Bad Carburetor Power

[Wood Doctor]

I ran into something today that I had not seen before. A customer asked me to fix a Stihl MS460 that would start and then die when close to idle. Whether on choke or not and during initial start up, fuel was being blown out of either the decomp valve or the muffler, so much so that I thought the tank housing was leaking fuel. Turn the saw sideways and fill the tank, and fuel leaked everywhere.

I discovered that the impulse hose was disconnected at the cylinder. So, I reconnected it and then I replaced the fuel line, the pickup body, and the decomp valve. It has a good spark and high compression, so I tried to start the saw again. No luck, it pumped fuel right through the exhaust this time. Seems to me that it needs a carb rebuild or a new carb, but frankly, I have never run into this problem before. Has anyone else? If so, how did you solve it? TIA.

 

[ammoaddict]

I ran into something today that I had not seen before. A customer asked me to fix a Stihl MS460 that would start and then die when close to idle. Whether on choke or not and during initial start up, fuel was being blown out of either the decomp valve or the muffler, so much so that I thought the tank housing was leaking fuel. Turn the saw sideways and fill the tank, and fuel leaked everywhere.

I discovered that the impulse hose was disconnected at the cylinder. So, I reconnected it and then I replaced the fuel line, the pickup body, and the decomp valve. It has a good spark and high compression, so I tried to start the saw again. No luck, it pumped fuel right through the exhaust this time. Seems to me that it needs a carb rebuild or a new carb, but frankly, I have never run into this problem before. Has anyone else? If so, how did you solve it? TIA.

Maybe inlet needle not closing?

 

[Wood Doctor]

Maybe inlet needle not closing?

Perhaps, but I have a hard time imagining how that could cause fuel to be driven right out the exhaust and lots of it. Frankly, I have never obseved this in the past. Carb failures usually mean that insufficient fuel or the wrong mixture is supplied. WDYT?

 

[pioneerguy600]

Time to pull the carb, check the metering diaphragm first to see if its stiff, if it is it will hold the needle off its seat and allow the engine to flood. If the diaphragm is still flexible the needle seat could be dirty or worn badly allowing the needle not to seal tight.

 

[Totembear]

The saws flooding maybe from a bad check valve. But its flooding...

 

[Wood Doctor]

Time to pull the carb, check the metering diaphragm first to see if its stiff, if it is it will hold the needle off its seat and allow the engine to flood. If the diaphragm is still flexible the needle seat could be dirty or worn badly allowing the needle not to seal tight.

Thanks so much. I never witnessed a carb that could pump that much, choked or unchoked. So much fuel was leaking onto my workbench that I thought the tank housing was cracked. Apparently this carb is filling up the combustion chamber and exhaust like a fuel pump. Filling the fuel tank then becomes a leaking nightmare as you turn the saw on its side to fill it.

I actually tried an old carb that needed repair and got the saw running, but the engine refused to run slowly and died. So, I installed the original again and the same massive flow appeared again. It is truly amazing how a carb can turn into a pump.

 

[huskihl]

Thanks so much. I never witnessed a carb that could pump that much, choked or unchoked. So much fuel was leaking onto my workbench that I thought the tank housing was cracked. Apparently this carb is filling up the combustion chamber and exhaust like a fuel pump. Filling the fuel tank then becomes a leaking nightmare as you turn the saw on its side to fill it.

I actually tried an old carb that needed repair and got the saw running, but the engine refused to run slowly and died. So, I installed the original again and the same massive flow appeared again. It is truly amazing how a carb can turn into a pump.

The fuel in the tank expands and tries to vaporize when you jerk the rewind. The needle isn’t closing and tank pressure is pushing fuel into the engine

 

[pioneerguy600]

Thanks so much. I never witnessed a carb that could pump that much, choked or unchoked. So much fuel was leaking onto my workbench that I thought the tank housing was cracked. Apparently this carb is filling up the combustion chamber and exhaust like a fuel pump. Filling the fuel tank then becomes a leaking nightmare as you turn the saw on its side to fill it.

I actually tried an old carb that needed repair and got the saw running, but the engine refused to run slowly and died. So, I installed the original again and the same massive flow appeared again. It is truly amazing how a carb can turn into a pump.

Get it out and all apart, let us know what you find as there are several areas where fuel can leak past , deteriorated gaskets and diaphragms, have even seen a hole in the pump chamber diaphragm allowing fuel to enter the crankcase via the impulse line. Many variables but you can find them step by step though.

 

[stihltech]

I ran into something today that I had not seen before. A customer asked me to fix a Stihl MS460 that would start and then die when close to idle. Whether on choke or not and during initial start up, fuel was being blown out of either the decomp valve or the muffler, so much so that I thought the tank housing was leaking fuel. Turn the saw sideways and fill the tank, and fuel leaked everywhere.

I discovered that the impulse hose was disconnected at the cylinder. So, I reconnected it and then I replaced the fuel line, the pickup body, and the decomp valve. It has a good spark and high compression, so I tried to start the saw again. No luck, it pumped fuel right through the exhaust this time. Seems to me that it needs a carb rebuild or a new carb, but frankly, I have never run into this problem before. Has anyone else? If so, how did you solve it? TIA.

Pretty common repair on this model. The inlet lever is too high. Bend the lever so it is level with the top of the metering side of the carburetor. If you have a pressure tester, check the inlet needle at this time. If it does not seal, you at least need a new inlet needle (valve).
If it holds, reassemble the gasket with the diaphragm on top. Pressure test. Should hold. If not, check lever height again. Also the button in the center of the diaphragm should be almost flat. If it sticks down it is the wrong diaphragm. Usually get 6 or so a year.

 

[9050lx]

Diaphragm packed with crud?

 

[Wood Doctor]

The culprits from what the forum has told me:

I have to agree. The metering diaphragm checks out OK. Still flexible.

 

[SteveSr]

The culprits from what the forum has told me:
View attachment 961405
I have to agree. The metering diaphragm checks out OK. Still flexible.

Pressure test the fuel inlet in the carb. If it doesn't hold 7-10psi indefinitely you have found your leak. I doubt that it is an adjustment unless someone monkeyed with it as the saw was running correctly.

 

[PV Hiker]

Pressure test the fuel inlet in the carb. If it doesn't hold 7-10psi indefinitely you have found your leak. I doubt that it is an adjustment unless someone monkeyed with it as the saw was running correctly.

This and few other post above.

Fuel tanks pressure themselves to the carb as noted when remove hose at carb it will fuel geyser. When needle is stuck open, fuel will get past and flood out even if the saw is just sitting.

You can pressure test carb in place for quick test.

 

[Wood Doctor]

I checked the carb's inlet needle and the inlet control lever. Both were OK. Still seemed like almost instant flooding and fuel was being pumped right out the decomp valve after a few choke pulls. In short, I became an expert at flooding this engine.

So, I let it sit for an hour and tried again at fast idle. This time it acted like it wanted to run. I continued to pull and she fired right up and pumped smoke out the exhaust for a minute or so. Then it cleared right up and I tuned the carb. Then I replaced a needle cage bearing that was causing the chain to hiccup.

All systems are go! Thanks forum. You guys are great, as usual.