Topic: Performance Question CB750

[John G]

I purchased a 1975 CB750 which was in the stages of being rebuilt. The cam that came with the bike was a new race cam with high performance valve springs and performance shims. I just wanted a nice running motorcycle to use on sunny days. Relive the younger days. Haha. Can anyone tell me what I would be up against with a high-performance engine for street use. I'd love to hear from you as it seems a waist to have these new parts and not use them.

[madmtnmotors]

You need to determine what cam you have. Either brand and part #, or at least lift and duration. Too much lift and you may need valve pockets enlarged on the tops of the pistons.

[John G]

This may help

[Old Scrambler]

You have basically the equivalent of OEM parts. Good ones........but plain vanilla

[John G]

Hi Old Scrambler. Are you saying this is not a racing cam. It's a stock cam?

[Jerry Rxman Griffin aka MuthaF'er]

Yes, you are correct. There is nothing "racing" about that cam grind. Basically close to stock.

[newday777]

I purchased a 1975 CB750 which was in the stages of being rebuilt. The cam that came with the bike was a new race cam with high performance valve springs and performance shims. I just wanted a nice running motorcycle to use on sunny days. Relive the younger days. Haha. Can anyone tell me what I would be up against with a high-performance engine for street use. I'd love to hear from you as it seems a waist to have these new parts and not use them.

That cam you have is a Web Cam #41 and is marginally better than the stock cam(it's not a hot race cam) so you won't have any mechanical problem running it on the street. You can even run a hotter cam on the street, as well as building it up more with bigger bore, porting the head for better breathing to allow a better cam and displacement. The big bore 836 motor is a very dependable street motor and can be babied around or get on and have more fun when you want to twist the right hand more.

Here is a good explanation of the comparison of the #41 Web Cam vs the stock cam.

https://www.tapatalk.com/groups/hondachopper/webcam-41-or-stock-cam-t56222.html

[John G]

Once again I'm impressed with the help within the SOHC/4 Group. So knowledgeable and helpful. To bad I didn't ask this earlier before I bought and installed the new .025 over pistons. I suppose I will run the Kibblewhite performance valve springs and spring shims. Now I'm very excited to get this done. Just in time for WINTER. Burrr.

[PeWe]

No need for race springs in a stocker.
Fresh +0.25mm pistons will run better with a fresh head.

Check if valves wobble sideway in guides when wiggle it with fingers holding the "disc",  lift ca 10 -15mm.
You should not feel a lash, a softness ok.
Exhaust guides will go bad first.

A high power engine can run very smooth if carbs and ignition are tuned OK.
Both my bikes have Dyna-S ignition and advancer springs shortened. 
I had points on my K6, but dyno graaf looked not good at higher revs.

K6 970cc - 107.5Whp (done twice on dyno 2 weeks between runs),
K2 748cc (+0.50) - 58Whp.

If you want more power, higher compression pistons with deeper valve pocket is a must. Then ready for higher lifting cam.

Or cheap pistons from Cruzinimage for a little bit more compression only. 836cc or later K7 (392) pistons in an earlier engine. My K2 has +0.50mm CI K7 pistons only. 58+Whp on dyno. But still rather tame, it take time between gear shifts during WOT.

An 836 with ported head, bigger in valves and modest cam still using stock carbs is a huge difference.

Better pistons and hotter cam  will bring ported head since the old head might have worn valve guides, exhaust especially. If engine has done over 50- 60.000 km, very likely.
Restore a stock head replacing guides, valves and cut all seats is one big step needed anyway.
A port job at the same time will help.
If you plan to use cam with higher lift, the guides must be replaced with new that is sunken deeper in the head.

Either stock guides with a new cut stop groove for lower placement or CycleX shorter iron guides.
If not, engine will drink lots of oil when valve keeper groove will enter the guide seal sealing lip.

Foto of an oil drinker with stock modified guides below.
9.6mm between seal and groove. It was sunken but not enough. (Measured an old stock head, ca 9.2mm)
It need running lash + at least 0.5mm, my head >11.3mm

My cam lift IN ca 9.8mm at running lash.
 At higher revs valve can go a little deeper.
This was found rather soon so the shop could remove guides and install them even deeper and back in again. Guides marked so they went back in exact as they sat.

Improve power need more engine jobs.

Maybe a complete restore of the engine is needed? Total teardown, check all chains, tensioners probably in need of replacement. New clutch etc. Rebore with new pistons, cheap Cruzinimage or better.

Forummember Don R has recently got a set of NOS 812cc pistons he might sell?
Find another old stock cylinder and bore it to 64mm.

Photo of the oil drinking seals when keeper grooves enter the lip.
Aftermarket "race" valves should have a shorter groove. -1.5mm would help.

[MRieck]

If you are using .25 over pistons that cam is a good choice as the piston valves reliefs cannot accommodate a larger lift cam. Just changing the springs will give you a 10 to 15% increase in power secondary to good valve control.

[PeWe]

OK, so stock springs are not that good even with close to stock cams.
My K2 with stock K7 cam got Kibblewhite springs to be prepared for a KH-D cam that unfortunately did not end up in OK valve-piston clearance.

I rev some extra with harder springs I thought could steal some power in stock config.

[johno]

Re springs, Mike is spot on as usual, I haven't seen a set of original valve springs that measured up to spec for many many years. They are standard spec around 50 to 60 lbs at installed height but time has caught up with them and they are usually around or less than 40lbs and don't like big RPM. If on a budget put a shim under them to get back to std pressure, better than nothing.
So John, lucky you: run the new springs and the Web cam. Its a nice cam and as you stated how you intend to use the bike it will give you a lot of fun.
Pewe is right too about checking the valves being OK and seated well, guides, seal etc. Check the top tip of the valves as the adjuster wears through the case hardening and pits the valve end, replace valve if that's the case.
Generally if people want to get their youth back, what you have will give you plenty of opportunity John.
ciao John

[PeWe]

I agree with the Pro's,
Play with the good cards you have. Going for more power starts a domino effect of expences.

Sheer luck that my stock engine got better new springs.
Need another set for a stock head restore I have planned.

The bike will feel stronger with 17:48 drive sprockets. (Stock 18:48 on K2-K6)
Gearbox feels like it was designed for it is my experience.

Not much more revs around 60Mph on 5th gear, only 250rpm extra.

[John G]

A real novice here talking. I'm hoping this makes sense to someone.
I have a Grind # 41 which We Cam doesn't appear to sell any longer.
The paperwork in the box says the valve lift for the Grind 41 is Intake 0.360 and the Exhaust is 0.360
I go to Web Cam's web page and the have cams listed for this engine as 341 and 341a and other.
The 341 cams say good for a stock engine and this cam's valve lift is .340
The Highest cam they sell is a 363b with a valve lift of .349 which is .011 lower than the Grind 41 cam that I have.
This 363b cam says:
Strong mid and upper end performance for mild track / strip. Requires Exhaust System, High Compression Piston / Big Bore, and Performance Valve Spring Kit. Check ALL clearances!
Follow me now. I put the Grind 41 cam in my lathe and run a dial gauge on the loabs and the highest reading I get is .328 and .331
Where am I going wrong? Dazed and confused.

[Don R]

 I'm not sure about the 41 grind itself and the obvious differences in numbers, but lift is only part of the story, duration, overlap, valve timing and ramp speed are also a big part of the profile.
 If it were mine I'd throw it in, time it, check valve to piston clearances and run it.

[John G]

Thanks Don. I'm working on it. Some things don't make sense to the average guy. Wish I knew more. Cheers

[richmagee7]

John,
I believe most cam manufacturers list 'valve' lift which is gross valve lift to include the effect of rocker arm ratio.  I had the same questions after measuring 'lobe' lift on a performance cam I acquired.  I measured the rocker arm ratio on my 75 750 to be 1.1 to 1.  Adding 10% to the measured lobe lift resulted in the valve lift number the manufacturer had listed.

So, taking your measured 'lobe' lifts of 328 and 331 and adding 10% would give you 'valve' lift of about 361 and 364, close to the advertised 360, 360.

Rich