New CV740 doesn't reach RPM at full throttle

c5vettster

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I am repowering Gravely Z 252 mower model 992071. It originally came with a 27hp CV740 and I found a NOS replacement that is exact with exception of newer ignition module. The new engine is CV740-0020. The older engine is CV740-011. I had to come up with a new wiring harness. The carburetor + ignition box are now powered from a tractor pin that was marked for carburetor but was previously unused. The other engine carb was wired to the regulator line + the starter wire. I think I wired everything correctly. The engine starts easily and shuts down with the key. However, it doesn't go up in RPM. I've check and both the throttle and choke are wide open. The limiter screw appears to have no effect so to speak. But the engine only goes up to ~2500 RMP at WOT. I can't figure out where my problems are. Do you think the voltage on the ignition box maybe unstable? I tried wiriting it directly to the positive terminal and it didn't make any different. Is there any setting on the governor or a different hole on the lever that I need to try? Really puzzled here.
 

c5vettster

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enter your serial number in order to locate service manual & wiring schematic for your mower.

The serial number for the engine is 3507009331. The tractor is 000612. Do you think that engine model needs more than just 12V on the carb and ignition box? Thanks
 

c5vettster

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enter your serial number in order to locate service manual & wiring schematic for your mower.

https://www.ariens.com/en-us/manuals/download

scroll down page for manuals, enter 992071 model number & 350700 serial number.

Thanks! It looks like the manual for the tractor.

I don't understand why the new engine only runs at half the RPM. Is there anything special about wiring the ignition on this kohler engine? It has 12V on the box. I left white and yellow wires hanging. It runs, but only at half rpm and appears to runs too rich. It is almost as thought it only fires the spark plugs every other cycle.
 

c5vettster

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A very odd thing. I took the muffler off to make sure I have no obstruction. The muffler was clear. However, when I run it w/o the muffler, only the left cylinder was firing - I could see the blue flame out of the exhaust port. The other cylinder was dispensing un-burned fuel fix. Naturally I took the spark plug out of that cylinder thinking that the engine would stop. However, after a moments hesitation, the other cylinder started to fire. I used an ignition tester and on one and then on the other cylinder and both were showing robust spark. I think I have an ignition problem whereby there is a lack of synchronized ignition between two cylinders.
 

PTmowerMech

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I am repowering Gravely Z 252 mower model 992071. It originally came with a 27hp CV740 and I found a NOS replacement that is exact with exception of newer ignition module. The new engine is CV740-0020. The older engine is CV740-011. I had to come up with a new wiring harness. The carburetor + ignition box are now powered from a tractor pin that was marked for carburetor but was previously unused. The other engine carb was wired to the regulator line + the starter wire. I think I wired everything correctly. The engine starts easily and shuts down with the key. However, it doesn't go up in RPM. I've check and both the throttle and choke are wide open. The limiter screw appears to have no effect so to speak. But the engine only goes up to ~2500 RMP at WOT. I can't figure out where my problems are. Do you think the voltage on the ignition box maybe unstable? I tried wiriting it directly to the positive terminal and it didn't make any different. Is there any setting on the governor or a different hole on the lever that I need to try? Really puzzled here.

Have you tried pulling the governor rod to see if it idles higher? if it idles up, then it could need adjusting.
 

tom3

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Swap spark plugs, see if the dead cylinder goes with the plug. Does the engine have two coils? If two coils maybe swap coils and see. Or one single coil with two wires to the plugs? If one single the timing would have to be ok if the other cylinder is running right.
 

bertsmobile1

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Pull off both kill wires and see if it suddenly works properly.
If this is the case the diode between the two coils to prevent them talking to each other has failed
 

c5vettster

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Pull off both kill wires and see if it suddenly works properly.
If this is the case the diode between the two coils to prevent them talking to each other has failed

I just took the mower to the kohler dealer for warranty repair. I think something is wrong with the ignition system. It has that digital ignition setup and there is a TSB on how to convert it to regular two-coil setup. They pickup the mower today. I will keep you posted on the progress.
 

c5vettster

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I just took the mower to the kohler dealer for warranty repair. I think something is wrong with the ignition system. It has that digital ignition setup and there is a TSB on how to convert it to regular two-coil setup. They pickup the mower today. I will keep you posted on the progress.



The shop has called. They are puzzled as well. Their idea is that the engine has a damaged crankshaft causing timing issues with the valve train. Kohler has authorized them to install a new crankshaft. This really sucks. It seems like I will be down for another 2-3 weeks. I wish kohler would authorize the engine replacement since it reflects poorly on their quality control - a brand new engine with a broken crankshaft.
 
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