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2 Cycle Carb Problem


Bigwind's Avatar
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06-16-06, 01:50 AM   #1  
2 Cycle Carb Problem

Hello All & Cheese: I have an IDC 500 Supreme on the bench. The carb is a Walbro WT149A. I have rebuilt the carb and the unit cranks, adjusted easily and runs extremely well! PROBLEM: Gas is being siphoned out through the carb after I shut the engine off? I've gone brain damaged and can't resolve this problem! I rebuilt another WT149A and it does the same thing? Have I put something in backwards? The valve seems to be seating O.K. but I am considering trimming a cue tip down, using a little lapping compound and twisting it in the seat to get a better seal. I can remove the gas cap and this doesn't happen. Normally I would just switch a cap out and check it but this gas cap had to be an odd size? What's your best advice or guess? Anything will help at this point. How far north will I have to go before I can find a Hemlock tree to chew on? Thanks, Ron!

 
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06-16-06, 10:34 AM   #2  
Bob

Ron:


It sounds like the venting of the gas cap is the problem. After you shut off the engine the gasoline continues to heat and expand which builds pressure in the tank. I would say it isn't a siphoning problem but the fuel is being pressure fed from the tank through the carburetor. The checking device in the carb will only hold back so much pressure.

That theory is supported with your removal of the cap to remedy the problem.

Hope this helps

 
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06-16-06, 10:58 AM   #3  
Posted By: marbobj Ron:


It sounds like the venting of the gas cap is the problem. After you shut off the engine the gasoline continues to heat and expand which builds pressure in the tank. I would say it isn't a siphoning problem but the fuel is being pressure fed from the tank through the carburetor. The checking device in the carb will only hold back so much pressure.

That theory is supported with your removal of the cap to remedy the problem.

Hope this helps
and shouldn't there be some sort of backflow diverter in the fuel line itself? The bulb looking object inside the tank?

 
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06-16-06, 11:00 AM   #4  
It really does help and I appreciate it very much. I have found that I'm missing a gasket in the fuel pump side according to the Walbro diagram, HOWEVER, I have found that I have 4 of these carbs an they are ALL are the same. None have the gasket they are showing? I have only rebuilt two of them so that's a mystery. This just adds credence to your thoughts! Thanks again, Ron

 
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06-16-06, 12:54 PM   #5  
Bob

Ron:

I can't think of any backflow device that would be necessary since the flow of fuel is toward the engine from the tank and has to be regulated accordingly. Most of the devices in the carburetor do that while allowing a mixing of the fuel with the outside air.

Is the bulb in the tank attached to a short length of fuel line? If so I'm sure its sole function would be to filter the fuel and wouldn't serve to check fuel flow. Any flow checking device would simply be locked into an off position by pressure in the tank.

There are some other ways I can think of to handle the tank pressure but a vented cap is the most common. Venting into the intake system would affect the fuel/air mixture. Let's see if some of the other guys posting on this forum can share an insight on this thing.

Hope this helps

 
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06-17-06, 01:41 AM   #6  
Hello Ron!

I agree, it does sound like you may need a cap...but...as far as I know, there is a gasket with the diaphragm on all 2-stroke walbro carbs. If that is missing, it could cause a clearance issue and cause the diaphragm to put pressure on the needle arm, holding the needle slightly open. I think that gasket is probably around .020" thick, so it would have an effect it seems. If that doesn't stop it, I do the Q-tip thing myself, usually with success.


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God bless!

 
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06-25-06, 12:37 AM   #7  
The inlet needle is set too high, it should be level with the floor of the carb.

 
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06-26-06, 02:06 AM   #8  
Not necessarily. The absence of a gasket would cause the button on the diaphragm to be closer to, and put more pressure on the needle tab, acting as though it was set too high. A stiff diaphragm can also do this if it is stiff enough to hold the needle open instead of relaxing out of the way.

Did you get it figured out Ron?


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God bless!

 
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07-01-06, 10:33 PM   #9  
A missing gasket on the pump side would not directly affect the inlet lever.
If the same problem occurs with other carburetors you`ve rebuilt then you may be installing the gasket over the metering diaphragm instead of the gasket and then the metering diaphragm.
Or these are Walbro WT`s? If you`ve used compressed air to blow them out you`ve damage check ball inside.

 
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07-02-06, 12:37 AM   #10  
Will not affect the inlet lever? How can it not affect the lever when your moving the diaphragm button .020" closer to the lever? It's not like there's 1/8" play in there. Leaving the gasket out would put the diaphragm button in contact with the lever, which in turn could relieve enough pressure from the needle to break the seal. Especially with a slightly stiffer diaphragm.


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God bless!

 
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07-02-06, 12:25 PM   #11  
Cheese/ bigwind
I now exacatly what your saying but your talking about the metering diaphragm. If you read back you`ll see that "Big Wind" said he was missing a gasket on the pump side which is the opposite side of of the inlet lever.


big wind ,
If you meant you were missing a gasket from the side with the inlet lever, thats called the metering chamber. The pump is on the side with one screw holding the cover.
Please verify the side the gasket was missing from.

 
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07-02-06, 08:30 PM   #12  
Mike,

After re-reading this, I see now. You are right, the metering diaphragm is the one that would interfere with the inlet needle lever, not the fuel pump diaphragm. Ron did indicate the gasket was missing on the fuel pump side, but I overlooked that. My apologies. You even emphasized "fuel pump side" in your last post, but I didn't pick up on it. 3:30 am isn't my brightest hour, lol.

For some reason, I got stuck on the wrong side of the carb. I guess I was thinking of what missing gasket would cause the carb to leak, and the gasket on the metering side could cause a leak if it were missing and I went with it from there. I have tunnel vision and it takes a smack to get me out of it sometimes.

Ron, did you get this resolved?


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God bless!

 
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07-04-06, 11:24 PM   #13  
Cheese
Any of this can be misleading. I`ve had to go over several of them to understand the problem in question. I expect you to correct me.
Were here to help each other.
Mike

 
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