32mm vs 40mm Bing

[goblush]

Hello!

I just moved to Germany from the US and brought my R 100 GS/PD Classic with me.

I just dropped it off at the local BMW Niederlassung (who will handling getting it through the TuV, not something I expect much problems with), but the chief mechanic was less than impressed with my 32mm Bings that come in US models. He thought that I had less HP because of them and might want to consider mounting 40s instead. I've found a few pairs for sale on ebay.de for reasonable prices, but I wanted to ask what the consensus opinion of doing that might be.

Thanks!

[Slope'r]

From what I've read its a good upgrade but more knowledgeable inmates than me will probably chime in.

[Rob Farmer]

I had the US bike that was imported to the UK by Givitsum (of this hamlet) obviously it was fitted with 32mm Bings. My current 100GS PD has 40mm's I think the 32mm had better midrange acceleration. If I was you I'd keep it just as it is but ditch the air system if you can. To get the most out of the 40mm carbs you will need higher compression.

[Airhead Wrangler]

Snowbum mentions that switching up to 40mm carbs is not necessarily an upgrade as far as power is concerned. The 32mm bings maintain higher velocity through their throat and thus mixing of fuel and air is more complete leading to better combustion. For more info refer to snowbums intake and exhaust tuning article:
http://bmwmotorcycletech.info/InExTuning.htm

[Rob Farmer]

Originally Posted by Airhead Wrangler

http://bmwmotorcycletech.info/InExTuning.htm

I've just lost the will to live. Does anybody ever read articles like that?

[bgoodsoil]

uh....I read it I get very motivated to read stuff like that when I'm supposed to be doing my homework.

it doesn't really say anything about the carbs though. Here's what Kevin Cameron says about it:

Like water spraying from a hose pipe versus dumping from a 4 foot sewer--smaller aperture may flow less overall but what it has is moving faster.

at low RPMs the 32 gets better fill and better power but at high RPMs it doesn't have the volume so the 40mm wins out on peak WFO horsepower.

So, according to Kevin Cameron, that mechanic is right.

[lkchris]

BMW has traditionally matched the carb size to the valve size, and most R100 engines that came with 40s also came with cylinder heads with larger valves.

This was most clearly demonstrated in 1977-78 when R100/7 had 32 mm carbs and smaller valves than the same-year R100S and R100RS that came with 40mm carbs.

Understand that putting on larger carbs is NOT the same thing as supercharging. Given natural aspiration, the engine will continue to injest the SAME air/fuel until if/when valve sizes are correspondingly increased. "Restriction" is an advertising hype term only.

[goblush]

Thanks to all the responders!

I think I will keep things as they are. Of course, I do believe I will take his advice on my next tire change. He says the Michelin OEM tires are lousy on wet streets, and we do have wet streets over here from time to time. So, it'll be Metzelers next time around.

[Readymeal]

Originally Posted by goblush

Thanks to all the responders!

I think I will keep things as they are. Of course, I do believe I will take his advice on my next tire change. He says the Michelin OEM tires are lousy on wet streets, and we do have wet streets over here from time to time. So, it'll be Metzelers next time around.

And i m sure a French mechanic would have said the opposite . At least both of them are cheaper in Europe. You ll enjoy riding around.

[AntonLargiader]

For street use the 40s are probably better. I have them on my R100R (same motor, when stock) and my impression is that the 40s are better on top and the 32s are definitely better on the bottom. However, a well-tuned GS motor with 32s can still do OK. There are a lot of little variables involved.

How are you going to be riding?

[Beemerboff]

The Bings are CV Carbs , the CV stands for CONSTANT VELOCITY, so they are not too sensitive to oversizing.

I think my 40 mm carbed bike responds better to the throttle than the 32 mm carbed 900, but that may just be down to better fine tuning, or the lighter flywheel.

32 mm carbs will flow enough for at least 50 hp per carb so no reason to go to them for top end power on most bikes.

[krehmkej]

Ah memories. In 1988 I moved to Germany shipping my new California-spec 1988 R80. Went through TüV. They said the CA emission carbs were not allowed. I rode over to the Bonn Niederlassung shop (full of green cop bikes). The shop foreman just pulled the carbs off a cop bike and installed them on mine. Over to TüV. Inspected. All good. Back to Niederlassung. Swapped my carbs back and that was the end of the story. They didn't even charge me, IIRC.

I switched to '84 heads on my R100/7 that had 32s on them. I then realized that the intake ports were too large for the carbs. I called Luftmeister to see if they had an adapter and spoke with Matt Capri. He said to not do it and those heads really needed the 40s. He happened to have a set on the shelf (Mikunis were the rage of the day) so I bought and installed them. I would guesstimate a 5 hp gain from what I had.

[chasbmw]

If you are going down the 40mm carb route, try and get the later 40mm bings as i am told that they are 'better' than the earlier versions that would have been fitted to many BMWs up to '84

[goblush]

It is running fine (usually), but now is missing/bogging down when accellerating from about 60 mph on up. It doesn't last long, and doesn't always happen, and then speeds on up, but it is definately not normal. I am taking it back to the Frankfurt BMW Niederlassung, but I wondering if anyone had some ideas on what might be causing it. This is after the mechanic supposedly cleaned/adjusted/synced the carbs (after the bike sat for almost a year with some Stabiled Texas gasoline). The gasoline has been diluted somewhat with some good German juice, but is not completely out of the system.

Thanks for any input!

Gary