Topic: '78 CB750K project, where to start.....

[gearsoup]

Hello all, first post here!
Since I've finished up my '78 CB400T project (engine rebuild, replace missing pieces, get it running), I've been planning on my next project. I picked this bike up for quite cheap ($500) and I'm wondering; where do I start.

Known items missing:
Seat
headlight bucket and fork mounts
factory airbox

Known issues:
rust, lots of it (PO had it sitting in his field of project vehicles for a few years)
wiring is a bit of a mess, there's even some household wire nuts!

Unknown:
Carbs. They're there, but not installed (for some reason)
Engine. It's not seized (kickstarter does move) PO claimed it ran before he started trying to fit pods to it
Transmission. No idea if it shifts thru the gears.

It seems to me that the first thing I should do (after checking oil) is to pop open the carbs (minimally) and make sure nothing has decided to build a nest there. Quick carb clean, install them and see if I can get the motor to at least run. Is that a bad idea?

Ultimately, it's going to be a complete frame-off rebuild. Not interested in bringing it back to OEM specs, but somewhat of a "resto-mod". Double disk front brakes for sure

[willbird]

Kind of faced the same order of operations issues myself recently.

Basics of our four cycle engines are fuel, spark, compression.

So if how is the fuel tank inside ? Then petcock, then clean carbs, with them off you can easily clean each circuit with compressed air.

What is in the oil tank and engine oil pan ? I’d drain both, have fresh oil and filter ready. If it sat a long time all the oil might be in the oil pan by now.

I pulled plugs and squirted some oil in each cyl before kicking engine over. Good opportunity to check plug cap resistance, trim wires 1/4” and reinstall caps, my caps were total trash, amazing the bike ran when parked. Might as well get new plugs imho.


How does the points cam look if it has been sitting around ? My K3 “shed bike” it was badly rusted, probably totally ruined. Then I’d check spark advancer for smooth action, if any issues there mark breaker plate index with fine point Sharpy and disassemble advancer, clean and grease it (note how it is assembled, there is an index mark, it imho can go together a right way or 180 out) reassemble and then check points gap.

Once carbs are back on and oil in all of the basics are covered. I ran mine a few heat cycles on some 10-30 I had around then changed oil and filter but I was the PO (a younger me) and I knew where it had been for the last 30 years since it was last ridden. Wild card PO I’d install new filter prior to fire up.

Bill

[HondaMan]

If you need some fork ears for a headlight from a 1977 K7, I have a set. Cheap, too!

[gearsoup]

Thanks will and HondaMan for your replies!

Tank looks rather decent inside, some surface rust, but nothing that can't be cleaned up. I've been swamped with work, so I've not had time to explore much more than that. Truthfully, on top of work, my "bike work area" needs a bit of tidying up before I bring another bike in.

HondaMan: yes, interested in the K7 fork ears, assuming they fit? (I wager you would know if they do). Also will be buying your book shortly. From what I've gathered, a lot of your expertise is with the older generations, but I assume a lot of the info just carries right on over?

[bryanj]

Good luck with those carbs, they are pd type and notorious for clogging passages.
Pilot jets are a press fit but need to come out and if i were you i would NOT strip the carb bank into individuals but pay sombody with a big ultrasonic tank to clean them as one unit as there are so many fiddly and unobtainable bits in there

[PeWe]

If engine is not to be pulled, taken apart and restored/repaired.
Open all engine covers to look for rust before attempt to start it.

An engine can have been really good when parked. During decades of no use, steel can rust.
Some parts can rust very much as clutch steel basket.
If starting it in that shape, rust flakes will come off and grind the bearings. Engine can be damaged.

If very rusty inside, pull engine and clean everything. When there replace chains and tensioners. Crank bearings too if worn.
Only tensioners if engine is a low miler.
Primary chain slack can be measured. 65mm when new, service limit 70mm.
See CB750 Shop manual.
If closer to 65mm, low miler or replaced.

[gearsoup]

Bike has 24k on it. Duly noted on the presence of rust, I will pop covers and see what there is to see.
Yes, I plan on taking the motor out, the frame could really use a sandblast and powdercoat. That said, I feel like I really should start it up before diving into the motor, no? Compression, valve noise etc? Or would you just not even bother and figure on needing pistons/rings/valves etc etc?

Regarding the carbs; I've not read up much on the carb system. Are there common upgrades (different banks/brands etc) that still have part availability? Does JetsRus not have components for the '78 series?

[bryanj]

Dont know about jetsrus, i am in uk but i doubt it.
Only place i found with any parts at all was Sirius Consolidated in Canada, other than what honda have left.
The PD carb was hondas answer to the Californian emmissions law and run very lean, i do not know of any honda mechanic who likes them, if a bike sat in the showroom for 4 weeks a minimum of drain out and float bowls off was required and if months a majour amount of work required, so notorious that in uk any salesman who did not start and run any bike with them at least every 2 weeks got fired

[PeWe]

If engine is to be taken apart no need to start it.
You will see if bores are worn, pistons bad. Check if rings have play up-down in piston ring grooves.

If bike has been ridden hard direct when cold pistons will wear.

Remove valves and check seats for wear. Valves mating surface too.

Valve in guide, lifted ca 10mm. Grip disc and wiggle it sideways. You should not feel or see a play, a softness only.
Exhaust valves with guides wear faster.

24k miles should not have caused that much wear if handled, serviced and used correct oil.
I'm sure that guides will wear quicker with thinner oil as 10W-40. 
CB750 must have 20W-50.

I do not understand why rings are replaced on ild pustons. Piston ring grooves can be too wide so better to bore oversize with new pistons.
My old Action Fours 836 cast pistons have worn ring grooves after ca 40.000 km.

Not bad enough to pull engine for but not good enough to assemble it.
This is typical when restoring... not good enogh to use when assemble an engine.
 It cost to take it apart again. Oil, seals and gaskets.