41mm Mikunis - will they work on a bored 90S?

[The Bigfella]

Can anyone tell me whether 41mm Mikuni TX Pro Series Flat Slide carbies would suit a '74 R90S engine that has been bored to take 100S pistons.... please?

Ian

[hal]

Others with more knowledge will chime in, but what I've understood - they're way too big. Hear people talk about 34 mm.

[The Bigfella]

The original carbies for 900cc were Dellorto 38mm pumpers. With an extra 100cc ????

[DanielMc]

Others with more knowledge will chime in, but what I've understood - they're way too big. Hear people talk about 34 mm.

Looked into buying some Mikunis for a R100S from this guy here in the UK - http://www.allensperformance.co.uk/prod ... ts-vm.html

He recommended 34mm or 36mm – the 36mm would help gas flow by reducing the step between carb and spigot on the 70 bhp big valve engine although he believed that the 34mm is probably a better carb for most eventualities.

It's not all about engine size - other stuff like inlet valve size, gas flow, cam duration and how the bike's going to be used are large determinants of what size carb is going to be ideal - and bear in mind that that the R90S heads have smaller inlet valves than the 70 bhp 1000cc motor.

[Roy Gavin]

I would check on the price of needles , needle jets, slides and the rest of the jets for these things before you went too far - at Oz prices even if you went straight to the correct setting the replacements would cost at least as much as a pair of a of new preset round carbs which are around $250- on ebay right now.

And they wont be any better unless the are spot on.

Add a couple of hrs dyno time to dial the flat slides in and you wont be far short of $1,000.00 if you pay $400- for the carbs.

Nice bit of bling though.

A pair of 32 mm carbs flows enough for around 90 HP, which should be sufficient for most road bikes.

[chasbmw]

I am using 38mm dells on a plugged ported 1070cc bike, they work fine with close to stock jetting. 41mm does sound a bit big but I think that lots of people used 40mm dells in the past. Not sure of the results!

[Roy Gavin]

I would think that that to get any benefit from 41 mm carbs you would want some pretty trick inlet stubs and inlet porting, to keep the intake velocity up.

I have some recollection that Supershaft claimed they made some trick items in his SJ days.

A little basic matching and cleaning up made a reasonable difference on my bikes, which makes me suspect that with a bit expertise and equipment good gains could be made without a lot of expense.

But flat slides are a race carb, and 41 mm is a race size, and my bikes are road bikes, so I have no expertise on the subject.

However, I ride with both sizes and the 32s on my 1000 G/S work better for me than the 40s on my GS.

[vanzen]

Given a relatively stock R90 or R100 (large valve) engine.
I will safely assume that 38mm, as per BMW's choice, will be something of an optimum
to produce a bit more top-end power
while retaining much of the (expected) low RPM tune-ability & tractability.
Keeping in mind that the R90S w/ 38mm Dels –
was BMW's street-going SPORT / PERFORMANCE model ...

Carb sizes larger than 38mm will tend to skew that "balance"
even further to the advantage of top-end power delivery.
This may be a worthwhile concession on a race machine
that does not see traffic congestion, traffic lights, parking lots ... or speed limits,
and race engines built to pack more fuel-mix and spin beyond the stock redline
as BMW Corp never intended.

Back in the day, CC Performance sold Mikuni-RS 40mm carbs, and 41.5 Mikuni Racing carbs for BMWs –
Purchase of either set required a call to CCP "to discuss suitable application".
A carb sized to flow more fuel than the engine is able to suck –
will have performance suffering on both ends.

[chasbmw]

Back in the day, people often fitted an after market head spigot for R90/s's, the purpose of which was to improve low speed running, it was done by reducing the internal diameter of the spigot by a tad thus speeding up airflow.

I was told that it worked.

Mind you on the other hand 40mm bings were used on a wide variety of the more sporting BMW variants, but maybe CV carbs are more adaptable than slides?

[Roy Gavin]

In my experience CVs are a bit self compensating and will usually run reasonably well , even they are a little out of tune or oversized.
You dont have to be far out with a Amal round slide to get a stumble , flat spots, etc. One groove out and they will hardly run on the needle.