it seems like if I spray on to either the top or the bottom of the throttle shaft the trimmer will stall. Has anyone else experienced anything like this? I think that there may be seals on the shaft..do they wear and leak? Is it more likely wear on the carb body or the throttle shaft itself?
> 
> There is only a Idle speed adustment screw on this carb...and it sits right If I below the throttle so that when I spray I may be hitting that a little also. Do the Idle screws or the matching threads in the carb housing wear and leak air?
> 
> ...


Go to the Walbro site they have lots of info for your carb. Have a good one. Geo


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## paulr44 (Oct 14, 2008)

In addition to Geo's sound advice about the service info. on Walbro's site, I'll add that any butterfly (as opposed to CV, or rotary barrel carb.) will allow some carb. cleaner to meter past the throttle shaft at idle (which also happens to be when intake vacuum is at it's highest). A worn shaft/boss will obviously allow more, but if you spray the shaft of even a new carb. at idle it may allow a bit of bypass and alter the RPM.

If you suspect the shaft as being worn, wiggle it. It shouldn't wiggle but a an amount so small it should be barely perceptible by eye. Butterfly carb. T-shafts wear over time depending on the application. In other words, if the unit has fixed speed settings rather than a trigger operated throttle control, the shaft will probably outlast you.

As for the mixture screw threads, no they don't wear. That is, unless you're messing with them every time you use it --- never seen them be a wear issue though. The surging at idle may be a restricted idle or progression port, or as you indicate a vacuum leak.


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## champagne_charly (Jun 19, 2013)

After wasting hours (and money on parts) guessing and blindly trying to fix similar issues.. What i have learnt is to have a cheap fuel pressure pump gauge a blanking pannel to hand for the carb run a vacuum and pressure test. In my case it took me 20 mins to do on 2 badly running machines and left me a very clear picture of where to move on to..You can also check the carb for airtightness after rebuild along with testing your fuel lines, vent and tank. 

As for the leak around throttle post, I have a brand new zama carb and it leaks there too. My problem 'may' partially be this however i have an iffy decomp valve so wont know for sure until that is fixed.


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