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Tecumseh 5.5 hp: won't restart unless primed


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02-23-15, 11:34 PM   #1  
Tecumseh 5.5 hp: won't restart unless primed

I picked up a Craftsman lawnmower with a Tecumseh 5.5 HP engine. I went through the carb (part no. 640174) just to clean it and everything looked pretty good...I didn't replace any parts, yet.

I got it running, but found that I have to prime it on every start/restart. I've read some people say this is normal, but every lawnmower I've used you are able to restart it without priming.

I mainly just have a general question about these small engine carbs. In order for the engine to restart without priming, the gas has to remain in the emulsion tube and not run back into the bowl (correct?)...but how is this possible if air can get into the bowl from the primer bulb? (i.e. the bowl is not airtight so gas can flow back into the bowl).

 
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02-24-15, 05:02 AM   #2  
I would prefer it to be that way. There is far less chance that you would flood that engine. Does the carb have a float bowl? Something tells me that it doesn't.

 
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02-24-15, 06:56 AM   #3  
Thanks for the reply. Yeah it's not that big of a deal but I'm mainly just curious if it is operating as designed. My experience with small engine carbs is pretty limited, but just from taking it apart I see that the bowl is vented via the primer bulb. So it makes sense that the fuel runs back into the bowl, unless I am misunderstanding something.

The carb has a bowl and float to regulate the incoming fuel. That seems to be working fine... no flooding.

I found a good cross section of the carb I'll have to dig it up and post it.

 
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02-24-15, 07:05 AM   #4  
Personally. I would leave it alone. Why look for trouble?

 
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02-24-15, 09:11 AM   #5  
Just out of curiosity. The mower was free so I am just tinkering. According to the lawn mower's manual, it says priming on restart is not usually necessary.

I'm wondering if the primer isn't sealing like it should and is allowing air into (or out of) the bowl when it is not being pressed, which allows the fuel to drain back into the bowl out of the emulsion tube. I've tried starting in immediately after stopping and it won't...I feel like there should be some fuel left in the tube to get it going again, but it appears to drain very quickly.

The carb looks like this:


I don't think it is the exact same, but cross section #4/5 looks pretty close:

 
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02-24-15, 02:08 PM   #6  
It's common to have to reprime these. It's just the way they are. If you run the mower for a long time and have it at full operating temp and you shut it off, you may not have to prime to restart if you try to restart soon. Gas doesn't remain in the emulsion tube. The engine needs a prime when it's below operating temp and the oil is thick. Once the engine is up to temp, the combustion chamber is hot, the spark ignites a leaner mixture more easily, the engine turns faster while cranking because the oil is thinner, etc... and it just doesn't take a rich mixture to get it running.


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02-24-15, 02:53 PM   #7  
Did you tighten all the screws when you cleaned the carb. If you didn't, I bet that you will find some loose screws. We all have some.

 
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02-24-15, 03:25 PM   #8  
It's common to have to reprime these. It's just the way they are. If you run the mower for a long time and have it at full operating temp and you shut it off, you may not have to prime to restart if you try to restart soon. Gas doesn't remain in the emulsion tube. The engine needs a prime when it's below operating temp and the oil is thick. Once the engine is up to temp, the combustion chamber is hot, the spark ignites a leaner mixture more easily, the engine turns faster while cranking because the oil is thinner, etc... and it just doesn't take a rich mixture to get it running.
Thanks for chiming in. It makes sense that the gas won't stay up in the emulsion tube just from the way the primer bulb works.

My other lawn mower (John Deere with a B&S) has the type of primer bulb with a hole in it so when you put your finger over it you create a seal to pressurize the fuel bowl. I am able to stop and start it without any repriming...maybe the hole in the primer bulb is small enough that the fuel in the emulsion tube doesn't drop very fast.

All bolts are tight, but I will double check everything.

 
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02-24-15, 07:35 PM   #9  
Well I figured it out. I didn't have the air filter mount on the end of the carb while I was testing it, and after looking closer I found that the hole for one of the air filter mount bolts goes into the cavity for the primer bulb. So it was allowing even more venting of the bowl. I put the bolt in and now it fires up without repriming. Even after 10 minutes it fired up fine.

That problem is solved. Now I just have to adjust the RPM...right now it is running around 4000, a bit higher than a normal lawnmower (~3500). It looks like someone messed with the return spring.

 
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