Gasoline Motor Trouble - need advise

[jteneyck]

The motor on our chipper/shredder (B&S 8 HP) started running poorly and eventually it got so bad that I had to look at it.  I took off the carb, took it apart and cleaned it (didn't really find anything), put it back together, reinstalled - and it ran great.  For about 30 minutes, then started doing the same thing.  Run OK, then struggle and revs. drop, recover, repeat.  I changed the spark plug, air filter, and fuel line.  No difference. I drained the gas tank and put in brand new gas.  No difference. 

I looked at it more closely and thought for sure the carb. had an air leak through the throttle shaft side seal.  I bought a cheap replacement carb., installed it, and it ran great - for about 3 minutes and then the same symptoms started again.   So it seems like the problem is somehow related to the engine warming up because it starts and runs great when cold. 

What could it be?  Head gasket, valves?  Any advise is appreciated. 

John

[fishhh4]

I bet ya it is still starving for fuel  .Fuel ,spark ,air .. in that order .

[Snipe Hunter]

I bet ya it is still starving for fuel  .Fuel ,spark ,air  .. in that order .

To add to this.

It does sound like it's starving for fuel. Loosen the gas cap a little when it starts running rough. The vent in the cap may be clogged and the carb cannot pull fuel from the tank because is in a vacuum condition. If it runs better with the loose cap, clean out the vent in the cap.

[jteneyck]

Thanks.  That's what my wife thought, too, so I replaced the old fuel line with a new 5/16" one from the tank to the carb.  Fuel at the carb inlet flows out fast enough to run a car. I loosened the tank cap; no change in flow.  The carb inlet is part of the new carb so there can't be any restrictions there.  The symptoms are consistent with variable fuel flow but I don't think that's the problem.

New air filter and nothing in the inlet duct to the carb., so air can't be the problem.  New spark plug but who knows about the actual spark.  Doesn't sound like a spark problem though, it's not cutting in and out.   

John

I tried loosening the tank cap when the problem occurred while the engine was running. No joy.

[AnHonestOpinion]

Get a spray can of carb cleaner.  When it starts running rough spray the carb and mating surfaces and mounting flange.  If it is a vacuum leak this should help identify it and the location. hopefully it is not a crack.

Good luck.

[jteneyck]

You know, that's how I found what I thought was an air leak on the throttle shaft of the old carb.  I'll be out there with my can of carb. cleaner later today to look further.  Thanks very much. 

John

[Roly]

If it has a electronic magneto system, that is suspect as they normally fail when the engine is hot.  Roly

[jteneyck]

If it has a electronic magneto system, that is suspect as they normally fail when the engine is hot.  Roly

I couldn't find any air leaks at the carb. or manifold connections, so this looks like the next most likely root cause as I think it does have an electronic ignition.  I'll have to check the manual to verify and see how difficult that is to replace, or if I should pull the engine off and take it to a small engine repair shop. 

Thanks very much.

John

[MstrCarpenter]

If it has a electronic magneto system, that is suspect as they normally fail when the engine is hot.  Roly

That was my first thought too. A coil can show the same symptoms but it usually just misfires a few times and then quits 'till it cools down. Tough to troubleshoot sometimes because it starts to work again just about the same time you try to check for spark. I believe your ignition module is external, or at least not behind the flywheel. If you do have to remove the flywheel, and you have a charging system, don't use the big screwdriver and hammer trick to pop the flywheel. You'll knock the magnets off the inside (they're just glued in/on the flywheel).

[Cecil]

Adjust the valves.  https://www.youtube.com/watch?v=1-XlET33VHM.  Not hard to do and takes less than 1/2 hour.

That is what I did to my chipper/shredder/vac with a Briggs and Stratton engine.  It ran poorly and started hard and sometimes would not start hot.  Starts first pull now and runs great.

Like you, I started with the Air Filter, cleaned the carb, replaced fuel lines, etc.  I always run ethanol free fuel, 91 octane in my small motors too.