Predator engine high compression at start

[Stuf28]

Hi. Hoping someone out there can help with an issue I have with my predator 420cc, 13hp, used on my Kart.

The engine is brand new and was running well, no issues until I had a mechanical failure on the kart which caused the engine to lock up and stop suddenly. There wasn't any load on the kart as it was running on the bench.

Since this issue the engine wont turn over through the full cycle using the pull start or the electric start. I haven't had the engine running since the mechanical issue.

I have taken the spark plug out and the cam cover off to have a look.

With the spark plug out the engine turns over with the pull start and by hand without any issues, no abnormal noises or restrictions.

The valves open and close and as far as I can see the push rods show no signs of damage (not bent, but visuals is limited). The tappets are not lose, nor have they moved (lock nuts have not moved).

As the valves and engine turned over okay with spark plug out I put the plug back in and watched the vales go through the cycle.

I noticed that when the inlet and exhaust valves are closed prior to the piston coming back up for the power stroke the compression is too high to allow the cycle to complete.

Main problem I see is that compression at start is too high. At this stage I don't believe it to be a bent valve or push rod.

I pushed down, slightly on the inlet valve tappet and the cycle can continue. Pressing on the tappet slightly released excess pressure.

The questions from me are,
1. Has anyone had a similar problem?
2. I understand that the camshaft has a mechanism that helps reduce compression during start, is this correct? Can it be damaged if the engine stops suddenly?
3. If a valve drops would it cause high compression? I don't think so, in my thought process it would be reduced compression.

I intend to take the crank cover off to look at the cam and take the push rods out to ensure they are straight and not damaged. If this doesn't show any signs of the issue I will take the head off.

Does anyone know what the gap should be on the inlet and exhaust tappets so I can set them up correctly once moved?

Thanks for your help,
Stuart

 

[KartFab]

What caused the engine to stop suddenly?
You can visually see the compression release working if you rotate the engine. Should be a slight dip in the exhaust valve on the compression stroke.

 

[Stuf28]

Thanks for the reply.

Okay, thanks for confirming that the exhaust valve is the one that opens. I can double check whether if I press on the exhaust valve tapped the cycle can continue.

The engine locked up when the clutch broke apart, my error when assembling it.

 

[KartFab]

Hmm hope it’s not something worse. An engine suddenly stopping like that could have sheared a flywheel key or broken something internally. You can check valve lash pretty easily, should be somewhere between .002 and .004 for most engines.

 

[Stuf28]

Thanks for the info. Both the flywheel key and the key on the crank drive end are still okay.
I will check the valves, push rods and the cam.

Do you know where I can buy any spares required?

Thanks

 

[KartFab]

You can order spare parts from harbor freight.
A lot of the 420 stuff is interchangeable with GX390 like springs, cam, lifters, flywheel, connecting rod. There are differences in the head, bigger piston, and different valvetrain though

 

[Stuf28]

Thanks for the help.