Topic: Revolutions and MPH

[Thousandson]

So I have a cb750 78. It red lines at 8.5. When I ride down the highway, I'll be in 5 gear about 5,000 RPM, and 65 MPH. I have driven large trucks for a long time, low revolutions. When I ride my bike going over 5,5000 RPM to get to 70+ MPH for long periods of time makes me a little worried if the engine can sustain the higher revolutions for a long time. My question is can I stay at 6,000-6,5000 RPM for possibly and hour or more and not cause internal damage?

It sounds like a dumb concern, its a 37 year old bike, I ride mostly in New England(temps will be around 50-100F). I have taken apart the engine, and reassembled it myself. I really enjoy my bike, I don't want to see her go to the scrap yard on my dumb mistake. 

[Jerry Rxman Griffin aka MuthaF'er]

Just keep your oil changed and keep doing what you are doing. You can always change the sprocket ratio if you desire to lower the rpms but it probably won't be drastic and you'll lose effective power. These engines were made to rev.

[MCRider]

Yes they were built/ designed for it. You shouldn't drop below 3000 or so without gearing down. Cruise at 6000 should be indefinite in length. Testing went 8000rpm for 10+ hours.

Change the oil every 1500 - 2000 miles and examine the filter and housing for debris. Age is our problem now. Extended periods of non-use allows hardening of certain rubber engine parts.

[70CB750]

If you keep her running at 6000+ rpms, she will only run better and better.  Nothing keeps the CB750 SOHC in better shape than riding the bejesus out of her.

[PeWe]

My CB750 K6-76 had no problems with 6000-8000 rpms all day long on German Autobahn. Mostly 140-180kmh. Full speed on 3:rd gear was 160kmh (9500rpm) when accelerating from pit stop or when passing cars. 9000rpms on 4:th gear. My engine has harder valve springs, 9000-10.000rpms no problem except for the std rods that still keep together... Had a cam in earlier this year that gladly passed 10.000...
Mostly 17-48 gearing back in those days. Today back to 18-48 (530 chain and sprockets)

I had to refuel every hour when driving constantly 160-180 kmh. Fuel, coffee/food and a smoke, then hit the road again. (30 minute pit stop, longer when eating)
Sometimes the speedo hit 200kmh.
The tourings were 5000-6000km in total. The oil was changed after 7000-8000kms. Mostly Castrol GTX2 and similar.

I use even better oil today I cannot see why I should change oil and filter more often than 6000kms intervals.  New pistons or opened engine a different thing.

EDIT: Correct ignition and carb jetting is the most important thing to verify. Not too lean or way too rich. Pinging can be too lean fuel mixture. I have had that, retard ignition then will make engine sloooooow.

[Deltarider]

I used to ride my 500 om the Autobahns and autostrada @ 7000-7500 rpm (140-150 km/h). Advantage: a vibrationfree ride and well cleaned combustion chambers. After such a ride the engine always felt younger and responded better.

[Desert-SOHC]

Sounds like your sprockets aren't stock, my 78 with stock cogs at 15/41 and the 630 chain is doing 75mph at 5,000rpm, at 6,000rpm I'm at close to 90mph. Even at a buck she is barely singin at 6600rpm and really doesn't seem to care much......sounds HAPPY! still with some throttle left there.

[750K]

As said above, you're good to go. The cb750's love to be higher in the rpm's, it's where they shine in my opinion. They can putt along but love to be opened up, or maybe it's me that loves it lol.

[HondaMan]

After the first 140,000 miles on mine, with that kind of gearing, the engine was happy as a clam! I finally rebuilt it, just now getting it broken back in again.

Here's some interesting insight from Honda's testing rides in 1968: the 5 test riders took the first of these 5 show bikes to Nevada (after the expo show in New York), where they were ridden back and forth between the Boulder Dam and Lighthouse, with strict instructions to "not leave the red zone" on the tachs. They were told to gear down as needed, and were allowed by the Nevada State Police to run as fast as they wished, all week, on this stretch of road. None of the 5 bikes broke, nor broke down, and all were reported to be running considerably faster at the end of the week. The only 'casualty' of the tests were the drive chains, which had to be replaced several times. I have a copy of this magazine story somewhere in my 'pile' of old stuff.

[MCRider]

After the first 140,000 miles on mine, with that kind of gearing, the engine was happy as a clam! I finally rebuilt it, just now getting it broken back in again.

Here's some interesting insight from Honda's testing rides in 1968: the 5 test riders took the first of these 5 show bikes to Nevada (after the expo show in New York), where they were ridden back and forth between the Boulder Dam and Lighthouse, with strict instructions to "not leave the red zone" on the tachs. They were told to gear down as needed, and were allowed by the Nevada State Police to run as fast as they wished, all week, on this stretch of road. None of the 5 bikes broke, nor broke down, and all were reported to be running considerably faster at the end of the week. The only 'casualty' of the tests were the drive chains, which had to be replaced several times. I have a copy of this magazine story somewhere in my 'pile' of old stuff.

That was the "testing" I was alluding to in my post. I remembered that they ran really fast for a considerable time in the desert, but I couldn't remember the details.