Carburetor Jet Sizes

[Julio A.]

I'm ordering new jets for my 1981 R65, The repair manual specification charts called for 140 mains, 2.66 needle jets, 46-241 needles, and 45 idle jets.

Cross referencing at the parts from realoem.com, the main jets were supposed to be 148 to 138 depending on the year. When I was looking around some airhead forums, I happen to find upon some claims putting 135 mains on the carb gives a marginally better performance than the factory specified sizes, so I'm thinking on considering a set of 135 mains instead.

I don't really know anything about carburetor and jet sizes so I'm completely lost about this.

[Barry]

Julio,

There should be no need to replace idle jets and mains unless they are damaged as they don't wear out. Only the needle jets and to a lesser extent the needle wear. If you want to change the sizes that's different of course.

The usual story on jet sizes is that the main jets did not impact at all on  emissions testing so the factory typically tended to go slightly rich rather than weak which is at least safe if not always achieving maximum power. That suggests there is some scope to come down a size. You would have to test it though as modern fuel is working in the opposite direction making the mixture slightly weaker than would have been the case back in 81. On the other hand typical riding rarely involves full dependence on the mains as on CV carbs you are mostly on the idle jet and  needle jet.

I did try smaller mains on mine dropping from 123 to 105 which was the size used on the 81 - R45's. That was much too big a change and the bike was all but unrideable above 1/4 throttle. What this taught me was that in spite of what you hear about mains affecting only 3/4 throttle upwards, a big change will also produce a weaker mixture lower down as the fuel still has to pass through the main jet to get to the needle jet and there will be some increase in pressure drop.  

One day I might try drilling out the 105's to somewhere in between.

[steve hawkins]

Purely conjecture, but slightly leaning out your fuel air mixture slightly might well give you a small boost in performance although I would not kick the shit out of it - pardon my french.  Quite often when people start mucking about with aftermarket exhausts and air filters etc they start to worry about jets and of mistakenly go larger.

This is what I did when I changed my silencers and went to K&N filter and then went to individual pod filters.  I up jetted my 1979 slightly.  But it developed a flat spot/stumble at 70 odd mph.  I did not get to the bottom of it till I returned to standard jetting, and the I adjust my needle position to make it slightly leaner still.  The stumble disappeared and the bike runs well.  

I was expecting the bike to be more freer flowing, but the opposite was the case - at least with the exhaust - noisey as they are the spoon baffle in them creates quite a bit of back pressure.  I feel I have gained some midrange, I think, but lost out a little at the top end - which is irrelevent on today's roads (purely annecdotal). So I am happy.

If I were you I would buy a set of standard jets and the leaner size and give the leaner ones a try - you can always go back to standard.  Check your spark plug colour and see what you think.  You could just buy a standard set and just perhaps play with your needle position to lean it slightly, although Barry seemed to think that the needle position adjustment is quite coarse, in a recent thread that is definately worth a read.

Are you changing your jets because you think they are worn? Are your spark plug sooty or dark brown, or are they a light tan when you check them?

Light tan/grey is where you want to be after a run.  If you are already there and have no stumble or other peculiarities , then it might indicate that a change of jets is unnecessary.

Cheers

Rev. Light

[georgesgiralt]

Hello !
If you have the exact user's manual which was shipped with the then new bike, you should have the correct numbers for your bike and marked. Otherwise, RealOem with your serial # will give the sizes (you'll check with the values on the bike and know if she had been tampered with) Bing also has the jetting if you have the part number of the carbs.
As other told you above, this was correct 30 years ago. Now fuel have changed and the bike worn out.
As others, I'll suggest you change all rubber parts in the carbs, and the needle and needle jet. Change also the springs (throttle, choke and above piston) it will save you some headache. And buy a couple of cans of carb cleaner to clean all passageways. Buy the part at BMW dealer. They are quite cheap and, of course, perfect for the job.
Then ride her up to see how she cope with the modern fuel we have.
Read this up and use it to adjust the jetting to the style of riding of yours and the fuel :
http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,lower_rpm_engines.html#1.%20Top%20end%20%28full%20throttle%20/%207.5k%20to%20redline
Bear in mind that all settings interact one with the others. So follow above instructions and notice that a needle position change of one is roughly equivalent to 3 size of jet needle size.
Last but not least, BMW used 2 springs above the pistons in Bing carbs. One weak, one stronger (post 1983). So you have 3 choice here : No spring, weak spring and strong spring to fine tune to your liking.
You'd be surprised with the bike with cleaned and overhauled carbs. Smooth as baby cheek and going like the wind ! Changes everything.

[Julio A.]

Actually, I'm having horrible mileage from the bike right now and I suspected the carbs need an overhaul. When I opened it up! There were rust deposits everywhere, worn seals and stuff and the jets were badly worn and the main jets slightly damaged. Likely from the PO Or my dad trying to dislodge a blockage way back.

My plugs indicate slightly rich mixture, slightly black-reddish brick color which looks like a good enough indication. So I am looking for the original jet sizes. I don't have the original manual but I do have a factory repair manual which contradicts what realem tells me. The difference is quite large, 140 at the repair manual and 148 at realoem. Jets are not readily available here so it'll be quite the hassle to experiment with jetting.

[Barry]

Changing needle positions is a coarse adjustment. Changing the needle jet much less so which is not surprising when you think about it. Needle jet incremental sizes are 0.02mm or 0.8 thou - that's a very small change in jet diameter compared to quite a big change in the needle area resulting from a one slot adjustment.

Mathematical analysis here if you can stomach it. http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1360055079/30#30


Weaker = better mileage is what I generally think too but only up to a point.  My bike runs a bit rich low down as evidenced by the ability to  start and pull away almost instantly at any temperature above 10 Deg C with no use of the enricher.  I know that isn't right yet I get very good mileage. Last tank 69 MPG (imp). Also when I was experimenting with "half needle positions" to eliminate a mid range flat spot richening the mixture actually resulted in better mileage. I think there is an optimum mixture which is not excessively rich but not too weak either.

Depends how a bike is ridden of course but for moderate use where a lot of the time is spent at 1/4 throttle or below I find mileage responds mostly to idle mixture settings, needle jet/needle position and then main jet in that order.

[georgesgiralt]

Hello !
So you have the bike, right ? So look at the carbs, (just under the top hat) there is a number stamped on it 32/64/something you should have consecutive numbers for left and right (32/64/307 and 32/64/308 for example ) This should gives you the correct jetting by looking at the Bing site. Last but not least, look at what's inside your carbs. Jetting could be the correct one. I do not think it has been tampered with.
As per the poor mileage, it is not uncommon to eat up a lot more fuel with the carbs clogged by years of storage and previous use.  Add to this the wear of some key and precise components and here you go at the petrol station.
Overhaul your carbs, then, if mileage still poor, you'll have a very decent base to investigate and tinker with settings.  

[Barry]

Georges makes a good point.  Gunge from the engine breather tends to accumulate around the needle jet mixing tube which in time completely blocks off the emulsion air supply. That's a killer for fuel consumption. When I cleaned mine mileage improved by over 10% and only one side was badly blocked.