# STIHL BG 75 Revival Carb Issues



## dingdiddydong (Jul 22, 2019)

Im reviving this dead bg 75 and it's proving to be stubborn. Replaced fuel lines cleaned out gas tank ultra sonic cleaned the carb. Gaskets seemed reusable and the diaphrams were pliable. Primer bulb is not cracked. Cleaned off the air filter too and replaced starter cord. The carb is a walbro wt 413. Not made anymore and seem to be rare. Walbro says start at 1.25 turns out for the adjustment screws in their manual. Stihl says 1 turn out. The engine was flooding nonstop with both of those settings. So i tried tightening the screw clockwise about a quarter turn. Unflooded engine cleaned spark plug. Engine starts after first pull. I fully tune the carb and set idle speed to correct rpm. Put it in my car on its side. I come back and their is a puddle of fuel on my floor mat. The carb, air filter box, plastic housing, fuel lines, and gas tank were covered in gas so I cant pinpoint the leak. Let it sit upright overnight and didnt notice any leakage at all it seemed to mostly dry. Anyway i tried starting it again and it kept flooding probably from the leaked fuel. I had to tighten the adjustment screws more to get it to start and quickly open them more to avoid running lean. Anyway the thing wont stay running unless the idle low screw is about 3/4 turn out and same with the high speed. Since this contradicts both companies manuals Im apprehensive to run this thing adjusted at 3/4 because i dont want to burn up the engine. The only part of this blower I luckily havent had to crack open yet. Anybody have any tips here? Should I just not worry about it and run it like this because it seems to run right. Or is this carb or gaskets the culprit and not to risk it because itll burn up the engine. I guess I'm wondering if I could tell if its running lean. It just goes higher in rpm the leaner it goes so i dont want to try and push it to see if it does because 7k rpm with no oil will hurt this engine.


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## paulr44 (Oct 14, 2008)

Two things.

First, you're describing a leaking inlet needle valve. Any diaphragm carb. should hold about 10 psi at the inlet valve. The carb. has to have the pump diaphragm cover installed to test it. You'd need a mity-vac or similar device to test it, but it usually can be rectified. Polish the valve seat using a baby q-tip and valve lapping compound. Stick the q-tip in a cordless drill if you can. Clean everything with an alcohol based cleaner or ultra-sound, and of course use a new needle valve. You should buy a complete rebuild kit, K10-WAT. Do NOT use any xylene cleaning sprays, it can deform the nozzle check valve. Sonic as you used is best, but may not act as a surfactant as will the alky or the like.

Second, the pre-lim adjustments STIHL suggests are just that, initial settings. You'd have to know how to tune a 2-stroke to finalize it. Summary for settings: warm up engine, turn idle mix. screw in till it starts to die, back it out 3/8-7/16 turn. High speed, well, that's tougher, but with a blower the full load is there all the time so I can make this fairly easy. MUST BE warmed up. Turn it in till it smooths out. You have to understand the sounds, else it'd be simpler. Then back the screw out until it runs rough. Then turn it in until it JUST cleans up. Here's the catch, if you just come down from lean drop-off you'll end up too lean. You have to come back from too rich till it cleans up else you'll burn it up. When cold, it should stumble at high speed until fully warmed up then clean up and run smooth. It's ok to bounce down in the too-rich zone until warmed up. But if it runs smooth at WOT when cold it's set too lean. This is more an audible setting than anything else, unless you're setting by RPM which we often do when specs. are available.

7500 RPM sounds about the max. for any blower...they don't list a spec. in the service manual. Don't know what you mean by no oil, a 2-stroke has to go about 18K before the rod bearings starve for mix oil.

And for what it's worth, mounting gaskets are cheap and should always be replaced if aged.


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## dingdiddydong (Jul 22, 2019)

paulr44 said:


> Two things.
> 
> First, you're describing a leaking inlet needle valve. Any diaphragm carb. should hold about 10 psi at the inlet valve. The carb. has to have the pump diaphragm cover installed to test it. You'd need a mity-vac or similar device to test it, but it usually can be rectified. Polish the valve seat using a baby q-tip and valve lapping compound. Stick the q-tip in a cordless drill if you can. Clean everything with an alcohol based cleaner or ultra-sound, and of course use a new needle valve. You should buy a complete rebuild kit, K10-WAT. Do NOT use any xylene cleaning sprays, it can deform the nozzle check valve. Sonic as you used is best, but may not act as a surfactant as will the alky or the like.
> 
> ...


Thats alot of good info thank you. If it were a customers machine i would've replsced the gaskets. Im going to try and tune this thing leaning slightly rich today. At that point if theres still a fuel leak ill get a gasket kit and if its not passing the pressure test ill do a rebuild kit and refinish the inlet needle seat.


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## paulr44 (Oct 14, 2008)

While it is rare, carburetors can leak from other areas. I am playing the odds which usually favor the inlet needle.


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## geogrubb (Jul 28, 2006)

I have a BG75 very nice blower but heavy.
My guess would be the ultrasonic cleaner destroyed a check valve or something, I have had terrible luck with a ultrasonic cleaner and small carbs, maybe it's just me. My new rule if BrakeParts cleaner won't clean it then replace it. Save yourself a lot of time and money and get a new carb on ebay, link below. Have a good one. Geo

https://www.ebay.com/itm/For-Stihl-...=123837678192c905352ac7e647d6aa15aa7e7992bdf6


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