Topic: Carb sync

[jimbo]

My bike isn't running great so I was going to sync the carbs tomorrow.  Only problem is I can't find a manometer around here anywhere.  Can I use a automotive vacuum guage and take readings from each carb, use one for reference and sync the rest off of it one at a time?  I know this isn't the best way but I'd really like to get some serious ride time in this weekend.  We don't have many more good weekends left in my area (NE KY) before cold weather hits.

[Gordon]

It may be possible, but I doubt it.  When synching the carbs, every little adjustment to each carb affects the others, at least on every 4-carb setup I've worked with.  You can probably get them in the same neighborhood, but not much better than a simple bench synch can do. 

[grumburg]

  I synched mine with a UniSyn from my SU days. Had to go over them 3 times to get them right, but they are spot on. Maybe it was beginners' luck. 

[DammitDan]

I was using a 4 carb manometer, and every time I adjusted 3-4, 1-2 would change.  And every time I adjusted 1-2 to get them even, 3-4 would change.  And what's worse, when I adjusted 4 by itself, all the others would change.

Boy was that frustrating...  But I finally got it.

I can't imagine trying to do that with a single tuner!

[bryanj]

Yamaha used to do a single gauge with a rotary valve and multiple connectors IT WAS CRAP!!! when you adjust one carb the others will alter also, at least in 95% of cases, so you need to see all four at the same time.

[jimbo]

Sounds like I just need to order one from JCW.  I'll do that today.  I can still ride it as it is so the weekend's not a total loss.

[techy5025]

Trust me...

Unless you are a person with infinite patience...go with four readouts.
Dials, mercury manometer, or carbtune all should work OK.

I have a single dial type guage mounted to a manifold that allows
selection of any or alll of the four carbs.  It mounts under the left
side cover and  is good for a quick check but not much else.

The old K0 carb set had TWELVE  adjustments and the new ones have
NINE.....all interact with each other.

Anyone who has ever tried to converge and old color tv set will
appreciate the problem. 

Jim
 

[6pkrunner]

I have quite a few articles and road tests as I have every magazine I bought since 1966. quite a few road tests and several Big Bike magazines dedicated to the 750 mainly.

[southbound]

Well, now that i'm a proud owner of a '74 CB 550, i'm sure i'll be spending quite a bit of time on this thing, so hello to everyone.

Now to my first question:

I started syncing the carbs tonight- one of 'em was off from the others- and once i got them done, i revved it up- oops, forgot to tighten the locking nuts.  now i can't even get it cranked, and the screws are all out of whack.  any recommendations for starting from scratch with carb syncing?

thanks.

[bistromath]

Well, you could try setting the screws to the same height above the lock nut, I don't know if that would get you started or not; but the best thing to do is to take the airbox off and bench sync by looking at the slide heights.

That said, try giving it a little throttle when you're starting it and that will probably get you far enough to get a basic sync done.

[southbound]

I'll give it a shot again on saturday.  the problem last night was that i killed my battery trying to start it, killing my momentum.  i think i flooded it in the process.  since it'll sit for a couple of days, probably not a bad time to do the tappets and cam chain.  anybody else got advice?  it's my first time sync'ing carbs. 

[pmpski_1]

i killed my battery trying to start it

Get used to the kickstarter. If it's not going to start or is hard starting you'll drain the battery every time. (Yes, I learned the hard way). Plus, you'll get some excercise 

[TwoTired]

anybody else got advice?  it's my first time sync'ing carbs. 

It's an air cooled motor.  Put a window fan at the front of the bike blowing air across the motor for extended stationary runs.  An overheated motor will give funny readings in vacuum.  And, the bike should be thoroughly warmed up before Carb sync.  This gives too little time to adjust all four carbs before you get into the "too hot" range without a good fan.

I got one of those big commercials fans.  Moves a lot of air.  It's almost like tuning the bike while riding.  Except the bugs are minced before striking you.

Cheers,

[southbound]

Ok, got it...  all i needed to do was let it sit for about 24 hours, and it cranked, idled long enough to sync 'em last night. 

wow, what a difference.  almost threw me off the back.  thanks for the help guys!

[eurban]

Glad to hear it worked out.  Just to add, I have found that alot of synching difficulties  can be avoided by initialy adjusting the carbs to all have an equal space between the bottom of the slides and the carb bores.  This would be a so called "bench synching" and seems to make the proper synching job on the bike go much more smoothly, particularly if the settings were way off.   Of course this is a bit difficult to do with the carbs on the bike but it is possible.  If the bike is running pretty smoothly and has been synched in the rencent past then there is no need to waste time with it.

[jdpas29]

 

i've got a 78 750K with stock air filter, 4-2 exhaust, stock ignition and shiny fenders on the front and rear  ...

my question is....  what RPM should i set the bike at to synch the carbs using 4 vacuum gauge setup?

also.... should the air filter be taken out to synch carbs?  i saw that somewhere and it got me wondering.

[TwoTired]

Synch at idle speeds.  Air filter installed.  And a fan blowing on a fully warmed up engine and after all the other tuneup items have been nailed.

Cheers,

[GroovieGhoulie]

I just sync'd my engine after tuning everything else and with the fan and everything.

Awesome.

Clutch rattle is gone.  The numbers were all way off each other and off the range in some cases.

Got her purring all sexy now.

[jdpas29]

thanks a lot guys...  i've got this carb synch tool coming in the mail and my airbox and stuff came already.  i was so excited when i got the airbox because i opened it up and it had a UNI already in it!  i was like weeeeee!!!  but then i took it out and the spongey stuff was coming apart in my hands.  ARGH!  but it's kewl... i had already ordered a stock honda filter.  the box is actually from an 78 750F, but i think it's going to work.  i gotta go get a couple of mounting screws for the box from Lowe's for the box.

hey.. one more question.   there are a few hoses that i'm wondering about on my 78. 

1.  there is a black rubber hose on the right side of the bike back near where the end of the exhaust is.  it's about a 3/4 inch around and has a cap in the end of it.  is this supposed to be capped?

2.  there are 2 hoses on top of the airbox.  one is the inlet for the warm air return i believe, but there is another black rubber hose that doesn't go into the box but is mounted in a bracket going across the top of the box.  it is about 1/4 inch around.  what is this booger?  it has been cut but if it's important i'll redo it to wherever it goes.

thanks for the help, boys!

[theunrulychef]

The 78 F's had some newfangled emmisions gear, I believe.  One probably connects for crankcase ventilation / warm air, the other might be a drain or return line.  There's a manual that will show the routings / what you can close off etc.  The same manual covers both K & F bikes, so it would probably be a good thing for you to have anyways.  I printed it off (at work  ) & put all the pages in plastic sleeves so I can have it with me while I'm working.

[jtb]

Send me your address and I'll put that manual on a cd and send it to you.

John

[JonnyHonda]

Ok I've had a search through old posts and I can't find a mention of this (Steve, prove me wrong). I'm looking for a set of vacuum gauges, and was wondering if it is possible to use a twin set, an balance one pair at a time?

After all the Pistons work in pairs, don't they?, and if you know what setting to set them to, It would work, wouldn't it?

Can you tell I don't what the 'eck I'm talking about.

[Bob Wessner]

Actually, I think this can be done. I seem to recall having read about someone who did this using gauges that he used primarily for a BMW. Did just what you are suggesting, started with two and always kept one, the same one, as the reference carb. I'll see if I can relocate it.

[eldar]

Might work but sounds like a pain. I just got a set from my local honda shop for $40. syncing is enough of a headache the way it is!

[Bob Wessner]

If you are interested in giving it a try, I found what I was looking for. It's actually for a product called TwinMax, but here are the procedures for more than two cylinders. I agree with the previous post only because of my love/hate relationship with my carbs, doing all four at once was bad enough.

BALANCING WITH THE TWINMAX

It is important on engines with more than two cylinders that you always balance all the cylinders against one which remains the reference or constant. Often there is one cylinder which is intended for this use-your manual will tell you this. It is suggested you use tube "A" for the reference cylinder. For good results, the engine should be near running temperature. On an air-cooled engine or a motorcycle it is advisable to sit a fan in front of the engine/bike to keep the exhaust pipes and motor cooled. On bikes, beware of exhaust pipes getting very hot and blistering nearby plastic fairings, etc. Keep an eye on engine temperature!

1. Ensure there is slack in the cable from the twistgrip or throttle pedal. If there is not a master idle screw, ensure all cable or rod adjusters have some free play-this makes sure that all carbs are sitting at idle, i.e., with their valves shut. Position the TWINMAX conveniently on the bike, such as on the saddle or on top of the frame and switch it on.

2. Connect the tubes to the carbs-one on the reference cylinder and the other on to one of the other carbs.

3. Turn the SENSITIVITY knob fully to MAXI.

4. Adjust the ZERO know until the meter needle is precisely through the middle of zero on the scale; this adjustment is very sensitive and very important.

5. Turn the SENSITIVITY knob back to MINI.

6. Start the engine and allow it to settle to a steady tickover. Don't be afraid of blipping the throttle-this makes certain the throttles are properly seated on their stops.

7. Slowly turn the SENSITIVITY knob until a deviation shows on the scale. The less out of balance the two cylinders being tested are, the more you will have to turn the SENSITIVITY knob to show an imbalance on the scale.

8. If there is present reference cylinder, quickly check each cylinder with one tube and choose the cylinder with the minimum deviation from zero on the scale as the reference cylinder.

9. Adjust the idle/slow running screw on the carb being adjusted (not the reference carb) until the meter shows an equal deviation on each size of zero. The meter is very sensitive, so care must be taken to use small increments of adjustment. Blip the throttle each time you make an adjustment to ensure the throttles are properly seated.

10. Repeat for all cylinders.

Once all the carbs are balanced at idle, they must next be balanced on throttle opening. As before, balance each carb against the reference cylinder.

Note that the balance must be checked as the throttle is opening and the engine is accelerating, NOT as the throttle is being closed and the revs are dropping. With some practice, you will be able to do this efficiently, and you will also be able to check the balance at a constant throttle opening.

You may find that a cylinder will balance fine on the opening up stage, but not on a constant opening; the skill lies in being able to interpret this and achieve a balance between one and the other.

11. Start with the SENSITIVITY knob about the middle of its' range. Run the engine and this time open the throttle gently and run the revs up to 3000 or 4000 rpm and make a note of any deviation on the scale; again, if it is not pronounced, increase the SENSITIVITY.

12. Now adjust the carb throttle actuator-this is either a cable or screw acting onto a plate or rod and may be hidden inside the carburetor top. Be careful to make only small adjustments. Note that if there are locking nuts on the adjusters, doing these up will affect the setting so double check the results and blip the throttle to settle things out.

13. Repeat for all the cylinders.

14. Adjust the slack in the cable from the twistgrip/pedal to manufacturer's specifications.