Engine runs 20 seconds, then quits

[jinman]

I believe that Soundguy is correct. My B&S engine on my John Deere would work fine when I removed the float bowl. It drove me nuts because I'd take the carb apart, put it back together, and it would run fine for a couple of weeks before the exact symptoms described by the OP. To prove it was the float needle, I would blow low pressure air into the inlet hose for a second (less than 30 psi; more will do damage). The engine would start and run normally for a few days. When it stopped, I'd blow into the hose again and I'd be back in business for a few days. I finally got tired of the problem and replaced the little plastic needle that came on the tractor with a brass one directly from B&S. That was two years ago and no more problems.

I just think the problem is the rubber seal on the jet is degrading due to ethanol and becoming sticky. The combined weight of the float and plastic needle is not enough to unseat the needle valve when it sticks. It is a hair-pullin' problem that will drive you nuts. Jazz-Dad, I'd try the very low pressure air at 20-30 psi to see what happens. That should really prove or disprove the sticking needle. Your carb float chamber has just enough fuel in it to start the tractor and let it run for 20 seconds or so perfectly. It then goes to running horribly and dies as the float chamber is empty. It's one of those things that makes you wish for Superman's x-ray vision.

 

[121parker]

Sounds like either a bad connection on your coil or just a bad coil it could also be a bad PC board.

 

[MoKelly]

I believe that Soundguy is correct. My B&S engine on my John Deere would work fine when I removed the float bowl. It drove me nuts because I'd take the carb apart, put it back together, and it would run fine for a couple of weeks before the exact symptoms described by the OP. To prove it was the float needle, I would blow low pressure air into the inlet hose for a second (less than 30 psi; more will do damage). The engine would start and run normally for a few days. When it stopped, I'd blow into the hose again and I'd be back in business for a few days. I finally got tired of the problem and replaced the little plastic needle that came on the tractor with a brass one directly from B&S. That was two years ago and no more problems.

I just think the problem is the rubber seal on the jet is degrading due to ethanol and becoming sticky. The combined weight of the float and plastic needle is not enough to unseat the needle valve when it sticks. It is a hair-pullin' problem that will drive you nuts. Jazz-Dad, I'd try the very low pressure air at 20-30 psi to see what happens. That should really prove or disprove the sticking needle. Your carb float chamber has just enough fuel in it to start the tractor and let it run for 20 seconds or so perfectly. It then goes to running horribly and dies as the float chamber is empty. It's one of those things that makes you wish for Superman's x-ray vision.

This is very insightful and something I believe is happening with my wheeled trimmer. Now I have an idea what to do to both get it working in a spot and fix longer term.

Tahnks.

MoKelly

 

[Gale Hawkins]

Does it have a Low Oil PSI kill device?

 

[JazzDad]

It has a low oil pressure light, which does not illuminate until the engine has stopped for about 10 seconds.

Jinman: I don't quite understand what the air is doing. Are you saying that the pressure blows the needle back off the seat? Each time I take the bowl off, the float is hanging down and fuel will flow. However, this sounds like the most plausible thing fitting the symptoms. I'll probably take the carb off and give it a good cleaning. Now, assuming the carburetor is on the bench and upside down; if you set the needle in its seat, can you feel any hesitation as you lift out out? This would be a great confirmation of the "stickiness".

I plan to get at it this weekend. I just hope it's not like my drive home today. 100 degrees on the car thermometer when I hit the driveway. I'll let y'all know. (If I don't die from heat stroke.)

 

[jinman]

Bret, the air pressure is just increasing the pressure inside the inlet hose to the carb. I removed the hose from the outlet side of the fuel pump and pressurized that line with low pressure air. Normal fuel pressure probably is 1 psi to 2 psi at most. The added pressure will push the needle off it's seat even if the float chamber is full. You'd think vibration would cause the float to drop and open the needle valve, but it just doesn't seem to happen that way. I think when you remove the carb and pull off the bowl, it's very hard not to disturb the float enough to make it open the valve. That's the only reason I can explain that everything looks normal. The low pressure air proved that it was not happening in operation all the time. Here is a link to my thread about the problem. The original needle looked like the photo below. The replacement was the same, but brass.

 

[JazzDad]

I took the carb off and just set the needle into the seat (without the float attached & the carb upside down). I tip it back over and the needle falls right out- no hesitation. The rubbery tip seems solid, not mushy and sticky. The seat is clean. I'm having a hard time going over to the the JD shop and paying $20 for that tiny little part. I also found an O-ring that is deteriorated. It's the one that seals the solenoid port to the "nozzle".

So, why am I posting? Just griping. I haven't tried the air thing, Jim. I would really like to make a seal around the gas cap and pressurize the entire tank. (The overflow vent would have to temporarily be plugged, though.

I will let y'all know what I find.

 

[jinman]

Bret, I DO NOT recommend that you pressurize the whole tank. If you pull the hose off the fuel pump on the carburetor side and turn the engine over, you should see gasoline gush out (catch it if possible). That will tell you that you are getting fuel that far. Then you can buy one of these kits for about $15 that has the nozzle with the red handle as seen in the middle of this set. Use that nozzle on a compressor set to 20-30 psi when your mower stops running. If it works, you need a needle. If it doesn't work, then your problem is somewhere else. See the needle ordering instructions below.



Next, if you need a needle, you can go to Briggs and Stratton online engine parts and enter your engine model off its label. Look on the parts list for 'float needle valve' or look on the parts diagram. For my engine, the part number is 797410 and is $9.35.

So, if you have to buy both items above, you've only spent around $25. It could be the best $25 you've spent in a long time. Believe me, I took my carb apart and put it back together about 6 times before I finally replaced the needle. I too didn't want to believe there was anything wrong with my needle. However, the compressed air is an easy test that confirms that the needle is sticking. Carefully give it a try before you write off the possibility. If it doesn't work with the air, don't bother with the needle. It's that simple.

 

[Midniteoyl]

Did you ever try swapping plugs? Had 2 do the same thing in one week..

As for the carb: Things can 'look' right and be totally bad. Since you have it out, get a rebuild kit and redo it all.. you'll be glad you did in the end.

 

[Egon]

Do you have a solenoid fuel shut off system? With a few more electrical items attached?