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[Michael S]

Have you ever had an older CO 350 just lock up while riding?

I've just pieced together my 1943 WD/CO (RAF Issue) 350 to running order. Engine is stock with no visible modifications. Gearbox seems non-stock as it is a hand shift system.

It starts and goes through all four gears just fine. After a few miles, with no warning, the back wheel will lock up. Pulling in the clutch allows the wheel to turn. After waiting a few minutes, I can start the bike and ride on like it never happened.

The lock up has occurred three times in the last (my first) 15 miles and does not seem related to load, uphill, downhill, or speed. The engine is running smoothly, with no hesitation or lack of power before lock up. Wide open throttle produces fine power, and idle is fine.

First time was after riding two miles through my neighborhood hills. Inaugural ride! Perhaps 20-30 mph. Sudden lock up of back wheel while cruising. Pulling in the clutch allows the bike coast. I may have been in 3rd. Within 15 seconds and two kicks the engine was idling smoothly and ran fine for the one mile home.

Second time: Rolling down a gentle grade on a cool evening, Perhaps 35-40 mph, it suddenly locked up in 4th. I pulled in the clutch and coasted to the size of a busy street. Within a few seconds I kicked the sick starter and the bike fired up and ran great for the U-turn and few miles to get home.

This last time, after riding about five miles thought all gears, up and down hill, with a full tank, I crested a gentle hill at 1/2 throttle and experienced the sudden rear wheel skid. Again, pulling in the clutch allows the bike coast to the side. This time, the kick starter was not easy to kick through. So, I waited three minutes, kicked the bike over, listened to it start right up, and gently rode home with full power, no smoke, no problems.

Engine is topped up with fresh oil. The gearbox also has fresh 90W and has never felt hot after the bike seized up. The bke runs with plenty of power, no smoke (after first few seconds.

Could it be the transmission? Is it likely the engine seizing? Something else? It's quite unnerving and I'm glad no one was right behind me.

Without significant dis assembly, is there a way to test/confirm what is locking up?

Thanks for any insights!

-Michael

[simon]

Locking up is definitely not something you want to do too much of as it puts huge strain right through the bike. My first check would be to pull the cylinder head off and check for witness marks on the bore. If they are there then it is seizing for lack of lubrication to the bottom end most likely so either oil pump or a blocked oil way or similar. If there are no marks then your problem is elsewhere. If the gearbox is seizing for any reason then you need to pull it apart but first I would check the rear wheel and the surrounding area although if the guard was snagging you would have to have spotted that by now. You certainly shouldn't go another mile on it until you find and cure the problem because the next seizure could be one that causes irreparable damage.

[PeteF]

If you can freewheel when you pull the clutch then it would suggest that the problem is in the engine. The obvious reason would be the piston nipping up.
I too would be looking at the bore/piston.

[Paul M H]

I think the other reply s are on the right track it rather discounts the gear box as you say you can coast to a halt when pulling in the clutch. Check cylinder as said but pay attention to ring end gaps. Place rings in cylinder and check gap may be a little tight so nipping up when expanding.

[Michael S]

Thank you Paul, Pete, and Simon.

After reading the shop manual that just arrived from Hitchcocks, it seems that the proper oil level for the CO is within 2 inches from the top of the filler. This was a surprise, but easy to comply with.

As the oil level was nowhere near this for the initial rides, I'll suspect oil starvation and will pull the top as you suggested and replace any scored or out of spec parts.

-Michael

[simon]

I'd suggest you also check the oil supply to the big end as well as they don't need a vast amount of oil in the tank to run without nipping up.

[Michael S]

Thanks Simon. Us Californians sometimes don't know the big end from the little end

To what do you refer when you say oil supply to the "big end?"

-Michael

[simon]

The big end bearing is the one at the bottom of the connecting rod and is fed with oil through the centre of the crank shaft via the quill that screws into the timing cover which also houses the oil pump. The splashed oil that has lubricated the big end is responsible for quite a lot of engine cooling so if it isn't sufficient in quantity the bike will be prone to seizing.

[Michael S]

Thanks Simon. Very useful hint there. I'll check the clearances for the crank bearing (big end!) and then pull the oil pump, check for need for lapping, plunger wear etc, and then clear/check all oil lines with compressed air. I have the piston out and see signs of seizing. Measures to be .030 over. Cylinder looks good, though I'll hone it again.