Topic: 750 K0 vs later carb rubbers

[Don R]

 Does anyone know the difference or importance of using the K0 rubbers or isolators on 4 cable carburetors and an early head?  I think I've been using the late (K3-4) rubbers on an early carb set since I've been a member here.
 They seem to be working but the new set of K0 rubber I got from Vintage CB750 got me wondering.

[69cb750]

The head nipples are shorter in 69 and 70, you can trim the 71-76 boots to fit the 69 nipples, otherwise they fit poorly.

[Don R]

The head nipples are shorter in 69 and 70, you can trim the 71-76 boots to fit the 69 nipples, otherwise they fit poorly.
  Thanks, now I know.  My blue/green 1970 bike has a newer crankcase, it must have a newer head too. Now I know the difference. The motor I'm collecting parts for has the early head, missing the center front bolt. Now I know what to look for on the head besides the fin supports and center bolt. 

[willbird]

The head nipples are shorter in 69 and 70, you can trim the 71-76 boots to fit the 69 nipples, otherwise they fit poorly.

I am getting stuff around to be able to CAD model the nipples and maybe some relevant parts of the cyl head. I'd love to be able to borrow a head as you describe in order to measure it as well. I do not have a CMM or anything fancy, will be using a height gauge on a small 18" x 24" granite surface plate.

[PeWe]

Here K0 rubbers on K6 head longer spigots.  (My VM 29 with shortened rack)
Bought the rubbers around 1989 found to be wrong early 2013 when trying to use them.
The K0 rubber is shorter on the head spigot side to match the early head.

Still waiting for a Sandcast....

[Don R]

 In my garage the sandcast has its original rubbers/boots/isolators whatever. L0L, my 11/69 build has early carbs on a late motor, the Nugget late K0 has its original rubber, and this latest #4 K0 with a 71 title doesn't have a head on it yet.

[BenelliSEI]

The last set of 1969-71 I got from vintagecb750 didn’t work at all! Could you even get the carbs on with the ones you received?

[69cb750]

69/70 Nipple=.375

[69cb750]

71 Nipple=.500"

[willbird]

71 Nipple=.500"

Hey that is cool :-).

Here is a nipple that I cad modeled in Fusion 360...it is a middle 70's nipple IMHO. Perspective is making it look more like the earlier one.



From that model I derived this one with no bead in order to be able to slide things onto it in order to trial fit.



This printed "carb rubber" is a prototype to allow mounting two CB500X throttle bodies (slight mods required to make into a 4 wide rack)  onto a CB750K head.



I derived my measurements from a used set of carb rubbers, want to get more precise measurements of the whole situation to get a better model.



The results from measuring the carb rubbers got me into the ballpark, as a machinist/ tool and die maker I want to be centered on Home Plate ;-).

[Don R]

The last set of 1969-71 I got from vintagecb750 didn’t work at all! Could you even get the carbs on with the ones you received?

 

 Honestly, I haven't tried but I'll dig out a sandcast head and give it a go. I usually put them on the head with silicone grease, then use a 2x4 and a tire iron inserted from above through the frame to insist the carbs go in the rubbers.
  That said, I'm used to old hard ones. It's only been recently that I have jars full of them that have been softened.

[BenelliSEI]

The ones I bought were off by +3/8”. No amount of “adjustment was going to get the carbs on. Curious what you will find…..

[Don R]

 Here are the Vintage CB750 K0 carb rubbers on a pre-26,000 head. They are almost as hard as old ones, but they are also cold. If I had time to put them in my ultrasonic cleaner for a few minutes and warm them up I'm sure they would go on.
  They don't look perfect but I'm betting they would work.

[BenelliSEI]

Here are the Vintage CB750 K0 carb rubbers on a pre-26,000 head. They are almost as hard as old ones, but they are also cold. If I had time to put them in my ultrasonic cleaner for a few minutes and warm them up I'm sure they would go on.
  They don't look perfect but I'm betting they would work.

Interesting. The ones I got were off a mile. I still have them. I wonder if they sent me the wrong one. Will check them on a later head (the Rickman has a K4).

[willbird]

Here are the Vintage CB750 K0 carb rubbers on a pre-26,000 head. They are almost as hard as old ones, but they are also cold. If I had time to put them in my ultrasonic cleaner for a few minutes and warm them up I'm sure they would go on.
  They don't look perfect but I'm betting they would work.

Interesting. The ones I got were off a mile. I still have them. I wonder if they sent me the wrong one. Will check them on a later head (the Rickman has a K4).

I have a set that I bought by seeking the cheapest avail, using for another purpose than dropping into an OEM as is bike. One of the ones on my CB750K2 split 360 degrees around during a ride. I call them "4 into 5" LOL due to who sold that set. I grabbed that set I had bought for another purpose and they were all too short to work properly....pretty sure they came from Amazon, they were like $18 for the set. I got the real deal from JT marks to get the bike working again.

Bill

[BenelliSEI]

JTMarks saved the day for me too.

[newday777]

I just received a set of seals the other day from JT Mark's and have some chrome bolts coming tomorrow.

[Don R]

 I spent some time looking at those intakes today, there is no way to line up the numbers or mold seams and have all four pointing the same direction. I tried laying a straight edge across the centerline of them and 1-4 were always off from 2-3. I had to match the ends up by rotating them by only looking at the face of them.
 I think with heat and lube the carbs would go on but I'm not sure if they will be oriented right when in the frame.
 
 

[BenelliSEI]

Yep….The final test wil be if the airbox mounting tabs are anywhere near the frame tabs too!

[HondaMan]

I suspect the ones that 4into1 are now selling are the 2nd-generation Tourmaster parts? They started out from Taiwan in their 1st generation and I got about 2 years from some before they cracked on my own 750 (on in 2016, cracked in 2018, then got some new-generation ones from Honda): then I got the later Tourmaster ones to replace them, which came from China. I installed them on that test engine in that September or October, never ran it, it sat thru the winter in the garage with carbs on it, and 2 of them had cracks in them the following summer. That engine was never even run, nor exposed to gasoline! I have another box of them just sitting in the parts bin, wanting to see if they will self-destruct in their boxes next. They both cracked where the head's rim (flange?) is detented-in on them.

[PeWe]

I bought "Honda OEM" 4-pack in one bag from Andy Cepok ca 2016. Found huge cracks in 2-3 after ca 4 seasons, ca 20.000km. Can have been cracked over a year.
Crack under clamp.  Carbs were not bouncing, strapped to frame with thick rubber hose stacked between carb and frame. Not any bending force to rubbers either.

I had used same clamps with stock carbs during the 80's. Carbs only attached with those clamps.  Used pods.  Those rubbers were only hard not a beginning of any crack.

Ordered new from DSS, Honda labeled bags. 1 rubber in each bag.
They got a nylon strap to fill the clamp groove. Cut and pre shaped to fit with a smooth surface against rubber.
Used wider clamps to cover more, 12mm wide. Old were 9mm.
Wider to press on a wider surface.

I have not noticed any issues yet after 3 seasons over 25.000km.
The rubbers are softer than old type of Honda from the 80's.

I have a new set in old Honda bags with paper label in each. Not cheap but feels better. A more greyish tone of black.
Feels like my K0 rubbers I bought 1987  in old style Honda bags. Paper labels with no bar code.

My K2 with stock carbs and airbox also got straps under clamps, 9mm wide version.

The cracked looked exactly as the new Honda in separate bags. Same markings on same locations. Only "date marking" differed.  Cirkle with numbers inside.

[newday777]

This is interesting Per.

"They got a nylon strap to fill the clamp groove. Cut and pre shaped to fit with a smooth surface against rubber.
Used wider clamps to cover more, 12mm wide. Old were 9mm.
Wider to press on a wider surface."

I don't know if I have seen nylon strap with a smooth surface.
Do you have pictures of the nylon strap you used?
Was it like a tiedown strap nylon web?
Or what was the nylon strap intended for?

[69cb750]

which came from China. I installed them on that test engine in that September or October, never ran it, it sat thru the winter in the garage with carbs on it, and 2 of them had cracks in them the following summer.

Not surprised at all.

Japan and Yamiya have the best rubber in the world.

[PeWe]

Also named cable ties. One side smooth, the other not.

[newday777]

Also named cable ties. One side smooth, the other not.

Oh okay. That makes a difference in material.

[PeWe]

When I did this. Hopefully an improvement.
Used old bad carb boots when I formed the straps with heat gun.
http://forums.sohc4.net/index.php/topic,180468.msg2147578.html#msg2147578

[PeWe]

NOS K0 boots $$$$
https://www.ebay.com/itm/196912444602?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=hol8dheps_y&sssrc=4429486&ssuid=XgCEg0CPSya&var=&widget_ver=artemis&media=COPY

[69cb750]

NOS K0 boots $$$$$$$

Set of Yamiya boots $100, new rubber  - https://www.yamiya750.com/index.php?main_page=advanced_search_result&search_in_description=1&keyword=insulator&inc_subcat=0&sort=20a

[Don R]

  I'm glad that we got to share the information, thanks. Yamiya seems to be the answer once again for K0 stuff. Everett powersports also has some regular K 71-76 carb rubbers.

[RAFster122s]

New rubber is preferred over old stock every day all day... Provided it is made from correct rubber compounds, yamiya and Honda can be trusted to make it correctly...

[HondaMan]

NOS K0 boots $$$$$$$

Set of Yamiya boots $100, new rubber  - https://www.yamiya750.com/index.php?main_page=advanced_search_result&search_in_description=1&keyword=insulator&inc_subcat=0&sort=20a

That's not a bad price! I think I paid $120 for the ones I'm using on my ride, 3 or 4 years ago, now.

[PeWe]

A good idea to have an extra set on the shelf for future needs.
As oil filter, throttle cable that suddenly snap off (pull wire close to grip) and other parts.

[Don R]

 I studied the roll angle of the new Vintage CB750 K0 rubbers and tried to match the down angle compared to the fins on another K0. I rolled the rubbers down so they angled the carbs similar to how it looks on a bike. They actually got better.
 The numbers and mold parting lines are even more wacky, but the rubbers appear to work, still nearly as hard as old ones though. I never got the carbs to pop in, I didn't want to risk breaking the K0 carb plate.