Can A TB Sync Indicate Other Problems ?

[pfb]

Background: I have about 1000 miles on new throttle cables, fast idle cable and D-box. The vibes were getting a little much around 4-5k rpm.

Yesterday I sunc(ed) up the TB's and all went well. Idle adjustment was right on but as I was increasing throttle to 4k the twinmax needle would pull to the right briefly as the rev's were coming up - around 3000-3500. At 4000 it would be centered again. Assuming the sync is proper at 4000 and even 4500 is it normal for a cylinder to act this way if the end result is what you want or does this indicate a problem?

Thanks

Peter

[DaveyMac]

Background: I was increasing throttle to 4k the twinmax needle would pull to the right briefly as the rev's were coming up - around 3000-3500. At 4000 it would be centered again.

 

Thats normal for a big twin, as one cylinder powers up as the other is in another stroke.(exshaust or intake).

Your twinmax is centered at steady 4500r's sensitivity up all the way your good-to-go!

D.

[ShovelStrokeEd]

Well, it could be normal but it is not right.

 

You are measuring vacuum in the two intake passages. A device like a TwinMax measueres differential. If the needle is centered during steady state running but swings to one side or the other during acceleration it means that one cylinder is working harder than the other to bring the engine to the new RPM. The harder working cylinder will produce less vacuum than the laggart.

 

This could be as simple as a slight touch up required of the throttles. It could be a difference in pumping ability between the cylinders. Things like valve adjustment, spark plug condition, buildup of carbon in the chambers, ignition timing variation can all effect this.

 

This is why you don't just do a synch on the throttle bodies willy nilly. It is the last step (short of replacing the tupperware) in a process that includes inspection/adjustment of all the factors I mentioned above.

[Guest]

Without a load on the engine, I think syncing at 2500rpm gives better results than syncing at 4500rpm.

[GordonB]

I noticed when I TBS, (after all the Valve/plug/etc adjsmnts) the needle will pull slightly to one side as the revs go up and then pull to the other side as they go further up. Found a happy medium and she purrs like a kitten.

[Ken H.]

Well, it could be normal but it is not right.

True, but I have to say it's a rare boxer that stays even during acceleration. If you can get one even at idle and even at a couple of different points steady RPM, you are doing well.

 

FWIW, although I am a big fan of water manometers, I bought a twinmax recently specifically so I could watch sync while riding, under load. Interesting, although not terribly surprising, it is different under load vs. at a no-load 'bench' RPM.

[ShovelStrokeEd]

Not unusual. The load required to maintain RPM in neutral is different from that to do the same, at speed, on the road. It will thus tend to point up the differences between cylinders more. I have actually gone so far as to try to synch with the bike in gear, on the centerstand and a dragging rear brake but gave it up as there is too much potential for a Three Stooges type mishap.

 

Ideal way would be to do the synch on a rolling road type dyno but few of us have access to such nice things. Could probably build one with some angle iron, a couple of heavy duty rollers, a timing belt and a small hydraulic pump with a couple of valves. Cost could run less than 1K or so for the really anal types.

 

Me, I'm happy with idle and 4K using my Carb Stix. I never did get around to buying that Magnehelic. Compared to my Blackbird, the 1100S feels like a cement mixer anyway.

[pfb]

Thanks everyone. Valves, plugs and sync were all done together over the winter. I attibuted my vibes to some stretching/seating of the cables. The bike runs much smoother now and I'll search the board for a good injector cleaner. I'm still learning the tuning ropes and I'm sure I will find that nice balance of tuning techniques. I wouldn't be this far along without you guys. I think there will be a manometer in the future.

Thanks again.

Peter