Help needed with R75/5 engine probs

[flashy_cj]

My wife and I are traveling with a BMW powered Chinese Chang Jiang and are stalled in Erenhot, Inner Mongolia. Here is the problem: the bike simply won't run smoothly and has virtually no power. I returned to the shop in Beijing that did the engine conversion but no matter what engine or carb setup we try (so far four engines and an endless supply of carbs including a set of old Bings) the problem persists. While I am no mechanic it seems to me that the problem has to be independent of the engine/carb combination.

I bought a set of working Bings from a friend in Beijing last night and the shop will install its last tested R75 today. If we still have the problem, what else should we look at (we have changed the coil, points, plugs, condenser, etc)?

Any help will be greatly appreciated.

[Tim Wood]

If you've tried different carby setups and it's made no difference, is the engine itself serviceable. Don't bother with any more carbs. Has it been assembled correctly? I assume that a BMW engine IS compatible with the Chinese host?

First check the compression with a proper compression guage. Every time it turns over, the compression should increase (double) until after about 6 turns it should steady at about 125 psi. That will tell you a lot if the rings are good and the valves are timed/adjusted correctly. If the valves are incorrectly adjusted, it will suffer from a lack of power. If that's OK, then look at the ignition timing VERY carefully. Turning the engine over slowly by hand and observing the valves and points operating will tell you if everything has been adjusted properly. You may need a lamp somewhere in the ignition circuit to monitor the points position. The spark occurs when the points OPEN and the lamp comes on.

It sounds like an ignition timing problem to me. As you turn the engine over (in the correct direction), stick something in a spark plug hole to monitor the piston position (don't let it get jammed as the piston rises). As it comes up on the compression stroke, both valves will be completely shut and the points will open and the timing mark (see manual) should all line up. As the engine turns over, you must identify what part of the cycle your on ie: inlet valve opens then shuts, compression occurs, piston starts to decend, then piston rises and exhaust valve opens. I'm not familiar with that bike, but this is common to any engine.

If all this fails, I'd get another mechanic (or two) to look at it. This should be a fairly obvious problem to fix.

Let us all know what happens. Good luck.

[Timo]

Aside from the above mentioned ailments (compression, spark, fuel - check the basics), one thing you didn't mention is the exhaust. Is it possible that you have a blockage in the exhaust? Just a thought - not sure what you are running for muffler or header pipes, but if this is blocked or constricted the bike will not run properly...

good luck!