Topic: who know's bmw's? 1979 r65

[Brandotheamazing]

Neighbors bike, he just picked it up, he's got an 84 bimmer too. but this one is actually pretty nice, got it for a song.
he took it for a ride, said it falls flat at fifty mph. My best guess without riding it or looking at it was spark advance stuck.

So who know's these bikes? what should I look for?

[Goofaroo]

I think you'll need to ride the bike to be able to diagnose the problem. I know a little about these bikes but "falls flat at 50mph" could be caused by a number of things.  Airheads are oddly simple engines so they are easy to diagnose.  Check the air filter first and make sure there's air getting through. Then see what the plugs look like. Then test the compression. Then check the valve adjustment, then check the carbs. They are CV type carbs with a rubber vacuum diaphragm in the top. The rubber can get stretched and still operate reasonably well but if they are torn or have pinholes it will wreak havoc on your power output.

You could be right about the advance but you can test it very easily with a timing light. There's a timing inspection hole on the left side of the flywheel housing.  Make sure it is timed and the advance is working properly.

If all of the above things are even remotely close to being in order this bike will run 90 mph all day.

The R65 is unique. Most airheads are basically the same and many parts are interchangeable between them. The R65 does not fit that mold and is also a bit physically smaller than the others.

As far as an R65 being different from the others, it is sometimes it's Achilles heel.  One of the great things about airheads is the ability to find any part you need due to the huge popularity of these bikes. The R65 can be much more difficult to source parts for.  Also be aware that they haven't yet gained in value due to their rarity.  They generally cost about the same or somewhat less than a "normal" airhead. 

Keep us posted on the problem.  He'll have a great bike once it gets sorted out. If you've never ridden one you need to insist that your neighbor let you take it for a spin. They are extremely nice bikes that have a charm to them that can't be matched. Take it out on the open road and you will begin to understand the attraction to the BMW airhead.   

[754]

 I always thought that the R65 had same lower end as 900 or 1000, and you could swap the jugs and heads. Is it really not true?

[Terry in Australia]

You can convert an R75 or R80 to an R100 by just swapping out the 1000cc pistons and cylinders Frank, but an R65 has a shorter stroke than the bigger boxers. I've got a feeling that you can convert an R65 to 1000cc, but there's a fair bit of engineering involved.

One of the members here (from Europe) races an R65 based 1000cc race bike, he "stroked" the crank by machining the pin a couple of mm's, and then bored the R100 sleeves a bit further as well. I'd be a bit worried about the undersized crankpin, but it seems to work OK. Cheers, Terry.

[Brandotheamazing]

Thanks, and I already have a "seat" on whatever bike he's not riding at the moment if we go cruise together, as both my bikes are non oped, and even at that they're not safe for someone to ride, bad wheels, leaky forks, cracked brake lines, that and having a newborn have put my riding the old hondas on hold for the time being.