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Honda Cb350 Twin

Honda CB350: 45 MPH at 5K RPM, 50 MPH at 6K RPM. Is that normal?

This is the first season I've owned a bike. I bought an old '73 Honda CB350 twin (325 cc). To get it up to 45mph, I have to push it to 5k RPM and to get to 50mph it's 6k RPM. This would definitely be a problem with a car, but I don't know if the same is true for a 2 cylinder bike from the early 70's since it redlines at around 9.5k. Any thoughts?

Dude,,,,Bikes may be New to You...

But Your Instincts/Hunches and Subliminal Grasp of MATH sound like an Ol' Pro to me

The Difference between 5K and 6K is 20%

If You Speed Up your MOTOR 20%
The Back wheel outa at least TRY to keep up,,Ya know??

Seriously ,,it's PROPORTIONAL

Your 20% Increase in RPM should be accompanied by a 20% Increase in Road Speed.

45 to 50 is Only a 10% Increase

Going from 5 to 6K RPM,,that's a 20% Increase

You should be going from 45 + 20%>>>to 54MPH

Your Gauges COULD EASILY be OFF,,,a LITTLE.

I know 350 Hondas fairly well.
They'll INDICATE 105~110mph at 10,000rpm

5K @ 45mph
extrapolates to
10K @ 90mph

Genuine,Real 95~100mph is what they would usually run.
On a Fresh one in Avg shape,,& decent avg tune.

They WILL(Or WOULD) Turn 10K in High Gear REGULARLY.

Of Course they would,,,the REDLINE on the Tach was Pegged at TORQUE PEAK.
9,500 RPM.
ALL of them would pull Hi Gear BEYOND That to at least 10K.

So as you can see,,the INSTRUMENTS were OPTIMISTIC.
Yours appears to be Pessimistic by about HALF the Difference in speed it should be

Ya Should be Showing a 9mph increase,,
Yet you're only showing 5mph gain.

And ,,Your RPM is HIGH at 5K.....according to Stock Gearing & Tire Size.

REGARDLESS of your actual overall gearing or tire size,,,
RPM Gain and SPEED Gain are PROPORTIONAL,,percentage-wise.

Rev Up 10%=10% Faster
Rev Up 50%=50% Faster
etc,etc
Twice the Revs=Twice the Speed.

.....................

I ran your figures on a MPH Calculator.

I ASSUME a 3.50 x 18 tire is a 25" Diameter,,,
and that was Original Size Tire

Your Overall Gear Ratio is the Total of 3 Reduction Ratios:

#1)Primary Drive---the Gears between Engine and Transmission INPUT
Yours happens to be 3.714:1 Ratio
That's 21 into 78 TEETH

#2) Gearbox Ratio.
Hi Gear is actually an Overdrive on Yours
0.9:1 Ratio
30 Teeth driving 27

#3) Final Drive>Engine Sprocket x Rear Wheel Sprkt
Standard ,original gear was
2.25 :1 ratio
16T on Engine x 36T on Rear Wheel.

So we get (3.714) X (0.90) X (2.25 )=
7.52:1 OVERALL Ratio

Engine Turns 7.52 Revolutions for each Single Revolution Rear Wheel Turns.

(Thats all in HI GEAR)

Ok,,,IF You have Stock Gearing,,,Stock Tire Size,,,
here's the MPH per RPM the Bike Mathmatically goes.
(In Hi Gear Only)

1000rpm= 9.89
2000rpm= 19.79
3000rpm= 29.7
4000rpm= 39.6
5000rpm= 49.5
6000rpm= 59.4
7000rpm= 69.3
8000rpm= 79.2
9000rpm= 89.0
10K rpm= 98.9

Notice the Figures are NOT exact,,due to "rounding off"
But also Notice the Linearity.

See the FIRST,,1000rpm = about 10mph?
And thereafter EVERY Other 1000rpm equals another 10mph

Your Bike,,,given it's Original Gearing and Tire Size
Produces a 10MPH Gain for Every 1,000RPM Increase.

That's Hard Numbers,Simple Math.

We can Look at Your Observation
5K Revs = 45 MPH

Thats about 5mph SLOW,,per Original Specs.
It "Maybe Geared Lower",,,that could Easily Account for that.

But at 6Krpm= 50MPH,,,
Thats WAY OUTA Proportion of the mathmatical Ratio.
By HALF.

It Should be UP 10MPH for the 1K rpm rise

From 5K to 6K is a 20% Rise
45mph Should Follow with it's own 20% Rise.
To +9mph,,,or 54mph

LOOK at what it's Sposed to be at 5K vs what is SHOWS
49,5 vs observed 45.
10% Slow

Look at the CHANGE from 5 to 6K you Observe.

That Change TOO is 10% Lagging
Speed Rise per RPM gain Should be same 20%,
But it only Rose 10%.
It's Short 10% again

All the Gears are STEEL,,so is Chain & Sprockets.
Tire is pretty well stuck to rim,,
And I doubt you got wheelspin at 45mph.

Only one thing that Slips in the whole driveline.

CLUTCH..
How Bad??

10% Slip

That's BORDERLINE Perceptible under moderate driving conditions.

What's happening is that when clutch is Supposed to be LOCKED-UP SOLID,,,
The DRIVE Plates are OVER-RUNNING the DRIVEN plates.

By How Much??
Same 10%

Every 10 Engine Revolutions,,,the Transmission is getting "Left Behind" by 1 Revolution

Cutch INPUT Turns 10
Clutch OUTPUT turns 9.

It'll begin wearing Faster,,and slipping Worse.
The rate of Deterioration is Exponential.
the Worse it Gets,,the Faster it gets Worse.

All while that HEAT from the Excess Persistent Friction is beating the hell outa your Engine Oil.

...................................................
The Only Fix is to replace the Clutch Plates.

If You have any mechanical ability,,,it's a rather simple job.
Nothing really Tricky to it.

On 350's,,,the gasket on the R/H engine Cover aka Clutch Cover has OIL PRESSURE plumbed thru it to feed the Crankshaft and the rather delicate Upper End of the motor.
Which makes it Absolutely Critical to Carefully Clean the Gasket Srfaces without scratching them,,,
Use Good Sealer on a New Gasket.

Cleaning Gaskets of those engines is a NITEMARE,,,very tedious,,the old gasket are extremely stubborn.

PARTS:
I'd strongly suggest
*A New Clutch Cable
*If Clutch HandLever is wallowed out at it's pivot ,,get a New One(Maybe even a New Pivot Bolt as well)
Unscrew the Lever and LOOK,,,You'll see what I'm talking about.
Difficult to get a Good Adjustment with a Sloppy Lever,,,which is Bad for your New Clutch.

* Clutch Plate Set,,,Probably only need the Friction Plates unless the Steel plates are Discolored from Heat.
If they aint completely Silver & Shiny,,,Replace them

*New Clutch Pressure Plate Springs

*Gasket & Fresh Motor Oil of course.
Do Your ENGINE and Yourself a Favor.....
Use straight 30W oil,,,if Not 40W.
I dont sell oil,I dont care what anybody puts in anything.
And I dont care what anybody sez or thinks about the matter.
Use 30W Valvoline or Pennzoil,,,if You have more than a few 100* Days in your region,,or stay in Hi 90's all summer,,,use 40W.

If You Do the work Yourself,,,there's a Centrifugal Oil Filter "Cup" on end of crankshaft.
It has a big circlip holding on it's cover.
Clean that thing,,,it's like a Dense Metallic CLAY,
So ya gotta Scrape it out,,a Bunch before even Trying any solvent to wash it.

Oil Pump in those engines is mounted in same area and driven off the Clutch,,of all things.
It has a fine mesh intake screen.
Take a look and make cretian it's not caked up with stuff.
Normally they're OK and need no attention.

Oh yeah,,get Yourself a good Service Manual if you dont have one.

.............................................
If You have any Doubts that what You are seeing is a Clutch Prob.....Go TEST it.
Drop down to 4th gear 5,000.
Chop Throttle Off Real Quick,,
Then Snap it Open quick as ya can to SHOCK the Clutch.
Watch your RPMs Flare while Bike does not much.

Or,,,Drag Your Rear Brake in High Gear around 3~4K,
and open the Throttle.
Listen to spin Up while going Nowhere fast.

DO NOT DO EITHER for any longer that it takes to feel how bad it's slipping,,,,just a few SECONDS.
And DO NOT Do It Repeatedly.

It can finish Off the Clutch Instantly,,,
and the LEAST it does is Fry your Motor Oil from Clutch Slip generated Heat.

Is that R/H engine cover a little Darker hue of golden than rest of Motor??
That AINT from AGE.
Unless 6~8~12 Months is OLD.
I seen a BUNCH of 6month Old '68's ,,as Toasty Gold as could be.

I also seen quite a few 10yr~15yr old 70's~72's~75"s still "original Silver" under Gin Clear Original Laquer.

>>>>>>>>>>>>>>>>
Or,,if You have a Shop or someone else do the work,,,
I'd strongly suggest on specifying,,
New Cable and Clutch Springs just arbitrarily,,,in addition to the "routine stuff" required for such a job.
AND to clean that filter gizmo,,even it's a few exta bucks labour charge.

Good Luck with it,,,thats a BAD problem,,,but Nothing Unusual or Terrible.
I'n serious as can be about the clutch slip overheating your oil.
I Cannot tell you "Dont Ride it Like That",,,but I gaurantee You'd be WISER not to.
It's a common repair,,,and will good as new when done

If You DO chose to ride it, put Straight 40W Valvoline in it asap.
That'll minimize the clutch's deterioration a bit,,but most importantly it'll help defend the engine while the slipping clutch tries to boil your oil.
Cheapest Insurance ya can get against an EXPENSIVE problem down the road.

Nice ol' bike,,btw,,,one of the All Time Classics.
As far as bikes have come since your 350's day,,,its STILL a great riding & Good Looking bike and Lotts of fun.

................
One last afterthought.
If you wanna do a shadetree verification of your speedo's accuracy...
Cruise at Steady 30mph,or 60mph for 1 Mile on the Odometer.
That'll take 120 Seconds,,or 60 seconds respectively

Or mark off a mile somehow,,,with a car,whatever
Cruise that at 60,,it should Show 1 Mile on the Odometer,,and take you 60 seconds to travel it.

IF Your does have Stock Size Tire>>3.50x 18,,and Stock Rear Gearing,,16Teeth x 36teeth,,,
At 60MPH you should be doing 6K rpm.

Expect it to be Off a LITTLE,,they all were,,,and always a LITTLE optimistic on the Speedo.
The Tachs were remarkably accurate,,pretty close.

Honda cb350 (brat style) flat tracker

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