Do You Agree With This Diagnosis

[mgilbert]

I posted about this a couple of times, and finally took the mower to someone with experience. I'd like to know if you agree with their diagnosis.

JS46 walk behind mower with 190cc OHV Briggs engine... Crankshaft bent about four years ago, but been running fine until this season. Started fine at start of season, but became harder and harder to start the last two uses. Hits, but won't catch and start - pfffft pfffft pfffft pfffft pfffft is all I get now. Continuing to try to start it results in backfiring. When it did start, it ran as good as new. Valve backlash OK. Flywheel key OK. Spark plug has only about ten uses on it. Fuel is fresh.

Here the diagnosis. Since it ran fine once it started, the problem is not with the fuel. Since it backfires, spark is fine, but the backfiring indicates that the timing is off, even though the flywheel key is fine. The mechanic believes there is an internal problem with the cam, or the gears that drive the cam, and that this is causing the valve timing to be off. Cost to prove this would be considerable. Cost to fix it would be prohibitive.

Guys, what do you think? Is there any chance this is something simple that he and I are both missing?

 

[ILENGINE]

I doubt it is a cam gear problem. I suspect there is a valve sealing problem, like loose valve seat. I have also seen where the valve seat wasn't square to the valve and wouldn't seal. If there is a camshaft problem it would be related to the lobe wearing down to a more round shape. That can be confirmed by removing the head, or the muffler and breather cover, and turning over the engine while watching the valves open and close. they may not open the same distance which will effect timing by how much fuel or exhaust it allowed to enter or exit the engine.

Now if we want to consider the most rare, that would be the camshaft is bent. Which will look like it is holding both valves open at what would be an odd time. Now take into account there is some valve overlap between the top of the exhaust stroke, and the start of the intake stroke.

 

[mgilbert]

I doubt it is a cam gear problem. I suspect there is a valve sealing problem, like loose valve seat. I have also seen where the valve seat wasn't square to the valve and wouldn't seal. If there is a camshaft problem it would be related to the lobe wearing down to a more round shape. That can be confirmed by removing the head, or the muffler and breather cover, and turning over the engine while watching the valves open and close. they may not open the same distance which will effect timing by how much fuel or exhaust it allowed to enter or exit the engine.

Now if we want to consider the most rare, that would be the camshaft is bent. Which will look like it is holding both valves open at what would be an odd time. Now take into account there is some valve overlap between the top of the exhaust stroke, and the start of the intake stroke.

A valve issue would make sense, as the problem got worse over time. The cam gear is plastic, so I was hoping to find it stripped, or that it has jumped a tooth. That, I can fix. I've never dealt with a bad valve/valve seat. How is that repaired?

 

[mr.farmall]

I posted about this a couple of times, and finally took the mower to someone with experience. I'd like to know if you agree with their diagnosis.

JS46 walk behind mower with 190cc OHV Briggs engine... Crankshaft bent about four years ago, but been running fine until this season. Started fine at start of season, but became harder and harder to start the last two uses. Hits, but won't catch and start - pfffft pfffft pfffft pfffft pfffft is all I get now. Continuing to try to start it results in backfiring. When it did start, it ran as good as new. Valve backlash OK. Flywheel key OK. Spark plug has only about ten uses on it. Fuel is fresh.

Here the diagnosis. Since it ran fine once it started, the problem is not with the fuel. Since it backfires, spark is fine, but the backfiring indicates that the timing is off, even though the flywheel key is fine. The mechanic believes there is an internal problem with the cam, or the gears that drive the cam, and that this is causing the valve timing to be off. Cost to prove this would be considerable. Cost to fix it would be prohibitive.

Guys, what do you think? Is there any chance this is something simple that he and I are both missing?

I once worked on a craftsman with a B&S rapid start. It would run only after it was up to speed by starting with a drill, then it seemed to run good. But after going through lots and spending a lot of time, a leak down proved the intake valve slightly bent and not sealing, even though the lashing was good. replaced the valve and it worked great.