Topic: Synching the $(*# Carbs

[Loudpipe]

Since Honda has managed to write the most incomprehensible manual in the world, what exactly is the process for synching the carbs?  The manual says to adjust the throttle lever 49mm off the stay... I know which bit is the stay, but what piece of the throttle lever is it talking about?  Anyone got a better diagram than they have in the manual?  Or better yet a picture off your own bike with little Windows Paint bits drawn for a simpleton like me?

[scondon]

 Set bike to idle around 1,200 rpm. Synch carbs so all measure even. Blip throttle a few times and watch it settle back down to new un-even levels. Repeat process until you either pull your hair out or resign to "good enough". Guess Honda couldn't get the correct Japanese/English translation for this

[Loudpipe]

Do you know where to get a 4-in-1 vacuum gauge so I can see the readouts for all carbs at once?  I've seen pictures of the original Honda gauge which is 4 analog vacuum gauges, and then I've seen others that are kind of a thermometer style gauge (except they measure vacuum, of course).

[Spikeybike]

get this one , it's a little pricey but well worth it ,after using other gauges that aren't worth jack

http://denniskirk.com/jsp/product_catalog/Product.jsp?skuId=&store=Main&catId=418&productId=p282760&leafCatId=41804&mmyId=

[PJ]

http://motionpro.com/motorcycle/tools/view/syncpro_sup__trade___sup_/

oops.. beat me to it:)

[chrislib]

X3 on the motion pro. I find that synching can be a pain in the butt....BUT the payoff is well worth it. Make small changes at a time and rmeber that adjusting one affects them all, so you kind have to go over them a few times till they are even.

[bozo4onion]

I love my Morgan Carbtune. $130 shipped from UK last month w/case. Buy the best, you only cry once.

[Loudpipe]

Cool.  Ordered the Synch Pro, but where should I even start?  I tried to get them all semi-even today by turning all screws all the way in, and then backing them out 1.5 turns, but that just made it so bad it won't run at all.  Where do I start to get it to run?!?  Also, when I had my Lisle gauge on any one of the carbs, it fluctuated so much it was hard to see where it was even reading (i.e. between 5 mm-Hg and 16 mm-Hg, it would just wave back and forth very quickly).  The bike has individual air cleaners and straight pipes with no muffle (one for each cylinder) and I don't know if this has anything to do with it or not.  How do I see what it's really reading?

PS: The mixture was set perfectly on each carb for a nice, light brown before I changed anything, so that really shouldn't be tooooo far off now that I've bothered with the synch adjustment.

[scondon]

Cool.  Ordered the Synch Pro, but where should I even start?  I tried to get them all semi-even today by turning all screws all the way in, and then backing them out 1.5 turns, but that just made it so bad it won't run at all.




PS: The mixture was set perfectly on each carb for a nice, light brown before I changed anything, so that really shouldn't be tooooo far off now that I've bothered with the synch adjustment.

 Confused as to whether your describing idle mixture adjustment or carb synching. Set the slides at equal heights for a baseline synch.

[martino1972]

yeah,i think you are confusing idle mix settings with sync. carbs...

[Loudpipe]

No, I'm not, although I have a habit of wording things confusingly.  Hope I never get a job writing instruction manuals!

I'm talking about the carb synch.  I don't know if that is affected by the mixture at all (which should be set fine in my case) but I'm referring to the carb synchronizing and the synchronizing screws.  Would 1.5 turns on all screws be too much to back them out to get a baseline synch (i.e. get it running well enough that it'll register correctly on the gauge)?

Also, is 1200rpm enough to get a proper reading?  Like I mentioned before, I put an analog Lisle gauge on each intake, and the needle swung wildly back and forth across the gauges, presumably as each cylinder went through the intake/combustion/exhaust cycle.  It made it impossible to see exactly how much vacuum it was making, unless you just take the highest reading on the assumption that is swings downward during combustion/exhaust when there is no air flowing through the manifold, and back up when the piston is on its intake stroke.

[eurban]

You should do a "bench synch" of your carbs prior to installation.  This involves using a convenient object like a 1/8th drill bit as a gauge between the slide bottom and the carb bore.  Adjust the idle speed knob until the object just fits between the slide and the bore in #2 (not adjustable) carb.  Now match the other carbs to #2 using the bit as a guide.  Search under "bench synch" or "bench synchonization" for more info.  You might manage to do this OK with the carbs on the bike.  Turning the slide adjustment screws out 1.5 turns does nothing to match the 1-3-4 slides to the #2 carb so it isn't good procedure.  You are wasting your time attempting a carb synch with a single gauge.  Changing one slide will affect vacuum on the others.  Use a quality 4 gauge setup (like the Morgan) and all will make sense. 

[Loudpipe]

Yeah... Jeff had done a bench synch before he sold the bike to me, but the nuts on the slide adjusters backed themselves off and everything came undone.

Also, the thin little metal "straps" which connect 1 & 2 adjusters and 3 & 4 adjusters broke when I tightened them back up.  Where can I find new ones/are they necessary?

[Bodi]

Those metal strap things... I'm not sure if they are just a locking washer of sorts or keep the two screws aligned, I think just for locking the nuts so they unscrew less easily. That apparently didn't work for you.
The balancing screws interact with the idle screw to keep the motor running, plus the choke high idle cam setting is affected. Bench sync is a good start. If the rack is set up fairly well, ie the idle screw has travel both ways from your base idle and the choke cam is working OK, don't move all the sync adjusters much at all. If you're starting from scratch... set the idle screw so it has some travel both ways and adjust the individual carbs so maybe a 1/16" drill slides with a bit of drag under the slides. Remount and try to set your idle to about 1200 - even if it's rough and stalls out. Now check the idle screw and make sure there's some travel both ways, if you had to change the idle screw a lot then screw it back to the starting position plus half as many turns as you went to get the idle to 1200ish. Then redo the 1/16 drill bench sync. This should get the idle screw OK once you reset the idle after starting.
Now use the vacuum gauge. Turn the idle up as far as you need to get the bike to keep running, you should have a good strong fan blowing at the engine to avoid overheating. You need a remote fuel tank, either long hoses on the stock tank or a small test tank (I use a 1 quart metal solvent can with a hanging hook, petcock, and nipple).
- Using a single gauge is a good way to have a nervous breakdown.
- The gauge needle waves wildly for the reason you guessed. You need a restrictor/snubber in the vacuum line to steady the gauge at an average vacuum level. Stuffing a rolled up cigarette filter in the hose, near the carb end, works pretty nicely. You can add aquarium air system needle valves in the line(s) and have adjustable damping too, the valve should just be closed until the needle steadies, it will just be cracked open a hair. Leave a good length of tubing between the snubber and the gauge to give a "reservoir" of steady vacuum.
Anyway, you want to look at all four carb vacuums and adjust each towards the average. This will give you a balanced rack with the idle setting just about the same as before syncing. Some racks have one non-adjustable carb slide and the other 3 get adjusted to match it, sounds like yours has all four adjustable.
Once you have it balanced fairly close you should be able to drop the idle and check it again. The sync adjustments are more sensitive at lower idle speeds.

[Loudpipe]

Well, none of the synch adjustment screws would go down far enough to close on a 1/16 drill bit, so I used a 1/8 and set it up with a slight bit of drag on 1, 2, and 4.  #3 would not screw down enough to even close completely on that, so I just got it as close as possible.  In order to get the slides to close on the 1/8, I had to back the idle adjustment screw almost all the way out.  The bike still wouldn't not start with it set up like this, and didn't even catch once (although it was cold; it used to catch on the first or second turn of the engine).  How do I adjust the slides down so the synch adjustment screws are in the middle, or nearly in the middle of their ranges on a 1/16 drill bit with the idle adjust screw screwed in a reasonable amount?

[scondon]

idle speed adjuster screw will raise/lower all slides simultaneously.

[martino1972]

something is out of wack then,you should be able to completly close the slides.!!!!
did you have the choke off..?

[Loudpipe]

Yep, choke is totally off (or open, whichever you prefer).  I'll take another look at it tomorrow.  Maybe the slides can be rotated down on whatever shaft they're on via loosening a screw or something?  I'm too fed up with it to mess with it tonight.  And it attacked me when I was trying to kick start it.

[eurban]

No I don't think he is out of whack.  What side are you measuring from; air filter side or engine side.  If you are doing it with the carbs on the bike, I guess it would be the air filter side   On the air filter side, slides wont close small enough to gauge with a 1/8th bit. On the engine side however, the gap between the carb bore and the slide should pretty much disappear when you back out the idle speed screw.  Most accurate technique is to gauge from the engine side (now I remember why I do this on the bench and not on the bike; thats why they call it a bench synch ) and upon reflection I whole heartedly agree with Bodi, use a 1/16" bit as your measuring tool since this should be fairly close to where you want your idle speed screw set when you are done.  The reason why your bike wouldn't start is that with the way you were measuring from the intake side you had your slides so far closed that no air was able to pass through the carbs and into the engine.  I guess with the carbs on you could try a 3/16" or so bit from the intake side to get all the slides at the same height and then back off the idle speed screw till it just starts to gap from the throttle linkage and then tighten about 1 1/2 turns.  This should get you to where your bike will start and idle.  Probably high . . . .

[martino1972]

good point.. .....that would explain alot..

[Loudpipe]

Yes, I was doing it with the carbs on.  I've not worked on slide carburetors before, so this is a new experience for me.  Only thing I've really worked on are the Webers on my Triumph, and I wasn't aware the slides are different heights (relative to the bottom) front to back.  I'll try a bigger bit tomorrow and what you suggested with the idle screw.  If that doesn't do it I'll just take the carbs off the bike.  I was trying to avoid that due to the unbelievable amount of parts all over my garage from the 750's motor, I don't really have a place to work on something else right now!

Anywho, thanks for the help fellas.

[Spikeybike]

well my 2 cent on that is , if you do end taking the carbs off definitely clean spot (i know you already knew that) 

lots of little parts that need to stay really clean   

[bryanj]

would help if you said what bike you are working on

[eurban]

I believe its the 78F on his list of bikes.

[Loudpipe]

CB500K0 this time.  Sorry, should have said that earlier.