Help :(

[newairhead78]

I recently bought a rebuilt 1978 r-100. After reading about the thousands of ppl that love these bikes, I was excited to buy one (owned a kawasaki vulcan 750 in the past) Although it started up fine, and ran fine on surface streets, as soon as I got it on the freeway it starts bogging down at 55mph. I took it to a BMW mechanic with high reviews. After 2 weeks he cleaned the carbs out (said they were a little dirty) changed the oil, replaced the throttle cable, and adjusted the throttle, changed the spark plugs. He said he took it out and got it up to 70mph with no problems, but he said that the exhaust could end up being a problem (previous owner took the baffles out of the pipes). So tonight I paid him $500 for the work and sure enough within 4 minutes of being on the road and as soon as I hit 55mph it started bogging down. In fact it died on me 4 times on the way home and is now running worse than when I took it in. Mechanic has offered to tow it back free of charge and make it right. Does anyone know what could be happening here? This is my first BMW bike and so far its been an awful experience.

[Mal S7]

Hi and welcome,

sorry to hear you are having a bad start to the ownership experience. It' s surprising and disappointing that your mechanic could take so long, and charge so much, to not fix your bike. An exhaust with no baffles will make your bike too loud, and won't help performance, but it won't make your bike bog down at 55mph.

It could be something simple like your fuel tank not being able to breathe. Your symptons sound like fuel starvation to me. Or possibly a carb diaphragm with a tear or pinhole in it.

Are you mechanically inclined? Capable? Its hard to advise you without knowing what will make sense, and what you can safely attempt.

One of the great joys of owning an old airhead, apart from riding it, is working on it yourself. They are old bikes now and there will always be something that needs attention, its not going to be an enjoyable experience if you have to rely on mechanics unless regularly emptying your wallet is not a problem for you.


cheers
Mal

[chasbmw]

Find yourself a new mechanic.

The engine is suffering from fuel starvation most probably due the float height being set too low, or possibly one of the carb diaphragms is torn.

[Mal S7]

To check your diaphragms (assuming its fitted with the original Bing carbs) is simple. You don't even have to remove the carbs.

Undo the choke cable where it connects to your carb tops.
Remove the four screws in the carb top.
Carefully lift the top off the carb.(Take note of which way on it is fitted.)
A rubber diaphragm will be attached, and a tapered needle. (careful not to damage the needle.)

Inspect the diaphragm for any tears of holes. You should replace them in any case but a quick check might reveal an obvious problem ..... though your mechanic should already have done this.

When I bought my 77 model, my first experience of a BMW, I unwittingly rode it for ten thousand kms with a torn diaphragm, I thought that airheads were just built dog slow!

Let us know how you get on!

[Seth]

I doubt the carb diaphragms are at fault, or else the bike wouldn't have died. It would have just not revved as it should. It should idle without a problem. I agree with the tank not breathing. Was the tank recently coated inside? Maybe they did this with the cap on. With less than 1/2 a tank, try running on the highway (smooth and straight) with the cap slightly unscrewed so you make sure it can breathe.

[Duane Ausherman]

Welcome to the "home" and we won't let you down. If we don't know the answer, we will make one up.

In addition to the excellent suggestions, measure the fuel flow out of the petcocks. Once you have done it a few times, you will learn what it should look like and then you can do this test in seconds without the beaker. Do this a few times a year and anytime you have a problem above 50-60.

Have you found the several excellent websites that tell you just about everything you could want to know? Find and start reading, dozens of hours of reading. Book mark the ones that explain in a way that you can understand and refer to them often.

Become the expert on your bike. Keep a log book on everything that you do, even trivia like changing a tail light bulb. Some of us forget this stuff and need notes with mileage and dates.

[newairhead78]

Thank you everyone these are great suggestions. Im going to try and get the 78 r100 manual anyone recommend a good site to find it on?. He had already replaced the left side petcock as the previous one was leaking slightly. He had the bike trailered back to his shop free of charge and said he felt bad this happened my first time using him and wants to make it right. He had replaced the carb gaskets, but not the diaphragms hmmm. I need to re-coup my $500 so Im really hoping he figures out the problem. I will let you know! Thanks again for all the suggestions and tips!

[Deleted User 62]

Get the Haynes or Clymer manuals, or both, at Amazon.com free shipping. The factory manual is expensive and a bit too advanced for a beginning mechanic.

[Zombie Master]

Get the Haynes or Clymer manuals, or both, at Amazon.com free shipping. The factory manual is expensive and a bit too advanced for a beginning mechanic.

Good advice! It's really worth buying the two manuals in order to compare explanations of procedures.

Don't forget to take digital pictures before and during dis-assembly.

One day you'll thank me.

[ME 109]

I'm in the low float level camp.