Walbro air purge routing - tank emptying into crankcase

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  #1  
Old 12-07-09, 08:18 AM
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Walbro air purge routing - tank emptying into crankcase

Hello,
I've been working on an old Homelite 320 snowblower with a Walbro HDC carb on it. I rebuilt the carb and have got it running well a few times. However, when I try to start it after it sits for a day or two, the crankcase floods and fuel ends up running out though the muffler until the tank is dry.

Thinking the carb was the problem, I rebuilt the metering system and reset the lever a few times, lower and lower. I've pressure tested it and it appears to be OK.

After testing it with the carb connected but not mounted, the fuel appears to be leaking not through the carb barrel itself but through the air purge lines. To make sure I'm clear, I'm attaching a picture. Line A goes from the primer bulb to the fuel pump cover. Line B goes from the pump cover then connected to the crankcase at the bottom of the engine, fuel passes through the engine to a twin connection next to it which flows to a t-connection at the bottom of the fuel line C (which is the lowest point in the connections). What it seems like is happening is fuel is passing into the carburetor through C, out through B, filling the crankcase, which then drains through the exhaust manifold onto the ground.



So what's going on? Are the A & B lines mismatched? Is there a one-way valve that is not functioning correctly? I've tested the lines back to the main fuel line and everything passes, and the gas cap is venilating correctly.

Thanks for any suggestions!
 
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  #2  
Old 12-07-09, 08:48 AM
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Check the metering lever height per the manual. Go to the Walbro website get the manual labeled “Diaphragm Carburetors” and the manual specifically for the HDC carburetor. If you continue to have problems post back.

Service Manuals

Manual for HDC carburetor.
http://wem.walbro.com/distributors/s.../HDCseries.pdf
 
  #3  
Old 12-07-09, 09:01 AM
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Hello,
I've been through the walbro service manual and through the HDC manual - and have set the lever as specified in the HDC manual. The reason I don't think it's the metering lever is that the fuel is not coming out the carb barrel - it's emptying through the purge lines which I believe are before the leveling meter comes into play.

I could be wrong about this of course - but the HDC manual doesn't address air purge systems, and the service manual shows the purge on the metering side, not the fuel pump side as shown in the picture.

Thanks for the reply.
 
  #4  
Old 12-07-09, 10:11 AM
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I am not familiar with the purge line you speak of and where it connects to the carburetor. I looked at some old HDC carburetors I have and they only have a fuel inlet line and no provisions for purging. This may have been a Homelite add on. What is the model number of the machine, UT-xxxx? Maybe the parts list will show it.

To rule out metering problems test as follows:
Remove everything from the metering side except the metering valve. Submerse the carburetor in water and pressure test carburetor through fuel inlet. If you do not have the correct tester, try an ear syringe or blow through a piece of tubing. The idea is not to exceed metering valve pop-off pressure.
 
  #5  
Old 12-07-09, 10:23 AM
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Originally Posted by blindjesse View Post
Hello,
I've been through the walbro service manual and through the HDC manual - and have set the lever as specified in the HDC manual. The reason I don't think it's the metering lever is that the fuel is not coming out the carb barrel - it's emptying through the purge lines which I believe are before the leveling meter comes into play.

I could be wrong about this of course - but the HDC manual doesn't address air purge systems, and the service manual shows the purge on the metering side, not the fuel pump side as shown in the picture.

Thanks for the reply.
Just one thought that perhaps the fuel lines are not connected properly...?
 
  #6  
Old 12-07-09, 07:27 PM
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Thanks for the responses. I had my first real-world test today and the snowthrower performed admirably, actually better than I was expecting. It ran very well, except when I shut it off and tried to restart it in a minute or two (if I restarted it right away it was fine).

I found out that if I turned off the fuel line valve from the tank to the engine, and (don't try this at home) turned the whole thing over for a minute or two to drain the case, I could get it to start up again. So now I'm wondering if this muddies up my original hypothesis about the bad air purge line.

As for pressure testing the metering value, I have pressure tested it but not in the way you describe (I've basically just tested the valve from the opposite end to see if it holds), but when the snow settles and I don't need the thing as urgently, I will.

Regarding the model number, I can't find anything along with a UT prefix. The snowthrower is a homelite jacobsen 320. There is a number on the crankcase - 95505a. The carb is a walbro HDC 74, with the number 4 548 on the edge.
 
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