Carb sync

[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.  

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.

[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?

[K2-K6]

As I was writing the above posts I was wondering the same thing. I can remember reading that method and was wondering just what happened at the factory during manufacture as I can't imagine the production line having to do a carb balance with gauges as a routine.

Well spotted about the paint though Julie. I guess you'd have to get knowledge about how Keihin set them up, which would seem like either drop the slides and mechanisms in then just lock the adjusters off. Or would they have run them on something like a flow bench that had a tapered four gang rubber socket to just push the carbs into and pull a vac through them to test and set parity.

Thing is, I've always in the past looked at them the way the manual says to and never really questioned it. But then I read that 5000rpm thing about a year ago and wondered if that was right. Puzzled over it abit (Yeh I know, a bit slow thinking time) and gradually realised what he was saying, and why.

Whatever the case, I think it's correct to set the carb's slides, as best you can, without the engine influence and leave them alone. The adjusters for the slides seem to imply this as you can only set them at one point for the whole range. There is no facility to alter them for progression over the range of use. You are just reliant on the production accuracy of the manufactured parts.

If you go for a moment to running an engine on miss-matched throttle slides, it's likely you'd get some differences in wear longer term from effectively running a different mixture through each cylinder.

I fully understand the view that there are manufacturing tolerances,  but let's face it, the more you look at these four cylinder Honda's the more you have to be impressed at the original production line manufacturing capability considering what else was being produced.

Sometimes when you stand next to someone's bike, you can just here how well it's setup. It'll be interesting to see what royhall thinks by trying it. Sometimes people tell you little things that previously you've not considered, but some of them, just sometimes, can make such a difference.

[Nurse Julie]

Interesting. I am no expert at carbs of any type but having had many bikes over the last 38 years with 1,2,3 and 4 carbs, I have always found the best way to get them performing ok is bench sync, make sure the floats are at correct height, there is no crud in them, put them on the bike and then just fiddle with the mixture screws. It may be considered a very basic and naive way of getting the job done but it works for me. Ultimately, it's usually us taking the carbs off the bikes for whatever reason, that messes them up in the first place. I have found generally but not always admittedly, that once the carbs are on and i have done the basics, they never need touching again unless a float holes or something gets blocked.

[kevski]

Every thing is was made on CNC's where every cutting tool is only used for a pre-determined amount of times minimizing the amount of difference between component sizes, all put together in the same way with the same settings, then one is tested out of so many to check it works within spec, and so on and so on, the machines then end up in dealerships where the final assembly takes place then run up before being sold any out of spec issues are then dealt with, the 3hg range is quite wide but acceptable, my 350 come to me with 5033 miles on with the yellow marks still on the carbs, i  renewed the rubber components in and around the carbs, also valve clearances, points and timing, fired it up and it was showing about 2hg difference on number one, within parameters i tweaked it a bit to give me around 1hg, CNC machining using best practice is wonderful but as of yet not perfect.

[Nurse Julie]

I bow down to you experts knowledge but  you are analysing the outcome of a minute difference of hg, what's the point?. If the bike runs, great. In my line of work if i got so hung up on a small variation on the human body, a majority of my patients would be in intensive care on life support just because their blood pressure was up a bit. If something works ok, live with it and enjoy it.

[royhall]

Thats us told then.   Problem is Julie, the 350F is a patient on the operating table again as I just rebuilt the carbs a second time (the way I should have done in the first place) and I am trying to learn from the experts the best way to set them up. Believe me, when the bike runs right I shall never touch the carbs again, I hate the bl***y things.  

I'm with you on that one though, if its running good leave it the hell alone.

The 350 is trying its best to die though. Whilst I was doing the carbs the battery decided instant death was the way forward. Never seen a battery go that quick ever, hence my other thread about batteries.

[Nurse Julie]

Oh dear Roy, it does sound terminal  I agree with what you said about learning from the experts, and I think it's great we have such knowledgeable members on this Forum who are able and willing to share their experiences which will help us all and future SOHC members. On that note, I will go and fiddle with the newly rebuilt carbs I'm putting on my CB550 and just hope my very basic knowledge does the job.

[K2-K6]

Certainly on here we are fairly well past consumer knowledge and into the back office stuff, if that's the right way to describe it.

I guess discussions about total strip and reassembly will naturally venture into more detail and it always seems more long winded written down than any face to face work through of something.
It's curious that people with low knowledge of any basics take a look at an engine and voice the opinion of how complex it looks,  will when faced with a problem start to adjust any part they can get a spanner on in the hope that they may improve it.
The bit that repeats itself fairly frequently is when someone arrives on forum with a poor running bike which is usually the result of impaired idle circuits, yet they think that a carb balance is going to sort it out. That confusion over which bit affects what is real easy to sort out if it's in front of you, but some of the problems go of for months, with the new owner getting very despondent about their latest bike purchase. If you go through the basics in the right order then it's really quite simple.
I get on some cars where for want of a little routine a problem could be solved, but they've picked out anti tamper seals, adjusted all sorts of stuff and completely lost sight of any rational diagnosis. Then you've got to check and reset everything before you can even see the original problem.
I'm happy taking the time to set anything up to run as it should, but when out using it I just want it to work, no questions asked.

[kevski]

Roy i went through your build thread for this bike and am rather surprised you had not rebuilt the carbs, i guess you are beating yourself for this, i personally have never restored a bike i have a preference to preserve and only do what is needed that always includes a carb rebuild brakes and suspension, if i totally restored a bike i wouldn't use it for fear of getting it dirty, so will you be using genuine  parts or after market, i got my rebuild kits from 4 into1 in California and just used the o-rings and needle valves cheaper and quicker than David Silver the valves let by but with every set of valves i have ever bought genuine or not i have had this problem, i then polish the tips and the seat and it always works,  really in short, set every thing as stated in the manuals there is no special formula, bench sync, put it back together and see what you have in hg across the rev range, if it all runs within 3hg you are in parameters and if like me you want it closer tweek it. The one thing i set different is float heights and only by 1mm on the 350 i have them set at twenty two. Having a duff battery may not have helped you with this as the ignitions do like a good battery. Good luck

[royhall]

Kev....... You must have missed the bit about me rebuilding the carbs in the restoration thread, it was quite a few pages long as carbs are a real struggle area for me for some reason    (http://www.sohc.co.uk/index.php/topic,8257.90.html)      They were given a really good clean, then done in an ultrasonic bath. They were replated where required and reassembled with new seal kits (the jets and needles etc were cleaned and reused)............ Everything was okay initially after I had sorted the petrol overflow issues, (they even synced well) but soon I developed problems on carb 4, in that it was running so rich it actually spit raw petrol from the exhaust. Thats when I decided to redo the carbs with full refurb kits......... Carb 4's problem was a way over size idle jet, looks like the PO had been cleaning it with a drill and made the hole huge, plus a chunk of float had broken away causing the float height to increase (never leaked from the overflow again though). How they synced okay like that I will never know, but the pictures are on the rebuild thread to prove it?..........So now they are rebuilt for a second time, only this time done properly.............The reason for this thread is to make sure I am setting them up as good as they can be, as they appear to be far more sensitive than the carbs on the 750. So just needed the views of the experts on here so I can learn something..........The real lesson in this is don't assume anything when it comes to carbs (or anything else), you never know what previous owners have been up to.

[kevski]

Will look through it again, and you are not wrong about assumption.