Which engine should I use?

[kjs5013]

Hello everyone!

I am a long time lurker, and finally got around to joining. I have used these forums for help over the last couple of years, but needed a more specific answer this time.

So, a few years back I decided that my back had finally decided that sport bikes were no longer an option for me. I wanted something sportier, and with a little more soul so I picked up a naked bike (KTM Super Duke R 1290). And as much as I loved the bike I wanted to get back into a Honda. The idea of a Cafe bike intrigued me. I wasn't sure if I was up to the challenge, but wanted to give it a shot. I ended up buying a 1978 CB750. The catch was that it came in about 10 different boxes. I was told everything was there, but I was definitely lied to. After a couple years of working on it off and on, I have a running bike. I have always been good with small motors like on lawnmowers, and figured it couldn't be that much more involved. Needless to say, I was wrong, but ended up overcoming most of my issues.

I found it easier to buy another bike to uses as parts rather than trying to source each and every part. Then found a couple more "good deals" on bikes. So now I have plenty of parts laying around. What I would like to do is start on a new project and take it a step further and completely rebuild as much as I can. What I would like to know is what motor should I use? I have acquired 6 different motors. I am planning on boring it, and possibly running a hotter cam. I don't think that I want to go too extreme, but have also been intrigued with the cycle x 1062 stroker kit.

I have 2 solid frames, so I could possibly build 2 bikes over time. Both frames are F models, and I have the matching engines for them. One is a 75, and the other is a 77. I have read horror stories of valve problems with the later F motors though. Anyways, I think I have deciphered the series of motors correct, but please correct me if I am wrong. Here is what I have

K3- CB750E-2209279
K4- CB750E-2347553
K7- CB750E-2705648
K7- CV750E-2714608
F1-CB750E-2510066
F3- CB750E-3106537

What 2 should I use as a power plant?

Thank you to everyone in advance, and sorry for such a long write up.

 

[Justafarmer2]

Hello. I don't have a lot of milage under my belt. But i would run the f motor. 34 vs 31 on intake and 31 vs 28 on exhaust side seems like simple choice. Cyclex has a solution to the f horrors. In different keepers n retainers

 

[Justafarmer2]

The f has a better bottom end as well. Hence the 9000rpm vs the older k 8500rpm.

 

[dirtdigger]

F and K is basically the same bottom end, couple small differences but big components are interchangeable. The first K had higher redline then the later...has nothing to do with the components in the bottom end.

 

[dirtdigger]

Going to depend on intended purpose, if you want just a good running reliable bike use the k motors, the earlier the better for the head and camshaft in the K's plus parts are easier to find. The K7's are a bit of oddballs and can have some f type parts mixed in the head. If you a good flowing stock head use the F but it will cost you more because more parts need upgrading etc. Just remember F head has larger combustion chamber to go along with the bigger valves. The K head can be made to flow as well fairly easily and you get the smaller combustion chamber. If you use the F head you will need to use F specific pistons to keep the compression up. Unless you are going drag racing I wouldnt really recommend the stroker setup for street...although cycle x says its fine but its a lot of expense that could be put toward more important parts like the cylinder head which is the choke point in the 750 motor. I can go into a lot of detail on recommended parts etc but its all going to come down to what you are planning on doing with it and how much you want to spend. I like Kenny at cycle x and he knows his stuff but on the stroker and really big bore stuff for a reliable street motor I would disagree a little bit....remember he is in business to sell parts! And they are good!