Petrol evaporates because of carb heat?

[schrader7032]

Micha -

I've never heard of this being a serious problem. I can't see how the heat changing the gas compound...maybe causing some of the fuel to evaporate more quickly, but that is corrected by tickling the carbs a slight amount to put more fuel in the bowl. Hot starting tends to be an "art".

You say that the magneto has been "overhauled"...a failing coil is usually the first place to look when a bike is difficult to start when hot. The bike must be in good tune...timing correct and magneto aligned for maximum spark when kicking starting. One of the biggest things is learning what your bike needs to start in this particular situation. I have the cold starting routine down on my R69S but that routine won't work when hot. I've been struggling lately, but I think I've found that my bike needs the throttle nearly wide open for it to start. When cold, it needs 4-5 counts of tickle. When hot, less tickle is needed but to prevent it from flooding or getting too rich, I have to open the slides to get more air in.

What is your starting routine, cold and hot? Maybe you need to look at that.

On my R69S, there are isolation spacers between the carb and the intake stub on the head. It's number 2 in this picture:

http://www.realoem.com/bmw/showparts.do ... g=13&fg=05

I would think your bike should have them also. Don't know if they're that effective as insulators, but they do separate the carb and head.

Kurt in S.A.

[Peter D. Nettesheim]

Micha,
I have a few BMW's and have never had to replace or for that matter repair a magneto. This is not your problem. Your problem is not any type of heat transfer in the carbs or cylinders. Your problem is the coil. Wait let me think about this......Ok, I'll say it again. Your problem is with the coil. If you are prepared to tell me you had it checked, please tell me how you did this as it is impossible without sophiscated equipment. If you just replaced the coil....replace it again as the one you just put in is bad. Ok, thats all. Good luck
Peter D. Nettesheim
BMWMUSEUM@HOTMAIL.com

[Micha]

Hi Peter,
What do you mean by "coil"?
The magneto received a rewound, and the condenser is new, also. I do not have these problems in cold weather, only when hot. The bike is a first kick starter at the morning, and so at the evening (I mean when the engine is cold).
The problems appear when I sometimes use it during a hot summer day, with lots of starting and stopping.

[Darryl.Richman]

The first sign of a failing magneto coil is that it is difficult to start when hot.

You can perform an experiment to see if it's the carb or the magneto coil. The next time you have the problem, pull out the spark plugs and lay them on the heads so that they are properly grounded. Then kick over the bike with the ignition on. Do it in the shade so you can see whether you have a strong, blue spark. If you do have a strong spark at both plugs, follow down the carb route.

It may also be that the engine is flooding. A lot of us have learned to turn off the gas a couple blocks before we know we're coming to a stop, to empty out the fuel line down from the tank. If the float needles (original float bowl covers) or the lever needles (new "lever top" float bowl covers) don't seat perfectly, then a lot of gas can continue to run into the motor and flood it, making the next start difficult.

If your motor is flooding, you can clear it by leaving the fuel petcock closed, then holding the throttle wide open and kicking it a number of times, with the ignition on. Eventually, maybe 10-20 kicks, it will clear and fire up. Then you can close the throttle and open the petcock.

[Micha]

Thank you all.

[EuroIron]

I guess you could make it rocket science to check a coil's output as if BMW's involved esoteric black magic

but it just ain't so

simple..... confirm largest open air plug gap it will jump when cool

then check it again when hot and not wanting to start

you could very well have carb's percolating fuel but if that were the case

holding the throttle open when kicking it would make starting it easier

and it would tend to puff a bit of smoke when it starts

also the exhaust fumes might smell "loud" or assault your nasal passages




Micha,
I have a few BMW's and have never had to replace or for that matter repair a magneto. This is not your problem. Your problem is not any type of heat transfer in the carbs or cylinders. Your problem is the coil. Wait let me think about this......Ok, I'll say it again. Your problem is with the coil. If you are prepared to tell me you had it checked, please tell me how you did this as it is impossible without sophiscated equipment. If you just replaced the coil....replace it again as the one you just put in is bad. Ok, thats all. Good luck
Peter D. Nettesheim
BMWMUSEUM@HOTMAIL.com

[VBMWMO]

I have noticed that riding at a very hot day is OK, regarding engine first start and running, but when stopping the engine for a minute or two after a long drive, I have difficulties starting it again. Magneto was overhauled and pistons are new, so, I think that heat is transferring the carb bodies and damaging the petrol's compound. What do you think? Are there some kinds of insulating spacers available? Where? Any other ideas? I thank you all in advance. Micha.