Topic: Pipes, AC and cam for a stock 78 CB750K

[maxheadflow]

I've been through a few places on this site and found a bunch of info on CB750s.  One thing I notice on the 78 CB750K is that the cam pretty much has 0 overlap likely due to smog.  Intake 0/40  Exhaust 40/0 This gives a nice linear torque curve but really limits peak torque and HP even when the intake and exhaust are opened up.  Can anyone suggest mild cam that can pep this bike up? While I've only ridden the bike for about 40 miles, it was pretty gutless compared to what I remember.  A good running K1 with pipes and AC and dyno tune would do 12.5 in the 1/4 back in the day.   While I really don't care for drag racing, I really want something with a little more pep for the mountains.

Looks like Webcam 341 or 341A or Cycle X CX1 would be the best bet.

[seanbarney41]

lots of the difference is just tune up stuff....k1 had more ignition advance built in, better carburetion, better pipes, better cylinder head ports and 17/48 tooth sprockets.  I don't think the cam shaft differences made much difference. All that k1 stuff can be fit to the k7.  Plus the k7 is heavier.

I had a k7 motor in a k5 frame with k1 carbs, advancer, MAC 4 into 1, 17/46 530 sprockets, 460 lbs and ran mid 13s my first day at the track with miserable launches that only slightly improved as the day went on.

I don't think a cam swap gains you much without some 10:1 pistons and all the 750 honda factory installed cams are not different enough to have any real advantage on their own.

`just my experience butt dynoing dozens of combinations

[scottly]

What is AC?? Air Conditioning??
Sean, I have compared a K1 advancer with a K7, and they have the same amount of advance.

[jgger]

Scottly, I thought the same thing when I saw that, pretty sure its Air Cleaner. Dang i scratched a few more hairs off the old bald noggin on that one!

[maxheadflow]

Air cleaner. KnN in the stock breadbox. Intake side opened up. Pods suck.

[seanbarney41]

What is AC?? Air Conditioning??
Sean, I have compared a K1 advancer with a K7, and they have the same amount of advance.

that is interesting...maybe you have to have a k0 adbancer?  Can you describe how to measure?  I believe I have at least 3 distinctly different 750 advancers judging by the size and shape of the stops.

[maxheadflow]

What is AC?? Air Conditioning??
Sean, I have compared a K1 advancer with a K7, and they have the same amount of advance.

that is interesting...maybe you have to have a k0 adbancer?  Can you describe how to measure?  I believe I have at least 3 distinctly different 750 advancers judging by the size and shape of the stops.

Many here seem to think that the ignition and cam timing changed in the early bikes but I suspect that nothing really changed until 77 when the K7s came out. Here are pics of the honda service specs  for the CB750 up to K5. Ignition and cam were the same. K7s and K8s got the neutered 40/0  0/40 which is shown in the tech section here.  No Overlap kills performance as cylinder scavenging is reduced severely.  If you compare to early cams, Honda added cam duration  on intake close and exhaust open to try and restore some of the performance.

It wouldn't surprise me if the advancer changed on those.  I suspect that the reason why they look different is that they came from different sources.  IIRC ND, tech, Hitachi.  Also advancers from CB500/550 and CB350/400 look the same and will fit. I don't remember if they were specifically marked.

 

[bryanj]

Yes they are on the flat surface but cant remember front or back, 500 is marked 323 early 750 300

[maxheadflow]

Yes they are on the flat surface but cant remember front or back, 500 is marked 323 early 750 300

Thanks,

I checked that spec book.  The 500/550 have the same advance range 1200-2500 but start at 5 deg then end up at 28-31.

[maxheadflow]

lots of the difference is just tune up stuff....k1 had more ignition advance built in, better carburetion, better pipes, better cylinder head ports and 17/48 tooth sprockets.  I don't think the cam shaft differences made much difference. All that k1 stuff can be fit to the k7.  Plus the k7 is heavier.

I had a k7 motor in a k5 frame with k1 carbs, advancer, MAC 4 into 1, 17/46 530 sprockets, 460 lbs and ran mid 13s my first day at the track with miserable launches that only slightly improved as the day went on.

I don't think a cam swap gains you much without some 10:1 pistons and all the 750 honda factory installed cams are not different enough to have any real advantage on their own.

`just my experience butt dynoing dozens of combinations

One thing to note about the 2 of the cams (3rd don't have a cam card) in the list is that they have intake close at the mid 30 degrees which means they build cylinder pressure better than the smog cams in the K8s which is 40 degrees.  Dynamic compression will likely be up if the pipe works well with the overlap.