Smoothbore carbs

Here's one you don't see very often. Keihin CRS 33mm smoothbore carbs with looong torquey manifolds. "Pulls like a train".



Legal for Classic racing (not flatlsides). Expensive and a tight fit, but you would think there would be more on the tracks.

Les's road cafe racer below.

That is a superb Seeley - needs the 6 speed box.

A friend is building a 750 Norton now and using the Keihin FCR 35mm flat slides, they sure are pretty.

Yes they do look good. The CRS carbs pictured start at 31 & 33mm and go up from there. They are legal on the classic road racing tracks because they are round slides. Both style of Keihins are smooth bore. The CRS round slides only need one throttle cable.

The FCRs are a flatslide carb and they require both an opening and closing throttle cable to keep from sticking open. You can hear the slide rattle a bit on this style carb (not to be confused with pinking). Unfortunately, 35mm flatslides are not legal for classic racing but they work well on hopped up and large bore street bikes.

Prices are going up on these already expensive Keihins. Here's a side view for those who want to fit their own.

A couple of small addendums to Jims comments:

Actually you don't need the 'push/pull' cables on the FCRs, they work fine with one cable.

The rattling slide issue concerned me before I bought mine, but I can assure folk that, although it is possibly true that the slides rattle, I think this is only an issue for non Norton riding folk... as it is impossible to hear any slide rattling above the exhaust pipe and engine noise of a running Commando engine !

LMAO

I've got a pair of these in 37 mm that we used on a serious SRX-6 singles racer. I'm thinking I might try them on the 1007 street Commando. If so, I'll post the details.

Ken

lcrken said:

I've got a pair of these in 37 mm that we used on a serious SRX-6 singles racer. I'm thinking I might try them on the 1007 street Commando. If so, I'll post the details.

Ken

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Now you're just showing off ...

Fast Eddie said:

A couple of small addendums to Jims comments:

Actually you don't need the 'push/pull' cables on the FCRs, they work fine with one cable.

The rattling slide issue concerned me before I bought mine, but I can assure folk that, although it is possibly true that the slides rattle, I think this is only an issue for non Norton riding folk... as it is impossible to hear any slide rattling above the exhaust pipe and engine noise of a running Commando engine !

Click to expand...

Is that what you use? One cable? Sudco (Keihin supplier) told me that two were required to prevent sticking. Is this paranoid overkill? They wouldn't even talk about one cable for fear of Lawsuits.

I have a friend with a pumped up modern Triumph twin with FCRs and he thought he was getting pinging until he let me take it for a ride and I told him it was only the carbs. They certainly worked well on the big bore, ported, Thruxton Triumph - as in perfection.

I am sure that if your carb(s) is/are operating from a bell crank, you should have a push/pull system. If that bell crank spring pops, dislodges or the like, it will not go good for you. Coiled springs internally on a round or flat side is much different.

pete.v said:

I am sure that if your carb(s) is/are operating from a bell crank, you should have a push/pull system. If that bell crank spring pops, dislodges or the like, it will not go good for you. Coiled springs internally on a round or flat side is much different.

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Internal springs are not fool proof either. I saw someone crash a single at a track when the carb top came loose and it went WOT into a turn.

The spring pressure on the Keihin smoothbores can be a little stiff. I modify the twist grips to ease that up a bit. I also tried drilling a new spring catch hole for the carbs on my test bike. At first I though there was a hesitation to return to idle but now it feels fine. The new hole is 2nd from the top of the bellcrank.