Topic: 1974 CB550K0 Carburation Issues

[58webbing]

My 74 CB550K0 is now on the road albeit with carburation issues. The carbs were supposed to have been cleaned and preperared by experts which I now doubt. The bike is in standard trim regarding exhaust, inlet filter and electrics and has been assembled using new seals etc. My problem is with the float levels a bit on the low side it is running very rich with a lumpy tickover and pickup, mid range it is behaving reasonably enough to inspire me for the next round of head scratching (bald patch watch out) So that I can think through the likely causes and solutions before the spanners come out I have some basic questions I would like some help with.
1. Is there an exploded diagram of just one of the Keihin carbs available? Trying to identify jets and fiddly bits on the WS manual 4 carb diagram is not easy.
2. Is there a list of all the jet, needle sizes available? The information at my disposal varies from source to source. I am hoping the PO has fitted bigger jets as this would mean an easy fix.
3. Are the Keihin jet sizes measured by the orifice diameter ie 100 is 1mm dia 80 is 0.8mm etc? I am anticipating maybe having to machine up some jets as an alternaive to tracking some down.

As a point of interest Yesterday saw the 1st test outing, with patchy ice on the roads and common sense saying "stay indoors" the temptation was too much so off I went. With the throttle not behaving it did make for an interesting ride with just a few back end twitches, but worth it. The biggest lesson I learnt was that I will have to put a switch on the side stand (health and safety) as with the stand down and heading towards oncoming traffic round a long left hand curve not being able to steer away on icy roads with a jerky throttle is not something I want to try again.

[calj737]

First order of business is to confirm your ignition timing, health of plugs, and the plug caps.
You can drop the float bowls and remove the jets to confirm their sizes without dismantling the carbs or bike. Attached is the matrix for your stock sizes.
It is possible that someone used aftermarket jet kits and needles, and these will not equal stock sizes, especially the needle. The taper is rather different, and the clip position is essential that it be in the correct slot.
If your floats are “low” you will run lean, not rich as the fuel is disrupted earlier.
Of course, you should also check the valves for proper clearances once the timing is known perfect.

[Deltarider]

My problem is with the float levels a bit on the low side it is running very rich with a lumpy tickover and pickup...

What are the airscrews set at? Are the carbs synced?

3. Are the Keihin jet sizes measured by the orifice diameter ie 100 is 1mm dia 80 is 0.8mm etc?

Yes.

I am anticipating maybe having to machine up some jets as an alternaive to tracking some down.

Don't. Standard is best.

The biggest lesson I learnt was that I will have to put a switch on the side stand (health and safety) as with the stand down and heading towards oncoming traffic round a long left hand curve not being able to steer away on icy roads with a jerky throttle is not something I want to try again.

Maybe you can fit a switch that will operate the turn signal buzzer.

It's my believe you should be able to have a decent runner, provided:
Airscrews are 1¹/₂ turn out, that is 360^o and 180^o. You can vary by adjusting ± ³/₈ of a turn.
Air intake is not blocked in any way in particular by anything under the buddy.
The main jet O-rings seal well.
Jets are not mishandled by using steel 'cleaners'. (Some cannot resist the temptation.)
You have the correct sparkplugs and no more than 8kΩ resistance.
The middle pic Calj posted, pops up here at many occasions. I myself find the pic (below) more ressembling the reality I have experienced with my carbs. Notice the difference in what the slow jet contributes.
I'd have the needles in the recommended position and tune with airscrews and main jets only. If you happen to have the slowjets #40, you could consider going #38.
Are you familiar with the Honda instruction manual on carburetion?

[58webbing]

The engine top end has just been rebuilt (to change weepy gaskets) and all settings are as perfect as I can get them. When I bought the bike from a large UK retailer they had sent the carbs away to be sorted by a company of "experts" and the carbs are the only part of the bike I have not examined relying on the experts knowing what they were doing. A mistake methinks! I have been fiddling with carbs on and off for the last 50+ years and every now and then something like my current challenge comes along to question everthing you thought you new about the subject. Conflicting data pointing to weak mixture in terms lumpiness with sooty plugs. On one start up engine wont idle next time it races so you check all possible influences hoping to find something to fix but so far to no avail. This is why I asked the questions so I can put everthing back to std. and start with a clean sheet of paper. Its nice to know that the good folk on this forum are there to hold my hand during this process. Its no comfort to know that experience has taught me the harder you work towards a goal the greater you appreciate it, being impatient I want it running correctly now. The idea of making jets is to make some standard ones, I have the tools and knowledge to do this but not the sizes at present. The other method I will try is the Gunson Colourtune gadget to see if that helps. Chatting to my chums at the pub the consensus was "I had a carb like that once"

[Deltarider]

The other method I will try is the Gunson Colourtune gadget to see if that helps.

Forget about the Gunson Colortune. It won't work on our bikes. I have tried it decades ago (you may do an advanced search on 'Gunson' and 'deltarider'). Moreover your model needs a rich idle that will not show 'bunsen blue' I'm afraid.
Here's the Honda Manual on Motorcycle Carburetion. http://cincinnaticaferacer.com/wp-content/Manuals/HondaCarbManual1975.pdf

[bryanj]

If you are in uk have you been on the UK forum? Much the same as here but busy at different times.
1 did you replace manifold to head O rings
2 are you certain both points gap AND timing are perfect
3 are the rubbers between manifold and carbs soft enough to seal
4 have you put your leggings (or such) under the seat which blocks the air intake---dont laugh i seen it done!!!
5 have you had vacum gauges on the carbs to set them
6 where in UK are you

[58webbing]

Thank you Deltarider for the Carb manual, some lite bedtime reading. It might trigger a line of investigation leading to the answer which I am sure when its found will be obvious. In answer to bryanj the answer is yes to all (except the leggings one) I live in Luton, Beds. I will take the carbs off the bike when A its warmer in the garage and B I have a clearer idea of how I will set about things. Then strip them down and see what the experts have done check if corrosion has caused any leaks swollen o rings etc etc. I was not sure about the UK site as I thought they were all bundled together judging by the international flavour.

[Deltarider]

Who knows what we can expect from the Brits these days...  Order! Ordeeeer!!!

[bryanj]

Have a look at sohc.co.uk we have probably less members but still the decent banter.

[58webbing]

Things are progressing, I found that the pilot jets that should be a 38/40 measured at 0.55 (even though they were maked as 38) so I hope that is the smoking gun I have been looking for. There were quite a few other minor issues that by theselves would not have made much difference but all together, who knows.
The next area of uncertainty concerns the throttle slide anti rotation pegs. The little (what appears to be Nylon) oblong pegs are only on the 2 inside carbs? The outside carbs have the oblong "pegs" sitting quite a bit below the surface which suggets that they have not been sheared off at some time in fact this arrangement seems to be deliberate. The linkage mechanism does a good job of stoppig the slides from rotating.
My 2 questions, are the anti rotation pegs on only on the 2 inside carbs standard? and if so why? To my way of thinking they should be on all 4 or not at all!

[TwoTired]

Slide alignment guides are normally on all four carbs.

Missing/ non functional guides may result is idle return issues.

Cheers,