Carb Synch Question

[rotorhead]

Going by the descriptions in both the Haynes manual and the LT Snyder/DesmoTimes maintenance manual, you're really only checking and adjusting (if required) Ducati carb synch while at idle. I saw no reference in either to hold a steady state higher RPM level and check balance.

Up until last year I'd been riding BMW twins for 23 years, and carb (or throttle body if fuel injected) synch adjustments on those bikes are made at both idle and at a higher engine speed such as 3k or 4k RPM.

For the first time since buying it a few months ago, I checked my 900SS's carb synch yesterday using my trusty ol' Carb Stix, and at idle there was a significant split in the mercury levels in the tubes. I brought the revs up to a steady 3,500 rpm and the levels became beautifully matched.

My question is, per the referenced manuals I should have made an adjustment to bring the carbs into synch at idle. But, in so doing I'm quite sure I would have then induced an imbalance at higher engine speeds. Because carb synch/balance at higher engine speeds is logically much more important than at idle I left it alone. Like on BMWs, are there actually separate adjustments on a Duc for both idle and higher speed carb synch?

[felix]

Originally Posted by rotorhead

... My question is, per the referenced manuals I should have made an adjustment to bring the carbs into synch at idle. But, in so doing I'm quite sure I would have then induced an imbalance at higher engine speeds. Because carb synch/balance at higher engine speeds is logically much more important than at idle I left it alone. Like on BMWs, are there actually separate adjustments on a Duc for both idle and higher speed carb synch?

The BMW had seperate cables going to each carb. You would adjust the balance at idle with the carb screws. You'd then check the balance at speed, and sync by adjusting the cable play.

The Ducati uses mechanically linked, twin Mikuni carbs, so there are no individual cable adjustments.

Have you checked for vacuum leaks?

[rotorhead]

Originally Posted by felix

The BMW had seperate cables going to each carb. You would adjust the balance at idle with the carb screws. You'd then check the balance at speed, and sync by adjusting the cable play.

The Ducati uses mechanically linked, twin Mikuni carbs, so there are no individual cable adjustments.

Have you checked for vacuum leaks?

Yep, you're familiar with the BMW twin set-up I see!

No, I haven't done anything else because I wasn't even sure there was/is anything wrong. Not having been a long-time Duc rider I don't know how these bikes are supposed to feel. On the BMW opposed twins it became pretty evident when you needed to synch up again. I recently did my first TB synch on the Strom and the same initial mercury split was evident from idle through 4k RPM. A tweak to one adjustment screw brought everything into line.

If there was a vacuum leak, wouldn't that also affect balance/synch at all throttle settings? I'm inclined right now just to leave well enough alone unless I'm convinced there is a problem with my bike that I need to or can correct. I was also wondering what method others use for their synch and what you see at idle vs. higher RPM.

[lotus54]

I always set the throttle body/carb balance at higher rpm first, then idle second (if has balance screws).

On Multi, I close both air bypass screws, run it up to 3k rpm or whatever. Balance the throttle bodies (it takes me awhile to get them as close as possible).
Then I'll balance using the bypass screws at idle. Typically I will leave one closed and adjust the other to get as close as possible. These are really supposed to be for mixture balance, but I haven't gotten around to making a special 'sniffer' for the exhaust analyzer.

I also prefer mercury type carb synchronizers- the long ones. I didn't care for the regular vacuum gauge type.

Mark