Topic: Honda Twin guy with a new to me 750f

[worst650ever]

Hi folks,

I'm another Honda person from scenic Michigan joining this fine forum. 

I just ended up with the rolling frame of a 1977 CB750F, and am slowly gathering parts and info to put it back together.  I have a daily rider '76 CB360T and had a '79 CB650, along with a rotating cast of CBR900s, Interceptors, and the occasional Italian bad idea projects.  I even have most of a disassembled 1986 Yamaha SRX-6 / SRX 600 in the attic, but I think a big thumper is anathema to SOHC/4, eh?    I spent most of my time over on Do The Ton and Honda Twins, but it seems like I have enough cylinders and few enough cams to stop by here. 

I ended up with a '77 F roller chassis and a '78 F frame that I took a lot of parts off of, and got what seems to be a '78 motor - the number on it is CB750E-3016610 (can anyone confirm that? The chart is confusing!).  The guy who sold it to me thought it was an "F" motor, but it was just painted black.  It has the two oil drainback holes under the rocker cover instead of just the one that the "F" motors have.  I didn't pay silly money for it, so I'm not worried, and now I don't have to worry about the keepers pulling through the retainers, right? 

In any case, I also ended up with a set of '77-'78 mechanical Keihin carbs that I just finished overhauling, and I have a new to me wiring loom on its way.  I need to find an F fuel tank and some other odds and ends like a front fender and that rear plastic flap fender thing that hangs down below the license plate / tail light bracket.  Any help any of you could provide with getting that stuff would be great!

I'll start a build thread as I get enough stuff together to get it going.  Thanks for already providing me with a wealth of information!

[mystic_1]

Welcome aboard!

Yup, CB750E-3016610 would be a 78 motor.  I do believe that's a K motor, since the chart indicates that F series 78 motors started at 31xxxxx.

Don't sweat the thumper, we'll forgive you for that.     Now, if was a 4-cam single, then we'd have an issue! 

mystic_1

[Stev-o]

Welcome from Texas.     Yes, appears your motor is a K8.

A lot of guys from MI here, what area are you in?

[worst650ever]

Thanks guys for confirming my suspicions.  I do have a fancy F motor valve cover and clutch assembly, so hopefully that counts for something    I spent part of this evening cleaning out the gunky master cylinder and trying to get fluid down to the calipers.  I suspect I have a clogged brake line - I was planning on replacing all the brake lines anyway. 

I'm about 40-50 miles west of Detroit - we do like our bikes and cars in Michigan, that's for sure.  Shame we can only ride for about 15 minutes every year...

[Stev-o]

If you are in the market for brake lines, Shaun is a member here and makes OEM and custom length replacements...

http://slingshot-cycles.com

[worst650ever]

Thanks Stev-o, I got a set for my '76 CB360 from those folks, and the quality and fit was great! 

I did find out that the set of mid '80s 37mm Honda forks I have would fit my Comstar front wheel and rotors.  That would get me high rise stock Honda clip ons, as well as two piston brake calipers and TRAK anti-dive, as well as a sturdy aluminum fork brace.  Hmmm, it's not even together and I'm already modifying it!

I also found that my cylinders are pretty worn out - they are at about 2.410" / 61.214mm, which is above the stock wear limit of 2.406" / 61.100mm.  My first clue was the 1.4mm ring end gap    It also explains all the ash on the top of the pistons, lots of oil burning. 

The rest of the engine innards look good.  The head is in good shape, as are the cam chain tensioners.  The camshaft gear bolts were loose, however 

Sooo, do I go 0.25 / 0.50 mm oversize, or do I go with a big bore kit?  I can get a big bore kit off eBay for $110 here.  What do you folks think?

[TomsK8resto]

Welcome from Tucson Az.

If it were me, I'd go big bore.  Good luck!

Tom

[Stev-o]

If it were me, I'd go big bore. 

+1!

[Jerry Rxman Griffin aka MuthaF'er]

That kit is not designed for the F2/F3. it would work but lower your compression which would offset the increase in displacement. Check out Dynoman. Maybe CycleX?

[70CB750]

Welcome.

[worst650ever]

Hi Jerry, I actually have a '78 "K" motor that was spray painted black.  I think the kit would work for that?

I measured the bores again, and ended up getting 2.4059", which is right on the edge of the 2.406" wear limit.  I'm leaning towards slapping a set of fresh stock sized 61mm rings on the thing and going from there, as it's $100 for the machine shop work plus $110 for the kit versus $50-$60 for the rings. 

I know that's a slightly sketchy way of doing things, but man is it the cheap way to do things 

[70CB750]

Hi Jerry, I actually have a '78 "K" motor that was spray painted black.  I think the kit would work for that?

I measured the bores again, and ended up getting 2.4059", which is right on the edge of the 2.406" wear limit.  I'm leaning towards slapping a set of fresh stock sized 61mm rings on the thing and going from there, as it's $100 for the machine shop work plus $110 for the kit versus $50-$60 for the rings. 

I know that's a slightly sketchy way of doing things, but man is it the cheap way to do things 

You could also find a cylinder set in better condition and go with standard rings.  That way you dont have to pay for the shop time and still end up with engine with decent life expectancy.  Somebody here would proabaly have one.

Btw, not sure what do you have to measure with, but measuring inner boar at home within 0.01mm is more like fantasy land.

[worst650ever]

Thanks 70CB750, that was also rolling around in my mind.  I have a somewhat fancy bore measuring device, but I agree that it is very difficult to get reliable measurements with a hand held tool.  It gets me in the ballpark, however, but when it is so close to go/no go like this, things get interesting. 

There's a listing on eBay here for $20 + $25 shipping, but no real info about the condition of the bores.  I'd be very interested if anyone here has a decent set of jugs (heh) they'd be interested in letting go of.  I have some spare 750f calipers and Comstars, as well as other trash and trinkets from years of motorcycle addiction if you folks are interested in trades. 

[worst650ever]

Build thread here.