sprocket ratio`s

[blacklav]

Ive just read a thread about different sprocket sizes.
I want to reduce the revs on my 1977 xs d type, as it is vibey at 60 mph and above, but ok at 55mph. 60 to 70 being my preferred speed.
It has a 33 tooth rear and an 18 front. Someone says an 17f and 30 rear is good.
Has anybody worked out the drop in revs for the different sizes, I`m sure there`s a formula for it somewhere.

thanks
chris r

[richard]

I run a stock front sprocket and a 32 rear which puts that smooth spot right at 45 and 60. anything over 65 it starts to come back, but at 70 it's still pretty good compared to the stock 34 sprocket. I would have to look, but I would say that 65 is about 3500 RPMs. 70 would be about 4000 RPMs I do have to feather the clutch a little from a stop though.

Dead on ignition and carburetor tuning will help with the vibrations a lot too. The down side to the 32 is going under 9 mph the engine is lugging, so stuff like cruising through crowded parking lots requires a lot of clutch action.

[blacklav]

Thanks for the reply Richard,
    it looks like I`ll be going with a smaller rear then. I`ll have a go at  working out the rear wheel revs for the different sprocket ratio`s and will post them- if I succeed that is.
Chris r

[headcase]

I went with stock front/30 rear, and Mike's od 5th gear. I love it and the bike doesn't mind either, but it does take finesse on the clutch lever to get things rolling smooth. 70 is nothing now compared to the stock setup, the vibes are MUCH better. But like Richard said, the tune is a huge part of the vibration issue. You'll get really familiar with the carb balance adjustment screw, if you get that dead-on (all else within specs too) it really smooths things out. Hopefully yours stays put better than mine does, seems like once a week I'm tweaking it back and forth.

[richard]

My tuning kind of creeps out of adjustment over time. Then I'll take a half a day to reset the valves and timing and all. After riding it tuned and it is so smooth I think Dang! why didn't I do that two weeks ago. It's like a new bike all over again when it's all tuned dead on. I run my carb mains one size rich because of the heat here and I carry a little screw driver under the seat just in case, but I haven't had to use it since I got new needles and springs.

[blacklav]

Thanks all,
I`ll get to checking the carbs etc at the weekend.
I have vacuum gauges for carb balancing, but I don't see any take off points on these carbs. Do you  do it by feel?
Chris r

[richard]

Mine don't have any vacuum ports on the 71 I have a ball type flow meter that goes up against the carbs, but I don't use it. I use my hand over the exhaust and the sound of each side. I also look for the even color on the head pipes and I have a laser temp meter to check running temps between the heads. The detonation pop of each cylinder should be equal at idle and under load. If one cylinder favors the other it will vibrate a lot more. If it had ports I would use the meter, but so far in thousands of miles it seems to be working just fine. I use a vacuum meter on the 80 XS and on bikes with more than two cylinders it's a must.

[headcase]

I have one muffler worse off than the other (leaky and a bit louder) and even with everything dead-nuts balanced, it still sounds 'off'. No doubt it affects the tune on one or the other cylinder, but until the exhaust is replaced with 2 equal pieces I just get the sound as close as possible, then try to tune out the ears and feel with the hands to finish it off. Idle screws, balance screw, valve lash, cam chain tensioner, repeat repeat repeat....

[richard]

I think that's what most people do. I have an 1980 that looks good and runs good, but the pipes sound terrible so I'm going to replace them when I get the time. I'm a lot more picky about the tuning on the 71 because I ride it more than the others.