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CH25 timimg

Xzander

LawnSite Member
Location
Murfreesboro TN.
Hello All,
In advance thanks to those who take the time to respond!

i just rebuilt a CH25 (kohler Spec #68562), nep pistons, valves, gaskets, seals, ignition modules, lifters. had the heads machined and valves done by a pro.
it will start and run but runs rough with random backfire. if i close the choke a little it runs better. if i reduce the gas it starts to run really bad and backfire a lot. I pressurized the cylinders at TDC and could hear the air leaking out the exhaust. There is no adjustment on the rockers, relies on the hydro lifters.

Could the cam gear be 360* out of sync? my understanding was just line up the marks on the crank and cam gears.

looking for any advice before i open it up again....
 

VegetiveSteam

LawnSite Senior Member
Location
St Louis, MO
Did this engine before you rebuilt it have a Spark Advance Ignition with a Spark Advance Module? If so what did you use for your new ignition modules?
 
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Xzander

LawnSite Member
Location
Murfreesboro TN.
it does have a Module Speed Advance. I bought the ignition modules on Amazon using the part number in the manual. 24 584 36-S
that part number replaces the original part which was either 24 584 03 or 24 584 11
i only replaced the IM after i ran it with them and had the same problem. checked all the wiring and made sure everything was connected well.

could the Module speed advance be bad? worked fine before the rebuild.
 

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VegetiveSteam

LawnSite Senior Member
Location
St Louis, MO
Okay good those are the correct modules so they aren't the problem. Yes the advance module could be bad. The fact that it would fail at the exact time you're rebuilding the engine is a little odd maybe. So probably shouldn't point a finger at it just yet but won't completely rule it out either. The speed advance module you would need is no longer available and has been replaced with a kit that includes two new MDI ignition modules and a new flyhweel. This is still a variable timed ignition but no longer needs the advance module. Kohler has that kit priced in such a way that it is only about 50.00 more than if you were to buy a new speed advance module alone. They flywheel by it self is over 200.00

Before you tear back into the engine check they flywheel key and make sure it isn't sheared. Contrary to popular belief you cannot just take the flywheel bolt out and look. You can see the top of the key and it may look fine but since it is a wooruff key the part you need to see is hidden from the top view. I always tell people to put the key in your hand. That is the only sure way to check it.

But if you think it could be out of valve time that may be the easiest thing to check and it only involves removing one of the rocker covers. I pick whichever one is easiest to get to and has room to see the rocker arms. What you are looking for is what is called valve overlap. Take the spark plugs out so you can turn the engine over easily. On the side you have the rocker cover off of you want to slowly turn the engine over by hand until you get that piston at top dead center on the exhaust stroke. Use a wooden dowel or something similar that can't mar the top of the piston and is long enough not to fall all the way into the cylinder so you can feel where the piston is. As the piston moves toward the head on the exhaust stroke the exhaust valve will begin to close slowly until the time the piston reaches top dead center. Right at that point the intake valve will open slightly. At that brief moment with the piston at TDC between the exhaust stroke and the intake stroke both the intake and exhaust valves are open just a tiny bit. If you roll the crankshaft about an inch or so back and forth the rocker arms should see-saw back and forth. Watch the exhaust valve closely when you bring the piston to TDC and just as you feel with your dowel the piston stop you should also see the intake rocker arm move. If you have valve overlap with the piston at top dead center your valve timing if perfectly fine. No need to tear back into the engine.

From there look at the flywheel key. If it is fine then I have to think the advance module is bad.

If definitely sounds like a timing issue and these three things are all there is to timing.
 
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Xzander

LawnSite Member
Location
Murfreesboro TN.
i have both covers off atm so i'll check that when i get home.
there is supposed to be .25mm minimum rocker gap. It doesn't appear to have any tho, that's why i replaced the lifters. I'm wondering if the valves are not closing all the way? but if they weren't there is no way to adjust them. will look closer at that too.

Thanks for the help, will reply when i check these things.
 

VegetiveSteam

LawnSite Senior Member
Location
St Louis, MO
The lifters are hydraulic and rocker arm gap should be Zero. The lifters need to be completely empty of oil when installed. A slight coating of oil on the outside as well as the Valspar grease on the cam side of the lifter but no oil inside. If there is oil in the lifters the valve very possibly will not close all the way. That could cause backfire too. And you are correct. There is no adjustment for the rocker arms. Again with the lifters empty of oil you want to torque the rocker arm screws to 160 in lbs. If you have an older manual it may tell you 100 in lbs but Kohler increased the torque value on the rocker arms to 160 in lbs about 10 years ago.
 
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Xzander

LawnSite Member
Location
Murfreesboro TN.
Just as you said at TDC the exhaust is closing as the intake is opening. So that rules out internal timing?
Key was there and looked good.
Any way to test the advance module? Or anything else I should check?

Do you have a link for the new advance module replacement kit?
 

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VegetiveSteam

LawnSite Senior Member
Location
St Louis, MO
There is no real good way to test the advance module without having a Spark Advance Module Tester. Kohler never had it as a required tool for their dealers to have so very few dealer had them. If you decide to go with the kit the part number you are needing is 24 755 308-S. That kit will come with 2 ignition modules, a flywheel, a wire harness and instructions.
 
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