I may have the chance of purchasing a Mikuni HSR 40 carb at a good price, was thinking of fitting it to my '98 S1. Anybody any thoughts on this ? would it be a waste of money and should I give it a miss, am I right in thinking the HSR 42 is the pumper to have ?

Go for the 42 cos you can share the jetting advice of others...

If you get the 40 and the jets are a long way off what you need you could be forking out cash that you could have put towards a 42...

Just my 2p

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08 Specialized Langster

What is your goal with changing it out? Getting the A/F mix right? Better throttle response? Better starting? More HP?

Mikunis have really good tunability and jet changes are about 60 seconds compared to 15 minutes. Easy to get a flat A/F because they don't vent the bowl into the intake stream. Easy starting. throttle response and power are great with the right associated mods, like a race exhaust and a tall Buell oval race air cleaner. Even better with SE536 or similar cams.

But truth be told, a CV40 is capable of higher intake velocity on a near stock bike and at low and mids. I've seen way too many stock bikes lose low end power with just a Mikuni carb swap. And the CV40 gives nothing up at 7000 rpms. Drilling the slide vent out to .125-.128" will help the throttle response and starting. Basically a free mod. The only thing else needed that costs money is jets and preferably a new needle with a better taper. No more than 25 quid, if I had to guess. A CV40 will support up to 100HP. Getting a flat A/F is next to impossible but the line is not too bad. But altitude compensation is a great benefit if you travel.

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Bob Krzeszkiewicz
2012 XR1200X - 2011 Kawasaki Z1000SX - 1999 Buell X1

On my 97 M2, I run the 40 with a Buell oval race air filter and a carbon V&H SS2R. Fitted the 40 straight out the box and everything has been great. Improvements everywhere.
Did think I might have needed to re jet, but did not need to.
Just my own experience..

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Today my pet mouse Elvis died ..................He was caught in a trap
There are two types of people in this world, those that can extrapolate

Thanks for the replies guys, the bike currently sports a standard carb with a Thunderslide carb kit, a K&N Forcewinder filter and full Buell race exhaust system, oh and it has a Millennium 1250cc big bore kit. I thought as I was getting the 40 at a good price and it's pretty near new I'd give it a go. Still unsure really.

No, don't be unsure. You'll do a bit better with the Mikuni with those goodies.

I found the thunderslide to open too quickly and bog down on my RSS1200 and S2. Needed a stiffer spring but I found stretching the spring to be a cheapass get by fix. The real problem on those bikes was the ignition mapping. A SE module for the sportster got me up to 75 HP from 65.

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Bob Krzeszkiewicz
2012 XR1200X - 2011 Kawasaki Z1000SX - 1999 Buell X1

Once again thanks for the input, have decided to go for it and buy the Mikuni 40.