bike carbs?

[craig]

HI

im thinking off putting bike carbs on my HC 1256, currently running a nikki twin choke. im thinking along the lines of a gsxr set of carbs, running two to each inlet of the head.

has any one one this yet? any info will be great.

[lambaj]

My pennies worth:

GSXR = 1100 ish cc ?? OK, bike is 4 pot, say 12000rpm, 1 carb barrel per cylinder. 1100cc divided by 4 = 275cc per rev, but its a 4 stroke, so half this again to 137.5cc per rev. Times 12000rpm = 1650 litres per min airflow.
Viva engine - 1256cc, 6500rpm max, 2 carbs per port, two cylinders per port: 1256cc divided by 4 = 314cc per rev, 4 stroke = 157cc per rev but 2 cylinders per port, so double up back to 314cc. 6500rpm = 2041litres per min. But, remember viva is old fashioned non cross flow 2 valves per cylinder head design, so you can assume a near 80% cylinder filling efficiency verses the 16 valve full crossflow bike engine with lairy cam = near 100% cylinder filling efficiency.
2041 * 80% = 1633 litres per minute which is very close to the original air flow requirement the carbs were built for.

So, roughly speaking you have the same carbs that are set up to flow enough air for a 1100 at 12000rpm running on an engine of similar capacity with significantly restricted breathing doing only 6500rpm (for 5% of the time). If you use two carbs per port I think you will end up using a lot of fuel for a seriously compromised setup that will be largly undriveable.
If you were to use 1 carb per port on the ohv, then that would be more like it, but two would be a waste of time, effort and fuel!

Thats my thoughts.....

Tony (HA21)

[pbottomley]

Tony for god's sake take a breath LOL

[Po79]

Hi Craig,

It sounds like a nice idea and would look pretty god too but I can't see it working in any shape or form.

Your engine is just far to small and inefficient to run bike carbs, the ports are to small to allow enough flow and so on and so forth.

You would also need to keep playing with jet sizes and have constant access to a rolling road until you get it set up right, if ever.

However nothing is impossible and some times things that shouldn't work just do, and once set up correctly bike carbs are far more reliable than most car carbs.

If you have the spare time and money (the parts are cheap as chips) then why not just have a play around after all the most fun comes from tinkering with our cars anyway

If you are serious though what would really pay and help heaps would be to get the head reworked with larger inlet valves (think the exhausts are ok?) and some heavy porting and polishing, maybe a mild cam, this alone would pay for itself with or without bike carbs then if the bike carbs work too you'll have a fast little number.

Po.

[lambaj]

Sorry Paul ! Could have said less, but got carried away trying to explain my reasoning ! Po's thoughts are valid about having a play, but if you just want a fit and work solution, there are far simpler options,

Would love to see the results though !
Tony

[rtlkyuubi]

im not using a viva engine but bike carbs are not as hard to setup as you think.

Now, obviously you do need to go to a rolling road ect to get them set up to work perfectly but you dont need to.

mine is a 2000cc 8v on 600cc carbs with the main jets drilled to 150:

[Po79]

That car of yours sound sweet.

I've got a couple sets of ZX7R carbs to put on my Pinto power viva when I get the chance, but I was told to use 1.7mm jets.

I'm guessing different carbs and engine combinations = different jets.

I know small engines can take bike carbs but I have only seen them on more modern 1300cc engines which compared the the smaller Viva engines are far better in very respect which is why I would look at tuning the Viva engine first to sqeeze every bit of power from it so you could make the most of the carbs.

It would be intresting to see how they do work on a stock engine though.

Po.

[craig]

HI Guys thanks for the answers,

i was just toying with the idea. only being 18 theres is not alot i can get insured on thats even slightly fast. so moding is the answer. i havea complete engine at home that i could have balenced, cam sorted, ect.

i just want something fast enough to have fun

[gtwannabe]

i know its an older post but here goes anyway ..there are advantages to fitting bike carbs to most engines ,you would only need 1 set of 4 carbs and the art of the job is in the inlet manifold ,which if you get it right could give a significant gain in performance .you would also have to get the ignition timing optimised to see a performance advantage .i have done 2 such conversions now with some rather stunning results .if you go the bike carb route and need any advice i am not too far away from you ,you are welcome to come up and i will give you a few pointers

[grandas_shed]

do fitting bike carbs make any difference to the fuel consumtion?