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Carb Sync

Carb Synchronization (Carburetor Synchronization)

Since I've never gotten around to it (those 'round toit's are hard to find sometimes!), someone else took the extraordinary step to perform the carb synch and document the process with pictures. The following information was submitted by SA750 Forum Member SmokeyTirez.

Hey everyone,

I synchronized my carbs today. Took pics and did a write-up while I was at it. The process is described below. Have fun, be safe, and good luck!

-Smokey

Click on a thumbnail to enlarge

1. Remove the airbox. Many other projects on the web page describe air box removal, so I won’t detail this part of the process here. remove_airbox.jpg (82821 bytes)
2. On the drive-side (left-side when sitting on the bike), unplug the spark plug wire on the rear cylinder. Then remove the engine fins on the rear section of the motor by removing the four hex bolts securing the assembly. One of these bolts also secures the choke knob. Carefully move the choke knob out of the way so you don’t scratch any chrome pieces. Reattach the spark plug wire after the fins are removed (you will need your motor to run later). remove_fins.jpg (102545 bytes)   fins_removed.jpg (132106 bytes)
3. Remove the vacuum plug screws and washers from each intake port (the washers will probably be stuck to the engine, so make sure you get them off). These screws are very tight so be careful not to strip them. NOTE: I use a screwdriver in the pics so that it is easier to see which screws I am referring to. I actually use an electric power drill with Phillips attachments to remove these tight screws. A drill will do a much better job providing the linear torque that is needed to remove these screws without stripping them. remove_vac_plug_left.jpg (108758 bytes)   remove_vac_plug_right.jpg (103226 bytes)
4. If you are using a carb sync tool, like a CarbTune, you will need to set this up now. Cut the included hose into equal sections. I cut the hose into four sections because it was designed to be enough hose for four connections. We only need two on the ACE. Cutting the hose into four sections, I assume, makes each length of hose the originally specified length. It may not matter how long the hoses are, but better to be safe then sorry. If you cut them into two sections then you’ll have plenty of hose to get the CarbTune wherever you need it. The CarbTune includes four restrictor plugs. Insert one into each tube, about ¾” into the tube. It does not matter which way the plugs are facing inside the tube, as long as they all face the same direction. Take the end of the hose that has the restrictor inserted into it, and plug this end onto the CarbTune. carbtune_kit.jpg (126611 bytes)   carbtune_tubes_with_restrictors.jpg (89638 bytes)
5. Install the included manifold adapters, one into each vacuum port. Then attach the hoses of the CarbTune to these adapters. adapter_installed_left.jpg (123583 bytes)   adapter_installed_right.jpg (101186 bytes)
6. Place the CarbTune on a flat surface with the top (the end with the hoses attached) elevated about 1” higher than the bottom. Using the vent hole at the bottom of the CarbTune, fill the reservoir using the included Mercury dropper bottle. MERCURY IS DANGEROUS. DO NOT LET IT TOUCH YOUR BARE SKIN.  
7. Now hang the CarbTune somewhere so you can read it while the motor is running. Don’t let the CarbTune or the hoses contact any hot surfaces (like your pipes). I had my wife hold the CarbTune while I did my adjustments. Wives are good for stuff like this.  
8. Start and run the engine for a while. I ran it for about 5 minutes. Observe the location of the mercury in the tubes while the engine is at idle. If your carbs are in sync then the mercury will “float” in the tubes at approximately the same level for each tube (each tube measurement represents the vacuum for the corresponding carb). If your carbs are out of sync then the measurements from one tube to another will be different. In the pic you can see that my carbs are out of sync by approximately 1.5 kPa’s. carb_before_sync.jpg (95176 bytes)
9. Shut off the engine.  
10. If you need to sync your carbs, then you will need to access the carb adjustment screw, which is located under the tank. The trick is that the engine will need fuel to run while you make these adjustments. You can use CarbTune’s auxiliary tank (p/n:08-0032). Or you can leave the tank connected and set it CAREFULLY next to the bike on a supporting surface. This surface will need to be strong (duh) and at least as tall as the top of the sprocket cover.  
11. To remove the tank, first unhook the tank vent tube from rear-right corner of the tank (little brown hose). remove_vent_hose.jpg (107594 bytes)
12. Next, remove the drive-side plastic side cover to expose the fuel filter and fuel line.  
13. Now unhook the vent tube where it exits below the pipe then slide it up towards the drive-side of the bike. Pull it out from around the fuel line and let it hang. remove_vent_hose_left.jpg (112100 bytes)
14. Now carefully slide the fuel filter off of the battery box hook. It should slide out very easily as long as you don’t try to angle it out. Just slide it straight out. unhook_fuel_filter.jpg (53919 bytes)

15. Remove the seat.

  
16. Now remove the tank by unbolting the rear 12mm bolt and lifting the tank carefully BACK and UP off of the bike. Set the tank down on the supporting surface next to the bike with all the fuel lines still connected. tank_nextto_bike.jpg (119761 bytes)
17. With the CarbTune hooked up to the bike, start the motor and let it idle for a couple of minutes. Note the difference in vacuum for each carb. Also note that the base carb (the one you will sync to) is the rear carb.  
18. At idle, adjust the carb adjustment screw clockwise or counterclockwise until the vacuum reading on the CarbTune is the same (or as close as you can get) for both carbs. If you cannot get these readings to sync up, then something else is wrong (eg. Vacuum leak). It will take some patience to get this adjustment correct. The carb sync adjustment screw is VERY sensitive. 1/12 of a turn can make a ton of difference, so go slow. I also noted that just pushing on this screw will affect the adjustment setting (since it is spring-loaded). So you will have to adjust, remove the screwdriver, observe the reading, then repeat as necessary. Don’t take the reading with the screwdriver on the screw because it will not be correct. adj_screw_access_hole.jpg (86622 bytes)   adj_screw.jpg (74646 bytes)
19. When your carbs are adjusted properly, the CarbTune will show something like this: carb_after_sync.jpg (83995 bytes)
20. Gently rev the motor to about 1000 rpms (guestimate if you don’t have a tach). 1000 rpms will be just a few hundred rpms above idle, so it won’t take much roll-on to get there. Then roll-off of the revs back down to idle. DO NOT SNAP THE THROTTLE OR YOU RISK SUCKING MERCURY INTO YOUR MOTOR. Listen to the motor as you roll on then back off. The rpms should increase smoothly as you roll on, and decrease smoothly as you roll off of the throttle. There should be no decrease or “dip” in the rpms before the rpms begin to build. The Carbs will not always stay in sync while the motor is above idle. The thing to look for is that the mercury indicators move at about the same rate, and that they do not move higher in the tube while the throttle is being increased.  
21. Congratulations, your carbs are now synchronized! Let the engine cool then reinstall the tank, the fuel filter mount, the vent line, the seat, the vacuum plug screws, the engine fins, the plastic side-cover and the airbox. Clean her up and take her for a ride. She should run a lot smoother now. The main difference I noticed was a smoother engine overall, and much smoother on downshifts. I get no backfiring anymore either. Have fun and be safe! ACE_clean.jpg (108716 bytes)
 Scans of the pages from the Honda Service Manual covering this procedure are located here: Sync1 , Sync2 , Sync3