Q: I have a 1999 Suzuki Intruder 1400 that I need to sync the carbs on. The manual says the idle has to be set between 950 and 1000 rpm in order to do it correctly. I’d like to know if there’s a way to sync the carbs with a lower idle setting. The bike has an aftermarket exhaust and has been rejetted, if that makes any difference. It just seems to me that the idle speed shouldn’t make any difference when syncing the carbs.

Lou
Via e-mail

A: Idle speed when syncing is important because it’s the only speed where the syncronization of the carbs is required. Once the slides or butterflies begin to open, the area of the opening that permits additional airflow rises quickly. It’s only at the very slight throttle opening required for setting idle speed that minute changes in opening are apparent. Remember, the throttle is almost closed. Very small changes in throttle opening affect the speed and smoothness of the idle. Once the throttle begins to open farther, these small changes are insignificant and irrelevant.

Don’t confuse syncronization (which is matching the throttle openings) with idle mixture setting, which is controlled by the pilot jet or idle air/fuel screw. The sync is set to a vacuum setting, which is determined by the suction created from the airflow across the throttle. This adjustment is a mechanical adjustment that’s affected by the throttle stops and the resulting rpm. Having the correct mixture determines the smoothness of the idle and the throttle response just off idle. So, assuming the mixture is in the ballpark (set to specs), first set the sync at the recommended idle speed and then trim the mixture screws (if they’re available — most are plugged per the EPA). The sync procedure usually requires going back and forth between each individual cylinder’s vacuum setting and the resulting engine rpm until you get the vacuum setting where you want it.

Over the years, I’ve often heard the phrase “syncing the carbs” used as if it were a performance issue. That really hasn’t been true for a long time. The phrase comes from an era when each carb or injector in a multiple rack was operated by a separate cable or linkage, rather than a common shaft (or, in the case of CV carbs, by vacuum alone). In those cases, you really did have to sync the carbs to ensure that they were even at part-throttle and that all reached full throttle at the same point. Not so today.

Doug Meyer

Carburetor syncronization on multicylinder bikes is an important service procedure that’s required at different intervals. Most manufacturers recommend that the bike is running at idle to do this test. The reason for this is that the intake track is stable at idle. If you have your carb sync gauges hooked up and you run the bike higher than idle, the gauges may become hard to read, making it difficult for you to set the carbs the same. At a lower idle, you may find that the bike is hard to keep running while you’re making changes.

An aftermarket exhaust and rejetted carbs will not affect your carburetor syncronization because the throttle plates aren’t being moved. The purpose of syncing is to get all of the carbs pulling the same amount of vacuum. This adjustment is essential for a smoothly running motorcycle, as long as everything is mechanically sound on the bike. Most technicians will do a carburetor sync after installing a jet kit to verify that the carbs are synced correctly. What you’re actually doing when you sync your carbs is adjusting the throttle plates so that, at idle, all of the cylinders are pulling the same vacuum. And since you’re adjusting the throttle plates, you may notice a rise or drop in your bike’s rpm at idle. So, the final step in the procedure would be to adjust the idle back to the manufacturer’s recommendation.

Chris Reo, Motorcycle Mechanics Institute RB

Introducing Chris Reo

I wear a few different hats. In addition to being chief instructor for MMI’s Kawasaki K-Tech Program, I’m also an instructor for Kawasaki Motors Corp. at the dealer level. I travel about 15-20 weeks a year teaching dealer classes for Kawasaki. I’ve been in my current job at MMI for nine and a half years (before that, I worked in a Kawasaki/Yamaha dealership). I also do a 7- to 12-minute tech segment on the television show Corbin’s Ride On, which airs on Speed Channel on Tuesday nights.

I’ve been on a number of TV shows over the years, starting with a show called Bike Week with Greg White. I did a tech segment where we installed accessories on a Kawasaki VN1500 and VN1500 Classic, and then gave the bikes away at the end of the season. I also did a tech segment on a show called Motoworld. Did about four seasons, and I answered tech questions online for the show. I was also on an episode of Discovery Channel’s Junkyard Wars, where a team from MMI had to build a dune buggy. Fun, fun!

As for my riding, I’ve been racing motocross for about 23 years (still at it!), and I’m an avid streetbike rider. — Chris Reo