CB450 Top Speed? and running out of gas...

[cellomaster]

*Went out riding today with my brother. We probably did 80-100 miles, just bumming around town and having fun. We wanted to test my top speed, and I couldn't get past 90, and that was on a downhill. I also seem to have a bit of "dead" space at the top of my throttle, in pretty much any gear, but especially in 5th. If I roll my hand one way or the other, its the exact same speed and power at a smaller throttle position.

This seems to be slow for this kind of bike? On uphill and straightway, not being able to get above 85 seems like there is an issue, rather than just the age. My bike has 5400 on the odometer, so its not wear I'm worried about. My air/fuel screw is at one turn out. Both carbs have been bench synced, it idles perfect at 1200, and I run 87 NE gas in it. Anyone have any tips?

The other thing that bit me in the butt was running out of gas and sitting on the side of the road for half an hour while someone went and got me some gas...I would love to put some sort of gas gauge on this, so I am not having to rely solely on what I think im getting MPG wise, and base that off my trip odometer. Any ideas on gas gauges that don't ruin the look, change performance, nor break the bank? Maybe I should just get better at learning the RES vs ON fuel selector...

 

[jensen]

What is the rpm @ 90 mls/hr ?

 

[stl360+450]

Front or rear sprocket changes could affect the top speed, but I would not be surprised to learn that you have stock sprockets. The tuning may just not be optimal.

I'll include a photo from the July 1970 issue of Cycle World when they road tested the CB450K3. The rider reached 103 mph in fifth gear, but appeared to ride fourth into the mid-nineties before shifting. Fifth gear is an overdrive gear, so is under powered at lower rpm. By eyeballing the graph, the test rider appears to shift into fifth close to 9,000 rpm.




I'm not a big fan of running these bikes at super high speeds — a lot can go wrong. Make sure the road is in good shape, the oil level is good, everything is buttoned up, all the cotter pins are in place, fasteners are secure, cables are working smoothly, etc.

If you run the bike with the petcock in the ON position, you should be able to make it to a gas station after switching to reserve. The tank will also sound different when you tap it as the fuel level drops. As you ride it more, you will attune to these things.

 

[EzPete]

In all honesty, a fuel gauge is an absolute waste of time. If you truly don’t want to be bothered with keeping track of where you’re at, just rely on your reserve as the backup.

Bike on = run
Bike runs out of gas, flip to reserve and go find a gas station.

The cumulative mileage is also a non-factor, not all miles are created equal.

List of things that could impact top speed

1. Riding position and rider weight
2. Timing advance off
3. Valves improperly adjusted (ie following a commonly viewed video and setting them 3X factory spec).
4. Clutch slippage (RPM increase without concurrent speed increase)
5. Airflow restricted
6. Fuel restricted

TBH, 90ish is probably realistic. The claimed 103 max is always a little suspect.

 

[WintrSol]

Definitely need your belly on the tank and chin on the bars with elbows and knees tucked in to get over 100, assuming all else is good. And that would be indicated by the speedo, not actual.

 

[jensen]

I got my CB450 K0 with 1200 miles on the odometer, and the max speed at that time was around 90 mph. I did rebuild the engine (low miles don't guaranty low wear), and after the rebuild I did put many miles on it. I ride the bike on a regular base above the 100 MPH, on a GPS (160 km/hr), in 4th gear, at maximum rpm (9500). I changed the sprocket (rear), with a smaller one (2 teeth smaller). The max speed I ever did on my GPS was above 170 km/hr, it was in Germany, early in the morning, moisture in the air, and on a "no speed limit" road (A3). The road was flat, no wind, legs as close as possible to the bike, arms in, chin on the tank.

90 mls is a good number, especially for the K1-K5. The K0 is a 4-speed, yours a 5-speed. So, it takes a flat road, cool temperature, moister in the air, and a perfectly adjusted K0 (adjusted on the dyne), to get there. Don't forget the tire pressure, quality of the rear (non-O-ring) chain (an old chain will consume more power, thus lower top speed).

If the bike is not perfectly adjusted (carburators, ignition etc), you won't hit anywhere near a real (GPS) 90 mls /hr.

So, I agree with @WintrSol, accept I did an actual speed measurement on the GPS.

 

[esmith250624]

Test Weight - Curb weight = 130lb Test rider

 

[cellomaster]

What is the rpm @ 90 mls/hr ?

Around 6-7K in 5th.

TBH, 90ish is probably realistic. The claimed 103 max is always a little suspect.

And if that's the case, I'm totally fine with that! Just wanted to make sure!

I think my timing advance for the ignition is slightly off, so I will check that next. I just went through with new oil, cam chain adjustment, valve lobe adjustment, new carb gaskets and full clean, new coils and condenser, I've replaced or cleaned just about everything at this point.

 

[cellomaster]

One other thing I'll mention, is after cold starts (and of course letting it idle for 2 minutes for oil to flow up), it tends to backfire a bit until I've gotten up to a high rpm a few times. After that, it's totally normal. Could that be ignition advance timing?

 

[EzPete]

One other thing I'll mention, is after cold starts (and of course letting it idle for 2 minutes for oil to flow up), it tends to backfire a bit until I've gotten up to a high rpm a few times. After that, it's totally normal. Could that be ignition advance timing?

Very possible. Could be a rich mixture as well.

 

[WintrSol]

Calculated rpm for the 5-speed for 90mph is 8000 rpm in 5th gear. For the 4-speed, it calculates out to ~8500 rpm The math was done using stock gearing and tire diameter, so there would be some deviation.
If you could reach 9500 rpm in 5th gear, it would be 107mph; the math says 100.7 for the 4-speed. So, the math is pretty close to jensen's results.

 

[cellomaster]

Calculated rpm for the 5-speed for 90mph is 8000 rpm in 5th gear. For the 4-speed, it calculates out to ~8500 rpm The math was done using stock gearing and tire diameter, so there would be some deviation.
If you could reach 9500 rpm in 5th gear, it would be 107mph; the math says 100.7 for the 4-speed. So, the math is pretty close to jensen's results.

That makes sense, given that I could only reach 90 on a downhill, and my rpms aren't going past 7-7.5K. I would assume some tuning issue, then? Still think it is ignition timing, I am not confident in how I did it after replacing the points. I followed the guys in Houston's video, and although i got it close, I don't think I got it perfect. Need to use a timing light next time.

 

[WintrSol]

I always use a timing light to verify timing, both at idle and full advance. It can be messy if you don't have a spare stator cover with a hole over the timing marks, though.

If your tachometer is accurate (not a given, but close), your actual speed at 7-7.5k is 80-85 mph; the speedometer MAY show 90 due to its known inaccuracy. My tach is better than my speedo, so I use it for speed; I just know that 5th gear is 11.4 mph per 1000 rpm, and do the math in my head. That's also why GPS gives much better numbers.
The speed in the other gears per 1k rpm is 4, 5.9, 7.5, and 9.6, based on my calculations.

 

[EzPete]

That makes sense, given that I could only reach 90 on a downhill, and my rpms aren't going past 7-7.5K. I would assume some tuning issue, then? Still think it is ignition timing, I am not confident in how I did it after replacing the points. I followed the guys in Houston's video, and although i got it close, I don't think I got it perfect. Need to use a timing light next time.

If you followed that video, it’s incomplete - there’s still setting the right side that needs to be done, which will certainly make a difference at higher RPM - particularly if it’s slightly “tight” on the right side.

 

[jensen]

As an engineer, I usually calculated the gear ratio's versus road speeds for the CB450 K0, CB450 K1-K7 and for the CB500T, here you go:

CB450 K0 (calculated with the 33 teeth sprocket instead of the 35 teeth standard sprocket):



CB450 K1-K7 (standard sprocket gear ratio's):





CB500T (standard sprocket ratio's):




So, as you cab see, 90 mls/hr isn't so bad, but looking at your RPM range you mentioned my guess would be that you were closer to 80 mls/hr @7-7,5 k rpm, as you can see in the second sheet.

 

[Jays100]

I built a speed spreadsheet also with a small difference, I added a small factor, 10%, for tire growth in top gear.
However, GPS would be the “last word” for speed.

 

[WintrSol]

My math comes up with 1.98 m for tire circumference, using the the SAE tire size of 18"x3.5" and reducing the value by 2.5% to allow for compression of the rubber to give the contact patch. It may actually be a little higher at high wheel rpm, so 1 or 2 cm more may be closer at over 1300 rpm at the wheel due to tire 'growth'. In any case, my spreadsheet mostly agrees, within a couple percentage points.

 

[EzPete]

I run a GPS Speedo on the 450 and I’ve touched 90 on flat ground. There was probably more throttle left, but that was enough for me on that bike. Lines up with the charts though. 45 MPH in 3rd sounds about right.

 

[jensen]

I built a speed spreadsheet also with a small difference, I added a small factor, 10%, for tire growth in top gear.
However, GPS would be the “last word” for speed.

I checked the calculated value's (rpm/ speed) with the GPS, the 2m circumference of the wheel is "reverse engineering", so including the tire size change at higher speeds. This also means that the lower speeds are probably a little off due to the smaller diameter.

When going full throttle, I keep the bike a 5 to 20 seconds at max rpm in fourth, before going back to 8500 rpm, it's not build as a racer, so I'm a little careful in those ranges. Sometimes I change gears too quickly between second and third, going full throttle in an in-between neutral, seeing the rpm needle going over 10k RPM, so theoretically I could go faster in fourth gear, but I don't have the balls for it. The bike is handling fine, and behavior is ok, but the risk of an exploding engine is a little too high.

If I want to do 10k+ rpm, I'll take the CB400F for a spin, that bike eats 10K revs for breakfast, if not careful, it easily goes deep into the red zone.

 

[cellomaster]

If you followed that video, it’s incomplete - there’s still setting the right side that needs to be done, which will certainly make a difference at higher RPM - particularly if it’s slightly “tight” on the right side.

I did do both sides, but I think due to the how the old style point system works, you can only get so close without a timing light!

Thanks for all the info and data on speeds, super interesting to see! Would it be worth it to drop my rear gear down to a 33 tooth? Does it gain that much speed, and does it increase drivability or do anything else?

 

[jensen]

btw, I created these sheets to find the optimal secondary ratio (sprocket set). In the original files I can change the value's in the yellow box. Since we have 5 speed-limits (30, 50, 80, 100 and 130 km/hr), it is nice to know at which rpm in which gear these speeds are reached. Usually I also determine the optimal shift pattern in relation to the speed limits.

 

[EzPete]

I did do both sides, but I think due to the how the old style point system works, you can only get so close without a timing light!

Thanks for all the info and data on speeds, super interesting to see! Would it be worth it to drop my rear gear down to a 33 tooth? Does it gain that much speed, and does it increase drivability or do anything else?

Dropping to a 33 might give you 4 MPH top end. Personally, I'd solve for the "dead top of throttle" issue before just firing up a parts cannon. If your points are at proper gaps, the next step would be a plug chop at high RPM to try and get an idea if it's a fuel or air issue.

 

[ancientdad]

I did do both sides, but I think due to the how the old style point system works, you can only get so close without a timing light!

When you adjust the points and set the timing according to the method described in the FSM, you balance the points gap within the range (.012" to .016") for timing the right side after setting the left. Here is the method described in words:

Start with turning the crankshaft until the points cam opens the left set of points to the fully open position and setting the gap of those points at .014", then turning the engine until the right set of points is fully open and setting them at .014", then adjust the left side timing by rotating the points plate so the left points break at LF. You then lock the points plate and check the timing on the right cylinder to see if the points break at the F mark and if they do not, you adjust the gap to get the right points to break at F. If after that adjustment is made, the gap on the right points is less than .012", you go back to the beginning and reduce the left points gap to .012", set that timing by rotating the plate to open at LF, then check the right side again and adjust the right points to open at F.

Yes, it can take a few tries to get both correct, but that is the procedure since there is only one plate that both sets of points are mounted on, so when you rotate the plate to advance or retard the left timing it affects both.

Would it be worth it to drop my rear gear down to a 33 tooth? Does it gain that much speed, and does it increase drivability or do anything else?

Part of gearing changes is whether or not the engine has enough horsepower to overcome the higher gearing. With the stock power output of these engines, higher gearing might look good on paper and lower the rpms enough to help with vibration at cruising speeds, but anything higher than stock gearing will not increase the actual top speed because the engine won't pull the bike near redline, or even power peak of around 8500, in 5th gear with stock sprocket gearing. Higher gearing will only mean lower rpms at the same speed, and going too much higher in sprocket ratio will actually decrease top end speed.

 

[cellomaster]

Here is the method described in words:

Excellent!! Thank you for laying this all out for me! I will definitely do this, maybe tomorrow.

Personally, I'd solve for the "dead top of throttle" issue before just firing up a parts cannon.

Totally agree! I was just curious more than anything! Not like I have money right now anyways...life sucks money FAST!

 

[MIBOWL]

As an engineer, I usually calculated the gear ratio's versus road speeds for the CB450 K0, CB450 K1-K7 and for the CB500T, here you go:

CB450 K0 (calculated with the 33 teeth sprocket instead of the 35 teeth standard sprocket):

View attachment 57058

CB450 K1-K7 (standard sprocket gear ratio's):


View attachment 57059


CB500T (standard sprocket ratio's):

View attachment 57060


So, as you cab see, 90 mls/hr isn't so bad, but looking at your RPM range you mentioned my guess would be that you were closer to 80 mls/hr @7-7,5 k rpm, as you can see in the second sheet.

Hi Jensen, I'm totally impresed with these tables, do you have any equivelents for the CB72 ?

 

[RobMan]

CB450 K1-K7 (standard sprocket gear ratio's):

Thank you for these tables. I copied the image for the CB450 K1-K7 and save it so I can easily use it as a reference. Both my speedo and tach get some flop at the high end so a bit of a guess.

 

[GMX]

The other thing that bit me in the butt was running out of gas and sitting on the side of the road for half an hour while someone went and got me some gas...I would love to put some sort of gas gauge on this, so I am not having to rely solely on what I think im getting MPG wise, and base that off my trip odometer. Any ideas on gas gauges that don't ruin the look, change performance, nor break the bank? Maybe I should just get better at learning the RES vs ON fuel selector...

Just ran across a pic I had taken a while ago of the petcock intakes residing inside the tank. It will give you a good idea of what is going on as the fuel level drops requiring the switch to reserve... (not sure what that blue blob is, but I did clean it out at the time)...lol

 

[ancientdad]

Just ran across a pic I had taken a while ago of the petcock intakes residing inside the tank. It will give you a good idea of what is going on as the fuel level drops requiring the switch to reserve... (not sure what that blue blob is, but I did clean it out at the time)...lol

That's a good shot of the On and Reserve feed tubes inside the tank for perspective on the fuel levels they function with.

 

[jensen]

Thank you for these tables. I copied the image for the CB450 K1-K7 and save it so I can easily use it as a reference. Both my speedo and tach get some flop at the high end so a bit of a guess.

I could send the sheet to you, so you can fill in the sprocket sizes and play with it ?

 

[RobMan]

I could send the sheet to you, so you can fill in the sprocket sizes and play with it ?

I would appreciate that. I can PM you my email address or perhaps you can share the sheets in the Technical Stuff section, I am sure others would love access to them as well.

 

[jensen]

I don't know if I can share the sheets in the technical stuff section. @ancientdad can I upload an excel sheet to this forum ?

 

[ancientdad]

I don't know if I can share the sheets in the technical stuff section. @ancientdad can I upload an excel sheet to this forum ?

No, I'm limiting it to just pictures to save server space. But, you could upload to any online storage and link it here.

 

[CarnivorousChicken]

Aside from using the reserve on the petcock, resetting your trip odometer -- or paying attention to the regular odometer -- will also give you a good idea where your fuel is, although it obviously can vary with traffic, what kind of riding you're doing, etc.

 

[cellomaster]

Aside from using the reserve on the petcock, resetting your trip odometer -- or paying attention to the regular odometer -- will also give you a good idea where your fuel is, although it obviously can vary with traffic, what kind of riding you're doing, etc.

Yeah, I think I average around 40mpg, based on my...ahem, spirited riding. I think I was just mad with myself the other day. It really isn't hard to keep a good eye on gas levels, and in the end, I don't *actually* want a gas gauge. Stock looks much better!