carburetor upgrades? Keihin, Mikuni Dellorto Etc..

[mfrias2nd]

So pulling up to a bike meet last night a nice gentleman, who use to work on bmw's back in the 80's, before introducing himself to me, said "those bings with those filters are killing your bike" I temporarily removed the airbox to fit the old style aluminum airbox but haven't found the right fitting elbows to meet the carbs so I've been running Uni Pod filters.

He continued to say that the bing carbs will work excellent with the airbox but if you're looking for a little more performance and throttle response go with Mikuni 36mm's He said he ran 38mm on his R80.

I love the thought of a little better throttle response and I read those flat slide upgrades would even help with mgp. What are your thoughts on this subject? Anyone recommend any particular carb, mfg, size jetting.

Much appreciated.

[Ed Miller]

Slide carbs give a different response to the throttle, but whether it's better is a judgement call.  If you snap open the throttle on slides, you can bog the motor if the revs aren't up high enough to deal with it.  The CV carbs don't raise the slide until the motor is spinning enough to use the fuel.  

I think the limiting factor of our motors is more how much can flow through the valves, not the air filter system or even the exhaust.  I don't remember where I read that, though.

[mfrias2nd]

This is a very interesting point. I'm not an aggressive rider and I don't run my bike like a race bike but the thought of better throttle response and mpg's is very interesting.

[nhmaf]

I doubt that you would see more MPGs with slide carbs.  Throttle response may be different, but I wouldn't anticipate a big change in normal road-riding performance - if you were on a racetrack it might be a different story.  I suppose if you ride up in the 5000 rpm range on the road all the time, the mikunis might be better responding - but your fuel mileage will also be piss-poor then anyhow.


The pod filters let in alot of dirt.  And water.  They look cool, but the reality is, particularly with the Bing CVs and the placement on the airheads, they don't give you any improvement in air flow, and generally do a poorer job of filtering out the grit.   Again, if you were on a track bike, no big deal, but riding roads with dirt, sand, and in the rain..

[tvrla]

I don't think any one brand of carburetor gets better mileage than any other. For best mileage, I'd think the bings are it. I've never found bings to have poor throttle response, but maybe I'm just an old fuddy duddy. My LS has the 850 kit which means it's got Mikunis. For a while there I wanted to convert back to Bings because the left side carb mixture adjusment is next to the transmission. A bear to adjust! But because of the head and intake mods, wasn't able to easily mount the bings so gave up on it.

That said, I do like the Mikunis now that I've lengthened the one side mixture adjuster so I'm not burning my hand or burning up the motor trying to adjust mixture. They're 34mm flat slides and I'm getting 50mpg or close to it pretty regularly.

[mfrias2nd]

that's great mgp! I'd like to think this guy knew what he was talking about but you never know. I would prefer to not run the uni filters on the carbs. but i'm having a hell of a time finding the right elbows to fit the R65 with the old style airbox.

[Barry]

Looking back a few years before the R45/R65 range was introduced all BMW's used slide carbs. Then they were upgraded to CV type carb's which were considered superior though more expensive so the base model R60 didn't get them. If our bikes had slide carbs we'd probably want CV's

I think with careful use at a steady cruise you can get excellent economy with properly set up slide carbs but throttle response and exceptional economy are mutually exclusive.

One of the great things about Bing CV's is they don't seem to wear out in a way that affects their performance unlike slide carbs which leak air due to wear in the slide. My experience with Amal slide carbs is they were fine when new but the idea they could still perform the same after 30 odd years and 100,000 miles is unthinkable.

[mfrias2nd]

True VERY TRUE! the grass is always greener on the other side, right? Working with my bings from the ground up gave me a deep appreciation for them and the way they work.

[tvrla]

I agree with Barry - bings just don't seem to wear out.

One of the problems with Mikunis is their independent floats. I've heard they can easily stick on, overflowing. In the two years I've had the LS, that's never been a problem, so I don't know. The Mikuni flat slides are a very simple carb just like the bings. I've heard the Keihans are a higher quality carb, and they certainly seem to sell for more.

[crocket]

Have gone into other carbs myself,but Bings seem best. My 1981 R65 has done about 160k and original carbs still giving around 55/59 MPG driven steady (4k rpm) Hardly ever need to set carbs.If needed cruises smoothly (for a boxer) at 80mph Dont think any mods needed apart from Twin brembos. Put progressive fork springs/dial-a-rides onand clamshell airbox on(for looks). Also still on ign points system.Only thing dont like is engine/exhaust noise is excessive when riding. Definately best of boxers.

[bubby-joe]

I doubt that you would see more MPGs with slide carbs.  Throttle response may be different, but I wouldn't anticipate a big change in normal road-riding performance - if you were on a racetrack it might be a different story.  I suppose if you ride up in the 5000 rpm range on the road all the time, the mikunis might be better responding - but your fuel mileage will also be piss-poor then anyhow.]

My old tired r-65 with Bing's got 38-40 MPG tops.  Lost speed on a longer hill with steady throttle position.  Around a month ago I replaced them with brand new 36mm Mikuni kit, milage went from 40 tops to 47 at excessive speeds (5000 and up) and at normal highway speeds to just under 50MPG so I'd say it's a fairly good improvement.  But to be fair the Bing's were very baddly worn and in the 200,000 KM PLUS coming very rapidly to the end of the next 100,000.  that's 4 thou less than 300,000 as of today.  And yes the engine and frame numbers match

With the differences I'd have to agree with the other old guy 36mm Mikuni's rule on my r-65 and Bings drool dribble slop burb pass gas all of that.... Oh yah and no longer idled at all.  You need not look back, just do it....you will like it.  Also of note my mikuni's are a pair of VM round slides not flat

[tvrla]

 Bubby - sounds like your old Bings needed needles, needle jets and possibly floats to get back the performance you're now getting with the mikunis. From what I've seen, the Bings don't tend to wear out, so replacing the worn parts is the more economical way to go.

I agree - I don't see how mikunis could give better mileage than the bings. The only real difference is vacuum operated slide versus cable operated. The fuel metering systems are pretty much the same otherwise. The vacuum carbs are better for touring as small changes in throttle position have little effect and that's about the biggest difference. The slide carbs are more responsive to throttle change.

[oz_johnno]

Just rebuilt my bing CV carbies, changed out the needles and the needle jets and the difference is like night and day.  bike is so much more responsive than it was.  Before you go investing a motza in new carbies, try rebuilding what you have.  The problems might just be something you can fix for a few bux.

OZ