Boxerworks.com

Hello everyone,

I've recently purchased a '71 R75/5 with the short wheelbase. The bike has 57k mi on the odometer, so I'll be going over the whole thing. I'm a former aircraft maintainer in the RCAF, so I know my way around machines, but I've only ever worked on Japanese and Italian types. I look forward to learning from all of you.

The first item to address is to return it to close to stock as possible. It had a Windjammer installed which I've since removed. I've started sourcing replacement parts for the headlight lens and reflector, which I'm upgrading to halogen. I've also found replacement signals and stems, which a previous owner had bent up 90 degree to allow for the Windjammer installation. I've performed a compression test. Number 1 shows ~120-125psi, but number two is barely making 110. Second test after adding a tsp of oil managed to make 120, so methinks it's the piston rings. So that'll be the next item. This is gonna be fun!

Cheers,

Matt

When you did the compression test, did you remove the carbs or hold the slides open. If not, this will effect total compression numbers. Welcome!

I didn't. I just did it with wide open throttle. No good?

Bamboo's right...with CV carbs, the throttle only controls the butterfly that is downstream of the carb slides. You must physically hold the slides up with something that doesn't restrict air flow or just loosen the carb rubbers and move the carbs off the intake stub.

And there may be a change in readings from cold to hot engine temps.

Kurt in S.A.

Thanks guys. I did indeed perform the test on a hot engine. This being said, should my next attempt show good compression, I'm still thinking of doing the rings because the cylinder base gaskets are leaking a fair bit, as is the oil pan gasket. I think the pushrod seals are a little sketchy too. So if I'm going to be digging that deep into the engine and pulling off the cylinders anyways, I figure the price of all new seals and rings is worth the peace of mind in the long run. I don't plan on riding this bike any time soon. I bought it as a medium-long term project to eventually ride across Canada.

My only guide - other than my general aviation mechanical skills, is a Clymer manual. It looks decent overall, but I wish it had more detail on the carbs. The last time I touched a carb was on my first car: a '79 Lincoln Continental about 12 years ago. So this will be a learning curve I'm looking forward to tackling.

Consider a lead-down test before teardown...it might provide other clues as to issues with the top end.

As for the carbs, they're really dirt simple...not that many parts. A manual can be had from Bing for maybe $10-15. They also sell a DVD which some have found helpful. About the only thing that gets somewhat difficult is dealing with the butterflies...removal is tricky and refitting them has to be done correctly. Unless there's some serious issues with the carbs, I'd leave the throttle shaft/butterflies along and just deal with the jets, etc. Don't dunk the entire carb into a cleaning solution...that will penetrate to the throttle shaft O-rings and then you'll have problems with them.

Kurt in S.A.

Well, I performed an additional compression test with the carb removed; lo and behold, the dial indicated 120 psi after 5 revolutions. I got about the same result when performing a leak-down test with the kick-start lever. Thanks everyone!

Be sure to watch the increase of each revolution. How that number increases is very important. Your first compression number should be roughly 1/2 of what you are going to end up with. Each additional one should be about 1/2 of the previous one. I only use 6 pumps as my standard.

The thing to watch out for is when each pump is roughly the same. Not good.

To be reliable, a compression test on these machines should be after the newly found bike has had a few hundred miles put on it. This cleans up the valve and seat. Then measure it hot, as you did.

Your numbers are in the ball park of a healthy top end as long as that number increased as described above.

As you probably already know, the bottom ends are nearly bullet proof.

Kriegmeister wrote:Well, I performed an additional compression test with the carb removed; lo and behold, the dial indicated 120 psi after 5 revolutions. I got about the same result when performing a leak-down test with the kick-start lever. Thanks everyone!

Thread delivers! Good in'it!

Clymers is good, but not perfect.

Haynes is also good, not the same as Clymers and also not perfect. It does provide another option and is free here...open and scroll down.
http://bmwairheadsdownunder.com/airhead_tech.html

This might help with the carbs.
https://youtu.be/hYr2_SO3V_Y