I put 40mm carbs on my R100R and I have not had any drop in my mileage, at slower speeds.
I does have more power and having more power I use the extra power, then my mileage goes down.
John

Oops, other way round: I had meant to say the 40mm carb will offer a BETTER performance at higher revs.

Hans

A couple of points,

for travelling you need good low to mid range power, I suggest that you will not be going more than 120KPh that often.

Dual plugs will give you. better fuel range, smoothnes and easier starting. The seibenrock conversion makes some its extra power from a higher compresion ratio. I would suggest that dual plugs would be needed on the seibenrock kit outside of europe. Mind you my dual plugged, gas flowed r100 Cs engines uses approx 15% less gas than a standard r100r engine.

Charles

I have a R100GS with 40 mm carbs. Can I reduce fuel consumption by installing smaller carbs, like from an R80?

Seems there is consensus that dual plugs increase mileage. From what i have read in the German Boxer Forum most people seem to burn 6 to 7 l/100km, which is very poor.

I have thought about this for my 1985 r80rt..originally i thought to use a sibenrock kit... note the higher compression ratio does not result in an equally higher compression pressure because there is restriction of the air intake using the original head and carbs. I will probably use 800cc replacement barrels and pistons and keep the 8.2 cr.. But... using a siebenrock kit with the cr reducing shim would give more low down torque if not much more hp. this would make the bike easier to ride at normal speeds. and smoother. you could always then remove the shims after your trip.

Alibabas questions:
1) Siebenrock have a “power-kit” (new cylinders and pistons) which increases the power with 20% and torque with 30%.
Is there anybody who has tried it?

2) Experiences on low-octane fuel? (Compression is increased to 9.5)

3) There is also an “enduro-camshaft”, 296°, any thoughts?

On The first question:
Been running a Siebenrock replacement set for about 6000km. Friend of mine have bought a R80 with the cc-up-grade kit.
Our experience is that the light-weight pistons gives a very smooth and rev-willing engine running. Power is said to be the same as BMW stock, but seems to give a touch more on our bikes.
Since it is cheaper to buy the Siebenrock kit than factory replacement barrels and pistons - I would recommend considering the "7-stones" option.

On the second question:
Low octane is a problem. Do not underestimate this. The 9.5:1 compression ratio with non-tuned heads is a no-no!
You can get far by grinding down all edges instide the combustion room, i.e. the head and the piston head - surfaces must be "smooth as a babys bottom" ! This is paramount - all sharper edges results in hotspots; if you see to that there are none there will be none.
However, there are two different options to cope with low-octane fuel:
a) Egyptian Police Special Edition of the R80
b) Dual-plugging
I have by misstake driven a couple of miles on 80% keroseen... at that time I had about 10.3:1 and dual-plugging. Top speed was 75-80km/h before the detonation sounds got too awfull... I strongly advice against testing it.

On the third question:
I have asked for Dyno-tests on the different cams. Surprice! Still waiting for replies...
Why do you want the 296' cam ? The 320' and 324' is said to be the best cam. I have the stock 308' cams.

Have you changed the gearing in the gearbox and choosen the best final-drive for your driving ?
Wüdo/HPN 5th and 1st sprockets is what I install in gearboxes right away when I get a new bike.

Been running dual-plugging since 1992. This last month, august 2008, is the first time ever that I have had the pleasure of driving the dual-plugging working as it is supposed to... see there is a bit of controversy on how this tuning should be done... I've read-up, been in contact with the "gurus", really done my homework, but it still has not worked... thing is it is very simple and *** complicated at the same time.
Installing a second plug will creat a fast-brun. The burn-time will be shortened by about 40%, this means that the timing of the ignition must be adjusted accordingly... and it is here that the problems start... If you get a ready-kit, e.g. Q-Tech Homepage , it is one thing, but if you start dabbling on your own (as I have done), it gets really complicated. Since you have an R80GS basic you may need to go with Daniel Dicke: Doppelzündung or OMEGA Electronic Ignition BMW repair modification restoration

Let us know what it is you are looking for in performance - makes it easier to recommend solutions.

The request was posted 3.5 years ago, a lot has happened since then.

I installed the Siebenrock-kit, ported the tops and installed a 296° camshaft and have used this setup for about 30kkm.
The result was stunning! Low rev torque has increased a lot and at 3500 rpm it is increased by 75% which was exactly what I was looking for.

I have not (yet) had any problems with low octane petrol but so far I have used mostly high octane.
This bike will not be equipped with double-plugging; if I get problems I will solve it in another way.
The gearing is original (for R80GS) except that the bike is fitted with an 18” wheel.

Great that you got what you were looking for -:)

Sorry I did not pay attention to the dates on the entries.

What did you do to the inside of the heads and top of the pistons?

Thanks for the feedback. I will maybe rethink which way i will go, although I very much doubt my next bike will be a BMW.