JS46 Bent Crankshaft

[mgilbert]

Well, it turns out that the JS46 that I've had hard starting problems with, has a bent crankshaft. It tries to start, and seems to be hitting, but just never catches hold and starts running, and it often backfires when I'm trying to start it. If I can ever get it started, it runs fine, but is a lot louder than I remember.

How can a bent crankshaft keep an engine from starting, assuming it isn't bent so badly that it locks up the engine???

 

[mechanic mark]

Balance, true, uniform, & timing have any & everything to do with a crankshafts manufacturing.

https://www.youtube.com/watch?v=ln4lY3Fk5e8

 

[redmondjp]

Well, it turns out that the JS46 that I've had hard starting problems with, has a bent crankshaft. It tries to start, and seems to be hitting, but just never catches hold and starts running, and it often backfires when I'm trying to start it. If I can ever get it started, it runs fine, but is a lot louder than I remember.

How can a bent crankshaft keep an engine from starting, assuming it isn't bent so badly that it locks up the engine???

It has nothing to do with the bent crankshaft. It has everything to do with the flywheel key which sheared at the same time that you bent the crankshaft. The key is made of aluminum and shears when the crankshaft stops suddenly and the mass of the flywheel keeps it turning, causing the key to shear.

This in turn causes the ignition timing to be off far enough that the engine often won't even start. So since yours starts, the key is probably only partially sheared.

In order to avoid eventual complete crankshaft failure, the crankshaft needs to be replaced, as well as the flywheel key. If you run it with the bent crankshaft, the additional vibration will literally shake the rest of the mower apart over time.

 

[mgilbert]

It has nothing to do with the bent crankshaft. It has everything to do with the flywheel key which sheared at the same time that you bent the crankshaft. The key is made of aluminum and shears when the crankshaft stops suddenly and the mass of the flywheel keeps it turning, causing the key to shear.

This in turn causes the ignition timing to be off far enough that the engine often won't even start. So since yours starts, the key is probably only partially sheared.

In order to avoid eventual complete crankshaft failure, the crankshaft needs to be replaced, as well as the flywheel key. If you run it with the bent crankshaft, the additional vibration will literally shake the rest of the mower apart over time.

I understand that. The crankshaft got bent three or four years ago. I didn't realize the key was sheared until the flywheel finally worked its way out of place far enough to affect the timing. I replaced the key over a year ago, and it has run fine ever since. It must not be bent badly, because the mower does not vibrate excessively. What I don't understand is why, after three or four years with a slightly bent crankshaft, it chose now to quit running. It just doesn't make sense to me that it would go from running perfectly with a bent shaft for four years, to not starting at all within a matter of a few days. I have a feeling something else is amiss, but I can't figure out what.

 

[DaveTN]

How did you determine a bent crankshaft? Sometimes the blade will bend
causing vibration and imbalance. Sometimes a bent blade will give
the impression of a bent crank. I've seen my share of bent cranks and I
never try to straighten one as it will probably break later, not to mention
being dangerous! Might want to check the valve and rocker arm clearances
to make sure the valve lash is wthin specs. OHV valves get out of spec
from time to time. Good compression? Hope this helps. Good Luck on it.
Then again I've seen multiple failures occur at the same time.

 

[redmondjp]

I understand that. The crankshaft got bent three or four years ago. I didn't realize the key was sheared until the flywheel finally worked its way out of place far enough to affect the timing. I replaced the key over a year ago, and it has run fine ever since. It must not be bent badly, because the mower does not vibrate excessively. What I don't understand is why, after three or four years with a slightly bent crankshaft, it chose now to quit running. It just doesn't make sense to me that it would go from running perfectly with a bent shaft for four years, to not starting at all within a matter of a few days. I have a feeling something else is amiss, but I can't figure out what.

Ahhh, so more of the story leaks out! You should have mentioned that the bent crank was years ago . . .

So what makes you think that they have anything to do with each other then?

You could have a failing coil/ignition module, or a gunked-up carburetor, or bad gasoline, or a fouled spark plug, or a clogged air filter . . . you'll have to start the troubleshooting process to rule each one out.

 

[mgilbert]

How did you determine a bent crankshaft? Sometimes the blade will bend
causing vibration and imbalance. Sometimes a bent blade will give
the impression of a bent crank. I've seen my share of bent cranks and I
never try to straighten one as it will probably break later, not to mention
being dangerous! MIght want to check the valve and rocker arm clearances
to make sure the valve lash is wthin specs. OHV valves get out of spec
from time to time. Good compression? Hope this helps. Good Luck on it.

When I rotate the blade by hand, the center bolt "wobbles" about 1/8" from side to side. I agree - I wouldn't try to straighten the crank. It would have to be replaced, and the mower isn't worth it. I did check and adjust the OHV clearance. It wasn't off by much, and adjusting it made no difference. I just don't think the bent crank is keeping it from running. That doesn't make sense to me. I could see it vibrating the engine apart internally, but that doesn't seem to be the case. I think something else is wrong. I hope the technician working on it can find the problem.

 

[mgilbert]

Ahhh, so more of the story leaks out! You should have mentioned that the bent crank was years ago . . .

So what makes you think that they have anything to do with each other then?

You could have a failing coil/ignition module, or a gunked-up carburetor, or bad gasoline, or a fouled spark plug, or a clogged air filter . . . you'll have to start the troubleshooting process to rule each one out.

I didn't go into all the detail in the original post, for fear that the post would get long, convoluted, and confusing. I don't think the problem has anything to do with the crank. That's my point. I've got someone with experience, parts, and tools looking at it. I'm just not sure I trust him. I have a feeling he is going to assume the problem is the crank, and just write the mower off.

 

[redmondjp]

I didn't go into all the detail in the original post, for fear that the post would get long, convoluted, and confusing. I don't think the problem has anything to do with the crank. That's my point. I've got someone with experience, parts, and tools looking at it. I'm just not sure I trust him. I have a feeling he is going to assume the problem is the crank, and just write the mower off.

I don't know how to tell you this nicely, but why in the world are you spending a dime on that engine if it has a bent crank? It is totalled, using car-lingo.

Throw a Harbor Freight Predator engine on that mower deck and get back to mowing! You're going to end up spending enough to buy one of those HF engines just to tell you why your bent-crank motor won't run.

 

[DaveTN]

That "backfiring" sounds like it's slightly off timing, which would also
make it hard to start. Even a slightly crimped key would throw off
the timing making it backfire. Backfiring through the intake valve on
trying to start it would be either an intake valve sticking or binding, or
timing off. Since you checked the valve lash, I would check for fuel
flow and clean the carb again and adjust it. Danged problem child! :thumbdown: