Topic: CB750K6 -76 project back to life with more updates

[andy750]

Per very interesting on the carb boot rubbers and I like your strap filler idea. Id used the band clamps previously but as I think we already discussed before the band clamps can be overtightened and combined with new carb boot rubber lead to cracking of rubber. So I am curious to see if this solution fixes the problem.

Good luck! Great documentation as usual.

cheers
Andy

[PeWe]

Thanks for your encouragement Andy

I have got many ideas on this forum so I'm happy to share found issues and what  I think will improve.

The clamp must have a smooth surface to rubber. The straps will fix that when the smooth side is faced to the rubber.

I needed to share my experience  of my last cracked boots that could have blown a piston or 2.

Even the newer Honda rubbers have became softer so the clamps that were OK before will harm the rubbers.

I remember Honda stock clamps with very fragile x-screw  that quickly became round or clamp fell in pieces.

I'll do my K2 too that can get its carbs synced at the same time when tank is off. Now only bench sync.

Always something to fix or improve!

[Jimcg]

I had done as the German made progressive springs specified last time...

Hi Per!

It's very interesting to read about your work and modifications and also educating and useful they way you write and often compare things you change and evaluate.

About the fork springs, do you have Wilbers progressive springs? How would you say they feel compared to the old stock?

/Jim

[PeWe]

Hi Jim,
Thanks for your interest of my bike project.
There are many ingoing rebuilds on this forum to get ideas from.

This K6 has springs from Wirth. It had US made Progressive earlier. I needed springs to my other K2 stock build so I bought new for this bike, my K2 build got the old Progressive not much used and still like new despute sitting inside fork in a barn for 20 years.

I compared them side by side. Looked similar with same lenght.

I do not remember the difference. First time worse handling when I used too long spacers 1989-90, maybe 15mm or so.  Corrected 24 years later

If stock are tired and cause sag, new progressive springs do not cost much.

I noticed a difference when replacing the old worn plastic piston rings inside, mounted on the pipe that is fastened by the M8 allen head bolt from under the wheel axle. I replaced the small rebound springs at the same time.

I have noticed that fork never bottom out even if it feels soft.

I'm curious about how it will act now on the road after a little more pretension.  Only 4.6mm direct upon the spring. And less oil. Honda recommendation for this bike.

I'll see if more is needed and a reason why spring installation guide recommended oil level 160mm from top, completely collapsed fork, no spring which I measured to be ~200ml (cc) when I drained the old oil.

[Jimcg]

Hi Jim,
Thanks for your interest of my bike project.
There are many ingoing rebuilds on this forum to get ideas from.

This K6 has springs from Wirth.

I hadn't heard about Wirth but I will read about them. A quick check gave me the impression they are about the same price range as Wilbers. Both are german spring manufacturers.

/Jim

[PeWe]

Really great that there are several alternative parts available today for these old bikes!

[PeWe]

Fitted the front wheel hub bearings.
!! Very close to a fatal mistake ending up looking for another hub!!

I began with bearing on right side where bearing is to sit flush with hub side. Heated the hub very well with my heat gun. Bearings from freezer, aimed and let it go. Bearing just fell almost 10mm too deep in. A little bit more and bearing inside hub not possible to take out.

I had to fix asap while hub still was warm. Used a punch, hammer on outer race in a cross pattern to get it back where to sit.
# Next time I'll make sure to tap punch marks in a crosspattern 14mm down from outside as a stopper.

Made sure that the spacer will fit. Had to bend the thin plate things a little.
Bearing from retainer side next. It stopped at a point where the spacerer still was loose.

I could make it to move further with a punch tapping on outer race in a cross pattern.
Before a metal socket with same size as bearing outer race.

This reminds me about the sitting front wheel ending up in same situation I solved by tapping in the right bearing around 0.5mm in need of a bearing shim on that side so inner race to be flush with hub.

I ordered extra bearing shims in various sizes after that experience for 15 and 20mm axles for front and rear. 0.1, 0.2, 0.3, 0.5, 1.0mm.

A press would be fine to have! More garage space needed!!


Old bearings other sides.

[Jimcg]

Good to hear you managed to save the bearing from getting stuck in wrong position.

About missing a press - isn't that a strange fact, there can never be to much garage space nor to much equipment? Only to little money. 😁

/Jim

[PeWe]

A press can be found for not that much. Cheap Chinese bench models.
The biggest problem is the space it takes and how seldom I really have need for it.

Need one wide enough where a complete wheel can be handled for future bearing jobs.
Around 550mm wide.
19" rim ~510mm.
$500US will fix that.

[PeWe]

This bike has a project thread so I have to do something on it.

Started top end tear down today.
Head and cylinder to go off.  I have just got a new RCS base gasket, 0.01" (0.25mm) thick, old double thickness.

This to crank up the compression a little and make piston crown outer edge closer to mating surface.
(The old got a leak after the previous head collapsed a little under nuts)



I like the "shoe stand tray". Perfect for tools and parts when wrenching. No dirt from floor.

[Gamma]

Now I know where to get finned covers👍. I had them on my bike that was stolen in 1980, so I’m tempted again

[PeWe]

Yes, I have seen new billet versions from Thailand.
Mine are old ones I bought from a friend. They match the billet block fins very well.

Maybe help a hot day too. This type match your build too!

[PeWe]

Cleaned piston tops before cyl off.
No bad surprises so far.
Easier to take off cylinder with less studs. No studs blocking when arrange rings during cylinder on.

Next step, clean surfaces, new base gasket and cyl on without pinching a flimsy oil rail.

I'll take close photos of the bores and pistons to show how a Billet block nikasil surface look like after 25.000km.
(Look like new).

[johno]

Nice work Per 
Good productive way to spend the winter !

[PeWe]

Thanks Johno.
Cylinder to go back on this weekend.
Another head (392 with correct look for a K6) goes on next week when hopefully ready.

That head has no thread inserts for cam holders. So I'll fix all threads.
Harder springs, heavy valves, high lifting cam and no thread inserts probably no good combination.

It would have been better before restoring/upgrading the head. Cover springs and all holes except cam holder threads with plastic and tape.

I need to inspect stock cam chain tensioner. If just a little aged rubber, I'll replace it with a new one. Maybe the modern oils help to keep rubber fresh longer.

[PeWe]

Cylinder back on after base gasket replacement.
The cam chain tensioner rubber wheels too hard so new Honda tensioner with the lower rubber fastener and slider too.
Good to have parts in stock

I had tried to get a race slider tensioner. The guy making them had not done them yet so next time.

I was very close to forget the rubber band around the inner 2 studs as a tensioner back stopper when cylinder went on.
I'll use a wooden stick next time as an extra finger.

All pistons moves nicely in bores without bad sound or making marks in oily bores.  Most oil wiped off.
Not sooting the new plugs!

All studs on, now waiting for the head.

[BenelliSEI]

“Rubber band” noted. Smart! Thanks.

[SOHC4 Cafe Racer Fan]

Big ole Jugs!! That's going to make some powah!

[PeWe]

OK power 101whp last year.
My goal was +106 to enter Mike's  hp result. But I got a little bit more torque than his published dyno graph

But, dyno results differ from one day to another and probably more between different dyno's, weight of rear wheel and tire pressure.

I hope for more next time. Another head with Kibblewhite 33.5/28.5 mm valves. The smaller valves hopefully not a restrictor. The port job might compensate for it.

101hp head with 34/28mm valves, K2 head. (Stock F2 in valves, Kibblewhite stock size.) Chambers fixed by myself to be more even, no shape to fit pistons better.

Hopefully a little bit higher compression without pinging and better chamber design. Last head gave 197.5-202.5 PSI with 125-75 cam. 0.25mm thinner base gasket reduce total chamber volume by ~1cc.

Planned head chambers volumes 23.7cc as the old head. Totally 1cc less. I'll see if compression tester will show +200PSI on all. Hopefully 205PSI or slightly more.

[PeWe]

The rubber band trick to hold the base gasket is from Mike Rieck.
To guide tensioner correct when fitting pistons 2-3, a wooden stick can be safer. I noticed it is too easy to forget the rubber band not visible from outside. I had to lift the block again and cut the rubber band while holding it.

Not lifting too much this time so rings pop-out as first time when tensioner did not want to enter the block. I pinched an oil rail when fixing the tensioner and too hurry to let block back on.
It took over a month to get new rings. Not available in Europe, only Dynoman and Summit racing had.
I ordered a complete set to all pistons since shipping was the highest cost.

That base gasket sit very tight around the "sleeves". I doubt it is possible to let gasket sit on case and let all 4 enter correctly without damage.
Sleeves od 78mm, holes in gasket 78.1mm.
The very thin gasket land between sleeves needed a thin pin to carefully press it down.

[PeWe]

Weekend job!
2.15-19 front
3.0-18 rear

120/80-18 rear tire is on its way too. Metzeler Roadtec 01 as before.
Front 90/90-19 also Roadtec O1

[Jimcg]

Weekend job!
2.15-19 front
3.0-18 rear

Always nice when gifts for yourself, I mean necessary investments, arrive. 😄 Looks really good!

/Jim

[PeWe]

Yes, It will be interesting how bike will handle with slightly wider rims on often used tire sizes.
I have to ride a lot to wear the new tire that sits on bike before these.
Maybe I shift earlier, my K2 get the wheels since its rear tire has not much life left.

This bike need road tests before new wheels so the difference the new wheels will make can be verified.
Fork has less sag and less oil than last years.

[PeWe]

Fetched the refurbished head today.
It look so nice when clean.

New stock guides, shortened on port side, lowered to accept higher lifting cams if 125-75 is not enough.
33.5/28.5mm KPMI valves, APE titanium retainers.

Ready to be dirty. This time with much better carb jetting from the first start.
Chambers 23.6-23.7cc, if I understood correctly.

(Old head with thicker base gasket had 23.7-23.8cc)

Gasket is 72mm MLS, chamber diam.62.5mm

[PeWe]

- Why do things in correct order?
Do the inserts before refurbish the head...... 

Prepare for next step, better threads. 12 Time-Sert 1:st step M6 inserts secured with Loctite 272.

I do not want to think about cam holder threads going bad when everything is working fine.

One stud had a partly bad thread in head, just a matter of time on that one. A few turns of alu threads followed the stud at removal.

I use an old set as guide when drilling.