[pumpman]

After an expensive tune-up by an excellent Ducati mechanic (Desmosedici trained in Italy) I still had the initial stumble when starting out, like one cylinder stops participating until more throttle is applied, and I decided to tackle it myself using a little gadget (pictured) I bought over 30 years ago for synching carbs on old MG and Triumph twin-carb cars. It measures actual air flow instead of manifold vacuum, and that seemed a better way to synch the throttle bodies. I closed the air bypass screws completely, balanced the TBs, and brought the idle up to 1100 rpm with the main screw adjuster on the linkage, leaving the air screws closed. The mechanic spent considerable time setting the CO, so I did not disturb that adjustment, although I probably altered it by closing the air screws. I also suspected the TPS was off, since cracking the throttle a tiny bit made the stumble go away, and I moved it back and forth until I found the "sweet spot" at idle speed. Considering that all the requisite electronic wizardry had been used for the tune-up, this procedure could be expected to produce a really rough-running engine, at least from a theoretical standpoint. Much to my delight, it no longer stumbles, and runs better than any Duc I've had! Now, I ask, does this mean that all those factory procedures, measuring vacuum, volts, and ohms, are not really a good measure of the situation, or did I just get lucky?

I forgot to mention the bike involved: 2001 996 with Termi half-system and appropriate chip.

 

[m90daytona]

if you want something done right, you got to do it yourself!

 

[safetyfish]

What is your fuel consumption like after your procedure?
What is the CO values now after the procedure?


SF

 

[pumpman]

What is your fuel consumption like after your procedure?
What is the CO values now after the procedure?


SF

I don't know fuel consumption yet, just started riding, but as for CO, who cares? It runs great, which, in my experience, means it is making better, smoother power, so mileage should be improved, if anything. I was getting 45mpg before, normal riding, and it ran fine except for the stumble, and now that is fixed. The whole point is that I did not use any diagnostic tools and got better results than the tech. How many times have you heard someone say they got their bike back from a service and it ran lousy? I already suggested that I may have just gotten lucky, but the results are all that matters to me.

 

[safetyfish]

Fuel sucked into the cylinder helps with latent cooling of the metal parts. A reasonable indication as to how well this is being done is by looking at your CO value. Too low a CO value and you run the risk of burning a hole in your piston, that is my reason for asking.

SF

 

[wdietz186]

The Uni-Syn is a cruder instrument than a vacuum gauge setup as you are introducing an air restriction not present in normal running.Plus that little bobber is hard to read.But you can get acceptable results if you mess with it enough.As for the TPS what you have likely done is advanced it so the ecu thinks the throttle is open further than it really is.This is having the effect of richening the mixture so your fuel consumption will be higher.So Yes you got lucky.

 

[pumpman]

The Uni-Syn is a cruder instrument than a vacuum gauge setup as you are introducing an air restriction not present in normal running.Plus that little bobber is hard to read.But you can get acceptable results if you mess with it enough.As for the TPS what you have likely done is advanced it so the ecu thinks the throttle is open further than it really is.This is having the effect of richening the mixture so your fuel consumption will be higher.So Yes you got lucky.

The Uni-Syn (you must be an old codger to know that!) does restrict the flow, that is how it works, but the restriction, whatever amount, is exactly the same for both TBs, so the measurement at each TB is consistent
(the "errror" is identical). It does take some patience, but it is quite sensitive and far from crude if used correctly. As for the TPS, it is interesting that you assume that the adjustment I made put it in the wrong position, rather than admit the possibility that it was wrong initially and I corrected it. You don't need a CO meter to tell if an engine is running rich or lean, there are other signs of those conditions. My 996 runs very strong now, all the way thru the rev range without the slightest sign that the mixture is wrong. The fundamentals are the same now as they were 50 years ago when I first began working on 4-stroke engines. I got lucky, so I guess the mechanic got unlucky?

 

[wdietz186]

Pumpman,Yes I am old and I actually like SU and even Stromberg carbs with the possible exception of triple carb Healys.Your TPS may have been out after it was serviced and you happened on the correct position.The only way to tell would be to hook it back up and see what the reference voltage is.The TPS being zeroed establishes the base reference for the fuel map so it delivers the right mixture at the right load/rpm.If you are within the tolerance then it will run fine, if not it won't.

 

[Andrew]

After an expensive tune-up by an excellent Ducati mechanic (Desmosedici trained in Italy) I still had the initial stumble when starting out, like one cylinder stops participating until more throttle is applied, and I decided to tackle it myself using a little gadget (pictured) I bought over 30 years ago for synching carbs on old MG and Triumph twin-carb cars. It measures actual air flow instead of manifold vacuum, and that seemed a better way to synch the throttle bodies. I closed the air bypass screws completely, balanced the TBs, and brought the idle up to 1100 rpm with the main screw adjuster on the linkage, leaving the air screws closed. The mechanic spent considerable time setting the CO, so I did not disturb that adjustment, although I probably altered it by closing the air screws. I also suspected the TPS was off, since cracking the throttle a tiny bit made the stumble go away, and I moved it back and forth until I found the "sweet spot" at idle speed. Considering that all the requisite electronic wizardry had been used for the tune-up, this procedure could be expected to produce a really rough-running engine, at least from a theoretical standpoint. Much to my delight, it no longer stumbles, and runs better than any Duc I've had! Now, I ask, does this mean that all those factory procedures, measuring vacuum, volts, and ohms, are not really a good measure of the situation, or did I just get lucky?

I forgot to mention the bike involved: 2001 996 with Termi half-system and appropriate chip.

You didn't get lucky, this is just an ancient tuning technique that has been largely forgotten in these modern times - fuel injection is very forgiving.

The sequence should be (in my opinion)
1. Set the TPS sensor.
2. Balance the throttle bodies (both for idle and acceleration synchronisation) - the latter may require a mercury gauge (rather that an SU carb volume meter).
3. Set the CO.
4. Adjust idle speed.

Step 1 is often over looked and step 2 is generally not done as either the types of tool you have aren't available or the mechanic is just not confident in this type of work.

These days I generally only maintain my own and the family vehicles - two and four wheeled (occasionally I return to my prior occupation). Yet two weeks ago I set the throttle bodies on my wife's MV Agusta using a mercury gauge bank (an Alfa Romeo tool), this was probably the first time in 10 years I'd used this tool.

I used to repair Italian cars - lots of them (often equiped with multiple webber carbs) and I learned how to balance carburettors. However I occasionally worked at other works shops and generaly they would not touch the adjustments on multiple carbs (except as matched units - i.e. turn all screws the same amount).