I am just learning about carbs, I kind of understand the theory, but why do the FCR, HSR, etc. give a reported 10 hp over stock? I know they have a bigger bore and are more adjustable to fine tune the fuel delivery, and that they are not vacuum operated like the CVs. Would they be a logical power progression after an airbox removal and good exhaust, before cracking open the motor? Oh, another question... I assume that without the proper pistons, oversized valves can not be fitted. Is that true? Thanks.

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Brian

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The primary difference in why some carbs make more power than others is what obstructions remain in the carbs when those carbs are fully opened.

In the CV carbs (constant velocity) our bikes come with a throttle plate and it's shaft will remain, though the plate will be turned sideways to minimize restriction. In most non-CV carbs the throttle cable pulls a slide up and out of the venturi, leaving essentially nothing in the way of air flow. For those carbs the difference is in just how cleanly the slide tucks out of the way and what other small protrusions, or more likely depression, remain within the venturi to disrupt smooth flow.

There are many other differences, but the ones I brought up are the ones that impart maximum flow, and therefore, maximum horse power.

From reports of those who have swapped their CV's for non-CV performance carbs, the results seem cost effective in comparison to opening your engine. If you decide to then open your engine afterwards, you'll have the right carbs for the job already mounted.

Oversized valves will not fit with stock pistons.
If you want these bad boys:



You're going to need aftermarket pistons.

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Bonnie cafefighter!

most of the people running those have other mods. on a stock motor its probably overkill. throttle response os usually better but to need all the extra air/fuel you really need motor work.the bigger the carb and less restriction also means slower air speed entering the motor

Quote:

On 2007-03-30 22:01, SPRINGER1978 wrote: most of the people running those have other mods. on a stock motor its probably overkill. throttle response os usually better but to need all the extra air/fuel you really need motor work.the bigger the carb and less restriction also means slower air speed entering the motor

I beg to differ, I picked up 10hp with my HSR42's over the CVK's.

Granted I have the normal mods on my stock 790, K&N pods and exhaust, I can assure you it's not overkill if you are out to make power.

Greg

Thanks for the responses. Looks like the discussion of whether carbs alone will pick you up power is still up for debate. I figured that was the answer about the valves, just checking... figures sweat would know... good luck with that, BTW.

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Brian

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No debate, carbs gain you power....period.

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Hated By All....cause thats just how it is.

A pair of 39mm Keihin FCR Flatslide carbs have just been fitted to my Thruxton. I pick up the bike on Wednesday from the guy who fitted them, and will be dropping it off for dyno runs, and having the flywheel lightened. I'll post the dyno results once I have them. Currently 71 rear wheel BHP - Hoping to be close to 80 BHP with the new carbs.

This is where the carbs were fitted:

www.brspecialtuning.co.uk/Keihin.htm

They will now be marketing a kit for the Bonnie/Thruxton range. UK OWNERS: What this space for their price.


[ This message was edited by: robnobrakes on 2007-04-01 01:32 ]

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Rob
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Quote:

On 2007-04-01 00:01, SCCTrim wrote: No debate, carbs gain you power....period.

Exactly. :upthumb:

There's no doubt flatslides make better power than CV (for the same size), but Springer makes a very good point. Simply sticking a bigger carb on a bike isn't necessarily going to make more power, and might cost you some. For every engine and desired power curve, there is an optimal size carb.

"The key to choosing the proper carburetor size is velocity – the same theory we used to choose the proper intake. It’s even more critical here since the carburetor supplies fuel to the engine by “measuring” the pressure of the airstream as it passes through the carburetor’s throttle bores. If too large a carburetor is fitted, a false pressure signal is generated and fuel delivery is imprecise which results in a poor running engine. "-O'neill Williams

Intake airstream velocity needs to stay high whether your engine is a lawnmower or dragster.

"But higher air flow numbers do not necessarily translate into more power, as many in the engine development field (including yours truly) have discovered. Ford's engineers were then vastly ignorant of the world beyond Michigan's borders. They had no idea Harry Weslake and Wally Hassan (who created the very successful Coventry-Climax racing engines) had learned years before not to take too literally what the flow bench said. They were narrowing intake ports to provide nominal gas speeds in the range of 350 to 400 feet-second, making good use of the fact that kinetic energy packing air into the cylinders increases with the square of it's velocity."

Michael