Forum bmwbike.com

I have a LWB 73' r75/5 with 56000 mi. It is only getting 25 mpg. I have completely rebuilt the carbs including new needles and seats. I have synchronized the carbs several times and set and rechecked the mixture and everything seems correct. Air filter looks like new. I thought maybe the odometer being out of wack might be showing as bad mileage, but the speeddometer reads high so when I'm going 40mph the speedo/ odo reads between 45 and 50mph which should be reading as more mileage. Anyone have ideas on why I'm getting the mileage I'm getting?

I assume you have the stock Bing CV carbs. How does the perform in general? Do you accelerate well? Can you easily go at highway speeds, like 75-80 mph?

Things that I would consider:

- torn diaphragms
- wrong jets and the jet needle is in the wrong clip position. The measurements are from the top or blunt end of the needle. Did you measure the distance the needle was sticking out of the slide to ensure that were at least the same left to right?
- you have the jet stack in wrong. The emulsion tube, the one with the little holes, is in upside down. When looking through the throat of the carb, do you see a small portion of the jet stack stick up inside maybe 1mm or so? It should.
- you have the carb slide in backwards.
- the butterfly plate is in backwards or not centered in the bore. There is a bevel on the edge of the plate and it must be oriented correctly. IIRC, there's a dot on the plate and it should be up and facing out, but that's just from memory
- the carb enricheners are assembled backwards. These are handed and cannot be swapped side to side. Since these are mini carbs in themselves, if they're installed wrong, they will constantly enrichen the mixture.
- the enricher action is reversed or set up wrong...you think they're off when they're not.

Kurt in S.A.

Hi Kurt, Thanks so much for your detailed reply. I've been very busy with christmas sales( I own a skateboard shop) and just checked for a reply to my question today. Most of the things you mentioned I have checked but not everything. I need to check that the carb enrichners are in correctly and that the action is not working backwards.
I am running the stock 9/10 late style Bing carbs and have installed new diaphragms, new needles and measured the length. The emulsion tubes were never removed and do stick up slightly. Triple checked the butterfly plates, they are right. The one other thing I'm experiencing is the bike won't start if it sits for more than a day or two, it's much worse in cold weather and reminds me of an old VW I used to have that had new but bad points.
I'm not sure the two problems are related but they certainly may be. I'm told you have to have just the right technique to start these bikes sometimes but what I'm experiencing is beyond that. Thanks again for your advice, I'll post back when I can work on the bike again. Phil