Bmw r75 carb serial numbers

[barryh]

what would be a symptoms of the main jets if they were to large?

You could try the basic main jet test:-

You need to get the engine pulling strongly on the main jets by using full throttle at 4000+ RPM in 3rd or 4th. A slight incline is best and obviously somewhere safe.

Roll off a little to 7/8ths throttle, If it pulls better you are too weak, if it hesitates a little before pulling smoothly you are rich. No noticeable change and you are about right.

[Jean]

This also assumes your diaphragms are in GOOD condition!!
Another indication is to check your plugs after a normal-speed run (for YOU) of around 60-100 miles.
Ther reason above for the 4000+ rpm is to make sure you are not running on the intermediate jet in addition to the main jet.
I'd go to 4400-4500 just to be sure since we (you) really don't know about the internal design of these carbs.

[Kurt in S.A.]

Are you sure about 4K RPM and the transition to the main jet? Bing's manual says that, beginning about 60% throttle, you are in the range of starting to use the main jet. This weekend on the ride on my /7, I paid attention to where the throttle was. I was running 5th gear, 4K RPM, around 65-67 mph, flat interstate. The throttle was only about 1/4-1/3 engaged. That's no where near being on the main jet. At that point, it had to be on the needle jet with some influence from the idle jet.

Kurt in S.A.

[barryh]

Are you sure about 4K RPM and the transition to the main jet? Bing's manual says that, beginning about 60% throttle, you are in the range of starting to use the main jet. This weekend on the ride on my /7, I paid attention to where the throttle was. I was running 5th gear, 4K RPM, around 65-67 mph, flat interstate. The throttle was only about 1/4-1/3 engaged. That's no where near being on the main jet. At that point, it had to be on the needle jet with some influence from the idle jet.

Kurt in S.A.

All of that is normal and demonstrates nicely that we rarely ride solely on the main jet.

To do a main jet test though you need 4000+ Rpm (more is better) at full throttle to be sure of being fully on the main jet that's why it's best to do the test on an incline so the engine is pulling hard. The other way is a flat out top speed run on full throttle but it's hardly safe to suggest that as the test method.

If it's not easy to achieve these conditions bear in mind that looking at the plugs is no use as an alternative indication of correct main jet size as the same criteria apply to doing a plug chop. The carbs have to have been on the main jet just the same.

[Jean]

All true.
Goes to show what sort of swivets one can get into when they try to fix what ain't broke.
I am SURE I've never been on the main jets all by themselves. BING 40's and a 2.91 rear end on the R100s.

[disston]

Main jets too large would show blackened plugs. Definitely darker than just tan.

Early /5 bikes were prone to various carburetor problems. Problem with the enrichners and other issues. I don't have one but have seen the discussions numerous times over the years. The best fix is usually to use a different set of carburetors like this this bikes PO has done. Boy Howdy, for once not a bad thing but looks like the PO did you a good deed.

Do the enrichners seem to work as they should?

I do have me Bing book nearby. I sleep with it.

Those carb numbers don't appear in the book for any BMW model. If the jets appear to be standard R75 sizes you should be fine.

[disston]

.

[lelocp]

i took the bike and did a main jet test and i have to say everything seems spot on but there's one thing i noticed about these carbs, that is that there is no spring at the diaphragm. It's really hard to get any information on these set of carbs. Do you think there should be a spring? Any way to tell by just running the motor and what effect does the spring have?

[Major Softie]

i took the bike and did a main jet test and i have to say everything seems spot on but there's one thing i noticed about these carbs, that is that there is no spring at the diaphragm. It's really hard to get any information on these set of carbs. Do you think there should be a spring? Any way to tell by just running the motor and what effect does the spring have?

Earlier carbs had no spring and later they all have springs. Once again, if it ain't broke....

[Deleted User 287]

i took the bike and did a main jet test and i have to say everything seems spot on but there's one thing i noticed about these carbs, that is that there is no spring at the diaphragm. It's really hard to get any information on these set of carbs. Do you think there should be a spring? Any way to tell by just running the motor and what effect does the spring have?

Earlier carbs had no spring and later they all have springs. Once again, if it ain't broke....

Yeah - My '79 R65 came with flat-top CV Bings - no spring on top of the diaphragm. They worked great as long as the carb was up to snuff.

OP - I don't know if your's are CV or not, as a lot of the earlier bikes (that I'm not that familiar with) had slide carbs. At least the R50 (& R60?) /5's.