+1 for factory carb sync procedure
 

Hello,

I just thought I'd write in with an experience and the results of a little test that I did.

As many of you know, the factory procedure for syncing your carbs is to set the idle to 1200 +/- 100 rpms, and then equalize the vacuum pressure in both carbs.

I sprung for the gucci Twin-Max carb-sync tool because I liked the concept that it has a single indicator that shows you the vacuum differential between two sources, instead of two indicators you have to compare (I tried a homemade solution and it was sloppy, and also bought a two-gauge too that was much better, but not nearly as precise as the TM).

The Twin-Max is great - it is super-sensitive (actually the sensitivity is adjustable), and it is plainly easy to see that I was getting a more precise sync than with the two-gauge tool I was using before (for sale, btw).

However, the TM is *so* easy to use that it made me start to wonder how the carbs were synced under various engine conditions other than idle. Ok, to be fair it was reall the TM manual that suggested that you want to identify the proper sync position at both idle and under power, and over time identify some point in the middle that is optimal.

I quickly discovered that perfeclty sync'd carbs at idle are decidedly *not* synced the rest of the time. It seemed to me that you really want them synced under "normal operating conditions," not at idle which is only when you are not really using the bike.

So, I tried syncing them at 3000 rpms. Easy to do w/ the TM. The TM reads dimensionless numbers, so w/ a diff of 0 at 3k rpms, I had a diff of a dimensionless "2" at idle. Figured I'd try it out for a while.

Well, those veterans of you probably already knew that I was barking up the wrong tree. But if you are new or were wondering, the results were that I got about 20% worse MPG and decidely more frequent "pops" (mini-backfires).

I also got some "klunks" when starting very cold, which at this point as near as I can tell somehow result from either one cyl firing off-time against the other, or just one cyl failing to fully compress under the weak power of the starter moter, and backing up against the other cyl).

So, I don't really understand why it is that you want the vacuums to be equalized in the one least-used and lowest-strain engine condition there is, but it seems that is the right thing to do. Perhaps it simply reflects the fact that the front jet is smaller than the rear jet, and that that is least noticeable at idle.

Anyway, so the TM is great, albeit expensive at $100. And be warned, it takes forever for the guy to get it to you b/c he individually sources each one from France, and doesn't mention that anywhere. Apparently the french are too busy running away from conflict to fulfill product orders in any hurry.

You've answered your question with your test.
The purpose of synchronizing the carburetors is to allow the engine to idle smoothly, nothing more, nothing less.
That is also the reason synchronizing the carbs is the last step, after adjusting the valves, in the tuning process.


Rex

I have to respectfully disagree here. My bike is incontrovertible proof that only a modest deviation in carb sync results in a) occasional but fairly severe feeling hic-cups from the engine, and b) noticeably worse fuel economy.

a) is decidedly metal sounding clunk that takes the space of one cylinder firing, and results in a shock you can feel through the whole bike, and an abrupt, albeit brief decrease in momentum. It feels how I might imagine getting a small piece of metal caught in your chain would feel if it happened while you were moving - noticeable hit when it got to the sprocket, but inertia is enough to destroy the object, at which point the chain continues on as normal.

I have been through many iterations with this. The clunk happens if the carbs are out of sync. It does not happen if they are in sync. The frequency with which it happens is proportional to how far out of sync they are.

The purpose of synchronizing the carbs is not to allow the engine to idle smoothly. Smooth idle is a byproduct of syncing the carbs. The reason to sync the carbs is to allow both carbs to take in the same amount of air/fuel. of course since your rear carb has a bigger jet, we are mostly just talking about the air. The benefits of syncing your carbs go beyond smooth idle. Better gas mileage, better throttle response, smoother acceleration,etc...

good job on the experiment with higher rpms. See how we learn. You have added some interesting info that will certainly assist others in diagnoses. Now you sync everyone's carbs. :D:D:D

Mine starts and idles well and runs strong all the way through the rpm range, except if I twist the throttle quickly, as at wheelie time, then it sometimes cuts out like someone turned the key off and on, like a hiccup. According to your info, mine must need synced huh?

Did you sync with the tank on as per the the vacuum line mod that sticks out the side? just bought the $4 fitting for the vacuum line to set it up.

I'll say this much - when I described my problem, everyone said "sync your carbs." I figured "yeah, right, like that is going to solve *that* problem..." but I knew I had to do it before I could ask for any more help/advice. Well, i did it, and then lo and behold, I didn't need any more help/advice.

Yes, I bought another vacuum fitting and ran an extension tube off of it. The I cut the petcock vacuum tube and spliced it with a nylon spacer from the hardware store, so I can separate it right next to the end of the other tube at will. The end of the extra tube and the splice sit just under the back left edge of the tank.

very cool! you know what's weird though. Mine always did a lot of popping on decel, some of that clunking sound and stalling until it got really bad one day and so took off the tank: did the air mix, tps, carb shims, foam air filt and it's been great ever since(over a year). Like i said, if i whip the throttle on, 1 millisecond hiccup and that's it. Screw it on normally and it's perfect.

I'm preparing to sync carbs, so it will be interesting. I got the one fitting from honda but will have to get under there and see what else I need. Never did it but will learn soon, huh.

When you cut the vacuum line, and added the new line to the fitting, did you make them the same length?