changing sprockets

[xsjohn]

Well I'm 18/29 and yours with 18/30 and mikes would be a hair higher than mine in5th and a hair lower for the other gears.........750 would pull it if you don't weigh a bunch..........remember you need to retard the ignition timing with tall gears..........or it just simply won't run right............and won't pull off in first gear like it should or could..........and after you get used to it and use it on the highway it would be hard to go back to the lower gears.........harder to pull double though...........mines 3750 at 70

xsjohn

 

[Trebor13]

thanXS
.

 

[nj1639]

I look forward to the days when the comparisons of the stock 650 to the 750 are posted by those that have run both, and then the same for the 8-1 compression/retarding/gearing by those that have done both.

 

[aldo5468]

xsjohn - I've got an '82 XS650SJ, I'm going to install a 31-tooth rear sprocket with 17 tooth front, and I'd like to retard the timing as you suggest - to fire 0.168 inch closer to the TDC mark on the timing tab. How much does the pickup head have to move counterclockwise to accomplish this? by the same 0.168 inches? Do you slot the hole in the top tab to its right side and rotate the head around the bottom screw? Or do you slot both screw holes? Thanks in advance.

 

[xsjohn]

Mark the pickup coil to the case before you start.........then grind off both break off screws down till they release the pickup coil....there will be a bit of the stud you can turnout with pliers...then replace with screws ...stator is threaded.......some pickup coils are already slotted some aren't........if not notch some on the right side of the top hole......bottom hole stays the same and is not slotted........slot the case so you can get a screwdriver on it without removing the case for easy dicking with........169 thousands would be maximum retard.....have to play to find what you need....and 17/31is still gonna scream on the highway.......

And get some longer screws and say 1/2 inch of small tubing like arrow shaft or something and stand off the round side cover an 1/8" or so..........180 plus degrees in there to 130.......water hurts nothing............

Oh........as far to move the pickup coil.......since it's a large base circle I gkuess you could also mark the sidecover from your original mark you made before you started and then pit another mark .169thou for the maximum retard mark......

Afterwords going around corners say at 2000 and does good......have to just ride and play for all the happy medium sweet spot.......and surely the taller the gears the more retard must be necessary.........18/30 is the lowest gears I have run onthis wone after the first day with the stock gears.........18/29 is on it now and the express is pretty good and 4th is wonderful for 40-60 without having to wind much..........

xsjohn

 

[aldo5468]

Thanks - very helpful information. 0.169 inch maximum on the timing tab works out to 5 degrees max of retard, if my math is right. Is it important to find true TDC first?

 

[xsjohn]

If I go past .168 retard on mine I start to notice it getting a little fat and may start to loose some power.........have to play to find the sweet spot.....that's why its good to notch the side cover so you can change it on the fly ........my TDC mark on the stator seems to be correct..........could just mark the coil to cover then place another mark .168 to the left of it........betting that the circle outside and the magnet location and the timing mark are real close...........and thanks for figuring the amount of degrees......honestly never did that......I just adjusted it for what I considered the best all around running for me and then used the timing light to guess that it was ..168.....or so it seems........I'm not terribly scientific sometimes........

xsjohn

 

[breahn riley]

I'm assuming this retarding process is a bit less difficult on a non tci bike?

 

[aldo5468]

Well, I'll feel better if someone checks me out on how I did the calculation. Here's how I figured it: measured 1 15/16 inch from crank centerline to inside (top) edge of timing tab - that's the radius; diameter is 2r = 3.88 inches; circumference is 3.14 x 3.88 = 12.18 inches; divide by 360 degrees = 0.034 inch per degree. 0.168 divide by 0.034 = 4.9 degrees; call it 5 degrees max retard in round numbers. With the stock timing listed as 15 degrees BTDC in my Haynes manual for this bike, which just sounds kind of aggressive for a road machine that needs a wide rpm range, moving it back toward 10 degrees BTDC as minimum static advance sort of sounded sensible. When all is said and done, you are right, it's not some particular number of degrees advance - it's how a bike runs in the kind of riding that you do that tells the tale. Thanks again for sharing your long experience with us.

 

[xsjohn]

The problem with the timing on these is that some people just dont know how much it effects the way these run....while they play with the pods and pipes and engine mods......I wouldn't ride mine around the block with stock timing.......it has to be where I want it to do what I to do.........and thanks for the kind word........xsjohn

 

[Viper]

Bandito, I have a 78 special, and it has a 36 tooth rear sprocket too. It's too much of a coincidence. I think they put 36T on specials. I'm pretty sure i'm going to switch to a 32T sprocket.

I would not have ever taken the bike on a large highway with the 36t.

 

[pamcopete]

Well, whenever these discussions emerge, I try to inject some numbers to dampen down the "feelings" factor. So, here is a little Excel spreadsheet that will yield RPM's vs MPH for all gear, tire size and sprocket ratios. The spreadsheet depends on a thing called "rolling distance" which is the distance that your bike rolls for one revolution of the rear tire. It's a very simply concept, and is akin to the effective circumference of the tire. The operative word there is "effective" because there is no way to calculate or measure the "effective" circumference of a motorcycle tire due to its circular cross section. You have to actually move the bike forward for one revolution of the rear tire and then measure the distance that the bike moved.

The chart does, however, have two typical rolling distances shown, one for 18" tires and one for 16" tires so you can use these numbers to get a fairly accurate display of all the other numbers.

The rolling distance and sprocket teeth are pull downs that will give you a choice of values. Just choose whatever value you want and the RPM's and MPH for all gears will be calculated and displayed.

http://www.amckayltd.com/xs650revs.xls

The problem with this chart is that it introduces facts into an otherwise emotional discussion. People want to talk about how much better there bike "feels" with an 18 tooth front sprocket, or how the vibes are less because the RPMs are way lower, like, 3500 at 65 MPH, but if they were to figure out their rolling distance and plug the numbers into this spreadsheet, they may find that the RPMs are more like 4200 and the reason they are claiming 3500 is because their tach or speedo isn't working right, but, like any good story, we don't want facts to get in the way of the punch line!!

So, just figure out your rolling distance, plug the numbers into the chart and see for yourself what the numbers really are.....you can still lie about them if you want to!

 

[xsjohn]

What ever those spread sheets say if you are on the highway with anything less than 18/30 for any length of time and don't know how to adjust the timing for taller gears and don't have the pilot jets right you will soon know what a bumble bee feels like.....

And if you move the compression ratio to 8-1 the vibrations will be all but gone and it will be pleasant to ride down the express .........fact.........

And mine turns 3500 at 65 with the 140 sized 16 inch rear tire........18/29

xsjohn

 

[Dot Heton]

If i go to 18 and 36 will it pick the chain up off the swingarm much? I'm running 1 inch longer shocks on a 73 frame and with it's short swingarm the chain just touches the pivot area.

 

[xsjohn]

You are under some constraints to gear these properly for the highway....if you really want the taller shocks then you will suffer with what I would consider (36 tooth) as a completely useless rear sprocket except maybe for hill climbing........the reality button is seldom fun to push.........

xsjohn

 

[Viper]

PamcoPete, I agree. I've checked out the spreadsheet several times. Pretty Good.

 

[xsjohn]

Look...I have been gearing Yamahas tall all for over 40 years......and for 10 years during that time these were selling I did it for a living.........plainly I know how to do it correctly...or as much as possible to make up for the poor engineering ............believe me or not at this point I don't care.......but if you want to do it sucessfully then listen or figure out a better way........carbs.....timing and compression is where you should focus...not how pretty the fashion statement is...if not then you may not be very sucessful.........how else can I put it...............

xsjohn

 

[Viper]

Look...I have been gearing Yamahas tall all for over 40 years......and for 10 years during that time these were selling I did it for a living.........plainly I know how to do it correctly...or as much as possible to make up for the poor engineering ............believe me or not at this point I don't care.......but if you want to do it sucessfully then listen or figure out a better way........carbs.....timing and compression is where you should focus...not how pretty the fashion statement is...if not then you may not be very sucessful.........how else can I put it...............

xsjohn

I'm glad that you've had 40 years experience with these bikes.

I only stated that I have a 36T rear sprocket, and that I am looking at a 32T. And that PamcoPete made a pretty good excel spreadsheet.

I am not questioning your abilities/experience at all, why not contribute to the tech articles on the site for the proper setup of xs650s?

 

[xsjohn]

I get wacko when the subject of gearing comes up.....32 would be ok if you didn't take trips.....

xsjohn

 

[retiredgentleman]

Viper;
My 78SE came to me with 17/36 gearing. If you like quick acceleration from stop light to stop light, the 36 rear was great. Not so good for the highway.

I changed over to a 33 rear and find its a good compromise between acceleration and lower rpm at highway speeds. I'm sure a 32 or 31 rear would also work well for you.