What exactly is a "soft sieze"?

[stopsign31]

Running down a busy road in town this evening, approx 40 mph for 5 straight minutes. Approached intersection for LH turn and scooter quit running. Puilled it over and let it cool off for 10 minutes. Fired back up and limped it home >10 mph. I have 200+ miles on a Taz 51mm bbk, V-8 pipe, stock carb, 110 jet, uni filter, 28:1 premix (removed and plugged stock oil pump). Was this maybe a soft sieze episode? Next steps?? Maybe compression check and plug chop?

[martynkim]

Running down a busy road in town this evening, approx 40 mph for 5 straight minutes. Approached intersection for LH turn and scooter quit running. Puilled it over and let it cool off for 10 minutes. Fired back up and limped it home >10 mph. I have 200+ miles on a Taz 51mm bbk, V-8 pipe, stock carb, 110 jet, uni filter, 28:1 premix (removed and plugged stock oil pump). Was this maybe a soft sieze episode? Next steps?? Maybe compression check and plug chop?

Your thinking sounds right. At 10 MPH you may have damaged it. I suggest replace plug look for air leaks and add more oil.

[Wheelman-111]

Greetings:

A soft seize occurs when the piston gets too hot. The aluminum expands a hair, using up all the clearance in the iron bore, which doesn't as much. The sliding friction increases and the engine stalls or even Locks up, as the piston sticks to the walls.

When it cools down, the piston once again has some clearance to allow it to slide, perhaps with a few scratches in its skirts.

Cruising at 40 builds up some heat in the head and piston crown. The big 110 main feeds enough cooling fuel vapor and oil to keep things stable in the Chamber, and the fan output at higher RPM carries the radiant heat away from the fins on the cylinder head. Stability.

But consider what happens when you close the throttle slide to slow down: The RPMs drop and the fan output decreases, so the head and Chamber below it warm up a bit. Even though the engine's no longer making as much heat through combustion, there's still a lot of internal friction as the revs slowly wind down. You can't pull in the clutch like on a motorcycle to allow it to go immediately to idle and blip the throttle.

Now all the mixture has to pass through the Pilot circuit, which on a stock carb designed to feed 50cc gulps is too small for 82 or so. Even worse, the mixture is the engine's only oil supply for that long coast-down. Insufficient fuel also means a momentary dearth of oil as well as Chamber cooling. CC temps rise abruptly, the oil on the cylinder walls thins out and may start to "cook", and you reach for your cellphone. What?! You ride without one ?!!

This issue makes the Stock carb poorly suited to supply any but the most modest BBKs. I ran my Contesta and Malossi 65cc kits with 70:1 plus the pump with good success, but they both had at least a 17.5 Amal Arreche or Keihin CV24 carb.

The Ruima bores seem to be particularly sensitive to off-throttle heat spikes. I suspect they're not as fussy as the Italians about measuring and matching piston-cylinder clearance. Or maybe it's the material quality. Then again, it might be Wheelman tuning "skills".

Anyway all of my 51mm seizures occurred while or after slowing down, despite big carbs and Pilot jets. The Char-Mo 50 has survived nicely, but by now I have learned to go very conservative on compression and to use sloppy-big jets.