Topic: high rpm VS mph

[johhny b]

I have a 1974 CB750.when I ride the rpm seems too high for the speed i'am going,I checked the sprockets, the front is 18 tooth, and the rear is 48. if i remember i'am showing about 7000 rpm and doing 60-65 mph in 5th. anyone  know whats wrong here.

[bowhunter]

On my 77, front sprocket is 15, rear is 41 tooth. Sounds like the PO wanted quicker acceleration. That would account for the high tach reading.

Bowhunter

[johhny b]

I thought 18 and 48 was standard for this model.

[Einyodeler]

Yes the stock sizes for the `72 - `76 750K`s are 18 tooth front and 48 rear.
That does seem like a lot of revs at that speed though,maybe someone here with that stock setup could see what rpm`s they`re turning.
On my 500 with stock sprockets I`m turning 5000 rpm at 65 mph.

[TwoTired]

Yes 18/48 is standard for the 74 model and 15/41 is standard for the 77 model.  The resultant final drives are nearly the same due to changes of the 5th gear ratio in the transmission between the two bikes.

69-76 Drive ratios, 5th gear .939,  Final 2.667

77  Drive ratios, 5th gear .969,  Final 2.733

As to your original problem;
Do you have smaller wheels/tires installed?
Is your clutch slipping?
Are your tach and speedometer both original equipment?

Is your assessment of RPM based on solely on tach readings or actual sound of the engine?

[Jonesy]

I'd check the items twotired just mentioned. My '73 CB750 turns around 5,000 RPM in 5th going 65-70 MPH.

Do you have a 16 inch Harley rear wheel? They were a popular conversion....

[Harry]

Am running a 500 using 550 sprockets. 6000rpm=120km/h in 5th, which acts as an overdrive, 4th corresponds to standard 500 setup in 5th ie 5000rpm=100km/h (60mph). 5th is good for cruising, but not for acceleration. 3rd redlines at 140km/h, and the acceleration is phenomenal.

[johhny b]

I have a 18 tooth front a 48 rear, an 18 inch wheel and a 100 link chain.the revs sound about right for whats indicated, the speed feels about right for whats indicated. the bike sat a long time before i got it, i don't know much about its history.every thing seems to be ok except the high revs.

[mph750k6]

I have a 1974 CB750.when I ride the rpm seems too high for the speed i'am going,I checked the sprockets, the front is 18 tooth, and the rear is 48. if i remember i'am showing about 7000 rpm and doing 60-65 mph in 5th. anyone  know whats wrong here.

[mph750k6]

I have a 1976 750 K which I've owned since 1977.  The stock sprockets are 18 and 48. The engine RPM's have always been just over 4000 at 60mph.

[Kevin D]

My CB750K1 is stock with 18/48 gears and 1000rpm is worth about 15 mph in top gear so
 
  2000rpm=30mph
  3000rpm=45mph
  4000rpm=60mph
  5000rpm=75mph
  6000rpm=90mph
  7000rpm=105mph
  8000rpm=120mph

It sounds like either your tach is out of whack or you've got a real bad clutch.

[Bob Wessner]

Likewise with my K0, as a matter of fact, right on the money with Kevin D's. Though I can only confirm up to 60 mph since I have had little reason to go faster than that, but since it is pure mechanics and gear ratios, I would assume the higher speeds would be consistent also.

[johhny b]

I adjusted the cluch and took it for a ride, it shows 6000 rpm at 55 mph in 5th gear.the speedo might be a little slow, i know i was'nt going 90 thougt. i hav'nt ridden this bike much, it sat for years before i got it , trying to work it in slowly. the thing that bothers me the most about it is the hard throttle, is there a fix to lighten it up.

[Bob Wessner]

Have you lubed the throttle cable? That might help.

[johhny b]

it's not the cable i think it's the spring, is there a weaker spring i can use

[bowhunter]

Maybe you should have someone pace you and get an actual speed reading. Then you'd know if it's the tach or speedo that's wrong.

Bowhunter

[TwoTired]

Just a thought. Are you sure the tach is for a 750 and not a 550.  It looks the same.  But, has a different gear ratio inside.

I had the opposite problem you have with your 750.  I got my original 550 with smashed guages.  The only ones I could find were from a 750.  Looked ok, the breaker said they were the same.  I put it on.  I could never get more than 7000 RPM out of that 550 even though it screamed at WOT.  Turns out the tachs have different multipliers. oops.  Don't know why that engine didn't explode...

The multipier is stamped on the back of the tach.  I think I have that 750 tach in a parts bin somewhere.  If you need it, I'll look and find the multiplier stamping.

Cheers,

[Clyde]

My 750's run about 4000rpm  for 60mph.
I think twoTired has got the answer. I bought a 550 tacho for my F1 and had to get it recalibrated due to the diferent ratios. I wanted the lighter green colour  for my 76 F1 so it gave me the right look.
The 550 tacho runs a 20:3 ratio and the 750 tacho runs at 4:1
That means if you have a 550 tacho on a 750 it will be reading 6600rpm when the engine is running at 4000rpm

[johhny b]

I just took the tach off. it's hard to read the numbers on the back.the top number is 057200-0720, i think this may be a part number , below that is what looks like 1580 tpm = 10000 tpm , hard to read, below that is a large 3 and another symbol that looks like an N. is this the correct tach.

[Bodi]

I just took the tach off. it's hard to read the numbers on the back.the top number is 057200-0720, i think this may be a part number , below that is what looks like 1580 tpm = 10000 tpm , hard to read, below that is a large 3 and another symbol that looks like an N. is this the correct tach.

No.

[TwoTired]

I just took the tach off. it's hard to read the numbers on the back.the top number is 057200-0720, i think this may be a part number , below that is what looks like 1580 tpm = 10000 tpm , hard to read, below that is a large 3 and another symbol that looks like an N. is this the correct tach.

Here's the number for the one I took off my CB550 (Read very low) and the parts breaker man said was from a CB750.  It has the light green face for the 76 model.
057200-0770
2500 rpm =10000rpm
9 N

Cheers,

[Clyde]

No this seems like it is of a 550. The ratio is wrong. It should be 10000:2500 or 4000:1000.
Are you sure it is not 1500, which gives 10000:1500 or 20:3

[johhny b]

It could be 1500, kind of hard to read. I guess I'll need another tach. thanks to all for the help, it's nice to solve a problem like this.

[johhny b]

one last thought, if the speedo is from a 550 ,would that be indicating wrong to.

[Kevin D]

To calibrate your speedo use highway mileposts, stopwatch, calculator. Drive at steady 60mph and check time in seconds for 1 mile. Divide seconds into 3600 for true speed.

                      3600/60sec    = 60mph