Carb Theory - Big Bore Kits and Jetting

[Phil-UK]

Hi

I've almost finished running in a CB72 fitted with the Rocky +3mm overbore pistons and the next step will be to fine tune the carburation particularly at 3/4 to full throttle. The carbs are the standard 22mm items and currently have #100 main jets.

My initial thoughts were that with a 10% increase in capacity ( 250cc -> 276cc ) I would need bigger main jets to maintain the air:fuel ratio. However, doing a bit of trawling around on the Net it seems people are saying that if you leave the carb size alone you will actually need a smaller main jet as you have more suck.

To counter that argument I have a 1971 issue of Motorcycle Sport Quarterly with an article takng a CB77 out to 350cc using the Webco kit, and that says Webco recommend going up @20% on the main jet.

Does anyone have first hand knowledge of what Webco or Rocky recommended for jetting, or knowledge of current carb theory to say which approach is correct.

I know I can just change jets and keep checking plugs but I would like to know what the science says.

thanks

Phil
( in a very soggy south of England )

[G-Man]

Hi Phil

I was talking to Ray Weissbrot about this and he says that his Read Titan 350cc big bore kit came with 110 main jets using the standard carbs.

My own 1961 CB72 came with a 350 kit and the mains were still 90s. Already I can provide conflicting evidence...... :-)

Just for interest, as the Keihin mains are based on jet diameter, rather than flow rate (Mikuni and Amal) then a 10% increase in jet size gives more than a 10% increase in flow.....

You comment about more 'suck' from a bigger engine is interesting. Assuming that the carb is there to meter fuel into what ever air passes through then the jet might stay the same to give the same air-fuel ratio. As with all Engineering problems, the answer depends upon what your assumptions are....

On balance I think that I might go to 100 or 105s to kick off with.....



G

[rrietman]

Phil; I built my CB72 about 4 years ago with those same +3mm pistons. after thinking about carb jetting waaay too much, I decided to start with bone stock settings and go from there. it ran fine like that. pulls strong, revs all day, and good plug chops. It still has stock jets in stock carbs, starts with one kick and runs like new. your results may vary, but worth a try.
Randy

[Phil-UK]

Thanks Graham and Randy - I've left the main jets at #100 for now and will see how it goes.

I guess my question is more theoretical than anything else. I can see justification for both theories but don't know enough about current carb "science" to figure out which is correct.
Most of the online "carb" resources cover how the carb itself works, rather than what happens when you depart from standard ... it really is "suck it and see".

Perhaps someone will come up with more info.

cheers

Phil

[leonvjames]

A long long time ago when I fitted a Read Titan 350 kit to my Cb77 there were bigger jets supplied, no numbers though. I had a problem with surging at part throttle, (where it spends most of its time) and had to build up the throttle slides cut away to cure the problem.
Leon, long time owner.

[G-Man]

Phil

Just a philosophical point to ponder.

The carb is designed to meter fuel into a moving air stream.

How does the carb 'know' if it's delivering fuel to a 247cc engine at 5000 rpm or a 275cc engine turning over at 4500rpm?

G

[brewsky]

More theoretical pondering........

At any given RPM, the larger displacement will be moving more air thru the same carb opening, which implies a faster air stream.

Some observations have shown that the faster the airstream across a given jet size, the leaner the mixture.

This would imply richer jetting may be required?