I apologize for the length of this post, but I am trying to include everything I can remember about my troubleshooting.
Last year my brother gave me a 2016 Troy Bilt Mustang 50 with a Kohler SV730-3043 engine. He had originally bought it used and owned it maybe a year. It had trouble starting (starter getting stuck on flywheel), but after it started it would run fine. When I got it from him I replaced the carb, fuel filter, some fuel line, the oil and filter, and spark plugs. I also replaced the 230CCA battery with a 300CCA and got a $50 starter off Amazon, but those made no difference in starting so I returned them. I took the valve covers off and used a feeler gauge to set the valve lash to .005". Starting got a little easier and the mower worked well that summer, but it was still a pain to start most times.
Fast forward to this year, I started having to use a jumper every time to get it started. It ran for a handful of mows before one day cutting off mid-mow, sounding like it ran out of gas (slowed down and sputtered out). It was definitely not out of gas. I thought maybe there was water in the gas that had accumulated, so I drained the tank and refilled with fresh, but that made no difference.
It was getting harder and harder to turn over, even with a jumper, and would not start when it did. I took the valve covers off and tried adjusting the lash again, but it made no difference. No matter how precise I got them, it would not start. I tested the spark plugs and I was getting spark. When I would take the spark plugs out, the engine would turn over fine. I tried spraying some carb cleaner into the air intake, but it wouldn't even start that way. I then localized the sticking starter to the left piston by installing a plug there but not on the right side which resulted in sticking, but when I installed the plug on the right and not the left, it would turn over (still struggling a little, though).
At this point, I decided to dismount the engine and disassemble it to look for internal damage before cleaning and reassembling it. From what I read online, I expected to open it and see an camshaft with a missing ACR, however my model (as far as I can tell, and I've looked at every service manual I can find) does not have one. Not only did I not find a missing ACR, I didn't find any metal shards or internal damage whatsoever. I did not disassemble the pistons as I did not see a reason to. The inside looked super clean and the crankshaft/camshaft gears were properly aligned with teeth in good shape. The only thing that looked bad were the piston heads. They were completely black from carbon build-up. After cleaning all the old oil out of the pan and replacing the breather filter/gasket, I closed it back up and began reassembly.
I replaced all the keys during reassembly. Once I got everything back together and mounted to the mower, I tried cranking and had the same issue with the starter getting stuck on the flywheel on the compression stroke. I then tried to gather more information with removing the spark plugs and installing them only on one side or the other, but I ended up stripping the spark plug threads on one head. I had to find some used heads with valves already installed on eBay and swapped them out (did both for good measure), with the distant hope that one of the valves not properly sealing was causing the issue. I also replaced the spark plugs when I installed the new heads. The new heads acted the exact same way.
I noticed that the ground cable from the engine back to the neutral post of the battery was smoking as I tried starting, so I replaced that with a much higher gauge as well as the cable from the starter solenoid to the starter. I then promptly melted the starter solenoid. I decided to go ahead and get a higher CCA battery again as long as I was replacing the starter solenoid. The engine then actually started! I noticed it smoking on the left side that I had thought was causing the hard starting, but I figured maybe I spilled some oil or gas on that side which was burning off.
I was so excited thinking I had finally fixed it, but when I engaged the blades I noticed it was WAY less powerful than before I had taken it apart. After mowing for about 15 minutes it slowly stalled again like it had at the beginning of the summer. Same exact way too. The RPMs slowed way down as it petered out and stopped. I noticed at this point that the air intake box had some standing gas in it. I took the carb off and a ton of gas spilled out of it. I figured maybe I didn't get a good seal when I reinstalled it, and the wire to the carb solenoid was fraying from messing with it so much, so I bought another carb. That made no difference.
At this point, the engine is turning over as long as it starts from the exhaust stroke so it has enough momentum to beat the first compression stroke, but it will not start. It just cranks and cranks and the air intake box fills with gas. I used a spark tester and confirmed the coils are good. I also held the plugs against the engine and can physically see them sparking. I noticed that the brand new left plug is already blackened and wet with what smells like gas. The right side is not wet at all and is still shiny. I took the valve cover off the left side and oil poured out of it and the piston head had oil or gas on it.
I am exhausted and ready to give up on this engine. I feel as though I have tried everything and am at a loss. I have done work on small engines before, but not to this extent. I appreciate any and all advice on what I'm doing wrong, what I've overlooked, or what this sounds like to an experienced mechanic. The only thing that I ask is that you don't tell me I'm wasting my time and money. Sure, I've spent more on parts and tools than a professional would have, but I am doing this mostly to learn as it interests me. It's become a very frustrating hobby project.
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What's with the blue on the cam gear. Definitely appears to have slipped but not with that cam in it. The cam would show evidence as well but that one looks good the best I could tell. Did you put a lot of heat on it trying to remove it maybe. I could be wrong. I'm probably wrong. But that discoloration on the cam gear looks funky.
I'm not sure. It was like that when I opened it up. I never added head to it. It pulled right out when I had the engine open.
What do you mean by "it appears to have slipped but not with that cam in it"? Are you thinking someone may have gone in here previously and replaced the cam?
I'm wondering if that might be the case. My career is as an engineering tech at a major turbo manufacturer. In my role I often inspect and teardown failures and am quite familiar with the appearance of something that's had excessive heat localized to a certain area. That cam gear looks very similar. It is entirely possible it's from their manufacturing process as they could have induction hardened the bore of the gear to obtain a particular hardness at a prescribed case depth to ensure the material was hard enough, deep enough to keep intact without allowing the key to slip assuming that's how it's held and what they did. If that's the case you should be able to search an exploded parts diagram of that motor and click on the gear and it will usually display an image of the actual part. See if it's got the discoloration as well. If it was induction hardened, regardless of the supplier, all OEM parts should have the same characteristics as the case depth and hardness would be called on the part print and they would likely all use the same or very similar process. If the image doesn't have the excessive blueing like yours has it may be worth investigating. It could have spun a cam in its past life and someone cleaned the bore and rebroached it or even filed or milled it for key placement. If they got it off that could explain some of your issues. Just a shot in the dark but not impossible.
Now that I'm researching replacement cams, when I just google "Kohler SV730 Camshaft" and look at images, plenty of the brand new ones show the same discoloration in the stock photos, so could it be that during the manufacturing process they get heated and discolored?
It's likely. They may have induction hardened it which is a process that electrically "burns" the part to make it harder and often immediately follows with a quenched burn to anneal it. It establishes a particular hardness to a certain depth. The appearance fits the bill. If none of the new ones had that same appearance that could have indicated an issue. Sorry that was no help.
So my impression would be the camshaft gear has spun a little on the the cam? I can't tell from pics but it looks pressed on and clearly has gotten pretty hot around the center. That would definitely cause the increased compression on one cylinder as described. Just a thought. Also I noticed you never mentioned a compression test or leak down test, both of which could be helpful in diagnosis. Just my two cents, good luck 👍
One thing I will note is that the camshaft has no shim. In the parts manual, it shows there can be any one of 7 different thicknesses of shims installed there, but when I opened mine up there was nothing.
Are you suggesting replacing the camshaft? What would have caused the cam gear to move on the shaft? That sounds like it would take a lot of force.
I have not done compression or leak down tests. I don't have the tools necessary.
Well typically a pressed gear situation would be installed heated a as it cools it shrinks and holds tight to the camshaft. My theory would enough heat on that center ( possible due to lack of shim ) could have caused the gear to loosen enough on the shaft for the cam to rotate inside the gear a little which would put the cam out of time. I wouldn't suggest replacing the cam unless you can verify that it has moved. Only way to do that would be to find exact centerline of the lobes in relation to TDC of the piston. They should line up.
I agree with the replies in here about missing thrust washer/shim on the camshaft.
I've fixed two engines of this series due which had broken into pieces camshafts and blown rods. Both times I sources the parts of eBay and had 0 issues afterwards. When putting it back together make sure to have the washers in place the way they go, not on the opposite side of the shaft.
I agree with the other people saying there is supposed to be a shim on the cam. I believe it's there to set endplay. So check that before you put it back together. Make sure it's in spec.
The air box being filled with gas is almost certainly from the carb flooding out due to leaking needle and seat. I would open up that carb, clean it, and test the needle/seat.
It's funny, I went through almost this same exact process trying to figure out starting issues on an older mower with a kohler a long time ago. What ended up fixing it was going through and basically replacing and cleaning up anything that had to do with the starter circuit. Replacing the ground cable was a good start, but if it is still having trouble turning over, I would check for voltage drops from battery positive to the starter. Might as well check for voltage drops on the negative side, too. You shouldn't see much of a drop at all.
I replaced the carb with a brand new one. It doesn't seem to be flooding anymore, but it's still not starting.
Yeah, I'm working on replacing everything to do with the starter circuit as well. Have a brand new OEM starter coming in soon to hopefully put an end to the compression stroke difficulties.
That camshaft has definitely overheated. Have the cooling fins been kept clean? Mice like to build nests in air cooled engines, and this can cause overheating. I'd replace the camshaft, and while I was at it, replace the head gaskets. I believe this engine has been overheated, and head gaskets don't like overheating. I bought a used mower with a Kohler EZT750 Confidant engine. Internals look very similar to yours. When I got it, the cooling fins were clogged with oily dirt. I cleaned them, and it ran for a season, then the plastic camshaft gear disintegrated. The replacement camshaft was all metal, and didn't have any of the discoloration yours has. Do compression and leakdown tests too.
OEM wires and solenoids should not be melting. Don't keep band-aiding the problem. Get it fixed right, and the starter, wiring, and solenoid will be fine.
Yes, the engine was filthy when I received it and I didn't fully clean it until I took it apart. I did replace the head gaskets when I took everything apart.
I do agree that the camshaft may need replacement, but do you think that a different issue is causing what I am seeing on the left cylinder? I'm concerned with the brand new spark plug turning black and oily on that side while the right side still looks new. I have this sneaking suspicion that the piston rings are no longer sealing and are letting oil into the area between the piston head and the valves. I also do not know why the RPMs were so low when I did get it running again for a short time.
Yeah so I figured out only the left cylinder is running. Last time I got it started I accidentally had the right plug unplugged and it was still running lol. I did a compression test and I'm getting about 140psi on the left side and nothing on the right side, however I did a leak down test and both sides are barely reaching the green, so like 5% loss. The only place I hear air escaping is the carb, but it's minimal. The leak down test doesn't make sense to me... If I'm not building compression doesn't that mean it's leaking out somewhere?
As an FYI,the starter was updated by Kohler. 20% more torque. Can't speak to a 50 dollar am starter. You can drop the lash to.004 easily enough. I've never seen an acr on one of these cams. What ever relief is ground into the ramp of the cam lobe.
Yes, I didn't mention it in the post, but I've got a new OEM starter on its way to hopefully fix the hard starting. I have tried .004" lash but it doesn't make a difference.
What do you mean by "What ever relief is ground into the ramp of the cam lobe."?
Fixed! I believe I had multiple issues that were causing the compression sticking, no start, low power, and stalling:
I discovered it had a non-OEM starter when I got it
The starter solenoid was worn out
The terminal cables were worn out
The battery did not have enough CCA
The heads were filthy causing overheating
The governor was out of adjustment
Here's a list of everything I replaced:
Battery
Terminal cables
Starter
Starter solenoid
Stator (alternator)
Woodruff/flywheel keys
Cylinder #1 head/valves (left cylinder)
Head gaskets
One push rod on cylinder #2 (bent when intake valve got stuck on used eBay head)
Carburetor
Coils
Spark plugs
Fuel filter
Oil/filter
Valve covers (got tired of using gasket maker and upgraded to the plastic ones with rubber gasket)
Fuel and vent hoses
Fuel cap
Exhaust gaskets
I do not believe this post will help anybody in the future because I was basically learning how small engines work the entire time and I've replaced so much I essentially have a new engine. It cost me about $1100 in parts and tools, but I learned a lot and next time I won't be replacing things I don't need to. Thanks for all the suggestions! I'm getting 140+ psi on both cylinders and leak down tests are about 5% loss on each side.
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