Topic: Update: Project CB750 Hemi

[HondaMan]

This was one project that was worth the effort, IMO.

As my bike is breaking in from it's recent rebuild, I am finding that it feels like there will be no end to the top revs. It winds VERY willingly, even in high gears. The little torque dip that is characteristic of the 750K at about 4500-5500 RPM is completely gone, replaced with a smoother, stronger torque from about 2500 RPM right through to that range (I have not passed 6500 RPM yet: it only has 300 miles on it as of yesterday). The surging that these engines often have, under light loads at higher RPMs, is also completely gone (that was a surprise!). 

My final compression ratio came out at about 8.9:1, and it runs on regular gas (always had to be premium before). I've had to lean out the jetting a bit because the breathing is better, but the carbs are the same size: this usually causes a rich condition. During this break-in, I am running #7 plugs to help keep it cleaned out in the HEAVY traffic I have to fight.   

My intent was to make my CB750K a better commuter bike. My "K"s have always been a bit high-strung and with the touring gearing I've used (18T/48T) for a long time, a little cumbersome in traffic. Now (17T/48T and new heads), it is considerably more nimble on the throttle, I can use higher gears over wider RPM ranges with less shifting, and it "feels" happier in the midst of 20-50 MPH traffic. Although I have used the 17T/48T combination before, the surging was worse than with the taller gearing. Now, it's smooth & hot!

[Pinhead]

Any MPG figures?
What jets did you finally end up using?
What other carb mods did you have to do?
Are you using stock or flat-top pistons?
Do you think it'd be possible to run flat-tops and shave the head for compression, or would piston-valve interferance come into play?
How much did you have to modify ignition timing? Advance? Retard?

[HondaMan]

I haven't checked MPG: I won't until I balance the carbs, which will happen if this rain lets up this weekend.

I'm going to end up with either 100 or 95 jets: it seems to be right in between there. Stock was 110.

I drilled the emulsifier tube's holes to .039", all 6 pairs.

The pistons are flat-topped. With the hemi arrangement, you could use domed pistons, but do the compression ratio math to figure out if you have to mill the head or not. I milled mine .010" to get back to almost the original CC volume: ended up with 0.6cc more than stock (for those numbers, go look into the "hemi" posts). One chamber was 0.7cc more instead of 0.6cc, a bit larger than the other 3, but well within specs. The original chambers varied by 0.3cc from most to least on the 2 heads I have.

I'm running +2 degrees static advance, which puts the timing slighty positive of the full advance marks at 2700+ RPM. More timing can be added, but I'm waiting for break-in first. I'd like to increase the spring tension a little and open the full advance up to reach 38 degrees instead of the stock 34 degrees, at about 3000 RPM. The original K0 had 40 degrees advance angle, but requires premium fuel of 95 octane to run up there. The slower curve will improve MPG and smooth out the torque jump at 2000-3000 RPM.

[Hockers Choppers]

Are your carbs the same as a 78 f model? I'm thinking of doing my head, any advise? Should I have someone experienced do it? I've built engines and "cleaned up the ports" on heads before but don't have a flow bench or anything. Want to do a cam this winter and maybe a big bore kit? Any advice? What size of drill bit do you have to have to get .039? Or do you have a set of jet bits?

Jesse Hockersmith
Victoria, Ks

[HondaMan]

Are your carbs the same as a 78 f model? I'm thinking of doing my head, any advise? Should I have someone experienced do it? I've built engines and "cleaned up the ports" on heads before but don't have a flow bench or anything. Want to do a cam this winter and maybe a big bore kit? Any advice? What size of drill bit do you have to have to get .039? Or do you have a set of jet bits?

Jesse Hockersmith
Victoria, Ks

Anyone can do it with a steady hand, moto-tool (like a Dremel), and patience. Be sure to measure ("CC") the chambers to make sure they are coming out even.

Small drill bit kits, as numbered bits, are widely available from tool supply houses. Hobby shops also sell them. You usually have to use a pin vise to drill with, which are available from both, as well.

The "F" carbs are a little different on the outside, but if yours are conventional slide-type carbs, they are the same inside as the "K" models, for all practical purposes. Some of the 750s have vacuum-piston type carbs: the rules for these are different.

[Hockers Choppers]

is .039 what they measure with a micrometer? I thought you had to have a flow bench to port heads correctly? Hell I have a bunch of die grinders and the long porting carbides for aluminum. And an extremely talented friend that works in a damn good machine shop. Am I set?

[HondaMan]

The .039 is the micrometer number, yes.

I did not flow this head, only reshaped the chambers. Although, the head was elaborately flow balanced in 1974, which is a separate issue. I'm not a fan of increasing the intake flow numbers unless the engine is bored out quite a bit. The stock passages work very well with the stock undersquare configuration: opening the intake ports without boring causes flat spots to appear in the throttle range.

[Hockers Choppers]

Thanks Hondaman,
I'm leaning towards the big bore and a cam this winter. Has anyone ever heard of a mega cam that is just for the 78 f model? It has .407lift and 266/268 duration. I thought you couldn't run a big lift cam with the late f head because of the larger valves? Is this the reason for the lower duration? As far as porting,  I'm probalbly just going to clean up and smooth the ports, not removeing much material.
Thanks again for your help,
Jesse Hockersmith

[MRieck]

Thanks Hondaman,
I'm leaning towards the big bore and a cam this winter. Has anyone ever heard of a mega cam that is just for the 78 f model? It has .407lift and 266/268 duration. I thought you couldn't run a big lift cam with the late f head because of the larger valves? Is this the reason for the lower duration? As far as porting,  I'm probalbly just going to clean up and smooth the ports, not removeing much material.
Thanks again for your help,
Jesse Hockersmith

That's a lot of cam especially for an engine with known valve train issues. I wouldn't do it. The f2 are a pain in the ass when it comes to modifying...period. You'd be better off using a K or early F  head and modify the drain holes on the F2 cylinders and upper case. This allows you to use a Wiseco 836 kit. You can get oversize valves, good springs, Ti retainers too. I like the 125-65 cam with the 836 kit. There is an online article about Jerry Branch and porting a CB head....it'd be a good reference.

[jtb]

Jesse Hockersmith
Victoria, Ks

Jesse,
Where is Victoria?  I grew up in Fredonia, in the SE corner.
John

[Hockers Choppers]

Jesse,
Where is Victoria? I grew up in Fredonia, in the SE corner.
John

Victoria is right on I-70 10 miles east of Hays.

Thanks Hondaman,
I'm leaning towards the big bore and a cam this winter. Has anyone ever heard of a mega cam that is just for the 78 f model? It has .407lift and 266/268 duration. I thought you couldn't run a big lift cam with the late f head because of the larger valves? Is this the reason for the lower duration? As far as porting,  I'm probably just going to clean up and smooth the ports, not removing much material.
Thanks again for your help,
Jesse Hockersmith

That's a lot of cam especially for an engine with known valve train issues. I wouldn't do it. The f2 are a pain in the ass when it comes to modifying...period. You'd be better off using a K or early F head and modify the drain holes on the F2 cylinders and upper case. This allows you to use a Wiseco 836 kit. You can get oversize valves, good springs, Ti retainers too. I like the 125-65 cam with the 836 kit. There is an online article about Jerry Branch and porting a CB head....it'd be a good reference.

I thought that sounded like a lot of lift. What can I do with this bike to improve it's performance without spending $1000. I'd like to get it to close to 100hp. Is this even possible? I'd spend more if it will get it there. I'm new to the sohc but have been riding and wrenching bikes for 25 years. any advise is greatly appreciated.

Thanks,
Jesse Hockersmith
Victoria, Kansas
hocker1970@yahoo.com

[MRieck]

100 is tough with a NA engine that idles etc. I'm in the low 90's with 915cc and work but I put more work into it and will dyno it soon though I still don't think it's 100 especially with the dyno it'll go on. 1000cc would do it with the 5mm valve conversion kit and .400 lift cam. You'd need other stuff though the 1000cc would do it normally aspirated. It's tough to get 100HP out of these engines AND keep it electric start that will idle etc.

[Hockers Choppers]

could you give me an idea about what hp I'd get with a 836cc, cam and light porting with this 78 f, dyna ignition, pods, mac 4-1??? I think its rated at 75hp stock, but it doen't feel like it.

[MRieck]

could you give me an idea about what hp I'd get with a 836cc, cam and light porting with this 78 f, dyna ignition, pods, mac 4-1??? I think its rated at 75hp stock, but it doen't feel like it.

It doesn't feel like it because it isn't close. Get rid of that head etc and with some compression, port work etc  you will be in the 70's

[HondaMan]

With a little carb work, that 70 HP will always be on tap, too. Mine is right around 60-62 HP (I lose a little at this altitude), but never lacks any response now. I lowered the compression this time around, improved the breathing and cleaned everything out from 100,000+ miles of old carbon, soot, buildups, etc. then, polished the new valves, dropped a little on gearing and tire size for some new-found nimbleness in the heavy city traffic I get to enjoy every day.   

Interestingly enough, I've had several people that I work with, and who have been entertained by this same traffic and watched me in it, ask me, "how much power does that old 750 make?". And, some of THEM are riding crotch rockets. 

[Hockers Choppers]

Whats wrong with the f head? It has bigger valves. But lower compression? Could it be milled down to raise compression or a thin head gasket?

[MRieck]

Whats wrong with the f head? It has bigger valves. But lower compression? Could it be milled down to raise compression or a thin head gasket?

It has the larger chamber. I forget how many cc's it is , maybe 26 or 27. The K is about 21 or 22cc. You could mill it but you'd to take quite a bit off the head...probably about .060 or more to get it down to the K chamber. If you deck the cylinder to get .040 of squish you'd probably be taking off a total of .080 or more. That's quite a bit. The larger valves in themselves aren't that big deal....you can get 33.5mm intakes (you can fit a 34mm on the stock seat) for the K and 28.5 exhaust. I think the larger, 31mm exhaust valve really isn't necessary for good cylinder evacuation. Plus there is the issue of the exhaust valve angle and the accelerated wear on the guide. Aftermarket retainers aren't available for the F2 and it really needs them if you start running a cam in the .375 or .400 lift range. As a side note OEM valves are not available anymore and the F2 heads I've worked generally have the tips blown away and/or the valve faces ruined by sloppy guides. You can get Kibblewhite replacement valves which are very good BUT you need to go through M3 Racing and he charges something like 60.00 or 70.00 per valve....Yikes. Like I said, the best valvetrain set up I've seen is the Kibblewhite 5mm conversion. I wrote down the weight differences of the valves, retainers etc but I can't remember where I put them. Anyway, the intake is 34mm, the exhaust 28mm and anytime you lighten that part of the valve train the better off you'll be. Problem is the kit ain't cheap but good stuff usually is expensive.

[Hockers Choppers]

OK, so I need a k head  to save some $ and will be better performing and reliable? Will any year work? I see them on ebay all the time. Any issues putting it on my cylinder? Just bolt on? Any advice on a cam? I want it to be streetable when I'm done.