Topic: Should I expect a higher top speed?

[DammitDan]

Right now I can get up to a top speed of about 85 mph on my '82 CB650sc...  (much) higher if I'm going downhill, but 85 is the highest speed I can consistently maintain.  I've got a larger-than-stock (+1 tooth) front sprocket and a smaller-than-stock (-3 teeth) rear sprocket, which I would assume would increase my top speed.  I also recently changed out my mains to the smaller stock 118 size, which evened out my mixture and actually increased my top speed (which was only about 75-80 with 120 mains).

I weigh close to 250...  Is this the limiting factor?  I haven't balanced my carbs in a while (they're not that bad), but I tuned the tappets and cam tensioner recently, so it's in pretty good running shape.

What do you guys think?  Would a carb balance improve my top speed?  What else can I do to help break the ton?

[ofreen]

It should do better than that.  Higher gearing will only give you higher speed if you have the power to pull it.  There are many bikes that will go faster in fourth gear than fifth for this reason.  If your engine is otherwise healthy, you may be overgeared.  How's the compression?

The variation you are likely to see in carb balance will have no effect at wide open throttle.

[DammitDan]

I have noticed that it pulls much better and more quickly in 4th gear (that's how I get it up to 85, but it's already running at close to redline in 4th at that speed)

5th gear it just poops out.  This can't be stock, can it?  Is it because I changed my sprockets, then?

[Jugghead]

Is it a possibility that your clutch is slipping and won't pull any more? 
Have you recently adjusted the clutch mechanism and done other Honda recommended service items?
Might be worth a look. Simple 5 minute tune up item, and effective at diagnosing what it is or is not.

[DammitDan]

Hmm, I hadn't thought of the clutch, and I haven't adjusted it in forever.  It already slips a little when it's cold and I really hit the throttle.  But if it was slipping, wouldn't it "jump" to a higher RPM when I'm in top gear and trying to make it go faster?

[ofreen]

5th gear it just poops out.  This can't be stock, can it?  Is it because I changed my sprockets, then?

If your engine is in a good state of tune, and has good compression, I'd say you are overgeared.  I don't know what the stock size sprockets are for your bike, but with those numbers you can calculate the percentage of change with your new gearing.

As far as your weight goes, it won't have nearly as much effect on your ultimate top speed as it does how long it takes to get there.  Drag, whether mechanical or aerodynamic, it what keeps us slow.  But even sitting bolt upright, you should be getting better than 85.

If your clutch is slipping, you should notice the higher RPM.  It would be particularly noticeable when you let off the throttle.  Rpm will generally lower abruptly before speed changes much.

[736cc]

Riding position will make up to a 10 MPH difference in top speed. Upright is slowest, flat against the tank is fastest.

[TwoTired]

I think you have a sick bike and not in the slang sort off way.

For comparison I have a 74 CB550 with a full vetter fairing that will do 90-95.
Any of my 550s that run properly will put the speedometer needle pretty close to the ton mark.
I have an F 550 running on three cylinders that tops out at 85 (it has one bad carb).

I'm not bragging, just illustrating that with 100 more ccs, I think you should be doing way better than 85 MPH.

What RPM do you have at 85 MPH?

Speculation:  You may have over geared the bike with you sprocket changes.

Your engine has a torque and power curve that begins at zero for zero RPM and increases to some peak as it gets up near redline RPM.
Generally you want to be at that peak power while geared for your top speed.  It is entirely possible to gear your bike in such a way that you cannot develop enough HP to overcome the friction and wind resistance that develops and accelerate the weight you are propelling.
At over 40 MPH, wind resistance increases exponentially.

In other words, you can gear your bike to equate 5000 RPM to 500 MPH theoretically.  But, if you don't have the power to overcome wind and frictional drag, you never will achieve 500MPH.

FYI, my 550's have stock gearing.

Cheers,

[DammitDan]

I've got a viper fairing on the bike, and even getting down under the wind doesn't seem to make a difference.  The RPMs don't suddenly increase when I hit top speed...  but perhaps it's just riding on the edge of slipping, and since I'm already up in the RPMs (around 7k @ 85) it's hard for me to tell the difference in rate.  I'll try re-adjusting the clutch cable to see if that makes a difference.

I was ticketed at one point this Fall for going 92 (and accelerating) on the interstate, so I know it can go faster, but it seems to be random when I can go above 85 or not (although the wind may have been a contributing factor on that one... i likely had a tailwind).  Even though the speedo only goes up to 85, I can tell when I go above it...  it slows down and tops out right before the "85" mark).  Is it possible that a weak spark is preventing a higher top speed?

[Klark Kent]

i seem to remember the sprocket change as an EITHER the front OR the rear proposition, as in either the 1 up front or the equivalent 3 or 4 in the rear.  you can try ust changing the one you can replace easiest and see whats up.   speedometer inaccuracy is also a possibility.
good luck
-KK

[medic09]

I've got a viper fairing on the bike, and even getting down under the wind doesn't seem to make a difference.  The RPMs don't suddenly increase when I hit top speed...  but perhaps it's just riding on the edge of slipping, and since I'm already up in the RPMs (around 7k @ 85) it's hard for me to tell the difference in rate.  I'll try re-adjusting the clutch cable to see if that makes a difference.

I was ticketed at one point this Fall for going 92 (and accelerating) on the interstate, so I know it can go faster, but it seems to be random when I can go above 85 or not (although the wind may have been a contributing factor on that one... i likely had a tailwind).  Even though the speedo only goes up to 85, I can tell when I go above it...  it slows down and tops out right before the "85" mark).  Is it possible that a weak spark is preventing a higher top speed?

Your numbers caught my eye.  I think TT is right (he often is) about something is ill.  Although an entirely different bike, 85 mph @ 7000 rpm is more like what my SR500 (single cylinder 500cc Yamaha) does, and that's still not it's top speed.  (Actually it makes 85 mph at a little less rpm.)  I should think a bike like yours would beat the pants off my little thumper at top end.

[DammitDan]

Well I figured it was sick... It's never been "healthy" per-se 

I'm pretty sure it was 7k, but it might be a mistake.  It's got a redline of 9.5k...  What kind of performance should I be expecting from my cb650?  I've always heard that this is a normal RPM range for this bike.  http://www.sohc4.us/forums/index.php?topic=9582.0

I'm running with the airbox cover off (which a lot of CB650s, including mine, had a problem with them on).  I figure it must have the power to go above this speed, since i can get it up that fast fairly quickly in 4th gear.  But in 5th it just drops off the chart.  It was bogging before with 120 mains with rich plugs.  Now with 118s it's running much more smoothly with even plugs.

Changing out the sprockets dropped the RPMs at lower speeds by about 1k.  But is the RPM scale logarithmic?  It seems like the RPM "gap" in the speed:RPM ratio closes as I go faster (like, at 50 the RPM will be ~ 3900, 60 it's ~ 5000, 70 it's ~ 5900, 80 it's ~ 6600, etc.)  Of course this is completely different in the lower gears.  Would changing the sprockets make such a difference on the top end as to totally kill the top speed?  Like it can't seem to "get over the hump"...

[Hockers Choppers]

Have you checked if you have a bad wheel bearing or dragging brakes? That happened on one of my bikes once.

[TwoTired]

I found a road test of a 1979 CB650:
http://www.sohc4.us/index.php?option=com_content&task=view&id=138&Itemid=52
Is the 82 that different?

Motorcylist Mag Street test statements/data:
 79 CB650 1/4 mile in 13.37 at 95.64 MPH . Trans final drive 2.74 :1  with 16/40 sprockets.

Most "felt" power above 6000 RPM.  Dyno test: 50 HP peak @ 9000 RPM ,  32.15 Ft Lb torque Max @ 7500 RPM

You reported 80MPH @ 6600 RPM.  The dyno chart says 6500 RPM  is 37.10 HP @29.88 ft Lb torque.  I think an extra ten HP would quicken the bike considerably.

Speed In gears @ redline (9500 rpm) 1st 42.22, 2nd 61.2, 3rd 79.1, 4th 98.2, 5th 119.3  (The Dyno doesn't have any wind drag to overcome.  But, this tells you how the stock bike is geared.)

You might be getting better economy with your current sprockets since you travel more distance per crankshaft revolution.  But, most likely, it is hurting your performance.  Since you are not able to accelerate into your engine's peak power RPM, you cannot apply that power to the rear wheel.  What I do not know is how much horspower you need to overcome the wind resistance and acheive, say 100 MPH.  If I did, I could calculate the gearing needed to accomplish that.

There may well be other issues with the run performance of your engine, too.  Further, the lack of RPM may also be keeping the slides in your CV carbs from fully opening and robbing a bit of power. (Just a guess, really)

Cheers,

[medic09]

Further, the lack of RPM may also be keeping the slides in your CV carbs from fully opening and robbing a bit of power. (Just a guess, really)

Cheers,

TT!  You guessed!  Educated guess, granted; but YOU OPENLY GUESSED! 

I didn't think you did that, really.  Thought it was against some engineering code or another...   

[Jonesy]

Further, the lack of RPM may also be keeping the slides in your CV carbs from fully opening and robbing a bit of power. (Just a guess, really)

Cheers,

TT! You guessed! Educated guess, granted; but YOU OPENLY GUESSED!

I didn't think you did that, really. Thought it was against some engineering code or another...

Kirk- "Spock... You.. guessed? That's extrodinary!!"
McCoy- "I think what he's saying is that he's more confident in your guesses than most people's 'facts'."
Spock- "Well, then I shall make the best guess I can.."

(Sorry, that scene ran through my head when I read those quoted posts... )

[medic09]

Further, the lack of RPM may also be keeping the slides in your CV carbs from fully opening and robbing a bit of power. (Just a guess, really)

Cheers,

TT! You guessed! Educated guess, granted; but YOU OPENLY GUESSED!

I didn't think you did that, really. Thought it was against some engineering code or another...

Kirk- "Spock... You.. guessed? That's extrodinary!!"
McCoy- "I think what he's saying is that he's more confident in your guesses than most people's 'facts'."
Spock- "Well, then I shall make the best guess I can.."

(Sorry, that scene ran through my head when I read those quoted posts... )

    
    

TT, forgive me... 

[BobbyR]

The real question is, do you really have to go faster than 85mph. If you have good acceleration and good fuel mileage you may want to leave it "overgeared". 

[Dennis]

Further, the lack of RPM may also be keeping the slides in your CV carbs from fully opening and robbing a bit of power. (Just a guess, really)

Cheers,

TT!  You guessed!  Educated guess, granted; but YOU OPENLY GUESSED! 

I didn't think you did that, really.  Thought it was against some engineering code or another...   

from the same guy who said ...........
 
        "engineeeeeeeeeeeeeeeeeers arre baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaad"         

[Hockers Choppers]

The real question is, do you really have to go faster than 85mph. If you have good acceleration and good fuel mileage you may want to leave it "overgeared". 

HELL YES!!!!!! 85mph won't even get you a ticket here in Kansas!! I feel like I can jump off at that speed! He said he could go 85 in 4th gear and it bogged in 5th. I'd say something is wrong with the chassis or engine. The gearing is probably OK?

[Ibsen]

I agree with ofreen. If the engine otherwise is sound, it should pull at least 105~110mph. With stock gearing I can pull 110~115mph on my 650. And as already pointed out, take a look at your gearing ratio. 1 tooth up on the front and 3 down on the rear sprocket is  equal to 5~6t down on the rear. I would say that is way too much, and becuase of the power loss it will cause, the top speed will go down rather than up. You have to keep in mind that the power in the 650 engine doesn't kick in before at approx 7000rpm.
If you put on stock sized sprockets you will probably see a huge difference.

[TwoTired]

Further, the lack of RPM may also be keeping the slides in your CV carbs from fully opening and robbing a bit of power. (Just a guess, really)

Cheers,

TT! You guessed! Educated guess, granted; but YOU OPENLY GUESSED!

I didn't think you did that, really. Thought it was against some engineering code or another...

I'm a RETIRED engineer.  And, sometimes my wild side escapes.
Cheers,

[Hockers Choppers]

OH, you engineer's and your WILD SIDES!!

[BobbyR]

The real question is, do you really have to go faster than 85mph. If you have good acceleration and good fuel mileage you may want to leave it "overgeared". 

HELL YES!!!!!! 85mph won't even get you a ticket here in Kansas!! I feel like I can jump off at that speed! He said he could go 85 in 4th gear and it bogged in 5th. I'd say something is wrong with the chassis or engine. The gearing is probably OK?

I have been to Kansas and I guess you really have nothing to lose by going fast - Just kidding. I think if you add in the wind drag of a 250lb man who probably has wide shoulders, coupled with too tall a gearing that keeps the motor from overcoming getting int the power range, you will have lower top speed. The tip off is he can go faster downhill. These bikes are not overdrive freewheelers like our cars, if you are maxed out on revs, you are maxed out. Changing sprockets is an easier job than most.     

[DammitDan]

I wanna go faster than 85mph to be able to keep up with my brother in law.  Sure I can kick his '70 Guzzi's ass any day of the week, but when hops on his '80 BMW R100s, he leaves me in the dust.  We were riding down a little tricked out lowrider on the backroads a few weekends ago, and even the lowrider out-paced me on the top end!  Of course he stuck with the guy the whole time (what kinda redneck would try to outrun a BMW r100 on the twisties, anyway?).  I kept up in the curves, but as soon as the road opened up, so did the throttles    Suffice it to say, it took me some time to catch up after my brother in law proved his point and the redneck scampered away

Ahem, and I said I was CLOSE to 250...  But I'm also 6'3, so at least I can try to explain away my incredible mass.  Erm, I mean weight.  Damn, there are too many engineers here

The reason I put the sprockets on was to fix my "running high on RPMs" problem back when I first started riding (back when I was concerned that the bike was riding sustained ~ 5k at 55mph...  HAH!  Let's just say I didn't know what these babies could do  )

The chain is in need of replacement soon anyway, so it looks like I'll be going back to standard sprocket sizes.  And I'm forking over the extra $$ for an o-ring chain this time, dammit.  I won't make that mistake again!