81 Liquifire Clutch won't engage under load

[IOWABOY81]

General Issue: Sled runs good and clutch engages with track elevated on stand in shop or if belt is removed from clutch.(no load) Under load engine will not rev high enough to engage clutch and propel sled fwd.
Issue: Last winter we were riding in Wisc and the sled developed a “bog” that would not propel it to go fwd. Under load the clutch was not engaging to move the sled fwd on flat ground. Sled ran well most of day and this developed late afternoon into the night. To get the sled home, one of my buddys had to push me with his sled to get up to higher travel speed and then the sled would go fwd under its own power as long as I didn’t stop and kept going at a high speed & rpm.
I can duplicate this issue now when trying to go uphill and getting sled under a load. When this issue occurs; If you take the primary belt off the sled’ the engine will rev up and the clutch engages normally. Likewise, with the belt on,, if you lift up the back of the sled and take the load off of it , it will rev up normally, engage and turn the track as designed to full speed / rpm.
I have made a lot of part changes to address this issue and while this issue does not occur on flat ground anymore it occurs going uphill under load conditions. If you stop on an incline under load, it will not rev high enough to engage the clutch.
To try and address this issue I have recently done the following:
New top end Piston/rings; wiseco – currently 120 compressions both cyls
New Stator/Coil Assembly
New Secondary Spring running in hole 3 – light blue (serviced secondary to confirm working condition)
New Primary Spring Purple – stock weights – (serviced clutch weights/springs to confirm working condition)
New Belt – Stock specs – Dayco HP 3022,
Removed Clutch and sprayed starting fluid on crank while running to check for a change in rpm or crank seal issue on PTO side. Could not identify any issues with that seal
New Carb rubber manifolds – Running VM36 powerjet stock carbs
Changed Jetting:
What it had in it last year from previous owner:
Main 120’s
Power Jet PTO 185, Mag 170
Pilot 40
What it has in it now:
Main 140’s
Power Jet PTO 110 Mag 105
Pilot 40
Air Screws out 1.5 turns
Jet Needle in middle position

Other items worth noting:
I am running air box
Stock exhaust in good shape
Stock Track studded with 96 trail studs
Primary Clutch is currently engaging fully on straight away in field full throttle (marker test) -engages at about 5000rpm
(Sled quickly tops out at about 70km/46mph) seems low to me.
Plugs are darker but not wet/fouled looking, tried new plugs as well

What to try next???

[AirborneX4Special]

If crank seals aren't known to be fresh they're very likely in need of replacement. Don't dance with that devil. Staring fluid regimen is only as useful as pounding on a half empty fuel tank to determine the fuel level.

[JoeRainville]

The PTO Seal on the Kawi Liquifire goes to the oil filled gear case, there is another seal behind that for the engine crankcase. Because both of those seals are bathed in oil, they don't fail first, the "dry" MAG side seal does behind the flywheel.

If you have never done seals is this motor, it's time.

-Joe Rainville

[Popcorn popper]

Id say your secondary spring is broke or weak. Pulling the belt down in sheave is same as starting in high gear