Topic: CB750 max RPM?

[Sam Green Racing]

Have you got no work to do

stuffy cnut

Sam.

[Terry in Australia]

I'm on vacation!

[Sam Green Racing]

I know, thats why I called you Stuffy.

Sam.

[Terry in Australia]

Ha ha, so back to max RPM's etc, my newly built 836cc engine is still holding together quite nicely, with the following mods:

Stock K6 crank,
RC steel rods, (modified Honda)
Arias 12:1 836cc pistons
M3 Cam Chain tensioner,
APE Studs,
Mike Rieck stage 3 head,
Megacycle 125/75 cam,
Keihin CR29mm carbs,
Dyna S ignition and 3 OHM coils, and
Unknown brand new 4 into 1 pipe.

I "gave it the berries" a week or so ago and @11000 RPM (approximately, my tach only reads to 11000) in third gear the speedo was reading 120 MPH. I ran out of road before I could do any more, and so have no idea what it will do in fifth, and to be honest, I was shiiting myself on a bike with 2 inch over front forks and pullbacks, and a lowered rear end, 16 inch wheel etc, the front was getting awfully "light" so I erred on the side of cowardice.

At those RPM's, the engine felt strong and was still accellerating, so I've no doubt that Mark's 14000 RPM  limit is achievable, with the engine in a state of "perfect" tune. (I still need to raise my needles another notch, as its still too lean, methinks)

Due, I guess, to the 125/75 cam the bike feels a little weak down low. Mike Kramer has sent a stock crank to APE for me to be lightened and balanced, so I'll install that when it arrives and see if that makes a difference, if the lightened crank makes the engine spin up faster as has been said before, that might make up for the slight drop in torque. If not, I'll try a "torquier" cam grind, at the expense of a little less "top end" BHP. Cheers, Terry.

[sparty]

Ha ha, so back to max RPM's etc, my newly built 836cc engine is still holding together quite nicely, with the following mods:

Stock K6 crank,
RC steel rods, (modified Honda)
Arias 12:1 836cc pistons
M3 Cam Chain tensioner,
APE Studs,
Mike Rieck stage 3 head,
Megacycle 125/75 cam,
Keihin CR29mm carbs,
Dyna S ignition and 3 OHM coils, and
Unknown brand new 4 into 1 pipe.

I "gave it the berries" a week or so ago and @11000 RPM (approximately, my tach only reads to 11000) in third gear the speedo was reading 120 MPH. I ran out of road before I could do any more, and so have no idea what it will do in fifth, and to be honest, I was shiiting myself on a bike with 2 inch over front forks and pullbacks, and a lowered rear end, 16 inch wheel etc, the front was getting awfully "light" so I erred on the side of cowardice.

At those RPM's, the engine felt strong and was still accellerating, so I've no doubt that Mark's 14000 RPM  limit is achievable, with the engine in a state of "perfect" tune. (I still need to raise my needles another notch, as its still too lean, methinks)

Due, I guess, to the 125/75 cam the bike feels a little weak down low. Mike Kramer has sent a stock crank to APE for me to be lightened and balanced, so I'll install that when it arrives and see if that makes a difference, if the lightened crank makes the engine spin up faster as has been said before, that might make up for the slight drop in torque. If not, I'll try a "torquier" cam grind, at the expense of a little less "top end" BHP. Cheers, Terry.

Terry,

The APE lightened crank makes a huge difference.  I am also running the 125/75 cam and it is not as weak down low.  When you do install it you will find that it zings the RPMs quickly.  Just hold on as the front wheel become very light above 8000 even in fourth!  It is funny that your are running lean.  I am still tad rich with the stock setting on my CR29s.

Sparty

[Terry in Australia]

G'Day Sparty, well you might be right mate, I raised the needles again (clip now in #4 of 7 notches) and I think that it's too rich, although I haven't taken it for a ride yet, but it's blowing a fair bit of black smoke when I give it a decent rev. Are your carbs 29 or 31mm? Cheers, Terry.

[sparty]

G'Day Sparty, well you might be right mate, I raised the needles again (clip now in #4 of 7 notches) and I think that it's too rich, although I haven't taken it for a ride yet, but it's blowing a fair bit of black smoke when I give it a decent rev. Are your carbs 29 or 31mm? Cheers, Terry.

Terry,  I have the 29s.  I am not blowing any black smoke and my clips are on #4 with 120 main.  I am running a cooler plug and the Dyna 2000 is set on No.4 curve.  I think my ignition advance is around 33.5.  I am running 95 octane which helps and K&N filters.  It seems like a good combo.  I have the Kerker 2" baffle which also helps.  I rather be a tad rich than a tad lean.
When Mike comes down in the Spring we will tune it some more.  Those CR are sensitive from zero to 1/4 throttle, you can't just wack it open.  From 1/4 to full you can give it the berries.  I have yet to go full.  3/4 is a ton of power already.

Sparty

[754]

Run stiffer fork oil if it is feeling light in the front in 4th..

[Terry in Australia]

G'Day Sparty, well you might be right mate, I raised the needles again (clip now in #4 of 7 notches) and I think that it's too rich, although I haven't taken it for a ride yet, but it's blowing a fair bit of black smoke when I give it a decent rev. Are your carbs 29 or 31mm? Cheers, Terry.

Terry,  I have the 29s.  I am not blowing any black smoke and my clips are on #4 with 120 main.  I am running a cooler plug and the Dyna 2000 is set on No.4 curve.  I think my ignition advance is around 33.5.  I am running 95 octane which helps and K&N filters.  It seems like a good combo.  I have the Kerker 2" baffle which also helps.  I rather be a tad rich than a tad lean.
When Mike comes down in the Spring we will tune it some more.  Those CR are sensitive from zero to 1/4 throttle, you can't just wack it open.  From 1/4 to full you can give it the berries.  I have yet to go full.  3/4 is a ton of power already.

Sparty

Thanks Art, are the 120's the standard mains that come with the carbs when new? I haven't touched the jetting yet, as I figure they have to be "ball park" for our engines, and I can fine tune them with the needles?

I'm taking it for a ride out to Steve K0's place in the sticks on Wednesday, (he's almost got his sandcast on the road so I want to check it out) so I'll find a nice quiet stretch of road and see what this needle position is like.

When I got the carbs, the needles were in the 6th (from the bottom) notch, can you remember where yours were? You're right about not being able to just whack the throttle open, it's a little disappointing actually? Still, once you get it into the "zone", it's scary fun! Merry Christmas mate!

Run stiffer fork oil if it is feeling light in the front in 4th..

Thanks mate, I'm actually running engine oil in them, but I think it was "light" because the wheel might not have been touching the ground at all times? Cheers, Terry.

[cafe750]

So, just out of curiosity, are there any perfomance gains to be had by swapping an f2 cam into an otherwise internally stock K3 750 engine?  It's got K+N Pods, and NOS HM300's. The only reason I'm asking is that I have a locked up f2 motor, and was looking at throwing a larger cam into my '73 anyway...

[HondaMan]

Yes. It will yield about 8% more breathing, bringing power on at 6500 RPM instead of 5500 RPM, as in the stock "K" engines. Typical HP increase is about 2 HP.

[sparty]

Terry,

You really crack me up my fellow down under CB 'er.  How is that Dyna S working for you?  Watch your revs pal.  Are those RC rods up to snuff for those high RPMs.  Can those RC rod bolts handle those loads for more than a few miles?  Or are you using after market bolts?  I'd hate to hear about a let go.  

You should consider getting into my coffee table book.  I am sure you can write up an nice story or two to go with pics of your rides.

Happy New Year!

Sparty

[Terry in Australia]

Terry,

You really crack me up my fellow down under CB 'er.  How is that Dyna S working for you?  Watch your revs pal.  Are those RC rods up to snuff for those high RPMs.  Can those RC rod bolts handle those loads for more than a few miles?  Or are you using after market bolts?  I'd hate to hear about a let go. 

You should consider getting into my coffee table book.  I am sure you can write up an nice story or two to go with pics of your rides.

Happy New Year!

Sparty

Coffee Table Book? What is this Coffee Table Book, of which you speak Art?

And don't worry about those old RC steel rods, they were fine back in the day, when nothing better was available, but the longest I spend at those kinda revs can be counted on my fingers, because at those speeds I'm well in the "Go to jail" range. If I was gonna race it, I'd certainly look for better rods, for sure.

I'll check 'em when I pull it apart again to install the APE crank, but I've been listening for any tell-tale "knocking" noises, but all I can hear so far is the rattle of tappets set at .005 for that 125/75 cam.

The Dyna S ignition and 3 Ohm coils are working fine, so far, no problems in the ignition department, I just need to play with the idle mixture on the CR's, and drop the needles a notch. I'll do that as soon as I get back home. Have a Happy New Year mate! Cheers, Terry.

[andy750]

So, just out of curiosity, are there any perfomance gains to be had by swapping an f2 cam into an otherwise internally stock K3 750 engine?  It's got K+N Pods, and NOS HM300's. The only reason I'm asking is that I have a locked up f2 motor, and was looking at throwing a larger cam into my '73 anyway...

This is what Im doing this winter - I have a F2 cam coming from Joe (Joelb) and will be putting it into my CB750K4 which had the top end rebuilt last year and mildly ported. I also have HM300s but think Ill be swapping these for HM341s and putting the 300s on my K2 (reason being I have small rustt holes appearing at the end of the 300s and the mildly higher compression K4 engine cant be helping). Im also putting in some HD studs to hopefully prevent any future oil leaks from the gaskets.

cheers
Andy