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2 stroke engine trouble shooting

auntiebling said:
will do
what benefit is that tho? thought 2 strokes kinda really need some backpressure
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that's the same type muffler used on power equipment/chainsaws. It could be clogged up inside from oil deposits. If it runs, there's the problem.
 
kuksul08 said:
I'm not sure we covered the obvious. Is the spark plug new?
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It is
So I went back to the old plug in the spares box and it's a little easier to get it to cough with starter fluid

The new plug is a multi electrode deal, probably needs a little fine tuning of the gap I am not I terested in messing with right now.

Old/existing plug is a single electrode
 
Such a simple engine, crazy when they create problems like this. You mentioned the carb is new, do you have the old one too? Are you sure the new one is jetted correctly? I used to race a KT100, different carb, but if the needle jets were off a bit it wouldn't start no mater what we tried.

I've never played with a Comer, but it looks like a pull start, so I'm guessing there is no where to plug an electric starter and have someone mess with opening and closing throttle while also partially closing off the incoming airflow to try and get it to fire up with drastically different air/fuel flows coming into the engine?

Have you popped off the head to make sure the barrel isn't scared or the piston partially seized?
 
Go back to basics. Find out what plug is correct and use that. NGK B8ES is a good starting point if you can't find what is original. Maybe post a picture of the 2 plugs that you have. Also, rob's suggestion about the carb is good too. Someone may have THROWN a bunch of replacement parts at this to try and get it running, but they may be all wrong too.
 
I don't see how the plug could make a difference in starting the engine when it's all cold. The gap makes a little difference. Is the ignition coil good and the spark strong enough? If it ignites with starter fluid then the question would be if the carburetor is able to get the fuel dissolved in the air stream, and if it is roughly the right amount.
 
I bet the timing is off. Get a dial gauge and verify the timing.
 
i cobbled the leakdown tester together today and it won't hold any pressure, just makes a kindof quiet darth vader breathing sound

wondering if i didn't have the piston in the right spot i rotated it down a little and the air started coming out the spark plug hole.

so.

crank case at a minimum doesn't seem to be sealing.

timing is static on these, and it does fire on starter fluid so timing might not be perfect but it's close enough for now. if i tear this thing down to do the crank case seals i'll be able to see the timing parts.

so at minimum the crank or case seal seems to be dead and if i'm reading this correctly they are on the inside of the case so have to split things open. this rabbit hole gets spendy in a hurry, it's probably due for a piston and rings, maybe a cylinder, according to one maintenance schedule i found. some teardown is in order in any case.
c51_schem.jpg
 
Put the spark plug back in, intake and exhaust plugged tight.
Intake or exhaust plug will have the valve to pressurize it.

Feel for where the air is leaking out or use soap/bubbles.
 
That's a cross flow scavenged 2-stroke with a magneto armature ignition. The crank is the pump, right? So you actually want the piston at BDC to test because at that point the transfer ports will be open and you'll test the whole assembly. I used to build test tools out of blood pressure cuff pumps and gauges. Blank offs for the ports you'll have to get creative. might have to make a couple plates. I suggest a blood pressure cuff pump, a flat plate on one side, flat plate on the other threaded for 1/4" NPT with seal, and a gas pressure test gauge like the ones at home cheapo. Get the parts you'll see what I'm talking about. Anyways leave the plug in and get the piston at BDC with the setup on there then put maybe 5psi in and check the leak down rate. If it'll hold 5 psi for 5 minutes your GTG. Don't use an air compressor! You'll blow the damn crank seals and then really have to split the cases. It was never made for that kind of pressure. Anyways just like augustiron said use soap bubbles to look. I actually use Windex a lot I always have some in the shop for my helmet/windscreen and it works great. If your leaking at the case then consider a new crank. Otherwise it could be the base gasket, head gasket, reed gasket... Speaking of reeds I don't see any there. What does that thing have for reeds? Don't see a disc valve setup either. It's gotta have some there someplace. Figure that out because a totally shot set of reeds will make it do what your talking about.
 
parts diagram doesn't show a separate key for the flywheel, it's probably cast in the flywheel just like a chainsaw. Pull the flywheel and inspect if its broken off. that will screw up your timing
 
I think JimE is correct about using bottom dead center, but based off my old man memory, if you have the intake and exhaust ports sealed it should not matter where the pistons is, since everything is sealed.

I agree with what someone else posted you only need about 6lbs for 5-10 minutes. It doesn't need to be tight as a drum and this motor isn't a reed valve or rotary valve, just a basic piston port.

So I will ask again, have you popped off the head, it is only 4 bolts.
 
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no, i'd tell yall if i'd taken the head off. need to try all the variations of the pressure check first.

i don't have a lot of time to work on this thing, so going in Least Teardown order
 
I put the piston at BDC, and got the same Darth Vader sound.

Next I think is trying to get to see what the crank seals.look like which involves removing the clutch and there is a special tool.for that, hopefully I can come up with so.ething close enough
 
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You do have spark plug in when testing, right? The motor must be completely sealed, intake, exhaust, plug. There is no way to pressurize the crank without a plug in the head.
 
bobl why you ask good question? :laughing

it was out for the TDC test and i din't bother repeating that one. it wouldn't have mattered as both ports are sealed by the piston right?

anyway, i put the plug back in for the BDC version and it held ~1psi for about 0.5 seconds. really any air that managed to get pressurized leaked immediately out/darth vader sound

still haven't made room to tear it down, it's still mounted to the kart in fact.
 
the piston doesn't seal anything except combustion gas. The pressure test can be done with piston in any position. Must have spark plug installed and tight. Exhaust port sealed perfectly. Intake sealed perfectly.
 
Just looked through a parts page for the C50 and the flywheel does have a keyway so the physical location of the timing trigger is correct.

The leak down results clearly show the motor has a leak. Crankcase seals would be one possibility but a cylinder base gasket or whatever goop the PO used to seal the crankcase halves could also be to blame.
The cylinder and the head are all one piece on that motor. Torquing down the 4 bolts that secure the cylinder to the cases could help close up a base gasket leak. More than 6 ft/lbs could easily strip the crankcase threads or snap the cylinder bolts.

Motor disassembly calls for a few special tools, can totally be done without them.

Piston stop
Flywheel puller
Clutch puller

Parts you would want to have are the crank seals, bearings, cylinder base gasket and some Hondabond or other crankcase sealant.
 
A couple more things. If you use a 2 foot long piece of heater hose (or similar) for listening, and pressurize the cylinder again, you might be able to pinpoint the exact location of the leak. Hold one end of the hose to your ear, and move it around until you isolate where the leak noise is coming from.
I see your engine uses a head gasket, so my second point is moot.
 
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