Rough engine

[dbergloev]

I have this older Craftsman driving mower with a 15.5 Briggs and Stratton engine.

For some reason it will not run properly. It worked fine a week ago, but while cutting grass, it suddenly shut off. When I turned it on again, it started running like this. In idle it almost run perfectly, but after a few seconds the RPM goes down, and the governor kicks in and manages to get RPMs back up. This continues again and again.

If I try to give it some grass, it starts hanging a limbo, just above the line from shutting off.

I have cleaned the carburator 3 times. The third time I bought a repair kit and changed most of the internals. I have also checked the ignition and changed the spark plug, along with fuel line, fuel filter and air filter. Lastly I have adjusted the valves.

I am out of ideas. Anyone knows what's going on with this engine?

 

[ILENGINE]

Sounds like two different issues with the carb. It is running lean at idle, but acts like the main jet has come out at full throttle causing a rich run issue.

I am wondering if you had one issue, but the carb cleaning lead to the loss of the main jet causing the other issue. Can't tell from the video but looks like the Nikki carb with the jet that falls out of the plastic emulsion tube. Also seems like one of the Walbro carbs used a rubber stopper in one of the holes in the bottom of the emulsion tube that would fall out and cause the same issues.

 

[dbergloev]

The Nikki carb has a rubber tip on the needle, don't think it should have a rubber stopper.

In general, this carb has very few parts relative to all the previous small engine carbs that I have seen.

 

[StarTech]

Also you should run a leak down test checking for bad head gasket since these engines are bad about blowing them. And as IL said the carburetor and if you didn't lose the main jet. The lost of the main jet would cause both carb problem but I also suspect a bad head gasket.

I repaired 310000 series where the head was badly blown it melted down the push rod plastic guides.

 

[dbergloev]

I did a pressure test on it, showing about 150 psi.

Also the main jet is not lost, it's still mounted in the carb

 

[StarTech]

Have you check the valve lift? A bad exhaust lobe is a possible. Just had one of the 793880 cams with a badly worn lobe. First one in ten years.

On the pressure test you are referring to a compression test? If so and was at cranking speed that is way too high as the ACR should have it around 70-90 PSI.

 

[dbergloev]

The valve lifts does not seams to have any problems. As for the compression, what causes that to be to high?

 

[dbergloev]

First assumption seams to be right. Went back in to the carburator, and the main jet is missing. Maybe it fell out during re-assembly, cause it was there after the cleaning. But then again, the way this carb is designed, is strange and seams cheap.

Also according to a few vids, the small whole in the big plastic thing, where the needle goes in, gets bigger over time, enough so that the needle does not properly block it. Thats the only thing I did not replace, so that may be problem 1, and then the missing jet problem 2.

 

[tom3]

Can you get a Chinese Ebay carb for this motor. Usually cheap if so. Might need to swap the main jet but could be the way to go here.

 

[dbergloev]

Okay, so I ordered a repair kit from one website, and a new carburator from eBay. It took some time to arrive, but I finally got both. And yes, the main jet had fallen out of the original, because I though those parts was screwed in, which they normally are.

But it did not fix all problems. I now have a brand new carb from eBay, and the original that has been fixed using the repair kit. It runs fine and smooth, but ONLY with full choke on. This is regardless of the carb I use.