Well I started pulling the engine apart but an emergency errand came up so I wasnt able to finish anything. I did learn that trash has been getting into stator area..

 

Stators that don't run in oil always seem to get dust and chaff in the cases, looks like you will have to add gaskets to the list of parts to get. I had completely forgotten, but that is a round door in the case behind the flywheel, you may only need to remove that to get the loose tensioner cap out.

 

Started pulling it apart this morning..


I have a puller for a GY6 but had to use the generic puller on this one. Of course it popped loose when I wasn't expecting and the starter clutch parts went everywhere.

That left this:



If anyone reading this doesn't have an impact screwdriver, they should order one now.

The plate had some sort of glue on it, I was worried I wouldn't be able to get it loose but the slide hammer got it off beautifully.

That was all the access I needed to get the cap out.

Reinstalled tensioner -

This is what it's supposed to sound like - video

 

I should note that I ran the engine without the rubber stop on the cam chain tensioner rod while the rod was installed upside down. The hydraulically locking ball check valve was not working at all and may have caused extreme forces on the cam and cam chain.

I took the video of the engine idling and then took it for a quick ride. The kart hesitated when I opened up the throttle. It was also popping on decel, something it has never done previously.

I'm pulling the buggy back into the garage and "KLUNK!" the engine stops instantly and the headlights go out. I figured it was electrical, I hadn't connected the yellow wire to the regulator, maybe it bounced around and shorted something? I'm still not sure what happened. I unhooked all the wires to the regulator to eliminate it altogether and let my son ride the kart. It started up fine but I could hear him open it up and the engine sounding terrible. I'm not sure if it was misfiring or what. When he got to end of the straight, the engine died and wouldn't restart.

The engine hasn't run right since. It will start up and idle perfectly but won't take throttle without an aggressive "KLUNK!" and shutting off immediately. I verified the timing - I aligned the key on the crankshaft to the cylinder and made sure the mark on the cam gear was at the top of the cylinder, along the cylinder centerline. I was thinking that maybe the cam chain had slipped a tooth but that doesn't appear to be the case.

I ohmed out the trigger and charge coil on the stator, both are good. I swapped to a different CDI, this did not help. I cleaned the carb, this did not help.

Finally I did a compression check just for kicks and I'm getting between 70 and 80 PSI. From what I'm reading about an ATC110 engine, this isn't even close, it should be closer to 100psi. This engine is basically brand new, has less than 10 hours on it. I checked the valve lash and both intake and exhaust are within spec.

I pulled the cylinder head just to see if maybe the valves were messed up but the valves appear to be good. I don't see any blow-by on the head gasket. I haven't manually opened the valves to check for damage but there is no indication that there has been contact between the piston and valves. The cylinder wall looks brand new. No scoring, you can see the cross hatch beautifully.

So if I'm not getting good compression that means it is a timing issue (I'm wondering if maybe the press-fit cam gear mount may have slipped?), a problem with a valve sealing (I plan to investigate the valves further), or a messed up ring? Any help would be appreciated. I'm really at a loss on what is going on and how it happened.

[edit] At lunch today I ran out and checked the valve seal. A leak-down test would probably be a lot more useful but I was curious if one of the valves was failing to close properly. I sprayed silicone in the intake and exhaust and watched for it to seep past the valve. Neither exhaust nor intake valve leaked.

 

The only thing I can think would produce the klunk is the flywheel come loose. Have you checked? If it is loose it will have damaged the taper. I remove the key and smear fine grinding paste on the taper surface, put flywheel back on and grind in, like a valve, until both surfaces have that dull grey finish when the paste is removed. Fit a new key and torque to the correct setting. That won't be the reason for low compression though. While you have the head off, take the cylinder off and check the bore for scores, rings etc.

 

Good idea about pulling the cylinder - There is no reason not to at this point.

The cylinder walls look great but I obviously couldn't check the rings without properly pulling the sleeve. I messed with the cylinder head a little earlier, a buddy told me to try using gasoline to see if it seeps past the valves. The exhaust valve held the gasoline back for over an hour without any sign of seepage. The intake valve wasn't pouring gasoline through but was definitely leaking. In 30 seconds you could see the area around the edge starting to darken, in a minute it was fairly saturated. The picture was maybe 1-2 minutes after I blew it dry.

I doubt this is why the compression is so low but I'd like to correct it while it is apart. I'm going to check the valve stems later on and attempt to lap the intake valve to see if I can get it to seal off better. May just be carbon buildup. I'm sure the cheap chinese springs aren't helping either.


Intake Valve Seep

 

I pulled the cylinder sleeve off, the cylinder looked fine. The rings were not clocked correctly at all. I corrected those and put it back together. I lapped the valves and did the gasoline leak test - Seems to be resolved. The engine is running insanely rich. I'm going to correct that and do actual plug chops once I get the thing running again.


Stock ring clocking

I did a bunch of research a while back on port matching and head porting. I made a flow bench and read through Vizard's book on head porting. That has been a couple years but I remember most of what I learned. These heads are so poorly manufactured.. They are apparently made to run but not to perform well. I did some work to the valve bosses just to smooth it out and cut down on their size. I spent 15 minutes max on each port but ended up removing a good bit of material. I could have spent 2 hours doing what I really wanted but I don't think this is the engine to go crazy with. A similar 125 with head work would make more sense based on my research, as the 110's were not designed to be high output.


Before rough port


After rough port

I did similar to the intake but it wasn't as pronounced as the exhaust. Cool thing is, if my porting is terrible, a new cylinder head is less than $50 shipped.

Anyway, I'm about to reassemble the engine. I'm going to verify that TDC on the cam is actually TDC (center of the lobes) to be sure it didn't get snatched out of center somehow. Also going to pull the flywheel and verify that the key is installed properly.

Very interested to see how it runs once I get it all back together. It made great power before but I suspect that after having corrected the leaking intake valve and piston ring clocking, performing the rough cylinder head porting, and properly jetting, I would think the increase would be noticeable. Maybe not. If it is noticeable I'm going to have to change the gearing because at WOT the RPM is almost too high in its original stock state.

 

I almost have the engine back together. I need to torque the cylinder head, reinstall cam sprocket, and reinstall the flywheel. The flywheel wasn't slipping on the crank but I'm not sure that it was seated fully.

Crank seat

does that look right?

 

1. Thought I was removing the oil drain... surprise!