Topic: Carb sync

[hymodyne]

evening all,

I'm interested in your collective opinions about carb synchronizing tools. Is there a preference between the column type (carbtune II) or the vacuum gauge model of the sort that davida sells in terms of cost and ease of use?

I live in a part of Massachusetts where shop mechanics stop working on "old bikes" after the 15th of april...tourist and travel season y'know...

hymodyne

[MetalHead550]

I've had good luck with the column type.  I have a mercury filled manometer but I'd go with a Morgan Carbtune or equivalent since they read like mercury without the poisonous liquid metal!  I've used the gauge type manometer and found that the needles bounced around too much.  The vacuum is indicated more smoothly and accurately with the column types.  Good luck.

[Bob Wessner]

The only other thing I would add per your inquiry re: cost, be aware that the Carbtune is probably the most expensive option, perhaps twice the cost.

[Clyde]

I used a set of vacuum gauges for a year or so, but then had trouble getting the bike to run smooth. I found one of the gauges had lost its calibration and I reset the needle, but then another gauge went.
The needle fluctation on gauges is a problem, but I used restrictors from an aquarium air supply line and screwed them down until the needles just moved.
Eventually about 6 years ago I bought a set of mercury columns and have had no trouble since.
No experirence with the CarbTune, but I like the idea and the fact that there is no mercury.
I would probably go for this if I was buying now

[MRieck]

I have always used the mercury gauge. Kowa Seiki makes a nice digital unit but it costs about 700.00 last time I looked.

[SteveD CB500F]

Carbtunes every time. Bought mine on eBay for £40

[rpyles]

Be careful with the mercury gauge - revving the motor can suck the mercury into the cylinders. Mercury doesn't compress too well.
 Richard

[dpen]

I bought a set of gauges from an Australian company for A$99 (about 50c American) and found them great. As mentioned before, replace the restrictors with some heavy duty ones from an aquarium.
They're more robust then mercury columns.
Urban myth-you'd have to rev the motor to blow-up point to suck mercury out of the column.

[TwoTired]

I've been using gauges I purchased in 1975.
about 15 years ago they all stuck due to the lube that was in the mechanical movements turning hard as rock.  I cleaned them and used a non-hardening lube.  (I forget what it was, though, maybe Lubriplate.)  Anyway, they've been fine since.  They came with the adjustable dampers to steady the needles.

I routinely check calibration amongst the four dials by hooking them up to a small manifold I got from and aquarium shop.  When all four are are connected to the same vacuum device, either one engine tap, or the MITY-VAC, it is easy to see if they need a bit of tweaking.  Usually they just need a minute or two of excercise (backing off the dampers) and they all match up nidely, again.  I only use them one or twice a year, though.

I prefer these to dealing with mercury.  I just know that if I had mercury sticks for 30 years, there would have been a mercury spill at least once.  It gets absorbed through the skin, you know.

[Jeremy]

Update

Have replaced plugs, caps, coils, checked timing, played with fuel screw(77 carbs)1.5 turns out. And still my one cylinder is running rich at idle. the other three are lean, probaly due to 4 into1 exhaust and uni air filter. Which way to adjust clip position? What would cause this rich condition in one cylinder? Air jet? Carb sync? Valves? please help

[Greg]

did you check the float height?  if the float was set high in the offending carb it would make that cylinder run rich at idle....

[TwoTired]

Let's review.
We have 4 identical cylinders fed by 4 identical carburetors.  But, instead of having 4 throttles we have just one that actuates all four carbs at once.  Given this senario, how well would you expect the engine would run if cylinder one was set for idle speed, 2 was set for 2/3 throttle, 4 was set to 1/2 throttle, and number 4 was set to 1/4 throttle.

Do you think all the cylinders would fire evenly?

This should point out the need for syncronization of the carbs.  If one carb is set for a cylinder firing speed equivalent to 3000 RPM and the other 3 are set for a cylinder speed equivalent of 1000 RPM.  The one cylinder is going to get more fuel than the other three, and you won't get a smooth idle, etc. etc.

If you can't or won't get 4 manometers to synchronize the carbs while the bike is running,  I don't think you don't have much choice but to set them mechanically, so they are at least close.

This is not to say that there aren't still some issues with carburetion.  If you weren't meticulous with getting all the passageways and orifices spotlessly clean, or you fed the cleaned carbs from a dirty fuel tank, there may still be some .016 inch holes that aren't flowing, too.  But, for sure, all the carbs must be aligned if you expect all the cylinders to behave in the same way.

Oh, by the way, for your carbs, #2 is NOT adjustable.  You will have to adjust the other three slides to open the same amount as #2.  Further, if you are going to change needle clip positions, you'll have to resync them afterwards, anyway.

Lastly, carb tuning is last on the list of things to do for a major tuneup.  Plugs, Points, timing, tappets, cam chain, and air filter must all be addressed before dialing with the carbs.

Be patient, take it step by methodical step, and it just has to run right.  Physics, it's very consistent.

Good Luck!

[carl550k]

 i give up with these carbs reving,have tried everything possible, am going to try bench syncing the carbs. i have no syncronizer so any suggestions would be greatly appreciated.thanx in advance, carl

[MetalHead550]

You should be able to get a murcury manometer for 30 bucks.  That would be my first recommendation.  There are insructions for making your own manometer in one of my manuals that I can quote if youd like.  Ive heard you can take a 1/8th inch drill bit and place it under each slide and set the position that way.  You would want to do it on the air filter side of the carbs though and this only works for carbs whos throat is higher on the intake manifold side than the air filter side.  So if you set the slide hights and theres a big gap on the intake manifold side of your carbs, then your boned and it wont work for your carbs.  Im sure TwoTired will be along in a bit to help ya out.

[carl550k]

 should have been more detailed, its a 76 cb550k with keihin carbs thanks for the advice.have to look at the carb throats though to see which side is higher.

[TwoTired]

The slides or" throttle valves" have cutaways that should face the air filter side.  The cutaways have the motto "we never close".  It is the engine side of the slides that really govern how much air gets through the carb.
I've alway felt that bench syncing the carbs is only good for coarse synching.  Because just tightening the adjuster lock down can make a inch or two of vacuum change at idle for the guages.  This can be the difference between a smooth idle...or not.
Also, worn slides can leak air that bench syncing can't compensate for. 

So, on the bench in the garage, I use the "Light" method.  This way I can't nick the slides or bores with very hard and sharp drill bits.  And, I don't like leaving signs that I have been "in there".  I shine a strong light into the carb bore at the air filter/ cutaway side.  While looking straight through the carb bore, I then use the big idle knob to close a slide until the light goes out. Then I back it off until I can just see a crack of light.  Now I use the individual adjusters for each of the remaining carbs to do the same, no light/light, to each in turn without moving the main idle knob.  When you are done, though, the main idle knob position should be the same for light or no light for any carb slide across the bank.   I then give the idle knob a twist or two to open the slide enough so the engine will start up.  Then put them back on the bike and make any final synch adjustments with the guages.  It's usually pretty close with low time carbs and close fitting slides.  But, worn slides or carb banks that were assembled from parts of other banks, often need greater tweaking on the guages.

I'll let someone else post a drill bit method.
Cheers,

[bryanj]

The drill bit method will work BUT you need a smaller drill than 1/8! Use the very smallest you can 0.5mm or 1/16 is the biggest you should use and check on the engine side of the carbs and then only if you are good at "feeling" the changes of resistance to drag, if you have problems with doing valve clearances dont even attempt it! Many years back I had a #$%* of a workshop jod on a 400 V4 that need a really odd screwdriver to get at the adjusters and eventually gave up ripped the carbs and did the bench settup then when replaced they were well within tollerance BUT EVERYTHING else had been set up first. Carbs are the very last thing to adjust

[dpen]

Mercury gauges are great-if you get what you pay for.
One of my apprentices bought some cheapies the other day-turned up pleased as punch. They didn't work for s%^&$t so we pulled them apart. Guess what-filled with vegetable oil.
You're better off getting a 20 foot length of plastic tube and filling it with auto. trans. oil as described in one of (I forget which) BMW sites.

[carl550k]

i have done everything ,valves,timing,cam chain,spark plugs, you name it.i know the bike has never been synced. the bike has been in family since new, also there is dabs of yellow paint on the adjusters. i was also wondering if the needles are adjustable on my carbs?

[TwoTired]

Yes, they are adjustable.  You'll have to remove the slides from the carbs to do so.

Definately have to sync carbs afterward.

[Quail "Owner of the comfortable k8"]

How do I attach a 4 hose Carburetor-Synchronizer to my 78k?

[kghost]

Bailing wire? Bubble gum?

Seriously, there should be a screw in each carb body forward of the butterfly (CV) or the carb slides.(forward means toward the engine side of the carbs). You usually remove the screw and insert threaded brass tubes to attach the hoses to.

[eldar]

ok say you are sitting on the bike, at the front of the carbs thee is a screw. It is a flat head style. Now on #1 and 2, this screw angles to the left on top of the carb, 3 and 4 angle to the right. Now all kits have 4 fittings that screw into these holes BUT some have longer fittings and these should go into the INSIDE 2 & 3 carbs. hook up the hoses to the sticks and you are ready to go. Just make sure you have the reducers in the hoses on the stick side. This is supposed to help with the mercury jumping around.

[Quail "Owner of the comfortable k8"]

Bailing wire? Bubble gum?

Seriously, there should be a screw in each carb body forward of the butterfly (CV) or the carb slides.(forward means toward the engine side of the carbs). You usually remove the screw and insert threaded brass tubes to attach the hoses to.

Where do I get the threaded tubes?

[kghost]

Should have come with the sync. tool. If not, you can get some brass tubing and a Die, then just make some.