High rpms in lower gears

The curved lines are:

At WOT (Wide Open Throttle, full throttle)
Driving force of rear wheel in lbs
VS
Vehicle speed in mph

Note that the driving force in 5th gear is about 140-150 lbs between 50 to 110 mph.
The expected accelerative force would be the difference between the driving force and the running resistance. Ex, at 80mph, driving force of 142 lbs minus running resistance of 60 lbs gives accelerative force of 82 lbs.

An accelerative force of 82 lbs applied to a 410 lb XS1B, with a 150 lb rider (560 lb total weight) would yield an acceleration of 82/560 = 0.146 G

1 G = acceleration due to gravity, about 32 ft/sec/sec, or about 22.17 mph per second.

0.146 G = 0.146 * 22.17 = an acceleration of 3.2 mph per second...
 
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On clutch slipping, it's more noticeable in the higher gears. Start with proper clutch adjustment. Properly lubing the cable and worm works wonders.
On the engine rattles I would start by doing a Major tune up.
Adjust cam chain. quiets cam chain noise.
Adjust valves. Quiets valve noise.
Adjust gap and timing on points, just check timing on TCI.
Do these things in this order, if not you may need to do some of them over.
This will fix most noisy issues. Once you get the mechanicals done then you can play with the carbs.
www.amckayltd.com/carbguide.pdf will help with your carbs. Once clean the "Tuning For Mods" section will help get them set up right.
Leo
 
Apologies if someone else has mentioned it already but I haven't time atm to read through the whole thread .

That rattling sound coupled with a high or hanging idle sounds to me like a very lean mixture together with detonation from the timing being out .

If you ride along maintaining a steady speed in any gear does the bike lurch ? sort of pick up then die then pick up then die in a continuous cycle ? classic sign of lean mixture at that particular throttle range.
 
Apologies if someone else has mentioned it already but I haven't time atm to read through the whole thread .

That rattling sound coupled with a high or hanging idle sounds to me like a very lean mixture together with detonation from the timing being out .

If you ride along maintaining a steady speed in any gear does the bike lurch ? sort of pick up then die then pick up then die in a continuous cycle ? classic sign of lean mixture at that particular throttle range.
Im not sure what you mean by pickup then die? Do you mean the bike stops pulling?
 
Im not sure what you mean by pickup then die? Do you mean the bike stops pulling?

no its more like a continual surging of revs up and down although you are keeping the engine at a steady speed and not altering the throttle .
Try riding the bike maintaining a steady speed and note what happens .
 
I do have my idle speed a little high. Have it set around 1500 at idle right now. I'll try more tuning with the carbs to get it to idle smoothly at 1200. l

If your idle speed is that high then the engine vacuum will be starting to draw fuel and air from the venturi, jet needle and main jet rather than the idle circuit so the engine will always seem to be revving too high between gear changes . It never gets a chance to drop to idle speed where it operates on the idle circuit

Why are you using such a high idle speed ? are you having difficulty getting the engine to idle at 800-900 rpms ?
 
If your idle speed is that high then the engine vacuum will be starting to draw fuel and air from the venturi, jet needle and main jet rather than the idle circuit so the engine will always seem to be revving too high between gear changes . It never gets a chance to drop to idle speed where it operates on the idle circuit

Why are you using such a high idle speed ? are you having difficulty getting the engine to idle at 800-900 rpms ?
It will idle at 1200, it just idle's better at 1500. I probably have to get my manometer back out for a carb sync and re-set mix screws.
 
On clutch slipping, it's more noticeable in the higher gears. Start with proper clutch adjustment. Properly lubing the cable and worm works wonders.
On the engine rattles I would start by doing a Major tune up.
Adjust cam chain. quiets cam chain noise.
Adjust valves. Quiets valve noise.
Adjust gap and timing on points, just check timing on TCI.
Do these things in this order, if not you may need to do some of them over.
This will fix most noisy issues. Once you get the mechanicals done then you can play with the carbs.
www.amckayltd.com/carbguide.pdf will help with your carbs. Once clean the "Tuning For Mods" section will help get them set up right.
Leo
...
the engine isn't operating on the idle circuit at 1500rpm ;)
.....
So maybe go up on pilots? I'm already on 30 pilots for my 78 bs38's

Robert, Before you get sidetracked on carb adjustment it wold be good to keep it on track to solve the issue that seems like a slipping clutch.............

one thing at a time.........
 
...

.....


Robert, Before you get sidetracked on carb adjustment it wold be good to keep it on track to solve the issue that seems like a slipping clutch.............

one thing at a time.........

Yea, I agree. I just thought that maybe carbs being out of tune would be the cause for high rpm's. From what I've read from the responses from this post, I don't think that I have a slipping clutch. People have been saying that a clutch slip would cause the rpms to shoot up with no acceleration. This doesn't happen on my bike.
 
the easiest way to confirm if you have a slipping clutch or not is to ride the bike flat out up a long steep hill in 3rd or 4th and put the engine under a heavy load.
if you haven't got a local hill then stick a passenger on the back.
You'll soon know if the clutch is slipping or not. :)

From your description I don't think the clutch is slipping either ;)

If you are using pod filters I would recommend that you start with 35 pilots in BS38s and work your way down.
I have a 78 special with 78-79 BS38s and am currently running 35 pilots . I tried 30 and 32.5 but I simply couldn't get a sufficiently rich idle with my straight thru exhausts and foam filters.

The symptoms were very similar to what you describe.

Let your idle mixture screws and plug colour guide you . If you have to have your mixture screws wound out past 2.5 turns then go up a size on your pilots.
When you have the mixture right your revs should return to idle immediately when you blip the throttle providing there is no other issues other than the idle mixture

I understand that 2 into 1 exhausts are notoriously difficult to achieve an even low idle. Someone else will no doubt advise on that.
 
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All that I get from this thread so far is that your bike is idling too high. If it won't idle at 1200 rpm you need to address that problem.

The 5000 rpm, I don't understand that. Can't you just shift sooner? What happens if you go through the gears shifting between 3,500 and 4,000 rpm?
 
thats terrible 2M .....sounds like an ol tractor lol;) :).........nice tick-over:thumbsup:

I don't think we can rely too heavily on the accuracy of crude 40 year old tachometers to be fair ..
I would imagine the tolerance range of their accuracy was probably 5% +- when brand new lord knows how inaccurate some of them are now !
I always gauge my idle speed not by the tacho but by ear when fine tuning. .

I would imagine that there were numerous reasons why Yamaha recommended 1200rpm as an idle speed one of which would probably be to maintain a small positive charging of the battery from the alternator at idle speed. With PMAs so prevalent these days that would no longer be an issue.
How do idle speeds of bikes built in the last decade compare ?
 
For instance: if I'm in 2nd riding at lets say 25-30mph, the rpms will be around 5k, then when i shift to 3rd it will stay around 4k until I shift to 4th. I guess what I'm asking is, Is this a normal range of RPM?

I don’t know what anyone else thinks but if I ride my bike in 2nd gear at 25-30mph, 5000 rpm’s sounds about right. I would never just ride around going that fast in 2nd gear though. My bike runs the best (lower rpm’s) in 4th gear if I’m going 25 to 30 mph. When do you shift? Redline? It sounds like you’re either just not running your bike in the correct gears for the speed you’re going or you’re waiting way too long in the gear’s rpm range to shift to the next gear.
 
I thought the 1200 RPM idle spec was rather high when I first got my 650. I was used to my old BMWs where I routinely set the idle in the 900 to 1000 range. The 650 just sounded "busy" at 1200, a bit too fast. I tried setting it down around 1000. It idled but would just up and randomly stall at stops. Moving the idle speed back up into the 1100-1200 range alleviated that problem. I've grown used to the sound of it now around 1200 and I just accept it as normal for this motor.

If you still use the mechanical advance then 1500 is definitely too high for the idle. The ignition is starting into it's advance curve at that RPM, the advance unit is beginning to fling open. As the timing advances, the RPMs increase. This is a self-feeding problem. More advance induces more RPMs which, in turn, induce more advance, etc. The RPMs can increase from 1500 all the way up to 2500 or 3000 all on their own, without you even touching the throttle. Having the idle set too high can also hamper the advance unit's return to the fully retarded position. The RPMs can hang and may never drop back down to idle speed.

So, your high idle setting may be the cause of your problem. Try setting it to the correct 1100-1200 (hot) speed.
 
I thought the 1200 RPM idle spec was rather high when I first got my 650. I was used to my old BMWs where I routinely set the idle in the 900 to 1000 range. The 650 just sounded "busy" at 1200, a bit too fast. I tried setting it down around 1000. It idled but would just up and randomly stall at stops. Moving the idle speed back up into the 1100-1200 range alleviated that problem. I've grown used to the sound of it now around 1200 and I just accept it as normal for this motor.

.

exactly ....thats it in't it ......1200 does sound high when you are used to an idle somewhere between 850 to 1000.
I suspect I have probably been setting mine too low because of the influence of other vehicles I have owned . To be truthful II think it just sounds more 'classic' with a slow idle..... less modern

I find I have to rev a bit to take off smoothly or the engine will not pick up immediately....theres a slight hesitancy. Next time I ride I'll try increasing my idle speed to 1200 and see if that has any effect on transition .
 
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