Right cylinder not accelerating

I have a newer to me 1972 R75/5 and having an engine issue. The past couple rides the bike has been idling ok but bogging when accelerating. I figured it was the carbs so I took both apart and they were perfectly clean and definitely rebuilt by the former owner. I then checked the valves and both sides within spec. Finally, I pulled the throttle from each side while idling and the right cylinder is not accelerating. The throttle cable is pulling and the carb butterfly opens and closes properly. To be sure the side wasn’t seized, I shorted the opposite cylinder with my spark plug extender and it does idle.

I did notice a lot of carb build up when I checked the plug. The right side looked the normal brownish tan color.

Any thoughts?

Have you measured the resistance in the ignition system? You should measure the number of ohms from the left spark plug cap to the right spark plug cap. You should be in the 15-20K ohm range. You should also measure each individual element. You have two spark plug caps/wires and the two coils. On the /5, you might only get 1-2K ohms from the cap to the end of the wire. Then if you put your meter leads into the two towers on the coils, it should read around 10K ohms. Be sure there's no corrosion inside the towers of the coils.

You might also want to start over with new plugs. What are the plug gaps?

I'm not an airhead expert so please bear with me. Does the bike have constant vacuum carburetors? I believe that the cable on a constant vacuum carburetor does open and close the butterfly but does not raise and lower the throttle slide. The change in the butterfly valve causes a change in the vacuum. There is a vacuum diaphragm on the top of the carburetor that does the actual raising of the throttle slide. They are known for eventually getting a hole in them which makes them less effective. The amount of effectiveness will be determined by the size of any holes in it.

This is my understanding and may be totally wrong.

Would somebody that knows more about vacuum carbs please help?

Try looking for an explanation.

http://www.omnilex.com/public/bmw78/cvcarb.pdf

Possible

I figured it was the carbs so I took both apart and they were perfectly clean and definitely rebuilt by the former owner.

Not to be rude or insulting are they reassembled correctly?

At my local shop, we have seen more than a few carbs with the diaphragms in good shape but out of position in the carb body. Not sure how it happened but it did. Also, other things in the carbs were assembled in correctly. I remember a bike came in with slider needles held in place with pieces of tooth picks.

As I said at the beginning of my note, not to be rude or insulting, by this I mean to you. Who knows what mechanical skill the previous owner had. He could have put the diaphragm on one side out of place and if you are like me, you reassemble things in the same order as they come apart. I must admit, sometimes I am surprised I am putting something together wrong because it was assembled wrong by the PO before I ended up working on it.

I personally believe you have a carb issue here. If the bike idles on both cylinders I would hazard to guess the plugs coils and wires will not be a problem. I could be wrong. Not sure if you could switch plug leads from one coil to the other. The spark fires at the same time for both cylinders a lost spark system. St.

I agree it seems to be the carbs. I have read that the diaphragms can cause vacuum leak. I plan to remove the carb and inspect it. As for the coils, they are both new but the spark plug wires are original. I was able to swap them out but no difference in performance. Thanks for the input guys.

kioolt said:

I'm not an airhead expert so please bear with me. Does the bike have constant vacuum carburetors? I believe that the cable on a constant vacuum carburetor does open and close the butterfly but does not raise and lower the throttle slide. The change in the butterfly valve causes a change in the vacuum. ....snip

Click to expand...

kioolt - you are correct. The R75/5 came with CV Carbs (The R50/5, R60/6 did not - slide carbs). If there's a problem with the diaphragm it can cause problems with throttle response.

Here is how I rebuild these carbs. It will show you what goes where.

13 BMW 1973 R75/5 Rebuild & Refinish Bing Type 64/32 Carburetors & Karcoma Petcocks

I hope that helps.

Best.
Brook Reams.

It's already been said, but diaphragm. I carry a spare on trips.

brook.reams said:

Here is how I rebuild these carbs. It will show you what goes where.

13 BMW 1973 R75/5 Rebuild & Refinish Bing Type 64/32 Carburetors & Karcoma Petcocks

I hope that helps.

Best.
Brook Reams.

Click to expand...

Brook:

I have actually saved this page and your YouTube videos to my favorites file already. I am grateful for you sharing your vast knowledge on these bikes. I have so much to learn but you have made it easier for us newbies to get up to speed.

Thanks again!

Calvin

So while I wait for the parts to arrive from Bing and Max BMW, I took the two carbs apart last night. The diaphragm looks newer and I did not see any damage. It has obviously been changed recently. I also verified it was installed properly with the notches lining up. Nonetheless, I plan to replace with original Bing ones once they arrive. The jetting is definitely off based on the chart below (the carbs are 64/32/11-12) so I ordered the 140 main, 2.73 needle and a pair of venturis since the two mismatched. It is odd that this has just started happening. The bike ran fine two weeks ago but starting stalling the past two rides. I did verify that there was no debris in the carb float bowls. Anyway, I will rebuild and report back. I appreciate the input.

So it was running fine and you didn't change anything or work on the carbs between it running fine and crapping out. In that case, I would think a clogged jet, but you checked that. Then I would think spark and replace the plug and plug wire and possibly the coil (unless a single coil fires both cylinders). Lastly, I'd check fuel flow to the carb on that side. To be sure, mismatched jetting might allow it to run fine but rich on one side such that it might cause that plug to eventually foul.