750 DOHC question

[d9canada]

I have a 81 750C (my profile picture) that I rescued last year just before it was to be scrapped.  I got it running well in no time even though it sat under a tarp for 18 years.  I've had lots of experience with the CB900C, my fave all round Honda 4, and the 750C motor is basically the same.

Last year I also picked up a bunch of RC04 motors that another guy had accumulated, and some a few 750K and C frames.  This winter I want to rebuild a few motors for future projects.  I've got the FSM for the 750 DOHCs and the Haynes #535 manual for all DOHC Hondas 78-84.  I've done a fair bit of reading but one thing concerns me - was there no different motor parts or specs for the 750F as compared to the K and C motors?  I mean, the SOHC F models are quite a bit different from the same year K models.

Did I miss something?

[TOOLS1]

Well you did miss something, the 75-76 "F" "F-1" had the same engine as the "K" model. Only the 77-78 "F-2" "F-3" engines were different. On the 750 DOHC models, they all shared the same engine.
TOOLS

[d9canada]

No - I was asking about DOHCs and if you'd asked about SOHC 750s, I have two and I'm well aware of the differences.  That's why I expected that there must be some differences on the DOHC F models.  But thanks for the confirmation about the DOHCs anyhow.

[Re-run]

Well, I think TOOLS did answer it. For the dohc years, the F,K, and C had the same motor. So if you got the specs for the motor for 1 bike, then you have them for the other 2. Parts will all be the same. So, in a way, when it comes to the model designation, you can leave the engine out. It only applies to the rest of the bike.

[motogrady]

Where then does the extra hp the f model has come from?

[Re-run]

Does the dohc f have any extra hp? I know the sohc F did, but I was not aware the dohc one did.

[TOOLS1]

Only the "F-2 and F-3" had more HP then the "K" model in 77-78. The extra HP came from a different head that had a smaller combustion chamber, and different valve angles, a different cam, and pistons. All other years/models were the same engines, and HP.
TOOLS

[shinyribs]

The single cam '76F from the factory had different pipes which freed up some upper rpm hp.  I don't know if that year model F used the same cam as the K or not. But I know my 76F would run circles around my 74K, and the K was a very light bike (chopper).

DOHC stuff, no clue. The 900F definitely used different cams than the 900C, but no clue on the seven fiddys.

[d9canada]

Shinyribs
Actually if you check the part numbers on Motogrid.com, the cams for all 900 models were the same in 81 (14100-438-010 intake and 14200-438-010 exhaust).  In 82 the part numbers changed (14100-438-020 intake and 14200-438-020 exhaust), but all I have heard is that the 81 and 82 cams were the same grind.  The 82 cams, part number and all were used in the 83 1000C.  

It's too bad that Honda's not more consistent in how/when they change part numbers.  Some things are the same year after year but the number changes each year while others do not.  Some things like side cover badges that are the same side to side have different numbers.  Go figure.

My original reason for asking about the 750 DOHC C, K & F was because I have several rebuildable 750F motors and also two 900F motors.  I just bought an 82 750C that I'd like to do up as a modest cafe racer.  I have two 73 750K SOHC in my collection so I was aware that SOHC F2 and F3 motors were hotrods.  I have a set of F2 pistons, that have a substantial dome as compared to the standard part.  I was hoping the DOHC 750F versions were hotrod versions of the C & K too.  

I should have known when I couldn't find a 750F DOHC spec separate from the C & K spec.  Oh well, the 900F motor will bolt into the 750 chassis so maybe I'll go that route with a Wiseco 985 kit.  1100F cams drop right into the 900, if I can ever find a set.  It's like putting 900 cams in a 750.

"It's all done with mirrors"

Thanks Everyone!

[Truck]

I'm planning the same build. Didn't think of a cam swap though, does it make a noticeable difference?

[d9canada]

Yes the cam swap is worthwhile to get rid of the flat midrange that is common to all the vintage Hondas. 

You don't have to rejet or anything, just be sure when you swap the cams into place that you don't drop anything into the bottom of the motor, and make sure you time the cams as noted in the FSM. 

Once you have the cams in place, pull the plugs out, leave the ignition off and turn the motor over by hand with a wrench on the rotor center bolt.  Turn it clockwise a couple revs and then check the cam timing marks again.  You may find it's slipped. 

This is usually because the A chain can get an extra couple of links caught up on the crank gear with all the fooling around.  You pull up on the chain and figure it's tight but as I say, it may have trapped a few links.  So double checking it can really make a difference before you get it all bolted into the bike.  Don't forget to adjust the cam chain tensioners and check the valves too.  Then, if it runs OK and the carbs are clean & working well, you can go on and do the vacuum carb sync.

On Thu, Sep 24, 2015 at 7:32 AM, Truck [via Honda CB750'S] <[hidden email]> wrote:

I'm planning the same build. Didn't think of a cam swap though, does it make a noticeable difference?

It's only illegal if you get caught.

If at first you don't succeed, use more lighter fluid

95% of Harley Davidsons ever made are still on the road... The other 5% made it home.

New Baltimore, Va '82 CB900c, 1980 CB985F/K 'Mutt"

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NAML

[shinyribs]

d9canada wrote

Shinyribs
Actually if you check the part numbers on Motogrid.com, the cams for all 900 models were the same in 81 (14100-438-010 intake and 14200-438-010 exhaust). .....

Whoops!  I said cams, but meant cases. Thanks for clearing that up,though.