Synching Twin Carbs

We were talking with a friend last night who was trying to get his dual Dell’Orto carbed Guzzi synched and idle speed set. As is often easy to do, we slipped into over-analyzing what needs to be done and how to accomplish it. Then after a a couple beer’s worth of continued speculation and jabber, we reeled it in and simplified it.

Synching a twin is easy. Here’s how to do it.

Some people are conceptual learners, needing to understand how something works and the approach behind tuning it. Others a procedural learners, concerned more with how something is done than why it works. Both are valid. I’m in the former camp. I’ve found that once you know how something works there are many different recipes for how to do it.

There is only one goal in synching your carbs: To have all cylinders contributing the same amount of effort at any time.

There are three steps to doing this on your twin:

1) Obtaining the optimal idle fuel mixture for both carbs

2) Setting the idle speed screws so both carbs run at the same rpms when the other cylinder is not firing

3) Adjusting the throttle cables so that the throttles move at the exact same time when the grip is turned.

There are some tools you can use to make this job easier or quicker, but they are not required. You can do a fine job of synching with just your ears and a screwdriver. A simple vacuum gauge like one you’d get at the auto parts store will make setting idle fuel mix easier. A motorcycle carb synch tool makes step one and two easier. A tachometer could also be used in place of a vacuum gauge or synch tool for the first two steps.

A note before starting:

Make sure your ignition system is tuned. Correct timing. Plugs gapped correctly. Points gapped correctly (if you have them). Ignition advance weights (if you them) lubed and in good repair. Battery and charging system in tip-top shape. People are constantly trying to fix ignition problems by adjusting the carburetors. It’ll never work, trust me.

Make sure your compression is good. If your cylinders don’t show compression readings that are within service limits within a 10% difference of each other, there’s no point tuning your carbs. Adjust your valve lash to specs. Tuning an engine always starts with compression, then ignition tuning, then fuel tuning. This is universal.

More about step one:

It makes sense that the optimum idle mixture is the one that yields the highest rpm at a fixed idle speed screw position, right? Your motor is working best and pulling hardest with this fuel mixture and it’s telling you plainly by running faster. You can tell it’s running faster by. . . listening with your ears or looking at your tachometer. Most modern timing lights have an accurate tachometer that doesn’t bounce around like a thirty year old cable-drive motorcycle tach. There’s another way your motor can tell you it’s idling at the optimal fuel mixture: High vacuum signal in the intake manifold. If the motor is pulling harder, it’s creating more “suck” in the intake. Makes perfect sense, no? So if you don’t want to use the rpm method, use a vacuum gauge or carb synch tool to tune the idle fuel mix until the highest vacuum reading is obtained.

How do step one: Turn both idle speed screws faster by the same amount so that your rpms are around 2000 or more. You are doing this because you’ll need to run the motor on one cylinder to do steps one and two. If you don’t increase the idle speed sufficiently, the motor will stall when you disconnect one spark plug cable. Disconnect one spark plug cable so the motor is running on one cylinder. You’ll be adjusting the carb that is on the side that is still firing (hopefully this is obvious). Adjust the idle speed screw until you get a normal to slightly fast idle (exact rpm not critical). Using the rpm method or vacuum method noted above, adjust the idle fuel mix screw until highest rpm or vacuum is obtained. Readjust idle speed screw until normal to slightly high rpms. Hookup the other spark plug cable and remove the one from the cylinder you were just working on. Repeat adjustment on this side. When you hook both plug wires up you’ll find that you have a very fast idle! This is where step two comes in.

More about step two:

It makes sense that the idle speed setting is the same for both carbs when either cylinder running on it’s own yields the same rpms, right? Or you can look at it another way. . . if the idle speed setting is the same for both carbs then moving either idle speed screw even the slightest bit would change the bike’s idle speed, no? If one cylinder were not pulling as hard as the other, turning its idle speed screw slightly higher would have little to no effect on the bike’s rpm. If they are both contributing equally, then messing with either one will have an immediate effect.

How to do step two: Even thought it’s listed a separate step, you’ve already started at the end of step one when you adjusted the idle speed screws. Using the rpm method, while each cylinder is isolated (by disconnecting the opposite plug wire) set the idle speed screws so that both cylinders give the exact same rpms. For the vacuum method adjust the idle speed screws until the same reading is obtained on both cylinders. Once you’ve got both cylinders running at the same speed, it’s time to hook up both plug wires and adjust the speed screws down BY THE SAME AMOUNT until the rpms reach the desired idle speed. You know that number from your manual, right?

About carb synch tools:

You can do steps one and two without isolating the cylinders and messing with spark plug wires using a carb synch tool (or two separate vacuum gauges). If you didn’t know it already, a carb synch tool is just a bank of two or more vacuum gauges so that you don’t need to isolate the cylinders. You can look at the vacuum for all the cylinders at once and make adjustments to fuel mix and idle speed until highest vacuum is obtained (mix) on each carb and they all show the same reading (speed) at idle. For bikes with a common plenum intake, this won’t work unless the carbs themselves have vacuum ports. Otherwise there’d be no way to see what an individual cylinder is doing.

Last step:

Getting the carbs synched perfectly at idle is worth jack shit if the throttle plates or slides don’t move at the exact same time when the throttle grip is turned. Different amounts of slack between the two control cables results in one throttle opening sooner than the other when the throttle grip is turned.

Some twins have two separate cables all the way from the throttle body on the bars to the carbs. Others have a single cable at the bars running to a splitter in the middle with two cables running to the carbs from there. In either case the adjustment is the same.

The cable adjustment barrels are usually at the carb bodies. Loosen both of them so there is a little slack when the grip is at rest (stop) position. Pick one carb to be your reference and set the adjuster so there is a little slack remaining but not so much that grip turns significantly before engaging the throttle plate/slide. Lock it down. This slack is so your motor doesn’t rev unexpectedly when you turn the bars and the cables move. You knew that already. With the air cleaners removed from both carbs so that you can see the slides or plates, lay a short piece of wire (paper clip? clothes hanger?) or a toothpick or a nail in each carb such that it lays underneath the slide or touches the plate and protrudes out the mouth of the carb. When the slide or plate moves, these will wiggle. Looking at your fancy new indicators, turn the throttle grip very slowly, taking up cable slack, until you see movement on one of your wigglers. If you’re lucky, they both wiggled at the EXACT SAME TIME and you’re done. If they didn’t, then adjust the remaining barrel adjuster until they do. Don’t forget to lock both cable adjusters so your setting lasts for a while.

That’s it. Really. Now your carbs are making equal power at idle and the throttles move in perfect unison.

If you did this right, your motor should now rev instantly and smoothly when the throttle is turned. The idle speed should settle back down immediately when released.

Don’t get too hung up in all this. You really can’t mess up too badly and it’s not that complicated. It boils down to you adjusting four screws in total. If you don’t get it right you can do it again. If you’re really the worrying type, note the initial position of all four screws by counting turns before you start.

Have fun. Learn. If you get bummed, remind yourself of how much better spending time working on your bike is than going to work or fighting the PPA.


9 thoughts on “Synching Twin Carbs

  1. Thanks, Tim.

    Unless I’m missing something (which is possible, if not likely, given the amount of sleep I don’t get lately) I don’t understand why you’d run two tachs on the same bike. Crank is common, not like one side can go faster than the other unless something is REALLY wrong ;-0

    Thanks for reading.


  2. I really meant to say using a dwell tach to see if both sets of points are set the same (being that the points affect the timing). Of course, this thought came out of a deep crevice of my brain w/out any editiors present to check ant validity whatsoever. To tell the truth I’ve never even checked dwell on an M/C before!! Just something from my old 1-8-4-3-6-5-7-2 chevy days.

  3. Aha!

    Makes good sense.

    I have a dwell/tach, but have rarely used it. Have just used point gap to date.

    Sounds like your idea, especially considering I have a single-fire ’64 timer with two points, would be precise and fast. Maybe I’ll try it.

    Never messed with cars. I do have a ’59 stovebolt pickup though. Points. Armstrong steering. Fred Flintstone brakes. Still a worker.


  4. Jason,
    I have balanced the rpm on both cylinders using my twin vacuum guages but when applying the throttle the RH cylinder steadily increses the vacuum but the LH cylinder decreses. Can you explain what is wrong?

    • Could be a number of things. . . one slide lifting off faster than the other? Different slide cut-away? Mixture diff? Valve clearance diff? Diff compression/leakage on each cylinder?

  5. When all set up is complete,and both leads are connected have you any idea why it revs at tic over and I can’t slow it down. Many thanks Harvey. PS I appear to have slack in cable and splitter.

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