TOPIC:

Sudco recommends only mechanical sync for smoothbore carbs.




Such mechanical sync is also known as bench sync, where the carb assembly has been removed from the engine and is on the work bench with the top caps removed to allow access to the throttle slide adjustments in each carb.

After the bench sync, would normally expect to do a "running sync" with manometer or vacuum gauges, but Sudco says among other reasons that the detached caps allow air leaks which adversely impact a "running sync."

In summary, Sudco says to carefully bench sync, followed by pilot air screw tuning of the idle circuits with the carbs fully assembled (i.e., caps attached).

Where a manometer or vacuum gauges are available, I suppose the manometer or gauges could be attached and watched while doing the pilot air screw tuning (caps attached).

The stock Mikuni carbs with side-located pilot air screws would be expected to have the same "air leak" as noted by Sudco with their caps removed. However, the Kawasaki FSM details the procedure for both bench sync (basically same as the mechanical sync described by Sudco), followed by a running sync with caps removed and manometer or vacuum gauges attached.

I'm as yet unconvinced why smoothbores should only be bench synced without the same later running sync used on stock Mikuni's as provided in the FSM. And I have long enjoyed success in syncing smoothbores by following the FSM stock carb sync procedure for Mikuni mechanical throttle slide carbs. The dreaded "air leaks" with caps removed hasn't been an issue. And in any event, running sync may be easily verified by reading the manometer or vacuum gauges after the caps are replaced and vacuum plugs reinstalled.

Let's say that bench sync has been done, and the carb assembly is now reattached to the engine, and the carb caps are off, and manometer or vacuum gauges are attached, and engine is idling, and synch screws have been adjusted to provide the best equal vacuums in all carbs, and the slide adjustments have been locked. Next, turn off the engine, remove the manometer or vacuum gauges, and replace the caps. Then re-start the engine and proceed with final fine tuning of the pilot air screws.

Remember to have a fan blowing on the engine to keep it from over-heating while the engine is at idle rpm during the running sync process and pilot screw fine tuning adjustments.

Here's a tip about bench sync that might not be mentioned in the FSM.
When setting the slides to equal heights (with paper clip, drill bit, wire, or whatever), be sure to have the idle adjustment screw threads in approximate mid-position or at least in a position to assure that the slides may be lowered during the running sync. Neglecting this step may result in being unable to sufficiently lower the idle rpm. For example, when fine tuning the pilot air screws gets the idle rpm too high, it's important to have some threads remaining on the idle adjustment screw in order to reduce the idle speed.

wiredgeorge wrote: The VM family of carburetors including the smoothbores already mention have mechanical slides. That is, the throttle cable pulls up on the throttle bellcrank which is bolted to the slide rod as are all four carb slide mechanisms. The tops being on or off make absolutely NO difference to the function of the slides... You can run the bike without the tops on any VM carb assembly including smoothbores. . . .

www.kzrider.com/forum/3-carburetor/85578...-carb-sync-how#85676

wiredgeorge wrote: Synchronizing a carburetor assembly means all four slides open at the same time. There is no "main slide" only "a slide" in each carburetor. The smoothbores, like all VM series carburetors are happiest when synchronized while the bike is running using a manometer. If you are trying to bench synchronize these carburetors, bend back the holding tab on the hex head slotted screws in front of the sync screws and loosen the hex head slotted screws so the sync mechanism is free to move. Then loosen the locknut and adjust the slide height using the sync screws. There is probably a spec somewhere for setting these where they are barely open... like .5mm or something along those lines but I like to use long heavy-duty paper clips under each slide and open the slides with the idle adjust knob and then close it down on the paper clips. Then I like to adjust the slide height so that when pulling out a paper clip, each has the same feel when pulled. This adjusts the slide height at about 1mm but the slide can be closed again when the paper clips are out of the way. You can use bigger tools to set the height up to a point. At that point, the slides will be adjusted too high across the board and the idle adjust knob won't be able to shut the slides sufficiently to get idle down.

www.kzrider.com/forum/3-carburetor/920-v...ncronizing-hight#983

If only bench synced and pilot screws tuned with caps attached -- as recommended by Sudco -- and manometer or vac gauges readings are acceptable with a stable idle, I would leave well enough alone and not attempt to further adjust slide heights with caps removed.

Good Fortune!

Must be when you have a new or rebuilt engine.
I have bench sync my 29mm on my kz900 then check idle with my meter and all four cylinders where way off.
I do sync my carbs while bike idling at 2k rpm then recheck with top caps on.
Perfect idle every time.
Have a set on my z1r with pots and now a set on my kz900.

Sudco's recommendation against the vacuum sync doesn't make much sense to me. If nothing else, one could check the vacuum sync with the caps on to see if the carbs are truly in sync. Sudco' comments about carb circuitry being throttle position sensitive (Item #3) leaves me wondering what they consider syncing the carbs. I cannot imagine a way that the carbs would be "operating on different circuits" considering that syncing carbs is done at idle rpm speed with the throttle closed. Fine tuning carb sync really only affects idle since the slides are closed down. With the slides closed minute differences in their height has an affect. Once the throttle is opened up the affect of any minute differences in slide height are negated by the fact that the opening is now so large. The carb sync would have to be pretty far off to have a noticeable affect on how the engine runs when the throttle is opened up. Ed

Each circuit has it's own job to a varying degree. The pilot will overlap all the circuits all the way to the top, to a varying degree. The carb sync is not only for the idle circuit, one would also raise the RPM's to check if the carbs remain steady. Those who have done this will know what I mean. Once you do this a few times, you can "dial in" each carb to fall where it is supposed to be along with the rest. It's not about setting and leave it, that's why the fan. It takes time.
Steve

I've sync'ed carbs numerous times. I follow the book (see below), and it doesn't have you rev up the engine at all. The entire process is done with the engine running at idle speed, Trying to sync the carbs with the engine running at higher rpm will not work. Ed

I don't agree.
Steve

Thanks for posting. I'm doing this for the first time

If you raise the RPM's, don't go more than 3000 and do it slowly.
Steve

When the manometer shows equal vacuums at idle rpm, the throttle may be lightly blipped to assure that the equal vacuums remain the same when the throttle slides click back down.

Be very careful not to over-rev, as over-revving can suck mercury into the engine.

A safer method could be to simply turn off the engine -- no throttle blipping -- while the manometer is showing equal vacuums at idle rpm. While the engine is off, open and close the throttle a few times, allowing the slides to click down. Then start the engine to assure that the equal vacuums remain the same at idle rpm.

The above is applicable when using a mercury manometer carb sync tool.







Good Fortune!

It can be maddening trying to sync the carbs. You adjust one another changes and so on. There normally is a "lead" carb that is not adjustable. That's the one you match to. I have a bolt and a extended tube on mine after I lost all the mercury when it fell over and I didn't notice it. It also acts as a restrict or so when I raise the RPM's they won't climb so fast. I used a male street elbo with a hose barb on one end. It threads in, not just pushed in like it came. I bought them in 87.
Steve

I had 29s on my LTD for a while prior to the Turbo. Syncing was easy and straight forward as Patton describes in detail. I would sync at idle then give the motor a little steady raise in throttle to insure the sync was maintained off idle as well as it would still be in sync once returned to idle.
This may be over kill to some , but it worked like a charm and bike ran great.
Now once I got into snowmobiles the sync process was more critical as each carb had its own cable coming from a block where one cable from the throttle pulled three cables at the carbs. For this application you synced at idle plus off idle to make sure all carbs were being opened at the same time accounting for any play in the 3 cables ( Polaris triples ).
Now I am spoiled to the one 42 mm carb bolted to the turbo , so no sync needed or possible
Hope this helps
Dave

I'll do a video when I get a new cam for the bike. I have my eye on one like the one I have now but the mount is better.
Most of the complaints of engine noise is caused by the carbs being out of sync. I can make them stumble and record the noise. Might stop someone from tearing apart a motor for no reason.
Steve