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Keihin CR29s or Mikuni RS34s?

CafeAddict

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Hi,
New to the forum, but have done a couple of rebuilds before, so not a complete newbie. I have three projects at present; Cx500 Cafe, Thruxton 2008 Cafe and now a 1975 CB750. The engine number suggests it is a K5, but the livery suggests it is a K2 as it doesn't have the K5 decals, etc.

The CB750 is in very 'original' condition (even has original owners manual under the seat) - only one owner and due to him aging and passing it on to other family members has not been running for approx 5 years. It has 30,000km = 20,000 miles. It does have a 4n1 exhaust, although I am not sure of the brand. If anyone recognises it in the pic, I would be pleased to find out....

I did get it started, but wouldn't idle and one carb was leaking fuel out of the bowl. So the carbs need a rebuild. As it will cost me approx A$150 plus a lot of time to rebuild the original carbs, I am considering upgrading to aftermarket carbs to get the 10% to 20% performance improvement, etc, etc.

So, I am tossing up between the Mikuni RS34 and Keihin CR29. Although the bike is stock 750cc at present, I will want to install a big bore kit at a later date to either 836cc (most likely) or possibly to 915cc. So I want to make sure that if possible, I buy carburettors now that will work well with both the current 750cc engine as well as a future big bore kit. So I am not replacing the carburettors in 2 years time.

Does anyone have any experience to enable some comparison between the two carbs? For example, I heard the Mikunis have some binding on a trailing throttle? I also heard that CR29s are actually faster than CR31s, especially for stock engine and road or non-race applications.

I have seen some other posts, but typically low on detail and without any comparison. Some just like the Keihins, while others like the Mikunis. One big benefit for me is the Mikunis are much cheaper, which makes a bigger difference in A$. But i gather the Mikunis have a rectangular body, so don't retain the 'stock look'.

Does anyone have any other recommendations or advice?

Many thanks

IMG_5049.jpg
 
I am no expert on the SOHC but I suspect the Mikunis will likely be a bit too big until you get bigger motor with more overhead on it. They can be too big on the later DOHC 750 for one, not working better until at least 900 or bigger.

Dirtdigger, a very common poster here who has built hotrod SOHC in a lot of different ways will likely be along to modify that as to his own experience.
 
I would rebuild the stock carbs till after you do the engine work. Around here the mikunis are $750 and cr's are over $1000. In my eyes the mikuni's will be too big unless you go to at least 900cc but people do run them on stock motors....probably not the best running. The CR's do look more like stock and available in several sizes. I run the CR's on my bikes, they seem to work well and very easy to tune. If you go 836cc then CR29 would be about right and could possible run decent on stock size motor. If you went 915cc I would almost be inclined to go with CR31's but the 29's would still work well especially if you didnt do much for head work and a conservative camshaft. You need to decide what direction you want to go with the engine size, 836 is the least expensive, if you go 915 you seriously need to look into upgraded rods. Both sizes will greatly benefit from head porting work and an appropriate camshaft. The head is where the HP is in the 750. 836 wont get you a whole lot of hp without head and cam and the 915 with a stock head would be less then spectacular. Lot of things to look into when you go to upsizing the 750, need to consider upgrading several areas internally to handle the extra power.

That exhaust kind of looks like a Kerker brand.
 
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