The compression vent hole is a tiny hole located above the exhaust port. Less than 1mm in diameter. It just bleeds off some compression at kick starting speeds.

210 psi compression is fairly high for a 300. It would certainly provide lots of resistance for the kicker and estart. Many GG engines come with too wide a squish measurement. Maybe yours is a rare one with tight squish and high compression.

My 250 tests at 200 psi. It kicks over fine (and cranks OK on the estart - engine warm). The 250 piston has less surface area than a 300 piston.

Maybe some 300 owners can chime in on compression readings. And yes there is variation between models of testers. Mine has a fitting that screws into the plug hole with a 6" hose to the gauge. Compression readings are only valid for a specific engine as a way of monitoring the health over a period of time.

If most 300 owners said their machines all came in at 180 psi, then maybe high compression is your problem. If fits the scenario.

Google the method for taking a squish measurement with solder. Try it and let us know what you measurement you get.

If the ignition timing is too far advanced you could get a "kick back" happening. This could happen by kick or estart. There is an oval washer that locates the stator plate in it's mounting slots. If this washer is missing, the timing could be way off.

The only way to find out is to remove the cover and pull the flywheel (a special puller is required).

I don't think that your problem is compression.A little higher comp. would be a problem while trying to start the engine cold with the starter and would cause engine problems when using bad fuel and for longer periods of use from what you are saying, not keeping you from putting the kicker down and brake them like that.
I would suggest to try some things before you end up dismantling pieces.
1st open the the radiator cap and see the level of your coolant because there is a chance ( seen it with 2018 250 model recently) coolant gets to your combustion chamber and causing hydraulic pressure (effect).
Solution open your cylinder head and inspect the o-rings and while you are there inspect the cylinder for cracks( rare occasion but could have happened for many reasons)
2nd take the spark plug off (observe it) and try turning the engine with your hand over the plug hole to see if anything liquid comes of other than gas (not much) Maybe for some reason your engine overflows with too much fuel.(not so likely)
3rd something is binding. Take the front pipe of and watch your piston for anything unusual. (scorches, broken piston rings or out of their place).

Report what you find and hope you find it soon.

p.s. In case you need cylinder head o-rings try from KTM same bore.
Gas Gas o-ring weren't the best at least the older ones but KTM's are thicker and work great with less problems.

So if it the engine turns fine without the spark plug in, what happens when you put the spark plug in? Is it immediately difficult to turn the engine?

Did you check the coolant level? Are you losing any coolant?

Have you started the engine? If so how does it run?

The more I think ahout this problem, the more I think it will be something very simple to fix, the difficult bit is finding the problem... :confused:

Good luck, Dave.

I was just thinking...

Are you sure it's not something crazy like your carb overflowing into the crankcases, flooding them with fuel, then causing a hydraulic lock in the combustion chamber?

If this was happening then I'm sure you'd smell petrol strongly and when you removed the spark plug it would be spraying fuel out of the spark plug hole when you turned the engine.

Really starting to grasp at straws now...:confused:

You said you've broken 3 kick starters...WOW that must ge some big force you're kicking against... :eek:

If it's not a mechanical restriction and something is really causing a very high compression, then I wouldn't run the engine... you could easily damage the crank.

Good luck, Dave.

How did the engine sound when it was running? Free reving or struggling?

I'd expect to break a kickstart lever you'd need a broken piece of clutch plate floating around in the right side case. It would have to jam the primary gears.

Using the wrong transmission oil can break kick levers.
The kick start sprag gear can bind from not sliding together.

Some answers for very good suggestions:

- The coolant level is ok. Hydro lock would have been a good answer, but not this time.

- After putting the plug back the engine gets hard to turn right away.

- No extra fluid (water / petrol) coming out of the plug hole when turning the engine.

- No extra noises (as said before) when running, but really crappy running at lower rpm's and strong kick later, like my TE125 (original needle, and through this forum I know what to do, thnx).

- Have to look for the vent hole. A new one for me too, but if there is one it could be the reason (the starting got worse and worse until stopped totally).

- If the oil is to blame then the original oil is wrong one. The first kicker broke around 2 hours of riding.

- The design of the kicker is bad. The cam on the kicker puts forces in place that can't hold it well. All three have failed the same way as the one in the picture (see attached pictures). Just today got a new one with different design, this looks like it will hold better (the third picture).

- Maybe I'll install the kicker, get a new battery and try once more.

It's normal for the heel of the aluminum lever to deform a bit. Your pictures show a poor match on the parts, I can see how those would break. The new lever and mount look much better.

I'd like to know what squish measurement you get. The motor should not be that difficult to kick over.

The compression vent hole is in the cylinder bore near the top. It's just a tiny little hole that allows some gasses to pass out into the exhaust. It's only effective after the piston rises above the exhaust port. Too small to mean anything when the engine is running.

You'll only see the hole when the cylinder head is removed. Maybe someone with their cylinder currently removed can tell us if the hole can be seen when looking in the exhaust port (from the outside). If so, with the piston at BDC you could see if a thin wire can pass through.