Topic: K7 and K8 Differences?

[Blasbo]

I am looking at a very reasonably priced 78 CB750 to use as a parts bike for my K7 project. From what I have read on the forum most of the parts are the same except for the seat.
Any other differences I need to be aware of?
Thanks

[Jerry Rxman Griffin aka MuthaF'er]

I'm surprised that seats are different??! You sure? I thought the only difference was in the engine.

Engine: Valve keepers and springs are different. The 78 uses the 77/78 F parts. Since the keepers are different, the valves are cut differently too but still the same size heads.

Everything else should be the same. Someone correct me if I'm wrong. 

[754]

3 or four carb differences.

Difference in striping on the paint

 I am not aware of seat difference?

Late 78s have a different engine case number boss area.

2 types of frames, some 77s I think have 2 removeable upper front motor mounts, rather than 1 welded to frame , and one that comes off.

 If you have found a F, the sidestand mount is in a different location than a K, stand is longer too.

[754]

77 carbs adjustable needle, 78 not

1 type has brass slides, the other, lighter anodized aluminum slides.

The plated steel hardware, ie the strip that ties the carbs together, and choke cable clamp, is yellow plated one year, and silver the  other year..

[Inigo Montoya]

The 78 has the silver slides. The seats are the same. Gas tanks are the same. Side covers are the same.
77 needles have slots. Mains on 77 are different.
My 77 has 2 removable upper mounts, I think the 78 does too.

[Blasbo]

Thanks for the replies. It sounds like I should be able to do an engine swap?
Not to pick nits , but to clarify, I believe the K8 has a stepped seat and the K7 does not.
http://www.sohc4.net/index.php?q=content/cb750-timeline

[cb750k77]

77 -78 are the same... EVERYTHING except paint and carbs are little different! thats IT!

[cookindaddy]

This has been interesting.

I am in the opposite situation, having bought a 77 K7 as a parts bike for my 78 K8.

I've been into the 77 motor top end and found the differences in the valves and clips mentioned. I had overhauled the cylinder hear on the 78 the year before and also had a spare head for the 78 to compare to the 77. I found the clips very hard to install on the 77. Those on the 78 are easy.

The seat seems to be the same although I'm not sure that the seat on my 78 wasn't "refinished" before I got it. It didn't have a stepped up back section although it looked like someone had carved the foam. I recovered the seat and it is now like new.

One difference I noticed that's not mentioned is that the 77 front brake hydraulic line has a junction block on the top of the triple tree which carries the brake light switch while the 78 line is one piece (no block) and has the brake light switch in the master cylinder assembly.

Everything else, except for the things mentioned already seem to be the same to me.

[cookindaddy]

I re-read the posts.

I don't think the valve springs are different? I measured all mine and they were within the spec for the 78 manual. Maybe I missed something there.

I haven't been into the carbs yet on either of the bikes so can't comment on the carb comments.

[GammaFlat]

The 78 is faster or the 77 - I forget

[Jerry Rxman Griffin aka MuthaF'er]

Check the valve spring part numbers for 77 and 78. The 78 is supposed to have the same part numbers as the 77/78 F engine. The springs should be stronger/heavy duty not necessarily longer. Stronger springs will help prevent valve float should a high rpm shift be missed like I did with my 75 750F. Broken valve obliterated my engine.

[cookindaddy]

Jerry,
Thanks for that info about the valve springs.
The 77 engine is back together now and chilling in the garage over the winter ready to be swapped in with the 78 engine coming out when it gets a bit warmer here.
What if I inadvertently mixed the springs up?
I remember that there were paint dots on the springs, but I was more concerned about which way around to place them, with the closely wound coils at the top or the bottom. The forum helped me but I can't remember now. I think it was the close coils at the top?
Oh well.
Maybe I just won't rev very high, or I won't miss a shift? Fat chance.

[Blasbo]

Thanks for the feedback y'all but it's a moot point now. I couldn't get there fast enough and it's been sold. 
Anybody here buy a K8 in Shreveport, LA for $200?

[754]

Buy first, then let others know..

 If its a toad, resell it.. its spring..they sell!

 There is a few 300 $ bikes on here that are ridden a lot..

[1080]

For the Canadian models the speedometer gauges are different.

'77 has Mph on the speedometers gauges & Km per hour just below that.
'78 has Km per hour & Mph just below that.

[eurban]

Just for clarity sake here, the 77 and 78 seats are indeed different.  The 77 has a mildly stepped seat while the the 78 has a seat with a much more pronounced step up.  The pattern of the covers are different as well.  Here's a pic of my 78K project with a stock 78K seat. Note the significant step up and the piping coming down at the rear of the seat.



And here's a pic of a 77K that I "burrowed" from the internet.

[BobbyR]

Other than the seat, the bikes are identical.

[eurban]

Other than the seat, the bikes are identical.

Well except for the previously mentioned, engine differences (valves and keepers), carb differences (different carburetor part #, different size main jet, different main jet needles), side cover badging, paint schemes and perhaps a few other things they are identical  

In general parts will interchange without issue.

[BobbyR]

Look here they did a good job listing engine  changes: http://www.hondachopper.com/engine/engine.html