RG500 Carb Sync

Performing an RG500 Carb Synch will make the engine run much crisper. The factory manual briefly describes a procedure which gets the carbs sorta close to synchronization. The steps described here get the engine running much more precisely. A major improvement over the factory procedure is using a flow meter to dynamically balance the carbs at idle and at running speeds. You will smile when you enjoy the improvement in rideability from properly synched carbs. 

RG500 Carb Synch Static Adjustments

Make sure that the cables are routed correctly to their respective carbs. The cables are numbered at the junction from 0-4 with 0 being the oil cable. 

The oil cable runs under the right rear exhaust pipe then across the back of the engine on its way to the pump. 

The cylinders are numbered LF=#1, RF=#2, LR=#3, RR=#4.

The longer of the two choke cables goes to the #4 carb.

Here is a video which shows the static adjustments detailed below. 

Run all the carb top cable adjusters completely in then verify all four slides are completely bottomed out in their bores. 

Back out the carb top cable adjusters 2/3 turn. Leave the locknuts loose for now.

Next up is initial adjustment of the Throttle Stop Screws, which affect the idle speed. The factory manual specifies adjusting the screws so the slide is lifted .5mm from totally bottomed out. Half a millimeter is about .020 inches. It has been suggested to use a .5mm drill bit. Stick the bit under the carb slide then adjust the Throttle Stop Screw so the slide is just barely lifted off the drill bit. You can tell the slide is off the bit when the bit can be moved back and forth a little. I don't have a .5mm drill bit so I used some .032 safety wire. It's OK to be off a little with whatever you use to set the static slide height because later on we are going to precisely set the idle by adjusting the Throttle Stop Screws using a flow meter. What matters now is having them all be set the same.

Figure out a method of holding the carbs partially open. One method is to anchor a bungee cord onto a radiator mount, wrap it around the handgrip, and attach the other end to the preload adjuster sticking out of the top of the fork. Another method is to wrap some 1/8” vinyl hose (or clothes line, or 12ga wire, etc.) 1.5 times around the throttle between the handgrip and the switch housing to act as a friction brake to hold the throttle stationary. If you just like using shiny tools you may want to use a vise grip lightly cinched onto the handgrip with the handle resting on the brake lever for this purpose.

Starting with the right side carb bank, open the throttle to raise the slides until the timing dot of #4 carb (right rear) is completely visible and just brarely touching the top of the carb throat. Make sure the handgrip is secure because all the other carbs are going to be synched to this one.

It helps to use a flashlight to properly illuminate the dot and throttle bore to dispel shadows when positioning the slides for adjustment

Now go to #2 Carb (right front) and use the carb-top cable adjuster to line the dot up with the top of the carb bore just like you did on #4. 

Repeat with the left two carbs. Note: The left side adjusters typically ride about a couple of turns higher than the right side. It's just how the cables are cut.

Check the oil pump and adjust the dash marks to line up on the pump and cable cam.

For bored carbs, I recommend a richer than stock oil pump setting since the slides are higher when the dots reach the top of bored out (taller) venturi. Here is a video about adjusting the oil pump. 

Remove whatever is holding the throttle open and let all the slides close.

Leave the carb top cable locknuts loose for now. They will be tightend up after completing the following Dynamic Adjustment step.

Adjust the throttle cable slack at the handgrip to have a 2-3 mm of play.

Dynamic Adjustments

Get a STE Sychrometer Flow Meter because balancing the carbs with a flow meter makes the bike run much sweeter. Here is a link to a source for them which has a good image of one. There are many other internet sources for these tools.

Cut a few mm off the Synchrometer's snout because otherwise it runs into the carb slide and cannot seal the carb's mouth.

Here is a video about the next steps, detailed below, which shows how to balance the carbs while the engine is running.

Start the engine. Warm it up.

Use the Synchrometer to adjust all four Throttle Stop Screws to make all the carbs flow the same with an acceptable idle speed.

Now do final throttle cable adjustment using the Synchrometer. Lining up the dots is as close as we used to be able to get. Now that Sychrometers exist we can sych the carbs much more closely. Using the throttle grip holding system you previously employed, get the engine running at a fast idle. Just get it running at a couple thousand RPM. 

Use the Synchrometer and adjust the carb top throttle cable adjusters so all four carbs flow equally.

Shut the engine off.

Tighten all four carb top throttle cable adjuster locknuts.

Start the engine again and verify all four carbs are still flowing the same while running at a fast idle.

Make sure all the the throttle cable locknuts are tight before pulling the rubber cable boots down into place over the cable adjusters.

Operate the throttle to confirm there is no bind or obstruction and that it operates smoothly.

Set the Idle Mix Screws to their default position: 1 1/2 turns out from seated. They are brass and soft which makes it important to be gentle when screwing them in to the seated position. Turning them hard as if they are a fastener can screw up the tips. Turn them until they are just barely seated, then back them out 1 1/2 turns.

Turn the Idle Mix Screws back and forth to find the setting which yeilds the fastest idle. In the video about dynamically adjusting the carbs I said there would be a seperate video about adjusting the idle mix screws. I have developed a method of setting those screws based upon initial pickup from riding along on flat ground with the engine just idling along. Problem with this method is finding a stretch of flat level pavement which is safe to ride on at a fast walking pace. Kevin Cameron made our lives easier by publishig a detailed article about carb tuning. His method of setting the idle mix screws is simply adjusting them to get the fastest idle. Which can be done in the safety of your garage. So I didn't make the video, I wrote this up instead :-) 

Carburator Idle Mix Screws are designed so three turns out from fully seated is the same as changing the pilot jet one size. Gamma carbs have it upstream of the carb slide which means it changes the amount of air going into the system. Backing it out lets more air in which makes the idle mix leaner. If the bike idles best with the idle adjuster screw all the way out then try going one size leaner with the pilot jet. If it idles best with the screw all the way in then try one size richer pilot jet. Most bikes work fine at the default setting of 1 1/2 turns out from gently seated.

When carb synch is complete it will idle like this.

The very last step is setting the needle clip position. Go for a ride someplace where you can roll along at 5 to 6,000 RPM in 6th Gear and roll the throttle open. If it just sits there without accelerating it means the needle is too rich. To lean out the needles raise the clip. There is a little washer on the needle which is used to make 1/2 clip position adjustments. Moving the washer from above to under the clip makes it just a little richer. Putting the washer on top of the clip makes it just a little leaner. Gammas are sensitive to clip position.

Note: There is a mistake in the video. My stroked-out brain said the washers are twice as thick as a clip positioning slot on the needle. What I meant to say is the slots are twice as far apart as the washer is thick. Putting the washer under the clip is the same as raising the clip one half slot; making the needle a little richer. Putting the washer on top of the clip is the same as dropping the clip one half slot; making the needle a little leaner.