Gearing Issue, Please Help!

[Sopersonic]

Hello Everyone! I recently purchased an 83 xs650 thats been turned into a bobber. im having trouble wrapping my brain around the gearing on this bike, it has an 18t on the front and a 30t on the rear. from my reading this should be geared for high end speed, my problem is that it will only go about 65-70 mph. surely im missing something?

 

[TwoManyXS1Bs]

Welcome to the forum, Sopersonic.

The original sprockets were 17T front, 34T rear. Your setup is more properly considered as "fewer rpms at hiway speed", often done to reduce vibrations.

See if your speed improves in 4th gear. If not, the engine may need some work...

 

[Sopersonic]

65-75 is top speed in 5th gear wot, the engine was just rebuilt and has about 200 miles on it

 

[650Skull]

That's 17/28, think you will find that is to low, try 18/32=17/30 or 18/34=17/32.

 

[Sopersonic]

I counted the teeth before I posted and they're 18/30, I feel like it should be much faster than 65-70mph. Its got plenty of power, I'm just running out of "gear". I wouldn't think that 1 tooth on either gear would make much difference. Ive also read that a stock XS650 should run in the low 100 mph range, I'm nowhere near that.

 

[fredintoon]

Hi Sopersonic,
18/30 sprockets let the bike cruise along at highway speeds at low rpm so the engine vibration won't shake your fillings loose but that high gearing stops the engine from ever getting it's revs up into the engine's power band except in the lower gears. Drop down to 4th or even 3rd and see how fast she'll go then.
And are you sure the indicated 65-70 mph is right? Yamaha speedos tend to slow down with age. My bike's speedo only reads 50mph when I'm keeping up with traffic in a 60mph zone but the odometer checks out perfect.

 

[Sopersonic]

The speedometer is brand new, but I'm mostly going by the guys I ride with when I say 65-70mph. I know that the only way to get to 65 is in 5th gear and the rpms are maxed out, the engine is screaming by then, and I can barely keep up with traffic on the highway.

 

[arcticXS]

Well, if a stock bike does 100 mph at, say 7000 rpm with 17/34 gearing, then yours should do 120 mph at 7000 rpm, all other things being equal. That is just mathematical fact.
Now, if you have a really small rear wheel, that would lower the gearing, but I cannot see a wheel small enough to cause a 70-75 mph top speed.
So there are only 4 options left:
-Your clutch is slipping badly
-Your engine is not tuned properly, i.e is not revving out as it should
-You are not revving as high as you think (Riding buddies on Harleys?)
-Your speed estimate is incorrect

 

[Sopersonic]

Haha now we're getting somewhere lol I brain keeps telling me that I should easily be able to do 100mph the way it's setup but that's not the case. I know the rpms are maxed out because I have it wide open, and I can't imagine 6 people all being wrong about the speed. I guess the next thing I need to check is the clutch then, it seems to shift up and down fine. What should I be looking for in the clutch pack to know that it's been slipping?

 

[pamcopete]

Soper...,

Here is a Excel spread sheet with all combinations of sprockets, gears, wheels and tires to show RPM vs MPH:

RPM vs MPH

Here is a list of things that couild be holding you back:

Bike goes slow

1. Low battery / charging system. A low battery will produce a low ignition voltage at the plugs which is most evident at higher RPM's.
2. Timing. Did you use a timing light to set the timing?
3. Stuck or no advance. When checking the timing with the timing light, did you check for maximum advance at 3,000 RPM.
4. Binding advance rod.
5. Fouled plugs. Did you install new plugs? Check them after you got it running?
6. Plug wire not making contact in the coil. Measure the resistance from plug cap to plug cap to ensure that both plug wires are making contact in the coil.
7. Brakes dragging.
8. Speedo out of calibration. You are actually going faster than you think! Use your GPS to check the speedo.
9. Tire pressure. Pump up to 32 PSI for high speed.
10. Drive chain too tight. That will rob you of power to the rear wheel.
11. Enricher plunger stuck partially on.
12. Enricher (choke) left on or partially on.
13. Weak or loose advance springs. Weak or loose advance springs cause the advance to use up some of the available movement because you have to essentially retard the advance to get it on the idle advance mark, so there is less movement available at higher RPM's.
14. Timing chain needs adjustment.
15. Cheap or bad gas.
16. Low octane gas.
17. Water in the gas.
18. Partially blocked pet cock or fuel filter.
19. Paper filter that has low flow when the tank is not full.
20. Try a run with a full tank of high octane, fresh, expensive gas.
21. Blocked fuel cap vent.
22. Incorrect sprockets. What RPM are you getting at 60 MPH.
23. Rear tire rubbing on brake rod or swing arm.
24. High wind resistance. Are you a large person?
25. Throttle cable not fully opening the throttles.
26. Blockage in the muffler(s)
27. Dirty or blocked air filters
28. High altitude
29. Very hot air, like 95+F
30. Very cold air, like 32F
31. Low oil
32. Really dirty oil
33. Gas in the oil
34. Weak TCI magnet in rotor. Unplug Reg/Rect to test
35. Cracked carb boots.
36. Hole in carb diaphragm(s)
37. Cheap pods covering air hole in carb inlet

 

[fredintoon]

Hi Soporsonic,
gotta tach on that bike?
"sounds like it's screaming" is a subjective thing, one person's maxed out is another's high idle.
I agree with the other posts which suggest that your bike's clutch is slipping.
BTW, nice looking bike but what's that winged eyeball on the rear fender?

 

[Sopersonic]

Man you guys are awesome! Looks like I have some direction now and can start some trial and error stuff. Like I said I'm a car mechanic and know very little about motorcycles, although I'm sure the operate the same. If come up with anything else I should be looking for by all means. I'll let you know what I find out.

 

[Sopersonic]

By "screaming" I mean I have the throttle open as far as it will go and I'm not gaining any speed, it's almost like I need another gear. No tach. When I bought the bike it was set up for foot clutch and hand shift and the winged eye was the shift knob. After I switched it all back the normal shifting and clutch I just cut the knob off and mounted it to the rear fender that I also had to make. It's just for looks, mounted turn signs on the wings are in the works lol

 

[fredintoon]

Hi Soporsonic,
did you ride the bike when it still had a foot clutch?
Did it act up that way then?
Perhaps when you switched back to normal clutch operation you didn't leave enough slack in the operating system?
Use the "search" button, type in "clutch slip" and "clutch adjustment" and do what those links suggest.
The XS650 clutch is just about bulletproof and it's more likely that it needs operating system adjustment rather than needing a rebuild.

 

[Sopersonic]

I didn't ride with the foot clutch, I've actually only been riding for about a month and still have my learning permit. When I switched it back to the hand clutch I modified the cable because I didn't know about the adjustment at the engine, insert forehead slap and groan here lol but Monday I put another, unmodified cable, back on and did find that the adjustment nut and bolt at the engine were not right. So I had to re-tap and die them and adjusted them the way they're supposed to be. I haven't ridden it since then yet so I don't know if it made a difference with how it rides.

 

[5twins]

I feel pretty much all these bikes need new clutch springs. The 30+ year old originals just aren't up to snuff any more. Weak springs mostly let the clutch slip in the higher gears where a greater load is placed upon it. I suggest getting some heavy duty aftermarket springs. I consider it just something that needs doing on these bikes and could very well be your problem. The engine may be revving out to the max but that power isn't being transmitted through the clutch.

 

[Sopersonic]

I'll definitely look into doing that, there are a few other things I need to do to it too, like fork seals, rear brake shoes, starting gear issues.... lol

 

[Sopersonic]

I'll definitely look into doing that, there are a few other things I need to do to it too, like fork seals, rear brake shoes, starting gear issues.... lol

 

[arcticXS]

Haha now we're getting somewhere lol I brain keeps telling me that I should easily be able to do 100mph the way it's setup but that's not the case. I know the rpms are maxed out because I have it wide open, and I can't imagine 6 people all being wrong about the speed. I guess the next thing I need to check is the clutch then, it seems to shift up and down fine. What should I be looking for in the clutch pack to know that it's been slipping?

You say that "I know the rpms are maxed out because I have it wide open"
Now this is not at all a fact. All you know is that you have twisted the throttle as far as it will go. In fact, you don't even know if your carbs are fully open.
As you bought the bike in a modified state, it could well have all kinds of issues.
If your engine is not tuned correctly, or has some kind of mechanical issue, "WOT" may give you only 3 or 4000 rpm. The ONLY way to know what revs you get, is an accurate tachometer, or even a multimeter with an oscilloscope function, used while actually riding the bike. Revving it while stationary may not show the actual max rpm when pulling hard.
For instance retarded ignition will limit your rpm. I once helped a buddy with a Honda XL100, which suddenly would only run 30-40 kph, when it easily ran 90 kph before. We finally traced this issue to a faulty CDI unit, that actually RETARDED the ignition timing, instead of ADVANCING it. With a replacement CDI, the bike ran 90 kph again.

So unless you can rig a temporary, good tacho,and check ACTUAL rpm, you will have to check everything, PamcoPetes list will be a good starting point.

 

[scabber]

Yamaha geared the bike correctly for the 40 HP the motor puts out. The mechanical advantage of the gearing is lost when the 18 and 30 is used in your case. You don't have the HP to run those gears. The clutch wasn't designed for it so it most likely slipping. Even if you cure the slipping you might not see 100. What good is it if it takes all day to get there.