R75 with siebenrock kit. What carb to use?

[Garnet]

I agree with Matt that the 1000cc kit: http://www.motoren-israel.com/product_i ... -1975.html is the way to go on a /5.

Somewhere there is a claim of 64hp with that kit with stock heads and stock sized carbs. That is pretty easy bolt on power.

Also avialable here. http://www.bmwbayer.de/en/POWERKIT-97mm ... -R90S.html

[Roy Gavin]

These kits will only give the gains stated if the squish clearance is correct, and it isn't a given that this will happen on every bike they are fitted too.

Unless things have changed recently there are no fitting instructions, so you have no way of checking apart from running the bike on a dyno.

Nice to someone making oversize squish band pistons for the early bikes, they were getting hard to get.

[mattcfish]

These kits will only give the gains stated if the squish clearance is correct, and it isn't a given that this will happen on every bike they are fitted too.

Unless things have changed recently there are no fitting instructions, so you have no way of checking apart from running the bike on a dyno.

Nice to someone making oversize squish band pistons for the early bikes, they were getting hard to get.

It will be way better than a 750cc. The Siebenrock pistons give 9.5:1 compression on pre 1976 R90/6 combustion chambers. I beleive R75/5 chambers are the same, they just have smaller valves. The pistons have a small step cut around the squish band to give clearance for the larger bore.
They are mounted to the block without any base gasket, just sealer and four orings (sold seperately) for the upper stud oil feeds.

[Roy Gavin]

They claim 9.5 to 1 , but those that check what CR they actually achieve usually find different.

The Wossner pistons in the kits are reputed not the match the Omega pistons the kits a originally used - I wasnt particularly impressed with the detail on supposedly high comp pistons I fitted to my R100GS, and it doesnt fell like a high comp motor. And I have one to compare.

But I didnt check the actual CR and in the absence of any guidance from MI set the clearance to 80 thou as detailed in the 336 cam installation instructions, so I shouldn't really be to negative.

[mattcfish]

They claim 9.5 to 1 , but those that check what CR they actually achieve usually find different.

The Wossner pistons in the kits are reputed not the match the Omega pistons the kits a originally used - I wasnt particularly impressed with the detail on supposedly high comp pistons I fitted to my R100GS, and it doesnt fell like a high comp motor. And I have one to compare.

But I didnt check the actual CR and in the absence of any guidance from MI set the clearance to 80 thou as detailed in the 336 cam installation instructions, so I shouldn't really be to negative.

Which clearance are you refering to? Valve pockets to valve? Squish band? How did you"set"it?
I just had my intake pockets deepened .028" (inch). I'm using a later advanced 336 cam. I plan to mill the heads until minimum valve clearance and/or squish band gap is achieved.

[Roy Gavin]

In the 336 cam installation instructions BMW also give their recommendation for squish clearance, and it is 2mm, or 80 thou.
I started from that end , runs fine better than before, it just doesnt have the snap of a high comp motor.

Udo Gietl confirmed that on his full house race motor anything less than 70 thou and his pistons hit the head, but his experience is probably not typical.

However, a road bike will have a carbon build up which will eventually tend to equal things out.

I have read of people who claim the 35 thou works really well, so if you think that BMW and Udo dont know what they are talking about you can always start from 35 thou and work outwards.

The PO had replacement valve seats fitted, and they were too far into the head, with a 40 thou step at the junction of the seat and the head. By the time that was blended out the heads needed re machining and skimming.

I set the squish to 80 thou with a 30 thou spacer under the barrels, so I am not done yet - let me know how you get on starting from the other end--------.