Welcome !

Unlike the Brit bikes you've been riding, pieces on the R75 are not a mirror image. Stand on the rider pegs and look down. You'll see that the cylinders are off-set, one in front of the other. This means some parts you wouldn't normally suspect are actually different lengths. This length difference is taken up in the throttle cables, choke cables, and the plastic air intake tubes.

So your problem is simply that you ordered 2 of one tube, rather than 1 Left side and 1 Right side tube. 

• While these air intake tubes are removed, you should also take the time to pull the LH air filter box off (1 screw) AND the top most Starter Motor Cover off (2 screws). The reason being that mice love to build winter homes inside these places. And when/if they do, the central air intake channel becomes blocked and the bike will not run correctly. It's so simple to check at this stage of the game. 

Hope you find this helpful.

Hello Richard,

Thanks for your response.  I have both right (13721258060) and left (13721258059) side tubes, both the old set and the new ones.  I have the tighter, more 90 degree bend on the right and the more sweeping bend on the left which appears to be as intended.  I have had the airbox off as well as the starter cover.  Both had just an expected amount of dust and dirt accumulation one might expect after the years and miles of usage the bike has seen.  I put in a new air filter and positive battery cable while in there.

I took the old tubes off the bike, but it was over 25 years ago.  It was a long time ago, but the fit well enough back then that I didn't note that they were obviously incorrect.

You are correct when you say that the Airhead is different from the Brit bikes, also the Japanese.  I guess that I have been spoiled up til now with the Airhead parts just fitting with little to no hassle.

DJinCA

There is a secret trick to this.  It is difficult to describe in writing, but stupid easy to demonstrate. Since I am not there, we'll do it the hard way. Looking at the picture you posted, try to visualize the entire assembly of the rubber sleeves, tubes, and the carburetor as an arc from the filter to the intake spigot of the head. What you want to do is loose assemble all the parts as an exagerated arc to the outboard and push the entire assembly toward the motor. You will need both hands for this.   If you are lucky, the whole lot just kind of jumps together. Be sure to have all the clamps loosened and placed where you want them. A little lubricant might be helpful.

Richards advice about the offset of the cylinders and the long/short tube is to be heeded as well. 

James, thanks for your response.  I will loosen everything and try again.  I originally got the left side together with the original (old) elbow and uneven connections clamped together, but the right was off by a lot.  In my photo, I have the airbox rubber pushed all the way onto the end of the elbow, that is, the ends are flush with each other.  The elbow is all the way onto the carb mouth and there is not a lot of extra hose between the carb and head.  I need ~20mm movement to be close to the airbox connection.  I am not sure where that can be made up.  

I understand about the cylinder offset.  As the pistons both travel to TDC simultaneously like a 360 degree vertical twin, but unlike one they travel in opposite directions and therefore cannot be on a common crank throw (e.g. forked rods) they must be on separate planes. 

I'll try to give it another go later today, the morning has been given over to taxes from which this is a welcome break.

DJinCA

I am going to go out on a limb here, and may perhaps invite dissent or even ridicule. 

It is my opinion that the rubber sleeve goes on the carburetor end of the air duct on bikes with the cast aluminum air filter chamber, and it belongs at the filter end of the tube on the flat filter air box. I have 1 of each style on the 2 airheads I own ('73 and '85) and that's the way mine are arranged. In the wild, you will see these assembled in every possible configuration. 

Try fitting the airtube alone to the flange on the aluminum air box. If it slides on to the flange by itself, try assembling the set up (as described earlier) with the rubber sleeve on the carburetor end of the tube. That O-ring on the stub of the aluminum airbox flange is there to seal the junction of the tube and the airbox. Think about it like this, referencing your attached picture, why would we need a rubber o-ring to seal a rubber sleeve on that flange?  Regrettably, the written word falls short of conceptual description in this instance.

Thanks James, I will try that.  I wondered why there was an o-ring on the filter housing if it was only to be covered by a rubber fitting/hose.

DJinCA

► Agreed. You must have the correct lengths statically, before re-assembly starts. That is, with the carb in position, the elbow should look like the exact shape and correct length. BMW had several airbox styles over the years, which may have led to different lengths and/or shapes with the air intake tubes.

► I loosen the carb mounting clamp and allow the carb to hang by the cables. Spray WD-40 3 places: all inside the carb-to-cyl rubber inlet hose, the carb's bell mouth air intake, AND the air box spigot. Then, I push the carb AND the air tube in unison back into place.

Hope this helps.

Sorry for the delay in getting back, I was tied up Friday and couldn't get back to the garage and yesterday was a BSA Club ride.  Thanks to both Richard and James for their help.  James, you got me pointed in the right direction, the tubes fit better right side up:-)

DJinCA

Glad you got it sorted.