Japan Meets Britain

[Vince Lupo]

Hi Vince,

I made an exel sheet for comparing the four speed to the five speed, remember ?
If you want it to see what happens when changing the secondary drive, it's yours.

Jensen

Jensen -- if you have the specs on secondary drive changes, I'd love to see it.

But very stupid question here -- I've heard that going up one tooth on the front sprocket is the equivalent of going down 3 teeth on the rear -- is that correct? Right now the bike is 15 front and 30 rear. I have a 16 tooth front sprocket that I have not yet tried (it is for a 530 chain, so if I wanted to try it I'd have to machine it down for a 520 chain). If I put the 16 tooth sprocket on the front, and went back to the 32 on the rear, would they basically cancel each other out, or does changing sprocket sizes front versus rear make a difference in terms of the bike's responsiveness?

[Hotshoe]

But very stupid question here -- I've heard that going up one tooth on the front sprocket is the equivalent of going down 3 teeth on the rear -- is that correct? Right now the bike is 15 front and 30 rear. I have a 16 tooth front sprocket that I have not yet tried (it is for a 530 chain, so if I wanted to try it I'd have to machine it down for a 520 chain). If I put the 16 tooth sprocket on the front, and went back to the 32 on the rear, would they basically cancel each other out, or does changing sprocket sizes front versus rear make a difference in terms of the bike's responsiveness?[/quote]

Vince,
That's correct, 1 tooth up on the counter shaft sprocket equals 3 down on the wheel sprocket, I used to keep a selection of counter shaft spockets in my tool box in case I had to fine tune the gearing on my racers at a particular track.
In terms of responsiveness, it shouldn't make any difference if you change front or rear sprockets.... it's the ratio that controls that but my opinion is that generally larger sprockets are better because it seemed like I threw my chain more often with smaller dia sprockets, not sure if that's a valid observation or not.

[ricksd]

Vince,
Your hunch is right- the 32/16 combination is the same as the 30/15, either will give you a 2:1 final drive ratio, 2 revolutions of the engine sprocket equal 1 revolution of the wheel sprocket, or wheel rpm is half of the output shaft rpm. To get the rear wheel spinning faster at a given output shaft speed you'll need to decrease the ratio- get the rear wheel rpm closer to the output shaft rpm, so a 30/16 combination would give 1.875:1, and give more speed if the engine is capable of reaching the same rpm.
In the past I would calculate primary reduction(crank to clutch), and transmission ratios, sprocket ratios, tire OD, etc, to determine gearing, but now I use online calculators, like:
http://www.gearingcommander.com/
They don't have a CB77 in their recipe book, but you can do it manually- if I dig out my manuals and get the primary sprocket teeth and transmission teeth counts I'll go through the 'Add Bike' process, so it's easy to experiment.
Also, I haven't tried it, but I'm told that 'Lab Metal' filler can take the heat of powder coat curing- could solve future pin hole problems. The forum at Caswell Plating, http://www.caswellplating.com/, is a good source of info.
Rick

[Vince Lupo]

Thanks guys for the confirmation - I think I'll probably just keep it at 15/30 for the time being. Here again, the combination of acceleration and highway comfort seems to work well with this gearing, though I do think that the visit to the dyno will happen at some point. I'm going to ride it and see how it does on the two-lane twisty roads as well as the highway.

[Vince Lupo]

The valves may be bouncing/floating. It may need more (or less) ignition advance at the top end with the slightly longer flame path. It's also possible that the fuel slope needs to be tilted.

So if the valves are bouncing/floating, how does one cure that? Mr. Ladd (who did the headwork) installed the new valves and the titanium caps, and set them to these specs: Springs 1.410 = 100 lbs @ .400 Lift = 250 lbs. And John re-checked the valves last week and he re-adjusted them.

Can you explain 'flame path'? And how does the 'fuel slope' get tilted? Does that have to do with the angle of the carbs themselves?

[G-Man]

Vince

Valve bounce (or float) is the valves leaving contact with the rockers / cams at high revs. You haven't changed to a 'hotter' cam and have used titanium caps so that helps. I don't think you are getting valve float. Less reciprocating weight and / or stronger springs cure it but the stronger springs will have a detrimental effect on cam wear. All engineering is based on compromise.

Flame path is just how far the flame has to propagate from the spark plug to the farthest part of the combustion chamber. You have a bigger bore plus a different piston crown shape so the flame has to go farther to completely burn the mixture the bike has inhaled. A sphere would be the theoretically perfect shape but bike combustion chambers are long and thin when it matters most. Sometimes you get 'pinking' or knocking where the flame front moves across the head compressing the unburned remainder ahead of it. If the temp / pressure gets high enough then the gas simply explodes with that characteristic noise.

As Teazer and Jensen have suggested, a dyno run will help tell you why you are losing top end 'oomph'. Your stock cam, carbs and valves may need a little more attention to match the extra breathing required of the big-bore motor.

By 'fuel slope' I think Teazer means the shape of the graph of fuel with respect to revs and load (throttle opening).

G

[Hoosier Tom]

I haven't tried it, but I'm told that 'Lab Metal' filler can take the heat of powder coat curing- could solve future pin hole problems. The forum at Caswell Plating, http://www.caswellplating.com/, is a good source of info.
Rick

Rick,

Lab Metal is what my powder coater uses.

Regards- Tom