Cylinder bore - Main jetting

[Bjoern]

Hi!

New in this group! First I'll start to appologise for my lack of skill when it comes to writing in english, I hope you guys will understand what I'm trying to say...

I bought this R75/5 since last summer, at first it didnt start at all but after som serious maintenance and work I managed to get it running. It had been left sitting for a long period of time. I overhauled the carbs using BMW original spares and stock needles and jet sizes. I also found a lot of information on the internet about my old BINGS and followed som tips and hints (for example meassuring the fuel level in the cups, choke problems etc.) It still lacks power at full throttle, it "hesitates" so to speak. Feels like it's running out of gas. This "error" is a bit less serious if the throttle is pulled slow and gently. It also turned out a bit better when I compensated the "lack of fuel" by adjusting the air-mix screws - however, it runs to rich on idle instead. I also got on other pair of carbs (newer model, /7) and there is no bigger difference between them and the old ones.

The bike's engine has been completely restored in the mid 80's, my thoughts now are: As far as I know it should be possible to fit for ex. R80 cylinders on the r75/5 engine and still use the R75/5 original head. The main jets and perhaps needle as well would therefore most likely also have to be bigger? Is there any way to identify what type of cylinders fitted to the bike (without removing the head and meassure the bore)? And, if there is, whats the std. main jet size on the R80?

Thank you and cheers from Sweden

[Kurt in S.A.]

Welcome! One thing you'll need to deal with...The R80 cylinders will not fit directly to the block of your /5. The engine case opening on all /5s and '74 and '75 /6 models have a 97mm opening while beginning with the '76 /6, the opening was increased to 99mm. So the cylinders can't be interchanged. The engine case opening can be opened up, but that's an expensive process. The newer cylinders can be machined, but that makes the cylinder weak and likely to cause problems down the road.

Kurt in S.A.

[khittner1]

Welcome, Bjoern! When you overhauled your carbs, did you check or replace the rubber diaphragms in the top of your carbs? If there are any holes or tears in those parts (pretty common if they're more than a few years old), the "hesitation" you describe at higher speeds can result.

[SteveD]

Willkommen.
1. Check your timing.
2. Check your valve clearances...
...then, check diaphragms aren't split or poorly located. Check float heights.
3. Check your carb tune.


Ensure the main jet, needle jet and atomiser aren't blocked.

The early /5 Bings were known troublemakers. Which ones do you have?

Whilst seated, look down onto the upper flat surface of the cylinders. What numbers can you see there?

[tsa]

Hej Björn!

Very nice bike, congratulations. You've already got some very good advice, follow that.

I'll only add that the iron sleeves of the R75 cylinders are fairly thick, and can be bored out to take R80 pistons. Then you even get the choice of choosing high or low compression ratio pistons, the latter may help if you experience knocking. I don't think there should be any need to change the carb jetting if taking the capacity up from 750 to 800 cc. There are "upgrade" kits available for the later airheads, 800 to 1000 cc, and these will work OK with the standard 32 mm carbs, although perhaps not if you're interested in absolutely max HP at 7000 RPM.

My little Bing users guide says the 64/32/3 -4 carbs had 140 main & 2.73 needle jets, whereas the 64/32/9 -10 ones had 135 main & 2.70 needle jets. Needle positions 2 (next to lowest) and 3 respectively. R80/7: 145 MJ, 2.66 NJ & N pos. 3. You got the needles in the right position?

I assume someone have fitted a new air filter?

[hal]

Velkommen Bjørn!

You'll find this a very nice community!
Lot's of good advice here, I would start with the diphragm if it's not allready changed.

Another thing not mentioned, are you running with both fueltaps open? and have you tried openenig the fulecap when it's surging? could be ventilation.
good luck!

[Airbear]

G'day Bjoern, and welcome aboard. That is a very pretty /5 and your english is excellent.

Can you tell us more about this "hesitation"? I'm wondering if your carb problem might be an ignition problem - simply because the new set of carbs didn't improve matters. Have you replaced the points and condenser? If the coils are the 45 year old originals you may have a weak spark that breaks down at higher revs and even slightly worn, dirty points will make this worse in my experience. With my R90/6 I had continual problems with 'missing' at higher revs. Cleaning or replacing the points would fix it for a while but new coils were the final solution. Perhaps a friend might have a pair of known-good coils that you can try.

Have you done a compression test? This is best done with the carbs removed or the slides propped up with something. If compression is in the 'good' zone and you have a good strong blue spark at the proper timing mark you can continue to worry about the carbs. Best of luck getting it sorted.