RZ/RD 350 & Misc. 2-Stroke Tech BBS

Hello, I'm hoping that there might be someone that has experience with this sort of problem. - I have a 1974 RD350. It normally runs good, but 2 times now while I was stopped and the motor was idling, the motor has suddenly revved sky high and it will not stop. Hit the kill switch, take the key out, nothing. It still keeps screaming. The only way to stop it is to dump the clutch while clamping on the brakes. So far I have been lucky that the bike hasn't got away from me. Obviously a runaway motor with no kill switch things could get real ugly real fast. Normally when I turn it off with either the kill switch or the key it will stop, so the electrical power is being cut off. The problem must be some sort of dieseling effect? Any help would be greatly appreciated.
Thank You, Gary

Sometimes carbon will build up in the combustion chamber. As it gets hot it can provide an ignition source. It sounds to me like there is another problem though. An air leak will cause the motor to rev. As will slides sticking in the carb bodies. Have you checked the kill switch to make sure it is functioning properly?

Sounds like you have a large air leak somewhere.
The spark plug is glowing red and firing the mixture.
Happens at idle because not much fuel is going thru.
To kill the engine, turn choke on or open the throttle wide open. Flood it.

The slides seem free. The bike was idling and it started racing on it's own without twisting the throttle at all. The kill switch and keyed switch both would turn off the electrical power when I tested them afterwards. Both times it didn't happen until the motor was good and hot. Actually now that I think about it, both times were also when I was tuning a bit and not getting much speed/airflow to cool things down. Maybe the heat allowed an air leak to open up? And the hot carbon was preventing it from shutting down even after the spark plugs were not firing anymore? I've got to do a leak down test and try and track down any air leaks. (I know, good routine maintenance that should be done anyway.)
Thanks seizer,
Gary

Thanks 2smoke.
Besides the leak down test, is there a method that works better than others for finding leaks? And if trying to find leaks by spraying, is there something safe to spray on the motor when it is hot?

Leak down test it, it will be an air leak I would say. I had this on an MT250.

Quick and dirty would be to spray starting fluid or wd40 on areas where there could be leaks while running. It should surge when you spray in the correct place. Flamable fluids so don't soak it.

To leak down test, take off exhausts and carbs (suppose you could plug carb intakes). Plug both sides. Use bicycle pump wiht a valve in one of the blocked ports to pressurize. Spray soapy water and bubbles will appear where there are leaks.

From the replies it sounds like you do not need electrical ignition as the plugs alone are hot enough to ignite mixture without spark thus why your electrical is working properly. Not actually spark causing ignition.

Brake clean is safe to spray on the engine.

Great, thank you all.
*Shouldn't all multiple cylinder motors have each cyl individually tested? I've seen instruction videos and even tester manuals where they tie all cylinders together with Y pipes or crossover tubes and do 1 combined leak down test. If the crank seal between the cylinders is bad it probably won't be detected that way would it?

The seal between cylinders is a labyrinth seal, it will pass air when static, but in much smaller amounts in a dynamic condition. You need to check both cylinders together on these engines, and best done with reeds, manifolds and cross over tube on.
*EDIT- speaking specifically about RZ engines, not certain on the lab seal on the earlier AC's.

Entire motor @ same time Amigo Here is a good read for you.
Harry is legendary & knows his trade ! He might still offer a leak down tester or you can buy ones on E-Bay fairly cheap. You can also make them, many tutorials. It's true how many motors have air leaks. If everyone in this forum tested their bikes, there would be raised eyebrows ...and of course pride driven denials... lol be on top if it, buy or make a tester.
http://www.klemmvintage.com/airleaks.htm" onclick="window.open(this.href);return false;

Thanks RuZty. I looked up labyrinth seal and how they work. And yes, the parts diagram for the RD does call it out as a labyrinth. So yes, it must be tested all together.
I've been wondering about this for quite some time, but never pursued chasing down the facts. Thanks a ton guys! *Just not too excited about what I will find out from the test...
Gary

Broken carb boots?

https://www.economycycle.com/shop/yamah ... ester-kit/" onclick="window.open(this.href);return false;

This is the setup that I use. It works well.

RuZty wrote:The seal between cylinders is a labyrinth seal, it will pass air when static, but in much smaller amounts in a dynamic condition. You need to check both cylinders together on these engines, and best done with reeds, manifolds and cross over tube on.
*EDIT- speaking specifically about RZ engines, not certain on the lab seal on the earlier AC's.

The Labyrinth seal passes air by design while restriction fluids. However, it doesn't pass enough air to accurately test both sides at the same time. If you only test using the left cylinder you won't always find a small right crank seal leak. That comes from my own experience in my shop. here is one example: I knew the right crank seal was leaking, but testing using the left cylinder didn't make it obvious. When tested using the right cylinder I could not get it to hold any pressure at all. As fast as I was pumping it in it was dropping to zero. When I used the left cylinder I was able to get it to 5 PSI and it held for some time. I always recommend testing each cylinder separately. It takes just a few minutes more, but will give you a true indication of seal and gasket health. If you're going to do it, do it right.