Seizing engine

[worntorn]

Sure does sound like valves tightening up. I had a bike that did this and it certainly made me think it was pistons nipping up the first time it happened.
This would always happen when climbing a mountain grade on a hot summer day.
It would lose power, slow down rapidly, then eventually shut down. Kicking it over at roadside immediately after the event found the engine free but with virtually no compression.
After sitting and cooling for a few minutes compression returned and the bike would start and run normally again.
I took it apart but found no damage. I surmised that one or both valves in each cylinder ( twin) was not quite closing  on its seat when the engine got really hot, ie on a long steep grade in hot weather. Luckily the valves weren't sticking in the fully open position.
A light ream of the guides solved the problem.

Glen

[RichardL]

Glen,

Does this mean valve(s) was (or, were) only stuck just before seating but otherwise opened fine? I suppose that would explain no damaged pushrods but still not running.

Richad L.

[worntorn]

Yes, the valves must have been sticking just off the seat but still opening and closing normally for the rest of the cycle.
 If they had been sticking in the fully open  or even half open position there would have been contact marks on the piston crowns as minimum , but more likely a bent valve or two.

I was very lucky.

Glen

[Triton Thrasher]

That is feasible JulianS. But wouldn't they be "taped" shut by the piston  .
Cheers

Pushed nearly shut and probably bent.

But a valve can't lock up an engine at road speed.

[worntorn]

A stuck valve can't lock an engine, but to the rider it feels much the same as an engine that is nipping up.

Glen

[Triton Thrasher]

A stuck valve can't lock an engine, but to the rider it feels much the same as an engine that is nipping up.

Glen

Any loss of power feels a bit similar.  Sometimes a seizure can make you hit the tank with your nose.

[worntorn]

My mistake , I missed the " locking up at 100 kms" part in the first post, . I wrongly thought this was more of a soft seize.
Quite right , there is more happening than a stuck valve.

Glen

[chotus52]

Thanks for the imput guys.
Some more info:
It was apparently blowing oil out past the valve guides & the guy made the new guides a snug fit during the overhaul. Compression is still good (160 psi in both cylinders). The bike came with a box of spares, including a 15/16" monobloc Amal carby fitted with 270 main jet. According to my instruction manual, it should be 1" & 270 jet for a Shooting Star. The carby fitted is a 26mm concentric Mark 1. The main jet size on the concentric is unreadable but it "looks" smaller than the 270 in the monobloc, so I'm putting the 270 into the concentric & have lifted the needle one more notch which is now fully lifted. Hoping this is the problem solver.

[muskrat]

G'day chotus52
No amount of carb changes will solve the problem of multiple seizures. The next one may send the piston through the barrels if your lucky or the rod through the cases if your not.
 Any seizure causes damage be it valve/guide or piston/bore. So it's had two or three, don't push your luck!
Cheers

[chotus52]

Muskrat, yes, I think that is the case. Top end coming off. I just find it strange th previous owner never had it nip in 4000km since the rebuild. He did tell me he never did 100kph on it. Cheers

[Triton Thrasher]

Muskrat, yes, I think that is the case. Top end coming off. I just find it strange th previous owner never had it nip in 4000km since the rebuild. He did tell me he never did 100kph on it. Cheers

Maybe he did have it nip up.  Maybe it hasn't done 4,000 km since the rebuild.

[chotus52]

It was definitely a nip up as I was a little slow to grab the clutch on the second seize & it locked the back wheel. Strange that by the time I rolled to a stop, turned off the ignition & kicked the engine over, it felt like normal compression & started first kick & ran perfectly normal. Same on all three occasions. Oh, I heard it start to detonate each time just before it seized. After the first time, I put 98 octane fuel in her & was doing up to 80mph on winding roads. I Guess every time I backed off for bends, it got a drink of fuel. The three nipups have been at steady 60 mph for 5 or more miles

[Triton Thrasher]

If the main jet is correct, but the needle is in too weak a position, the engine will be safer (from seizing) at 80 mph than at 60 mph.

[chotus52]

The main jet size marking was unreadable but I compared it with the correct 270 jet & it looked smaller. I've since fitted the 270 since the last seize. Running fine but not tested on highway yet. I wonder if that could have been the problem.

[Triton Thrasher]

Could be.