1967 CB450K0 Black Bomber Rebuild - By Royhall

[royhall]

No. I took the side panel off again and had a  close look with a mirror on a stick and a good light. It appears that there is a passage for air all around the filter, it just doesn't look like it without a really close look. Once warmed up I gave it a bit of throttle to about 7000rpm and it ran cleanly with zero hesitation. Got to be careful as 4 years on it's not run in.

The engine sounds good with no rattles or knocks, so far no oil or petrol leaks either. I now need to get a few hundred miles on it then see what the oils like. Last time I drained the oil it was way above the max mark and smelled of stale petrol, it also ran out of the drain plug like water. Severe petrol bore washing.

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[K2-K6]

I think you're on the right track Roy, with the removal of restrictive paper filter element, if I understand correctly.

At risk of teaching people to suck eggs  the CV carb works by negative pressure between the throttle plate and the main jet positioning under the most restricted part of air slide.

Opening the plate cause a drop in the chamber above the airline greater than that underneath the slide, this then lifts the airslide as those two are unequal, until they reach parity at that same throttle plate opening, the jetting calibrated around this balance.

Now if the intake air path is restricted at air filter, the above and below vacuum (both coming from the same inlet tract airbox side) will not allow that relative separation as it shouldn't be restricted there.  Without that, the air slide won't move upward against it's spring. This gives an odd /opposite affect to fuel metering though. With overall vacuum climbing and airslide not rising, it then starts to pull more fuel out of main jet even though the needle has not lifted, sending it significantly rich by that action. It's one reason, I  believe that CV carbs aren't generally "choked "with a plate on airbox side.

It seems imperative that intake from outer end down to carb venturi (incorporating the entrance to above airslide intake must be as free as practically possible for the whole thing to work.

[royhall]

I think that explanation is spot on, and may explain some of the reluctance to return to tick over issues as well. In this case the CV carbs do have a choke plate mechanism on the intake side. I find it's only required for initial cold start, and as soon as the engine fires it needs to go fully off. For some reason these carbs have a tendency to go rich at the slightest provocation. If it doesn't work this time I am going to buy a Mikuni conversion kit with cranked pod filters. Apparently it's like a magic bullet for Bombers.

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[AshimotoK0]

If you sadly do decide to go the Mikuni route Roy, at least you will have a half decent set of Keihin 14C carbs to pass onto anyone if you ever sell the bike.
I did hear that some people have fitted Suzuki T500 carbs.

[royhall]

Don't think T500 carbs are easy to get. I had a T500 some years ago that had snowmobile carbs fitted. I decided to fit the correct carbs but couldn't find any decent ones. Strangely, it ran great with the snowmobile carbs fitted it was just the airbox didn't fit correctly. As I was still in engineering then I made up some spacers, that sorted the problem. Wish I hadn't sold that bike, they are worth strong money now.

[florence]

I would say changing to slide carburettors is a good move.  Over the years I had so many problems with the CV carbs on my CB350, despite careful maintenance.  Often, the slides would stick, seemingly dependent on the weather, and suffered the same rich running problems you describe.  Sometimes I could 'nudge' a stuck slide by feathering the choke momentarily.  In the end I swapped to 32 mm Amal concentrics which oddly enough had come off my friends 1969 Black Bomber, a modification that had been done in the early 1980s to cure this same problem.  The bike now runs really well and is much more responsive at all rev ranges, all the hesitancy I used to experience is gone.

If I could afford them, I would use Mikuni slide carbs but these Amals seem to be working very well so I will stick with them for now.

[AshimotoK0]

I would say changing to slide carburettors is a good move.  Over the years I had so many problems with the CV carbs on my CB350, despite careful maintenance.  Often, the slides would stick, seemingly dependent on the weather, and suffered the same rich running problems you describe.  Sometimes I could 'nudge' a stuck slide by feathering the choke momentarily.  In the end I swapped to 32 mm Amal concentrics which oddly enough had come off my friends 1969 Black Bomber, a modification that had been done in the early 1980s to cure this same problem.  The bike now runs really well and is much more responsive at all rev ranges, all the hesitancy I used to experience is gone.

If I could afford them, I would use Mikuni slide carbs but these Amals seem to be working very well so I will stick with them for now.

I went to Newark a couple of months back and a bloke was selling an early  CL350 and it had been fitted with some Chinese carbs and was running really sweet. ..He races CB350's so must know a bit about them.  I bought his old carbs for £20 and the diaphragms in them were perfect so a good buy for me.
Somewhere in my stash I have a pair of CB750 carbs that had been modified to use on a CB350K (albeit a racer that was bored out from 325cc to 350cc)

[mo goldie]

I can get hold of a pair of T500 carbs... so will see how bike go`s tomorrow as out with a few mates and one is a T500 which he just got on the road, and he has the Carbs so will keep that in mind............ BUT like Ash I like to keep it with what it was made with ( Even tho they are a pain in the ass ) and like Roy said it don`t take much to upset the running of the bike being weather related or mixture related, anyway I will see how she go`s tomorrow fingers crossed 

if you do go down the route Roy with the Mikuni Carbs can you please keep us all updated on where you got them and how much the Kit cost and if it has helped.... I here they is a few changes to be made to fit them as the kit don't come with all the parts needed to do the Job.......but that's what I read anyway.

[royhall]

Unfortunately the Mikuni kits are about £475 including delivery. If they had been £175 they would be on the bike now. Also, they only fit K1 onwards and not K0 so you have to pay another £45 plus delivery for cranked pod filters to fit behind the bomber side panels. So they really are a last resort that I'm hoping I won't need.

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