Tach rpm

[mralex714]

Riding at 75 mph my tach usually reads about 3800 rpm. Now at the same speed it shows 4100 rpms. First I guess I should ask what it should read at 75? And am I looking at clutch issue?

[Eck]

Not sure if you by chance just changed the oil recently, but my first thought would be ...if you did change the oil, do you have the correct oil in it for a wet clutch?

If you have not recently changed the oil in it, then I would (as you are) suspect the clutch going bad and slipping.

I would think you could tell if the clutch is slipping, like when in 5th gear and you just give it full throttle (under a load in high gear), you should hear the engine speed up (increase in RPM's), but watch the speedometer and see if it goes up accordingly.

[videoarizona]

Interesting. On my 89 with stock tires, etc., 4100 rpm is about where I am at 75mph.

[mralex714]

Not sure if you by chance just changed the oil recently, but my first thought would be ...if you did change the oil, do you have the correct oil in it for a wet clutch?

If you have not recently changed the oil in it, then I would (as you are) suspect the clutch going bad and slipping.

I would think you could tell if the clutch is slipping, like when in 5th gear and you just give it full throttle (under a load in high gear), you should hear the engine speed up (increase in RPM's), but watch the speedometer and see if it goes up accordingly.

Oil changed about 2000 miles ago used Rotella 15/40. I didn't notice any lag in acceleration like the clutch was slipping.

[mralex714]

Interesting. On my 89 with stock tires, etc., 4100 rpm is about where I am at 75mph.

Before at 4100 I would be just over 80 mph

[Flyinfool]

Did you change tires?

Check your tire pressure?

As mentioned, in 5th gear at around 65-70MPH crack the throttle wide open, if the clutch is gonna slip, this is where it will happen.

[mralex714]

Did you change tires?

Check your tire pressure?

As mentioned, in 5th gear at around 65-70MPH crack the throttle wide open, if the clutch is gonna slip, this is where it will happen.

Didn't feel as if the clutch slips under load.

[frankd]

At an indicated 75MPH my 89 shows about 4100 RPM also. You're comparing your tach to your speedometer, and if the clutch isn't slipping maybe your speedometer changed. You can't really be sure which changed unless you check the speedometer versus a GPS. My speedometer reads about 3 MPH high from 20-90 MPH. My 83 is about the same.

[camos]

Riding at 75 mph my tach usually reads about 3800 rpm. Now at the same speed it shows 4100 rpms. First I guess I should ask what it should read at 75? And am I looking at clutch issue?

Perhaps you might also say what gear you are in. The difference you stated would be about the difference between one gear or another. Did you forget that you shifted?

[mralex714]

Perhaps you might also say what gear you are in. The difference you stated would be about the difference between one gear or another. Did you forget that you shifted?

 

All comparisons done in 5th gears.

[garyS-NJ]

The clutch if slipping barely would be on heavy load with an engaging clutch like a holeshot or changing gears. To have a clutch that's constantly slipping such that a higher RPM is required to maintain a fixed speed you would have a clutch that is getting so hot is would disintegrate quickly. This could happen if you clutch master (or more likely slave) got sticky keeping the clutch semi disengaged.

 

And not to change the thread but I also use the rotella diesel oil because I read that it has more zinc (lubrication) and good for older motorcycles. Someone posted (Honda CB750 forum) some time ago that oil should be specified for a wet clutch (adding that I should not use zinc additives) to preclude slipping. But the Rotella seems to be fine to me..

 

) (

Oil changed about 2000 miles ago used Rotella 15/40. I didn't notice any lag in acceleration like the clutch was slipping.

[rickardracing]

Are you riding two up during the higher rpm versus one up at the lower? Pulling a trailer at the higher rpm? Did you................................................................gain weight?

[Ozlander]

Interesting. On my 89 with stock tires, etc., 4100 rpm is about where I am at 75mph.

Same here.

[mralex714]

Same here.

I am running stock tire size. It sounds like I have a speedometer problem.

[videoarizona]

That or maybe you have a different rear end ratio!?!? Wonder if a different diameter front wheel would change speedo....Are you the original owner?

Edited November 17, 2015 by videoarizona
spelling

[Flyinfool]

The speedo is driven off the front tire.

So a different size front tire will change the speedo.

A different rear end gear or tire size will have no effect on the speedo but will change the tach reading for a given speed.

[mralex714]

That or maybe you have a different read end ratio!?!? Wonder if a different diameter front wheel would change speedo....Are you the original owner?

I am not the original owner. The readings changes almost daily or after riding awhile and stopping.

[videoarizona]

((The readings changes almost daily or after riding awhile and stopping.))

 

OK...that's a decent clue right there: It doesn't help me, but maybe someone else will recognize this issue.

 

I'm going to go back to the basics:

1: Speedo is driven by a black donut shaped unit mounted between the left front fork and the front wheel on the axle. The unit is somewhat round except on the top where two ridges exists. The unit is mounted such that these ridges are located between a ridge mount on the wheel. This is how the unit is "locked" in place. If the unit floats, your speedo readings will not be accurate. From there, the cable goes north to the speedo head...where it mechanically drives the speedo head. Simple system and reasonably accurate. I would take the front wheel off, being careful to note how the speedo unit is placed, take it off, clean it and grease it. Check interior parts for significant wear. If nothing appears wrong then put back together. Grease cable end as well. Make sure nut holding cable is secure to the speedo unit. Remount the speedo unit, axle and front wheel back together, making sure the ridges on the speedo "donut" are on either side of the ridge on the wheel and secure.

Now try and get some grease into the speedo head itself. Some have been able to put grease on cable end at top and shove it in and that works. Others have had to use some pressure to get the lube into the speedo head to grease the bearings. Either way...try to grease it.

 

That should eliminate any issues with speedo except the speedo head itself being bad. BTW, the electronic portion of the speedo signal also goes to the cruise control...but don't think your problem is there.

 

The tach is electronic/mechanical. Signal originates at the coil of #2 cylinder, (LF) and goes to the tach. It's a green/yellow wire from coil. You might dig down in there and check that wire for corrosion at coil or anywhere else it goes....although I can't see it going anywhere else in the simplified schematic. Corrosion at either end could cause different readings based on moisture/heat/cold... ie, resistance flucuations, etc..

 

david

[Flyinfool]

There is another possibility. A slipping clutch.

 

Get the bike going around 60 MPH in 5th gear, crack the throttle wide open. The tach and the speedo should both rise at the same rate, If you are getting a bit of slip or creep the tach will rise faster than the speedo.