SOHC.co.uk Forums > CB350/400

Carb sync

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kevski:
Completely understand your view on this, but as you said reasonable comparability is not absolute, assuming each carb will draw the same vacuum because the slides are at the same height is short sighted, small differences in machining tolerances during manufacture, wear during prolonged use, will create variations which will need to be ironed out using both ways of adjusting, i have recently done my carbs firstly on the bench using a small drill bit then on the bike although they were pulling to within spec, then using the gauges to improve this finally using the idle circuit to improve it a little bit more, with respect you do it your way and i will continue to do it the way i have been successful at for many many years.

K2-K6:
So it does work then if you bench set and fit the carbs, as you found them within spec. ;)

I'll make it clear that I'm not critising you but will offer a method alternative if I can't see how what you state doesn't completely answer the question posed.

It is a form of peer review that we are doing which is well understood in many sciences as a method to shake out best practice. Also it allows others to see a record of our thinking to either take that on board or themselves to offer a contrary view. There's nothing wrong with that, it has to stand up to others scrutiny for them to either try it or believe it offers some advance.

Still what I see in your description is potentially adjusting the tickover vac settings via the wrong mechanism. What the version I've described above adds to your assertion of tolerances is, by checking vac readings at 5000rpm it does exactly what you desire in that it does eliminate variances for casting, machining etc of each carburettor without the influence of the idle circuit, which is highly desirable for running each cylinder equally under load.

They are two separate functions also with two quite separate methods of achieving parity across a range of operating points.

To set a routine based on that, assumes that all other mechanical checks are completed;

1) bench set carbs to equal.
2) fit and run bike up to warm.
3) check vac at idle and adjust with air screws only to get within stated tolerance.
4) raise revs to 5000rpm to get vac reading (this is what tells you about cylinder parity) adjust slides if required.
5) IF SLIDES NEEDED TO BE ADJUSTED, check vac readings again at idle and reset idle screws to balance tickover with new slide positions.

As I said, I believe this is what Hondaman is saying and think it offers an important level of isolation in setting two differing components, if that is true.

I think royhall has set a very interesting and open question,  I'm not suggesting that people should take what I say as verbatim instructions. I view this thread as an open discussion for him to understand if there are any advantages to be considered for his own use.

Inadvertently it would also catch people who visit here with very little experience in setting up these carb sets. If you bench sync and then run the motor to find the idle vacs are all over the place it should prompt them to consider what the state of the idle circuits really are like. So many seem to get caught at that point, then change the main slide positions which just gives them more errors to frustrate them.
If you put on bench synced carbs and some of the idle screws show no effects when turned, then it should be a prompt to verify those idle circuits.

And stop Bryan having to tell them to get their idle circuits sorted. ;)

royhall:
Thanks for the input guys, getting a bit heated there for a while. :D 

I must say though that I tend to agree with the K2-K6 version of things although it does go against the advice of the official Honda manual. There appears to be many ways of carrying out this task, but ultimately there can only be one correct result.

I dont really see how altering the slide heights at tickover could possibly enhance full throttle running, surely the slides must open uniformly. As K2-K6 says, the low speed has to be balanced with the idle circuit so that the fast running is not interfered with. As most things like this require a compromise, it must be more important for the balance to be correct at higher throttle openings than at tickover as thats where the bike spends most of its life and where the performance matters. After a day of thinking about it, this now makes perfect sense to me. I will try it the K2-K6 (Hondaman) way and report back on how I get on.

So for now I am going back to basics and resetting everything from scratch using the 1.5mm drill method, and double checking the other settings such as timing, tappets etc.

Now the next question is. This picture refers to the setting of the carbs before sync using the throttle stop screw. The distance H being 56mm. It is not clear to me from this picture where the measurement starts and ends. My guess is its from the top of the cable nipple to the top of the bracket. Just to clarify, after this 56mm has been set, the 1.5mm drill is used as a plug gauge to set the slide heights with the four adjustment screws.

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For a simple procedure, this has got very complicated. :o

kevski:
Not really heated Roy, i just pointed out that reasonable comparability is not absolute, i am not poo pooing the method as there is more than one way to skin a cat, i am sure that anyone that does this job always bench syncs the carbs, for those that don't, put the tools away and go and watch the X factor, but even the bench set won't be absolute and as its a round hole if you are the slightest bit out with the drill bit it will show up, i always check mine across the rev ranges and try to get it them all within 1hg, the parameters are between 16 and 24hg though higher or lower is not important unless it's very low, then between carbs for these 3hg is the acceptable amount of difference, that distance H would be 56mm. Keep posting on this so we can see how it all goes.

Nurse Julie:
I have been trying hard to keep up with this thread but there is one thing that I cannot understand. How is it that Keihin built the carbs and bench synced them, put the little bit of yellow paint on them and sent them to Honda for fitting to the bikes and as the yellow paint is still in situ (I know this having seen NOS Keihin carbs on a few occasions) how come they work just fine when fitted to the bike without any extra adjustments?

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