Motor cuts out

[Holysmoke]

While I was working on the carb's and characterizing the cylinder performance, the engine would just die instantly, and be hard dard to restart. It seems electrical. Wait a few minutes, then it would start up. Plenty of fuel. Compression us good. Temperature related? I am blowing a fan, be it a weak fan. Switch issue? Any thoughts?

[Duane Ausherman]

How is the spark?

[Holysmoke]

How is the spark?

The spark looks good. Whole I was checking the left plug it fired up, probably coincidence. It's intermittent

[montmil]

Model and year?

[Holysmoke]

Model and year?

'78 R100S

[Holysmoke]

Now she won't start at all. Got a backfire and a few pops though. Checked for spark, no spark! Now we're heading in the right direction. Where do I start looking next?

[Duane Ausherman]

If you don't have any spark, you won't have any backfire or pops. Obviously you have some spark, but maybe you didn't test for it correctly.

You shouldn't just do the shotgun approach, you need a plan of logical procedures to follow.

You have told us nothing about fuel flow, compression numbers and method of deciding that it is "good", or valve adjustment.

If you aren't a mechanic, I suggest going through the standard tune up procedure and document every step of the way.

[Holysmoke]

Duane, you're right, I should write each thread so it can stand alone. That leads to my next question, is it better to have one thread as one goes through the process of bringing a bike bake to life, or to split it up, capturing phases of troubleshooting?

[Rob]

I won't pretend to guess what Duane will respond with, but I will tell you he is humble, and obtuse about his website.
While his website is devoted to mid-seventies and earlier airheads, much of it (like fuel flow rate) carries over through any carbureted BMW motorcycle.

Duane insists that newcomers get used to Googling "BMW motorcycles fuel flow" (like I did) - changing the suffix to whatever ails you, of course.
Somewhere near the top of the results, you will probably find a hit on Duane's site.
In this case: http://www.w6rec.com/duane/bmw/petcock/index.htm

Stroll around in there for a while. It is fairly user-friendly.

[Duane Ausherman]

Duane, you're right, I should write each thread so it can stand alone. That leads to my next question, is it better to have one thread as one goes through the process of bringing a bike bake to life, or to split it up, capturing phases of troubleshooting?

I just grab a loose leaf spiral binder note book from a pile where I ripped out the few pages of some long forgotten project. Now that I am downsizing, I am throwing out many of these.

If it is one that I plan to keep a long time, such as what you are doing. I make tabs for each chapter and just guess where they need to be. Each chapter is done serially. I leave the back half of the pages empty for the unknown aspects that will jump out later on.

I know, some of you will prefer to do it digitally. I hate using a pen now and far prefer the keyboard. How many keep the computer out in the shop running and ready? I think few do. With old fashioned paper, you can scribble easily and that means far less is forgotten.

Take a lot of photos, lots and lots of them. I try to take at least 2 to 3 times what I expect that I will ever need. Still, I always find that something didn't get recorded with a photo.

If this is a project where I don't really know what I am doing, I lay it out in order of being removed. I even take photos of the parts laying on a work bench.

I also start with printed out pages of articles that I found on the Internet. I high light the things that I think that I will need ahead of time.

This must seem obsessive, but I learned this while working in artificial heart research some 50 years ago. It has paid of handsomely for me.

Did I mention that I keep things clean? Especially the surface under where I am working. Don't ask how I learned this one.

Do not believe very much of what you read in officially printed books on BMW motorcycles. None are complete. They have errors and usually not the best and most efficient way of doing the job. I really can't think of a single repair job where we followed the factory procedure. We had better and faster ways of doing it all. Sometimes our method was much slower, but it was what was needed to get the result. Eg, fork alignment, tire installation and balance and the list goes on and on.

My shop had about 10 dealers within 100 miles and not a single one could do a decent job of a simple tune-up. Few even tried to do serious stuff like a transmission rebuild........ thank goodness.

You must learn to do this stuff yourself. I have to admit that I think that the state of the art is better today than back in my day, due to the Internet. At least we have many outlaw shops that do a better job than most dealers.