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Discussion Starter #1
After 2 months and 20 painstakingly logged jetting / needle clip / float height / fuel screw changes, the one performance characteristic that absolutely befuddles me with this bike (2001 M750, carbie) is that it absolutely, positively, never, ever accelerates cleanly from a standing start.

I can tweak top-end, I can tweak mid-range, but from zero to 20, it's a wheelbarrow of mud.

Seriously frustrating.

Always open to new thoughts.
Current setup (all new jets, new Factory Pro titanium needles, and emulsion tubes)
Main: 132.5​
Pilot: 42.5​
Needle position: 2​
Float height: 15
Fuel screws: 5​
 

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Discussion Starter #3
raise the needle?
Yup, tried that.

Also, took the Main up one size to 135.

No matter what, I have sluggish acceleration. I mean, awful.

I teach motorcycle safety courses and we have a stable of 250s that will out sprint my M750.

Now starting to take @ducvet seriously when he told me weeks ago to check ignition stuff...
 

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The Worst carb problems are usually caused by the ignition system. This is why I always suggest a “ Tune Up” before trying to find a problem that could just be some maintenance thing that didn’t get done. 19 year old bike, have you ever replaced the plug caps and wires ? Either plugs , caps, or wires should be resistor type to avoid interference. I always ground the plugs and just check for an even and consistant spark on both cylinders. If timing is retarded, low end response will suffer, so that should be verified as well. If I have that, I move to the carbs.
Have you checked the vacuum diaphragm action ? Do both slides raise together at an even rate ? Are the vacuum lines and valves in the lines clean and flowing air in both directions ? Mine were collapsing and the little cans were clogged. I replaced them with fuel line and fuel filters that won’t clog or collapse. If the diaphragm can’t move air freely it can’t raise the slide as it should. If there are any impediments or leaks the diaphragms won’t raise
the slides as they should, or maybe in your case, as quickly as they should. I would check the slide function for smooth and even movement, especially when you first crack the throttle. I think you may have slide issue.
 

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The Worst carb problems are usually caused by the ignition system. This is why I always suggest a “ Tune Up” before trying to find a problem that could just be some maintenance thing that didn’t get done. 19 year old bike, have you ever replaced the plug caps and wires ? Either plugs , caps, or wires should be resistor type to avoid interference. I always ground the plugs and just check for an even and consistant spark on both cylinders. If timing is retarded, low end response will suffer, so that should be verified as well. If I have that, I move to the carbs.
Have you checked the vacuum diaphragm action ? Do both slides raise together at an even rate ? Are the vacuum lines and valves in the lines clean and flowing air in both directions ? Mine were collapsing and the little cans were clogged. I replaced them with fuel line and fuel filters that won’t clog or collapse. If the diaphragm can’t move air freely it can’t raise the slide as it should. If there are any impediments or leaks the diaphragms won’t raise
the slides as they should, or maybe in your case, as quickly as they should. I would check the slide function for smooth and even movement, especially when you first crack the throttle. I think you may have slide issue.
REally excellent description.

Is the problem throttle response or is it leaning out off of idle (stumbling)? Or?
 

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Discussion Starter #6
The Worst carb problems are usually caused by the ignition system. This is why I always suggest a “ Tune Up” before trying to find a problem that could just be some maintenance thing that didn’t get done. 19 year old bike, have you ever replaced the plug caps and wires ? Either plugs , caps, or wires should be resistor type to avoid interference. I always ground the plugs and just check for an even and consistant spark on both cylinders. If timing is retarded, low end response will suffer, so that should be verified as well. If I have that, I move to the carbs.
Have you checked the vacuum diaphragm action ? Do both slides raise together at an even rate ? Are the vacuum lines and valves in the lines clean and flowing air in both directions ? Mine were collapsing and the little cans were clogged. I replaced them with fuel line and fuel filters that won’t clog or collapse. If the diaphragm can’t move air freely it can’t raise the slide as it should. If there are any impediments or leaks the diaphragms won’t raise
the slides as they should, or maybe in your case, as quickly as they should. I would check the slide function for smooth and even movement, especially when you first crack the throttle. I think you may have slide issue.
Plugs, caps, and wires are all new from MotoElectric.

Coils are new from California Cycle Works.

At the risk of wading into the obvious, how would I actually go about checking the diaphragm and slide? Simply removing the airbox and watching the action as I twist the throttle?

I’m afraid my frustration over this after all of the changes is affecting my judgment :)
 

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Discussion Starter #7
The Worst carb problems are usually caused by the ignition system. This is why I always suggest a “ Tune Up” before trying to find a problem that could just be some maintenance thing that didn’t get done. 19 year old bike, have you ever replaced the plug caps and wires ? Either plugs , caps, or wires should be resistor type to avoid interference. I always ground the plugs and just check for an even and consistant spark on both cylinders. If timing is retarded, low end response will suffer, so that should be verified as well. If I have that, I move to the carbs.
Have you checked the vacuum diaphragm action ? Do both slides raise together at an even rate ? Are the vacuum lines and valves in the lines clean and flowing air in both directions ? Mine were collapsing and the little cans were clogged. I replaced them with fuel line and fuel filters that won’t clog or collapse. If the diaphragm can’t move air freely it can’t raise the slide as it should. If there are any impediments or leaks the diaphragms won’t raise
the slides as they should, or maybe in your case, as quickly as they should. I would check the slide function for smooth and even movement, especially when you first crack the throttle. I think you may have slide issue.
And thanks again for all this incredible detail.

How would I go about checking the timing?

Now we’re getting into voodoo territory… Places I have never trod :)
 

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Plugs and caps should both be resistor type, verify.

You set the pickup gaps did you remove the timing plate they are mounted on?
If so did you change the advance when you did so?
do you know how to check?
Buy or borrow a timing light and see if the timing is right or pull the cover again.

full throttle would not be okay if it was a bad diaphragm probably.
Did you verify the pickup wire orientation at the ignition boxes? There is a typo in the factory manual iirc so compare to a known good wiring setup. I have seen a number of bikes come in with the wiring backwards after someone was playing with the pickup wires. This often leads to a bike that is way out of carb sync as well but capable of running... just not well. Also do not trust ANY aftermarket parts I find way more aftermarket bad than oem, simply double check with another oem and if no change use the new part.
 

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Discussion Starter #9
OK. Perhaps your guidance has led to a discovery...

After watching some extremely helpful videos online about vacuum diaphragms and slide action, I was able to verify that both have smooth and even action.

Also, using my air compressor with low pressure, I blew air into the vacuum ports on the carbs and watched the slides raise, both evenly and completely.

After reattaching the hoses, however, neither of them allowed for enough air volume (I only set the compressor to 10 PSI) to raise.

Removing the hoses completely, I learned that one was blocked, and the other was a mere wheeze.


Questions:
  1. Would this also answer why I constantly needed to set my fuel mixture screws strangely high?
  2. See attached photo. The triangular box that the hoses run to have 4 holes. Does it matter which ones the hoses run to, and... are the other 2 just for venting?
I have extra fuel hose. I can pick up some small inline fuel filters per your recommendation.

I really hope this is all I need. I kinda dread the possibility of misaligned sensors, draining the oil, removing the alternator cover... Argh. Electrical gremlins.

Thank you so much for helping me discover this. It's late and I teach class tomorrow so it may be a day or two before I can test to see if this solves her issues.
992919
 

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Discussion Starter #10 (Edited)
MY CARB EDUCATION CONTINUES...

I've posted the next "event" in this saga elsewhere in this forum (regarding vacuum line filters.)

Appreciate any additional input!
 
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