Carby Comparo ?

Say that money was not a factor in this comparison... for grins, let's just assume that the kits are the same price...

How would the JS 32mm Motorsport Flatslide carbs compare to the Keihin 35mm FCR flatslides?

Assume that both pairs were properly jetted for a stock (for now) MK2a 850.

Comparisons in the areas of: Throttle response.. off line, roll on, throttle off? Favored RPM band? Idle control? etc...

Also in relationship to possible mild / mild + engine mods... How do they compare in their ability to "tune" upwards if the engine is developed? say a cam, 920 kit, headwork, compression change, etc...

Would the FCR be a bit more "future proof"

Opinions are respected of course, but experience here would be much more helpful

THANKS!

JD

Greetings,
I own neither, but I can tell you from experience that because the FCRs are in a "rack" that carb synch issues are easier to do, probably are more stabile, and eliminate the need for a split throttle cable.

I can not say I have much experiance with JS carbs on a Norton, but I do have a lot of experiance with FCRs on Nortons and other bikes. There is no question they are superior in every way except clearance. That is why they are nearly standard on any modern racebike. Jim

I would have thought the JS carbs are probably a better choice, unless you have an all out race bike running in a class where after-market carbs are allowed.

does not seem to address the original inquiry
not looking for choice... looking for how they compare to each other so "I" can make a choice... would like to find insight to the specific areas addressed in th original post.

i.e. What are their performance characteristics when compared to each other, not neccesarily what one may think as a the best carb.

JD

Carbonfibre said:

I would have thought the JS carbs are probably a better choice, unless you have an all out race bike running in a class where after-market carbs are allowed.

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Well an FCR flows more air and fuel, it has zero slide stick from vacuum due to a vacuum breaker plate, it has no slide or needle wear due to wheels on the trolly that carries the vacuum breaker plate and needle, it will not loose synch, it has an accelerator pump for quick responce , it certainly has the potential to make more power on the right engine and it sounds cool at idle- the vacuum breaker plates rattle like a Ducati clutch. It only has one easy to get at idle speed adjuster that controls both carbs. Tuning is different from most carbs as they don't have different slide cutaways available. Tuning is done with needle selection and one main jet and one pilot jet per carb. You may end up with a pretty good slection of needles by the time you have it spot- on but they are readily available. A crossover tube is an absolute necessity to get good low speed running.
Downsides are:
A tight fit on a Norton,
No choke or fast idle provisions so you have to pump the throttle a couple times to start it and then baby-sit it for a minute.
Weighs a bit more.

The JS carbs are smaller, lighter, and an easier fit. Probably more practical on a daily rider. They have an enrichener with fast idle for touchless cold starting.
Like any conventional slide carb they will suffer from slide and needle wear- although nothing like an Amal. They will need to be balanced periodically and they will need to be jetted slightly richer in the midrange as they don't have an accelerator pump. Jim

Thanks, good info!

Wondering if anyone could provide real world "seat of the pants" experience in the comparisons?


JD

I have the dual Keihins on my Triumph as well (2003 Bonneville t-100) they are very good and have not needed any fine tuning -except when I pulled the stock exhaust. They needed rejetting, but were very simple to work on. I would love to put a pair on my Combat, but they are just too pricy for my wallet at the moment. I haven't heard one bad review of the set from JS Motorsports so I'm more than likely going with a set of those.

You can add at least one more option to the list. Dellorto PHF36's pumpers. The installation details and tuning would be up to you but the price is about midway between the PWK's and the FCR's, and you wouldn't necessarily need to get them from Mike's. Maybe there's someone out there with experience with these?

http://www.mikesxs.net/products-42.html#products

Dellorto are good carbs, I've had them on Ducatis for a long time. They don't show wear till high mileage as long as good air cleaners are used. They are easy to tune and hold their tune ok. I'm interested in JS carbs though, from what has been said here I think I'm missing out with a single Mikuni. Graeme

rpatton said:

You can add at least one more option to the list. Dellorto PHF36's pumpers. The installation details and tuning would be up to you but the price is about midway between the PWK's and the FCR's, and you wouldn't necessarily need to get them from Mike's. Maybe there's someone out there with experience with these?

http://www.mikesxs.net/products-42.html#products

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Will chuck a Jogenson artical on tomorrow , They seem o.k. for a 80 hp Engine . 8) :wink:

Thanks for the suggestion and appreciate the contribution BUT......

would like to keep the thread focused on the original post in comparing the FCR to the JS offering.


Thanks for the understandting.

JD

rpatton said:

You can add at least one more option to the list. Dellorto PHF36's pumpers. The installation details and tuning would be up to you but the price is about midway between the PWK's and the FCR's, and you wouldn't necessarily need to get them from Mike's. Maybe there's someone out there with experience with these?

http://www.mikesxs.net/products-42.html#products

Click to expand...

rpatton said:

You can add at least one more option to the list. Dellorto PHF36's pumpers. The installation details and tuning would be up to you but the price is about midway between the PWK's and the FCR's, and you wouldn't necessarily need to get them from Mike's. Maybe there's someone out there with experience with these?

http://www.mikesxs.net/products-42.html#products

Click to expand...

those are stock on the BMW R90S and were excellent on mine, great performance and no trouble at all over many miles

Jeffdavison wrote;

comparing the FCR to the JS offering.

Click to expand...

Jeff have the replies helped you decide on what you're going to buy?

If it's the FCR, let us know what you think. I'd be interested to know, but it is a lot of money for a pair of carbs.

It's apples to oranges, but I love them on my Thruxton.