MTD 20” PowerMore 139cc Chinese - Engine Missing

[evh]

I picked up this cheap used MTD mower. I show it has the Chinese made Powermore OHV engine. I have done a ton of work on this to get it to the state it is in. It is more of a learning experience and “hobby”. Anyhow, my only remaining issue is it seems to have a miss. Carburetor and fuel tank were both cleaned thoroughly. My best guess is I have an air leak behind the carb where it mounts to the engine. There is a big gasket/heat shield component between them. I plan to carefully put some black gasket maker caulk behind it to see if I can seal it up. Make sense? Any other ideas? Also, the plastic top housing is broken and I need a new one of those. Anyone have one laying around? I think it is part number 731-05692 / 931-05692. I don’t think running it without that would cause a miss.

 

[tom3]

Have you tried a new spark plug? That housing directs cooling air around the cylinder, might overheat badly the way it's broken. Might be able to repair that with epoxy and use as is.

 

[evh]

Thanks Tom3,

I did try a different spark plug, exact same results. I actually used some black gorilla tape and "reassembled" the housing and will use that until I get a replacement. I was worried about the overheating as well. My next step is to go through the carburetor again and see if I missed anything, and then maybe use the gasket sealer.

 

[bertsmobile1]

You check for an air leak by getting a spray bottle of some sort, filling it with WD 40 start the engine and then saturate the area between the carb & engine with WD 40 ( or similar ).
If there is a leak the goo will get sucked in, the engine will faulter & blow lots of white smoke.
if there is another pair of hands get them the throttle up & down as on a lowering throttle opening the manifold suction is higher.

The usual culprit is a short in that idiot steel plug cap.
I rip them off & fit a Honda plastic one.

 

[evh]

Bert, you mean a short in the spark plug cap?

 

[bertsmobile1]

Yes.
The plug wire shorts out to the cap then the cap will either arc to the head or allow the spark to leak back down the outside of the wire.
If you run the mower in a very dark place you can oft see the occasional blue flash on the outside of the wire / cap

 

[evh]

Well, I ran it in the dark and saw no sparks. While it was running I placed a screwdriver between the cap and valve cover. Nothing. No sparks, didn't change the issue. I also dosed the front and back side of the carburetor with WD-40. No extra smoke at all. My next step is going to be to replace the coil. As I mentioned before, this mower was in very rough shape when I got it. It looks like something heavy was dropped on it that cracked the plastic housing and also bent the recoil and coil. I have everything fixed and it almost looks new (sans the black duct tape on the plastic housing), so I have to get this running well! Again, a challenge and learning for me.

 

[bertsmobile1]

Before you go to that effort, remove the kill wire from the coil.
If the plug cap comes off easily, it will be fine but if the cap is difficult to pull off then you will need to make up a replacement kill wire so you can turn the mower off.
Run the mower and see if the problem goes away.
If so then the kill wire is shorting against the engine or at the switch .

 

[evh]

I just removed the coil and checked it out. It seemed too close to the flywheel. I put it back on with a business card between the coil and the flywheel (which was further than it was). No change. I will try pulling the kill wire, but I don't think that is it. But I will try anything at this point. I did not find a suitable replacement in my hoard of coils, but I have a few more intact mowers to check out. These are normal Briggs and Tecumseh mowers so I kinda doubt their coils will fit this Chinese mower. Thanks for all your suggestions, keep em coming and I will keep you posted. Note, the last time I had i running I moved the coil wire, boot and kill wire all over and it changed nothing on how it was running. I will try the kill wire and then may go through the carb one more time. Then ultimately order the replacement coil.

 

[bertsmobile1]

Worth remembering that the coils have a timing chip in them and that chip is polarity sensitive so if you put the coil on upside down it will either fire erratically or not throw a spark at all.
The actual switch is quite a crude affair & can come loose so it randomly shorts the coil.
And no , because of the afore mentioned timing chip very few coils are interchangable so they get to charge $ 50 for a $ 10 part because it is unique.