Boxerworks.com

I have a R75/5 that i tore apart this weekend (to a rolling chassis).
i definitely plan on adding the siebenrock kit on there but it got me wondering if i can keep the 32mm Bings or if i should consider 40mm Bings or delOrtos....

I like the idea of keeping the smaller Bings so the bike looks authentic (a sleeper). Can they be re-jetted to match the upgrade?

The key to the bike will be reliability. I can't have something that will require constant tinkering.

Also... will the standard 38mm exhaust work with the Sienbenrock kit?

Thanks!

-josh

Fine to keep the 32mm Bings, 32s were also fitted to 1000cc bikes, I would jet the same as a r100, you will be in the ball park.

Sorry, can't be of much help, not knowing exactly what the generic term "Siebenrock Kit" involves.
The most reliable option:
Ask Siebenrock about a rec'd carburetion.

My guess is that the 32s will work –
First, the "size" of a carb can be used to achieve desired power delivery characteristics
which is to say that a number of "sizes" will work on any given engine configuration.
The rule of thumb is that, within an acceptable range of sizes,
a smaller carb is chosen to improve mid-rpm torque delivery (acceleration)
and a larger carb will allow higher rpm and a bit more hp at the top.
The former, not the latter, will be the important gain on the street, IMO.

A 10 hp increase suggests an increase in bore diameter and possibly compression.
Without a change of porting, valve sizes, intake or exhaust plumbing
the 32s (guessing again) may require re-jetting,
or possibly just attention to the idle mixture, and needle position ...
Knowing the specifics of your "kit" and it's resulting displacement will be your best clue.
i.e. if resulting displacement is 800 cc – look to the carb & jetting of an R80 to start.

Very likely that someone here will have experience with this "kit"
and pop in to provide better / definitive info ...

All old BMWs (in fact all old machines) will require "tinkering".
This can be done up front or on the installment plan.
The effects of age, wear, and abuse trump engineering every time !

the kit takes the bike up to 1050 cc (or is it 1070).

More than one type of seibenrock kit, the standard 1000cc kit which includes new cylinders and pistons, and the 1070 kit that adds new conrods and a sportier camshaft. You should check which kits fit which bikes, as your 750 is fairly early.
40 mm carbs would make the best of the 1070 kit.

chasbmw wrote:...
40 mm carbs would make the best of the 1070 kit.

Or 36-38 mm Mikuni VMs, TMs, ... 38 mm Dellorto PHMs ...

vanzen@rockerboxer.com wrote:

chasbmw wrote:...
40 mm carbs would make the best of the 1070 kit.

Or 36-38 mm Mikuni VMs, TMs, ... 38 mm Dellorto PHMs ...

I hear good things about Mikunis, and over here in the UK a pair can be had new for around 60% of the cost of a set of Dell'Ortos.

A Mikuni (VM or TM) is just a carburetor.
As slide carbs they will have certain characteristics that are 'different' than constant velocity type Bing carbs
normally fitted on later type 247s.
Some of these characteristics might be called an advantage –
like crisper acceleration, and the absence of "diaghram lag".
Others might be termed disadvantages –
slide carbs often have difficulty transitioning from one circuit to the next – flat spots, and are altitude sensitive.
Excessively large slide carbs (esp) will 'bog' when whacked open quickly but provide more mix at WOT...

Mikuni / BMW baseline jetting chart

jjwithers wrote:the kit takes the bike up to 1050 cc (or is it 1070).

You can't put a 1070 or 1050 kit on a /5 without boring out the block openings from 97mm to 99mm. This involves stripping the block completely. Also a /5 block is not as sturdy around the front bearing web and has poorer lubrication to the mains, which makes it less desireable for a big horse power increase. Siebenrock now makes a 1000 cc kit that fits early blocks and heads (I just got one for my early R90/6). It doesn't show up on their web site but they make it. Motoren Israel and BMW Bayer also carry it. Eurotech in Cal. may also have it, but you'll need to contact Chris and ask. That will be a better option if you want to keep your original block. You may want to upgrade to /6 and up con rods though. They have a lot more strength than /5 rods.

Wow matt. Thanks for this. I had no idea. I just found out about these kits a few months ago. I wish they were available 6 years ago when i built my last /5 bike (maybe they were, but i knew nothing of them).
I think making it a 1000cc will be sufficient. I definitely want to keep my original block. I plan on doing a complete tear down of the engine and i'll look into getting some /6 connecting rods.