Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: royhall on December 14, 2016, 04:14:18 PM
-
Does anyone have a simple and effective step by step way of syncing the carbs on a 350/4. The manuals all describe slightly different ways of doing it. They appear to be very sensitive to each other, a quarter turn on one seems to disturb the others a fair bit, so I'm chasing my tail with it. All was very simple on the 750's, but not on this. Think I may have missed something somewhere. Cheers.
-
I have also found that the 350s and the 550s are very hard to get spot on, my 350 is to within 1cm hg and then fine tune using the air screw, i have always found it easier on the bigger bikes and more modern bikes, i think it's just a case of plugging away and using either number 2 or 3 as the bench mark carb.
-
On the 750 I usually take the bottom of the air box off and the air filter out before syncing. Is it required on the 350/400 to take off the airbox manifold first, as its a royal pain to get out. Or can I get away with just taking the filter out. Cheers.
-
I don't have a reference for the tolerance on these smaller fours, but the 750 is given as 40mm (1.6 in) Hg.
-
On the 750 I usually take the bottom of the air box off and the air filter out before syncing. Is it required on the 350/400 to take off the airbox manifold first, as its a royal pain to get out. Or can I get away with just taking the filter out. Cheers.
Not sure that's such a good plan Roy, the idea is to set the motor up to run properly as it will be on the road, removing the air filter to set them will be wrong when the filter is refitted, as with the filter out, all carbs get a straight supply of air, and when the filter is put back there will be resistance to airflow, but some carbs get a fractionally higher flow than others, messing up all your work! :o
-
I don't have a reference for the tolerance on these smaller fours, but the 750 is given as 40mm (1.6 in) Hg.
I think there is a parameter of 2hg on the smaller fours, around the 22 mark, but it can be any number as long as the difference between carbs does not go out of the 2hg parameter, the air boxes and filters should be left on.
-
I had them off the bike for servicing so I synched them on the bench using a 3mm drill under the throttle slide on the engine side of the carbs When I fitted them and fired the bike up with the vac gauges connected only one of them needed a minor tweak for them all to be spot on.
Don't forget that balancing the carbs is the last thing you do after setting the ignition timing and the valve clearances. If either of these are out you won't get them to synch correctly.
-
It does say in the official Honda manual to remove the air intake housing before syncing. Haynes and Clymer don't mention this. Honda book states 3hg tolerance. Have been through the rest of the settings first but I think I will go through them all again just to be sure. Raising pressure on one cylinder is dropping the other three quite a lot, more than I would have expected.
Sent from my Nexus 6 using Tapatalk
-
I find that odd as the air box stabilizes air flow to the carbs, maybe try without the filter at least then there is less resistance.
-
I agree Kev........ I would have thought syncing should be done as near to real operating conditions as possible. Thats really why I started this thread to try to find out how the pro's do it. This is the first time I have done this on the smallest four so need to ensure I am doing it the best way........ On the 750, airbox on or off doesn't appear to make any difference in the syncing (set them up with the filter and lower airbox off then rechecked with them back on)......Maybe due to the small size of the carbs I am making too big an alteration at one go. I will leave it for today and see what the replies bring. Thanks all.
-
It's something I've thought differently about more recently. I've owned a set of vac gauges for 30yrs or more and used them on many things. As I've said on some other threads, I do feel more now that there's an element of show with this method of syncing that maybe inconclusive at best.
Over many uses for syncing other's bikes I've experienced just that variability that seems to fall either way with the engine not seeming to run or perform any better than before the gauges were used. Logically the smaller bore carbs would appear to need a very finely resolved adjustment mechanism to achieve a very close parity. If the above posted tolerances are correct, then this I would see as an acknowledgement by honda of the physics involved.
The thing that really made me think it through again is something I read (I thought by Hondaman, but can't at the moment find it. Please correct me if that's not the case) and it was to check the sync at 5000rpm. I first thought that this was really odd, but then working back through it I can see why this should be logical.
If you've built the engine well and it has a reasonable comparability from one cylinder to another, then it's further reasonable that to run it flat out (wide open throttle) the slides should all be matched for height physically to supply the same torque from each pot. In other words, bench syncing accurately with a metal gauge in the carb should give you that.
Is there any reason to alter them based on a tickover that essentially has a method of equaling (the idle circuit and airscrew adjustment) dedicated to its refinement AT IDLE SPEED ? To me it just looks like if you've got all the other elements, timing, plugs,valve clearance etc then the main slides should be left alone and trim each carb individually to give even load to each of the cylinders at tickover with that very fine adjustment the idle screw allows you.
It should result in a better setup for the entire range of operation and overall smoother running.
I'm more than happy for people to critique this view, I'm trying to understand if it's a valid one too.
-
K2-K6 you have pretty much summed it up, the two words reasonable comparability across all four is good but not absolute thereby giving us the need to balance the four carbs, unfortunately there is no magic formula to do this and it is just a process of trial and error, myself when i cannot get absolute identical readings at idle but within stated tolerances i then see what it's pulling at the higher rev ranges, or we could take our bikes to a dyno and they could do it all for us, but where's the sense of achievement in that.
-
I'll respectfully disagree with you kevski on this.
If you've set the carbs on a bench to parity, then when you run the engine at idle it shows a difference when you measure the vac levels, the tool for modifying that characteristic is definitely the idle circuit. This will speed up or slow down that cylinder and change the vac reading you obtain.
Just for a moment if we consider what's happening there, the idle circuit if it has any differences from one carb to another (whether manufacturing or cleanliness) will because of its overall size have quite an influence over the idle speed of that cylinder. This is the right tool for taking any differences into consideration.
If we go back to the method of adjusting the main carb slide to correct the idle setting, then as you lift the throttle away from idle (and the idle circuit ceases to contribute to the mixture now used) that adjustment you've just made becomes a mistake from the terms of cylinder parity. The other end to this could, as there are other questions about this on here, be why is one of my header pipes / plugs running a different colour to the others? Because you've potententially just built in an error to main running mixture while attempting to get the idle smooth.
I believe the two parts need to be understood for what they do, yes, adjusting one will affect the other but you'll possibly end up with two errors that compromise the smooth running you're after in the first place.
-
How can you respectively disagree with something you said that i agree with, in my first reply on this topic I made reference to the fine tuning via the air screw for the pilot circuit, regardless of bench syncing there will be small differences that may or may not be adjusted with a combination of vac gauges and pilot circuit adjustments, either way it's still trial and error until we get running how it should.
-
I don't think you've got my interpretation quite right in what you're saying about it.
To make it clear, what I'm saying is that having set the carbs to have the slides equal, then adjustment of the slides should not be used to alter the idle balance at all. The idle screws aren't fine tuning of the main slide position, they are wholly and singularly the adjustment method to balance the cylinders at idle revs.
There is no conflict of what I've said so far.
It maybe trial and error, but I think to use the right adjustment is fundamental to achieving the ideal result.
-
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.
-
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. ;)
-
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.
[attachimg=1]
For a simple procedure, this has got very complicated. :o
-
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.
-
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?
-
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.
-
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.
-
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.
-
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.
-
Thats us told then. ;D ;D 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.
-
Oh dear Roy, it does sound terminal ;D 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.
-
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.
-
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
-
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 (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.
-
Will look through it again, and you are not wrong about assumption.
-
Kev. Starts at the link given, then the problems start at page 11. Will hopefully get the bike back on the road over Christmas. Will post how I go on. Thanks for your help. Cheers.