Topic: Questions on 750 F camshaft / cylinder head

[kwaltz6]

Hello all, new member here. I posted an intro in the new member forum and had a couple questions about my 750 F I'm working on. It's a 1978 that's pretty much bone stock. Last summer I was on a 3000+ mile trip and sprung an oil leak with less than 300 miles to get home. I'm still a little sore about that one. Anyway, the leak seemed to be coming from the spark plug hole on cylinder 1. My worst fear was that I blew a ring or damaged a valve/guide and had oil in the combustion chamber that was being forced out. I was able to get it home in the back of a truck and after pulling the motor (which is infuriating that you need to pull the motor to get the cylinder head cover off) found out that the leak was actually coming from the threads of one of the studs meant to hold the camshaft journal. Somehow I got some metal debris logged in there and what happened was the camshaft itself had a large grove in it, the journal was torn up, and subsequently it pulled a stud out just enough to let oil pass through the threads.

So here I am attempting to rebuild everything and figured why not just do a mild restoration on the whole bike? I've torn it down to the frame at this point and will post some pictures in the project forum later. What I am trying to figure out now, is what are my options for replacing the camshaft/cylinder head. Finding a 78 750F specific camshaft has been pretty much impossible, but I've found some for earlier years. I also wondered about replacing the entire cylinder head, but same deal. So a couple questions I am wondering are: Other than slight difference in lift/duration, what issues might I run into using an earlier model camshaft (either F or K model). I would obviously like to stick with F model parts if possible. Secondly, I know the cylinder heads themselves have some differences in size of combustion chambers and different oil passages, especially between the F and K models. I found a complete head from a 76 F model. Could I run this head and swap out the pistons for the earlier flat top pistons? I appreciate any information and look forward to discussions.

[newday777]

Welcome kwaltz6
What was the debris you found blocking the oil port? Was it the degraded cam chain guide and wheels?

As far as I know, the 75 & 76 F heads, as well as all years of the K model heads, are different than the 77/78 F heads.

[Jerry Rxman Griffin aka MuthaF'er]

If you use a 75/76 F head you will also need to use the corresponding pistons and cylinders (due to the different oil return holes). This "could" be worked around but it's not worth the trouble so just find a 75/76 F whole top end. You will lose 5 or 6 hp doing this swap. The 75/76 F cam is the hottest since 69/70 edit**** then came the 77/78 F cam, our hottest stock cam. Those would also be a good choice but being able to get an accurate ID on a cam is damn near impossible. Avoid the later year K cams after 70/71(?) due to detuning. However they will also work just not as good.

[robvangulik]

An F1 head on F2 cylinders/pistons doesn't work, pistons too high/combustion chamber too low, camshafts can be used from all years, but with different power output.

[754]

From what you described , there is no head damage. If you can't get a late F cam, buy a mild street cam.

[kwaltz6]

Thank you all for the info! I think I've got a plan sorted now. Jerry, your mention of the 75/76 F cam being the hottest since earlier years; I've found a 76 F cylinder head that I plan to just pull the camshaft and holders from and put in my 78F. With the exception of the lobe grinds being slightly off, I'm gathering no other major issues then? I figure if I just use a different Cam, I'm okay with loosing a few HP for exchange of being cost efficient and less of a hassle.

Also for those who asked. My current head is not damaged, other than the threads for one stud that pulled out. I will likely use an insert (not helicoil) and some sort of sealant to fix. So far I haven't found what they mystery debris is that got lodged in there to begin with, but I'll check the tensioner/wheel soon. I will post some photos in the next few days for everyone and hopefully this thread helps someone in the future.

[754]

Be real careful around the oil jets, you should pull them and clean the passage,  it's likely the source of your trouble..

[seanbarney41]

77/78f cam is hotter than the 75/76f...really, the difference between all the different factory cams is minimal

[Jerry Rxman Griffin aka MuthaF'er]

Yes. Looks like I didn't finish my thought. Edited it. Thanks Sean.

Yes, the 75/76 F cam will work for you.

[70CB750]

While you are there, check valve play in guides, 78 Fs are known for eating guides up.   

[PeWe]

It is always a good idea to check guides when head is off and bike has done som miles/km.

New seals too. The parts are old independent of usage.

I bought a K6 head that looked really good, not touched. All threads OK.
In valves and guides looked like very low mile while exhaust valves and guides were shot. Valve stems wiggled a lot in guides.

I had fresh stock ex valves and new guides so a quick job for my local shop nearby.

[HondaMan]

I have an F2/3 cam, I think, if you want to go stock.