Hi all...my son and I took my newly restored ZM21 outfit for its first road test. All was good the engine was pulling exceptionally well. However after about 5 kms when going up a hill the bike came to a stop. The engine was not seized, it was like there was no spark or the decompression leveller was engaged. At the time I thought the engine was locking up, however the back wheel did not lock, and the bike was easy to kick over, and it restarted after about the third try. We rode off again and came to the next hill, the same thing happened. I was thinking maybe a condenser issue, but why would it re-start immediately after stopping? Perhaps a tight engine, but I board it 6 thou oversize, and again why would it be able to be kicked over? This bike has been fighting me all the way. Has anyone had this issue, that could please help?

Try again with the fuel cap loose and he tank only 1/2 full

email (option): bsansw1@tpg.com.au

Bryce, do you say the engine was not stuck, but there was no compression ?
If so, there is a possibility your exhaust valve was stuck - opened.
After a minute or so, it will cool down and release.

It could be your ignition is very "late" or you have not enough play in the valve guide.

yes the engine was not stuck...with the usual compression. If it was the value would it not be missing and back firing etc?

This could be it Trevor...as I did fill the tank and I did fit a rubber seal on the fuel cap. I will try it tomorrow see if your idea works. You reminded me of a problem I had with a fuel cap some years ago with a WM20.

Took the bike for another test ride yesterday. The engine is now seized...bummer because its been hard to find a ZM21 engine and I have spent many dollars and hours on the project. I well back to the work shop.

I would say the problem was ignition timing (too retarded causing overheating) or an air leak (overheating because the mixture was too weak)...

Going up hills is when you tend to use a lot of throttle in a lower gear and that is commonly when the effects of both of the above are most marked...

The effects are clearly seen at night when the first six inches of the exhaust pipe will glow red....

In my experience incorrectly set ignition timing is the most common cause of problems of this type and I consider the only accurate way to set and then check it is with the cylinder head off the engine...

Oil problems are unusual, in fact pretty unheard of, with BSA singles of this type...

Your .006" bore clearance should be plenty if you have an original piston and you have followed the correct running in procedures..

However, I always recommend carrying out initial running in (at least 500 miles)without a sidecar fitted...That exerts a lot of extra load on the engine at a time when you really don't want it...

The engine must be treated very gently for the first 250 miles or so, not slogging it in too high a gear up hills, not accelerating hard from a standstill and cruising on a closed or rolled of throttle on level ground...

As the running in period progresses loads should be gradually increased for short bursts with maximum performance being achieved only as you get towards the 800-1000 miles mark....

If you have over worked the engine during the early stages the piston would be the first thing to fail....

I have built scores of these engines for myself and for customers when I did it for a living and the comments above are based on the experiences learnt while all those engines were running in...

Some people will tell you to drive it hard when running in...DON'T!!...Ian

email (option): ian@wright52.plus.com