Topic: CB750 Go Fast Engine Ideas

[RRRToolSolutions]

I have also done the Mach III and Mach IV route, but I like the Honda much better.

You could always add one these weedwacker things - I am simply amazed what it does to a simple 2 valve engine with a de-tuned cam have less duration and lower valve lift, as-cast ports, 7.8:1 compression, and only 738cc. 2,000~4,000 rpm is like your stock CB500 Four. At 5,000 + rpm it was the 2nd fastest bike Cycle World had ever tested in 1984, losing to the GS1100E only by .02 seconds and the pilot stated clearly that he could not master the launches because the boost was too much for the stock tire.

I have a bunch of bikes and my CB750 will likely be one of my all-time favorites. Built right, it can be riden at any rpm between 2,000 and 10,000 - whether that's a fun ride 2-up on a Sunday or out blazing the mountain roads with go-fast friends. My K-triples never handled the causual stuff very well.

Gordon

[chewbacca5000]

Wow Gordon!  That thing looks like a freaking Monster!  Are you sure it's still a CB750 and not some experimental military aircraft.  All you need to do is add a rocket booster and you can fly rich folks to the moon! 

[MCRider]

Wow Gordon!  That thing looks like a freaking Monster!  Are you sure it's still a CB750 and not some experimental military aircraft.  All you need to do is add a rocket booster and you can fly rich folks to the moon!

I don't think it is a 750. More like a bored out 500/550? hard to tell close up, but motor mount area doen't look it.

[Retro Rocket]

Points cover screws are in the right place for a 750....

[MCRider]

Points cover screws are in the right place for a 750....

Kinda looks that way, but look at the motormounts...

Which is it Gordon?

[RRRToolSolutions]

Not a Honda guys. I bought this one for a winter restore project last November after seeing one at the Barbers museum the month before. It is a very mild tuned 2-valve 738cc engine that becomes a monster when the pressure is applied. As the May 2011 issue of Cycle World editor wrote "you don't want to be eating a sandwich when the boost comes on". This bike did a 10.71 1/4 mile in stock form back in 1984.

[MCRider]

Pretty!

No fair throwing curve balls. 

[HondaMan]

To see Mikes work and see what a port job can do with new cam, 810cc pistons, stock carbs, stock rods, stock airbox, stock HM300s then go here:

http://forums.sohc4.net/index.php?topic=32093.100

Dyno results = 73.5 hp (rear wheel) - could be improved with revised clutch, new rear tire and new chain - and just discovered swingarm bushings are shot Pulls strong above 5000 rpm and my own personal top speed is 104 mph (by GPS) before I backed off - but had more.

Good luck
Andy
 

That should put you in the 80 HP family, Andy! The gearbox/bottom end typically loses between 5 and 7 HP on the SOHC4. This can be improved, though: use the harder ball bearings (like SKF, today) with the "extra clearance" option (0.0003" extra), fresh primary chains, a single-row bearing (K0-K2 early) output shaft, and chamfer the oil holes on the tranny shafts where they feed the centers of the gears (except those with the bronze inserts). Remove the 5mm bolts in the top end rocker shafts, even add a couple extra oiling holes to encourage those shafts to spin, then use the K0-K1 cam sprocket (or the post-1976 version) with the lightening holes, a K0 spark advancer (or make your own from another era's model, to 41 degrees advance) and raise the mainjets to 115 while dropping the carb needles a notch from where they started. Use the ND X24ES-U sparkplugs with 5k ohm plug caps, and that 810 kit will go to 10K RPM and find even more ponies, with no loss of reliability over the 736 engine.

I only wish the 810-811-812 pistons were easier to find. They seem to me to be the ideal compromise between longevity (100k miles) and HP.

[chewbacca5000]

Thanks HM I'd be more than happy with those numbers.  Seems like Andy's engine has more than enough power to bend a rod with the ability to spin up to 10,000 RPMs.  Not sure I have enough disciple to keep off the gas.  Pretty sure I will punch it once in a while.  I am thinking of selling some of my stock goodies to raise funds.  Much thanks lots of good food for thought.