Author Topic: 750 K2 Strip & Rebuild - help please  (Read 20953 times)

Offline K2-K6

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Re: 750 K2 Strip & Rebuild - help please
« Reply #150 on: July 01, 2022, 01:58:36 PM »
"Thanks Nige - all makes perfect sense. The only remaining question is ‘how much’ gap to finally leave wrt the “closed / running position”…. 01mm, 0.5mm? I do have the original ‘thread remaining’ figures, and so indeed could roughly set this."

This doesn't matter as you'll be setting the rpm as soon as you start it, so just eyeball it to nearly closed as it will mean you have to use the twist grip to start it running if it's just a little too closed.  The whole setup routine (Bryan method) establishes the components in the correct orientation one to another to give  potential for in use service adjustment in either direction needed. The final closed gap is always going to be dependent on rpm when running.

Offline K2-K6

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Re: 750 K2 Strip & Rebuild - help please
« Reply #151 on: July 01, 2022, 02:15:06 PM »
'(To labour the perfect sense bit; I’m left bloody wondering why I bought a set of vacuum gauges 😂, though am indeed happy to leave ‘em in the cupboard!)"

It's certainly become folklaw that using vacuum gauges is the right route to correct any sort of unspecified maladies  :) and very wrongly used in someone hoping that it's going to fix their particular problem.

It will of course only fix the slide balance from one to another IF it was wrong in the first place. Also conditional is that all other service items should be resolved first PRIOR to setting with gauges, else you'll establish a fault by wrong reading.
They are a convenient way of setting carb balance if used appropriately (and don't require a workshop to remove carbs to complete) but only with re-establish and accurate prior routine. Often given as a solution though from unreliable scources.

There's a different use though on one bike in these SOHC engine and that's the four cable carbs. They are used to set the individual slide position and balance at idle (there's no master lift parity on that system) after which the in service adjustment is done equally by the "clock face" grooves on the slide stop position screw to keep them that way.
Also at a higher rpm too when the parity of the cables is tested independently of their stop position, this to give full load parity (again because it's not controlled be the lift bar) and important for cylinder balance under maximum torque running. Certainly there's more value there from owning gauges, but most don't don't know why that detail exists in the Honda manual.

Offline Bryanj

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Re: 750 K2 Strip & Rebuild - help please
« Reply #152 on: July 01, 2022, 02:47:16 PM »
Using the small drill bit ses the slides at the same height which gets you close but the gauges then take into account all the minor manufacturing imperfections in the carbs

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #153 on: July 02, 2022, 02:00:06 AM »
Well Bryan, that’s thrown the cat amongst the pigeons in my wee brain…. I was ready to mothball the vacuum gauges.

Did Honda (ie dealerships) routinely vacuum gauge their bikes at the standard valve service intervals? I genuinely thought it was a ‘set & forget’ type procedure, which was never really necessary unless major works had been required.

I definitely like and respect the idea of mechanically matching the sliders once and for all, and ask whether valves set iaw the manual have a significant effect upon MPs and warrant de syncing those sliders? Would you not end up chasing your tail over the years?

Specifically, my 750 has been rebored, new pistons, rings, and a comprehensively serviced head… to the point that any (In/Out) valve could go into any (In/Out) seat (I never knew such tolerances existed). The carbs have been fully serviced. So, in my case here, is there a case for simply leaving alone once bench set?

An interesting topic - appreciated.
« Last Edit: July 02, 2022, 02:01:56 AM by Lobo »

Offline Bryanj

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Re: 750 K2 Strip & Rebuild - help please
« Reply #154 on: July 02, 2022, 07:26:46 AM »
I think carbs were supposed to be checked at first service and then every other major service or annualy and if everything else was adjusted first were frequently inside tolerance but rarely exact.
Always checked after major work and then at first service afterwards, or if removed/disturbed for any reason

Offline Sesman

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Re: 750 K2 Strip & Rebuild - help please
« Reply #155 on: July 02, 2022, 07:33:03 AM »
I have a set of mercury vacuum gauges from the 70’s. I can honestly state that all the multi cylinder bikes I owned in the 70s and very early 80s benefitted  from regular carb vacuum balancing. The improvements were very notable on the 400/4 and Suzuki 550 fours I owned. The mech sync will provide a reasonably accurate physical/dimensional sync, but the vacuum gauges iron out any differences in fluid dynamics created by manufacturing tolerances and inlet flow characteristics. I guess it depends on how fickle and fussy you are with the set up.

Offline K2-K6

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Re: 750 K2 Strip & Rebuild - help please
« Reply #156 on: July 02, 2022, 08:01:05 AM »
Original manual, 1970, for 750 calls for carb checking firstly at 3000 miles in service schedule, then check at 3000 mile interval. And that's for the four cable system.

Certainly by setting them mechanically but then putting the gauges on will confirm or dispel something of this topic just from the readings that are present.

I feel that review of this element is significantly skewed by not fully appreciating the role the airscrew adjustment takes in setting these up, which most contemporary workshops didn't even have significant understanding of when working with them.  The adjustment of most carb rack slide position has insufficient resolution to fully trim the cylinder parity at idle which by design is completed by the idle mixture circuit. That strategy is a sound one and transferred to fuel injection system on virtually all current engine, in that the flywheel pulse monitoring is used to trim fuel quantity for each cylinder to bring smooth idle irrespective of intake tract variances.  This is the same as these carburettor (they are particularly well designed) in working with cylinder energy output to give idle parity which then is ignored as throttle gets wider open.

Carb slide position (at idle) is not the right tool to compensate for cylinder to cylinder casting/flow variation as you'd need the maximum flow at full open to control that condition. It would likely give unavoidable poor refinement at idle though, if needed.


Offline K2-K6

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Re: 750 K2 Strip & Rebuild - help please
« Reply #157 on: July 02, 2022, 08:17:28 AM »
Essentially the vacuum measurement is only controlled (at idle) by the geometry of the space under the throttle slide, that's a small part of the venturi arc in combination with the underside of the slide itself. It cannot take into account most intake tract variability as they would lay outside the scope of influence of the above named parts.

Likely to be more affected by piston ring sealing (pulling the vacuum to be measured) more than anything else, and probably why Honda don't call for monitoring until running in is well out of the way.

Offline Sesman

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Re: 750 K2 Strip & Rebuild - help please
« Reply #158 on: July 02, 2022, 08:27:27 AM »
Interestingly the Suzuki 550 models of the era had a higher vacuum setting on the two outer cylinders than the inner two.

The rules for best results are to mech sync then vacuum sync with instruments. Then check at regular intervals….it works.


Offline Bryanj

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Re: 750 K2 Strip & Rebuild - help please
« Reply #159 on: July 02, 2022, 10:24:02 AM »
Now im awake--ish.
When at Meads i had to do a full sevice job on one of the 500 V4's and was trying to get the carbs closer, after 4 hours of peeing about cos we didnt have the correct double bent screwdriver tool i gave up and told the customer i was having problems and it would not be finished till next morning.
Came in fresh, pulled the carb bank and set them up with a 1/16 drill shank on the butterflies.
After reinstalling vacuum was well within book spec but not spot on so buttoned it up and reported as within service spec, customer went away and came back saying he was very happy and thought it was running best it ever had plus thanking me for being honest,

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #160 on: July 04, 2022, 05:09:57 AM »
Thanks all on the carb set-up info; all done & ready to be installed.

Todays problem is asking for tips to get the bloody rubber tray securely into the battery box. One of the pull-nipples long gone; the other with a nick. The rubber ‘lugs’ are suffering slightly from age (sound familiar?) but pliable enough I’d have thought. Despite applications of copious rubber grease they’re not showing any signs of compliance. Hands / knuckles now worn out…

Anyways, tip pls?
Ta.

Offline Johnny4428

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Re: 750 K2 Strip & Rebuild - help please
« Reply #161 on: July 04, 2022, 09:38:55 AM »
Piece of thin cord or string round the back of the nipple a couple of times and pull through as the cord unwinds hopefully pulls the nipple through. Haven’t tried this method but saw it somewhere.
1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1974 Honda CB550K1. Running resto,
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #162 on: July 06, 2022, 12:03:05 PM »
Thanks Johnny; did try this on another nipple problem using waxed sail thread… it basically cut through said nipple.  I may try it all again, but with a thicker / gentler cord.

(1) Today unlaced the hubs for polishing. When rebuild time comes back any comments on greases / anti seize lube etc to go onto the (SS) nipples where they sit in the chrome rim?

(2) the (front) disc mounting plate is to be repainted black. Should it be gloss, satin.. or matt pls?

(3) since I’ve got the disc in my hand, any ideas how to de-glaze it? Been through past threads here but not found much other than to buy a new disc.

Ta.

Offline Johnny4428

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Re: 750 K2 Strip & Rebuild - help please
« Reply #163 on: July 06, 2022, 07:07:46 PM »
Managed to refit mine with pull through nipples which were intact and a bit of rubber grease. Wasn’t too bad, but the disc pads on the sides of battery box were a pain in the butt! I think the inner disc plate is satin, but i think there is someone with more knowledge waiting in the wings that will confirm.
1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1974 Honda CB550K1. Running resto,
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

Offline Lobo

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Re: 750 K2 Strip & Rebuild - help please
« Reply #164 on: July 07, 2022, 12:44:09 PM »
Onto the wheels / hubs today. Can anyone pls advise which of the below will rust / quickly look shabby - certainly I can plate but prefer not to where possible.

#6 I reckon definitely needs a zinc coat as seems ‘straight forward’ steel plate; but the other pieces?

Ta.
« Last Edit: July 07, 2022, 12:45:57 PM by Lobo »

 

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