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Briggs single cylinder OHV intermittent dying

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Briggs single cylinder OHV intermittent dying

Postby Mr Mower Man » Mon Jul 01, 2019 10:18 am

I see these engines all the time on consumer grade lawn tractors: the 17.5 HP Briggs & Stratton single cylinder verticle. The model in question is 31P677-3373-G1. This thing is not acting like all the others, for some reason.

The customer complaint was that she could mow for about 15 minutes, and then it would sputter and die. She tried to choke it, and that would prolong the running for a few seconds, but that was it. The first time I got it, I replaced the carburetor, because it was running extremely lean all the time anyway, loaded and unloaded. It seemed fine when I tested it, so I thought that was that. But she called me back a couple of days after I took it back to her to tell me that while it runs better now, it still dies after 15 minutes of mowing. She lets it sit for about 5 minutes, and then she can get about 4-5 more minutes of mowing out of it before it conks out again.

I know what you all are thinking: ignition module. But I don't think so. When I test ran it last week, it died, acting like it starved for fuel. When I pulled the gas line from the carburetor, no gas was coming out of the line (this is gravity fed from the tank). I blew back through the line, and then gas started coming out. So I removed the tank, made sure it was clean, replaced the fuel line and filter (with an OEM 691035 filter and 792020 gas line), and made sure the elbow coming from the tank was clear. So everything in the fuel system was new: line, filter, tank (cleaned, not new), and carburetor. I also replaced the spark plug (which was a Briggs branded plug - perhaps the original one) with an NGK CS6 (1716).

I was testing it again this morning, and it ran good & smooth for several minutes. Then eventually (after about 35 minutes of running, 20 of mowing), it started to sound like the engine was laboring slightly, more than it should be I thought, but it kept going. Then it started to sputter and act like it was going to die. All I did was slow the tractor down, to reduce the amount of load on the engine, and it revived and continued running for a few more minutes, during which it would act like it was starting to stumble intermittently. After a few more minutes of this, it was consistently running kinda rough, like it was cutting out. Then it started to die, and when I stopped the PTO to reduce the load on the engine, it was still wanting to die. So I shoved the throttle all the way up to choke the engine, and it limped its way to the shop. Then it finally died then, and I could not get it to run any more.

I took the gas line from the carburetor again, and nothing was coming out. I removed the gas cap, wondering if there had been a vacuum pulled on the tank. But still, nothing came out. Then I blew back through the line, and then gas started to come out. What's going on here? I thought the section of fuel line from the filter to the carburetor felt pretty hot to the touch. Am I getting some sort of vapor lock? I hardly EVER have problems related to vapor lock. Do I need to wrap some kind of insulation around the fuel line to keep the gas cooler? I see these engines on these kind of cheap lawn tractors all the time, and none of them have this problem. What ideas do you have?
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Re: Briggs single cylinder OHV intermittent dying

Postby Arkie » Mon Jul 01, 2019 10:34 am

Good that you determined that it's a fuel delivery issue.

Is it gravity feed or fuel pump fed? (varies by tractor design)

Some eq has a fine screen inside the fuel tank and blowing back through the fuel line clears the screen enough tha tit will run ok for awhile, but the gunk is still in the fuel tank. On some that the tank is hard to remove to clean I just remove the screen that is in the bottom of the tank or punch a hole in it, clean out the tank then install a inline fuel filter with a manual shutoff. Also replace the fuel lines with new. Fooling with old fuel lines will be sending fuel line trash to the carb. Use a Red Briggs inline filter for gravity fed and a white for fuel pump type. (the white filter is too fine for gravity fed)
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Re: Briggs single cylinder OHV intermittent dying

Postby 38racing » Mon Jul 01, 2019 11:00 am

Daughter's jd had same issue last summer solved by blowing back into tank. Been lucky so far as it hasn't happened again.
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Re: Briggs single cylinder OHV intermittent dying

Postby KE4AVB » Mon Jul 01, 2019 12:27 pm

Yes it sounds like vapor locking at the fuel filter. I have seen MTD, Kohler, and Briggs to do it here. I even seen Kawasaki where the fuel line are wrap in heat shielding on the water cooled engines.

Today's fuel can boil with temperatures as low as 140F. The only solution it to heat shield the fuel line and filter. Plus that 40 micron filter is too small gravity fed systems. Needs to be 70+ micron for good fuel flow.

To know what temp of the fuel at the filter is use a temperature sensor to check it.
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Re: Briggs single cylinder OHV intermittent dying

Postby bgsengine » Mon Jul 01, 2019 12:37 pm

I Agree with Arkie - drain the fuel tank dry and check bottom of tank for a filter screen for any debris clogging it up, make sure it's new fuel lines from tank all the way to engine (with filter in between) . I had one that got clogged with pet hair (almost invisible against the fuel tank) , I put a piece of 1" PVC over the hole of the empty tank and a rag on other end and blew out the tank with compressed air (jets the stuff up to stick to rag) and found it - a big glob of white hairs (pet,dog, deer, whatever) that got in there somehow... Took tank out completely and flushed it , havent had a problem since.
How poor are they who have not patience. What wound did ever heal, but by degrees? - Iago (Othello Act II, Scene 3)
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Re: Briggs single cylinder OHV intermittent dying

Postby Skywatcher » Mon Jul 01, 2019 2:08 pm

Hi Mower Man

Had a similar problem with a B&S 331877 engine on a Sears DLS3000 lawn tractor. Turned out the cooling air from the cylinder was blowing onto the carburetor causing the fuel to boil in the float bowl. I ended up making a new gasket with a skirt for between the carburetor and the intake manifold to work as an air deflector to direct the hot air away from the carburetor. The engine still vapor locks occasionally when customer is working the machine hard in temperatures above 28ºC (83ºF or 301ºK). Just check to see how hot the carburetor is when the engine starts to falter. Best of luck,

Sky
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Re: Briggs single cylinder OHV intermittent dying

Postby 38racing » Mon Jul 01, 2019 9:16 pm

Does 10% ethanol change the boiling point of the gasoline?
Edit: googled it. Ethanol has higher boiling point at atmospheric pressure .about 78 c compared to 35
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Re: Briggs single cylinder OHV intermittent dying

Postby KE4AVB » Tue Jul 02, 2019 6:07 am

Kinda doesn't matter too much when you have 200C air temps blowing across the fuel line and filter from the muffler. Just takes longer to get the fuel up to boiling point. The ones I had problems with last spring were reaching 100C @ the filter after I turn the engine off according to my laser thermometer. Many riders have the muffler up front where many ZTRs it is on the rear hence I haven't the fuel heating problems on the ZTRs.

Now higher octane fuel does have a higher boiling point as well as Summer blends of gasoline as the fuel doesn't need the lower flash point temp as it does in the Winter.
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Re: Briggs single cylinder OHV intermittent dying

Postby Mr Mower Man » Thu Jul 04, 2019 10:26 am

Update:

I cleaned out the tank and made absolutely certain nothing was inhibiting fuel flow. I even re-routed the fuel line so it was a straight shot to the carburetor, whereas it previously had a gully in it. So I was running the engine yesterday, and after about 20 minutes, it started to run rough again. I was wondering if the fuel was starting to get hot, and the fuel level in the tank wasn't that high, so I wondered if adding fresh, cool fuel would temporarily improve running conditions. So I raised the hood and walked back to the shop to get the gas can. Before I got back to the mower, the engine had lined out and was running perfectly. I went ahead and added the fuel, and closed the hood. In a couple minutes, it was running rough again. So I raised the hood again, and after a few seconds it smoothed out again. I repeated this a couple more times just to establish a clear pattern. And it is a clear pattern: with the hood up, the engine runs fine. With the hood down, the fuel line and/or carburetor is getting too hot and we start having vapor lock issues.

So here's my plan, based on what KE4AVB said. I'm changing the 691035 filter (60 micron) to the red pancake filter (298090S, 150 micron), which is standard for many gravity fed fuel systems. This may allow little bubbles to get through, whereas they couldn't before. Just a guess. Then I'll test run it. If there are still problems, I'm going to wrap the fuel line in some kind of heat insulation to keep the fuel cooler. I could do both at the same time, but I want to see what factor makes the difference. Even if the carburetor bowl is getting too hot, I'm hoping that cooler fuel in the line will translate into cooler fuel in the bowl.

The thing that's really bothering me is, why is this a problem on this machine, whereas I have never seen it happen on others just like it? I think I can fix it, I just am perturbed about needing to fix it in the first place.
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Re: Briggs single cylinder OHV intermittent dying

Postby bgsengine » Thu Jul 04, 2019 11:17 am

what brand of tractor? where's exhaust located? Exhaust heat shield present and accounted for? Tractors with hoods often have air ducting (usually the top vents) to direct outside air into the rotating screen, which aids in cooling - just changing position of the hood you are altering the air flow dynamics of cooling air getting to the engine... Which is why I wonder about any missing heat shielding from exhaust area.. Likewise an exhaust leak could potentially be sending exhaust gas (low oxygen content) back to the air intake.. not to mention, those with the enclosed air cleaners are pulling their intake air from the engine cooling air flow, again a dynamnic altered by changing hood position..
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