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A question on small engine repair

A question on small engine repair

Old 08-04-2017, 09:22 AM
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I have a Briggs&Stratton twin cylinder garden tractor with a fuel feed problem. It will run at low speed all day but when powered up for mowing, runs out of fuel in about 3 minutes. It acts like it's getting no fuel once it warms up. I've even put a remote Briggs fuel pump on it with the same result.

My question is how is the crankcase pressure fed to actuate the pump? Is there a diaphragm or something in the crankcase that might be failing. It seems like the pump is not supplying enough fuel at high rpm to keep the float bowl full.

I'm going to look on-line for engine parts, but I'm not confident of finding out about the just by looking at parts diagrams. Most of the diagrams are sketchy at best.
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Old 08-04-2017, 09:35 AM
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What specific engine is it? What tractor is it on? That will determine how it all goes together. If its a gravity fed or level system there is a diaphragm in the carb. If its a remote system it sounds like a vapor lock issue. Do the fuel lines run close to the exhaust? Also, it could be a line blockage. Stopped up filter or a weak line collapsing under vacuum from the pump.
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Old 08-04-2017, 10:06 AM
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It's a 42E707 1631 01 vertical shaft twin cylinder engine on a Craftsman tractor built in 1997. It has a remote fuel tank that is lower than the carb and uses a diaphragm pump on the side of the carb. I have bypassed that pump and mounted a remote Briggs diaphragm pump in the frame rail. The pump delivers adaquate fuel while cranking with an open fuel line. The tank has been removed and cleaned, all rubber hose has been replaced, carb cleaned and fuel filter replaced. I'm using non-ethanol fuel and it runs very well at low speed. The problem only happens when the throttle is high for mowing. If you pull the choke out quickly, you can catch it for a second, but then it dies again.

As I've said, I'm thinking it's a crankcase pressure problem not actuating the pump fast enough. Maybe it's got something to do with the breather, IDK. When I power it up and start mowing, I can mow about 50 to 100 feet before it starts to die. I can drive it around the house at low throttle perfectly.

I haven't open up the crankcase breather unit yet because IDK what I'm looking for. Is there a reed valve or something in there?

Last edited by dave1123; 08-04-2017 at 10:22 AM.
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Old 08-04-2017, 10:26 AM
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This is really pissing me off! Because of it not running and all the time I've spent on it PLUS the heat and mosquitoes, my grass is waist high now! Actually it's not even grass but field weeds that I keep mowed.

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Old 08-04-2017, 10:40 AM
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is there fuel in the bowl when it stalls? If so then we can eliminate fuel delivery and the pump. If not, then im not sure. a remote electric pump should supply plenty of fuel, if the lines are good.
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Old 08-04-2017, 11:20 AM
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I'll have to pull the plug and check, but I doubt it.

What I find amusing is you and I are trading posts back and forth about fuel problems! Your jeep and my tractor. LMAO!
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Old 08-04-2017, 11:36 AM
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Originally Posted by dave1123 View Post
I'll have to pull the plug and check, but I doubt it.

What I find amusing is you and I are trading posts back and forth about fuel problems! Your jeep and my tractor. LMAO!
Haha yeap. Im a certified briggs and stratton tech. competed in small engine mechanics in HS and ranked 19th in the nation.
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Old 08-04-2017, 03:11 PM
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For the sake of argument we'll assume the float level is correct and the the needle isn't getting hung up in the seat. Then, there should be a spring and disc in the crankcase breather.
Deposits build up or the spring takes a dump then the disc no longer seals well hence no vacuum. Pulse pump receives no pulse and gravity can't keep up.
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Old 08-04-2017, 08:10 PM
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Thank you very much! That's exactly the info I want to know! I've been going thru this system in my mind and as far as I could tell everything was perfect. Originally I thought the float needle was hanging up or the float was set too low not allowing the needle to open far enough to keep up with demand. Then I started thinking "what powers the pump." I wasn't sure if it was pressure or vacuum but I knew it came from the crankcase. Looking at the parts, all it said was "breather assembly." I was going to take that out and examine it but made this post first.

Thank you with all my respect, Turbo!
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