Ducati.ms - The Ultimate Ducati Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
143 Posts
Discussion Starter #1 (Edited)
Note: Please forgive the fact that this is a duplicate post: I had edited a previous post with this info, but wanted to make sure it was actually seen. Thanks for your patience and attention.)


Title says it all, really...

PRISTINE stock carbs, totally rebuilt using FactoryPro jets.

Been goin' thru their famous "pink sheet" for the rejetting process.

Using a nearby rarely-travelled road, and using my 2nd gear pulls as baseline:
  • Selected jet pulled all the way to redline from 7500K up
  • Selected needle position allows for smooth pull in midrange over 3500K up to 7500k
  • Float bowl position allows for full throttle roll-on from 2500K without stumbling
But here's the mystery, all of that in 2nd gear is lovely. But when I open her up and shift to 3rd (doesn't matter when I do it, either) she just STOPS pulling at 6000K, despite a pinned, wide-open throttle.

Here's something I notice tho: When I do the full throttle roll on in 2nd (where she'll climb all the way to redline) there is a little hesitation at 6000k... then around 6200/6400, she keeps pulling.

Open to your diagnostics!
 

·
Registered
Joined
·
143 Posts
Discussion Starter #3 (Edited)
Hey @belter

Here's the rundown:
  • Brand new stock air-box with snorkels
  • New K&N air filter
  • New MIVV exhaust
  • Main - 137 (I've a hunch this is my primary issue...)
  • Pilot - 45 (Wondering if this isn't also too rich. Pulling away from lights isn't terribly snappy.)
  • Clip position - 2
  • Air / Fuel Mixture Screws - 3 1/4 turns out
  • Float Height - 14mm
  • Carbs balanced using MotionPro's carb balancing device
  • Idles at 1100 like a kitten
  • Starts flawlessly with choke
Other goodies done this summer:
  • New California Cycleworks coils
  • New pickup sensors
  • New belts
  • All valve shims at spec
  • Rebuilt fuel pump
  • Vacuum petcock removed and replaced with manual fuel shutoff
More experimental ride notes:
  • Upon recommendation, I applied choke after getting her up to speed. Bike ran worse. At WOT struggled to hit 5800 RPMs in 3rd.
  • Next, I pulled the air box cover, and wired the filter down. Now she'll pull to 7K, but no further.
 

·
Registered
Joined
·
776 Posts
But here's the mystery, all of that in 2nd gear is lovely. But when I open her up and shift to 3rd (doesn't matter when I do it, either) she just STOPS pulling at 6000K, despite a pinned, wide-open throttle.
Sounds like your new fueling set-up has placed you way down in power.

In third gear, most of an engine's power is being used to overcome aerodynamic drag. You can test this by getting into a tuck position to reduce drag to see if you can get better than 6,000 rpm. The drag increases with the square of the velocity, so in second gear aero drag is much less, allowing you to reach a higher rpm.
 

·
Premium Member
Joined
·
4,275 Posts
Defiantly too big a pilot jet.
Stock pilot should be correct jetting for the stock carburetor.

Main may be too large as well given a closed airbox and snorkels.

Before you chase jetting tough what did you set the pick up gaps too and what position of the timing plate?
Have you tried the oem coils and wires?
Did you swap out the ignition boxes?
What spark plugs? brand and model?

As strega mentioned the less gearing advantage you have in higher gears means you need more power. Still 3rd gear is not a overdrive so the bike should easily pull to max power in that gear (and beyond).

Are you sure your tachometer is correct?
What do you have for a tachometer? The bike is done making power somewhere between 7000-8000 anyway so depending on how accurate your tachometer is you could be simply at max power if you are struggling past 7000rpms. Will it pull to 7000 in 4th? this is close to a 1:1 ratio with the 5 speed (iirc). I do find tachometers reading off on a pretty regular basis when they are electronic.

I do not use Factory jet kits so I am no help with main jet sizes 99% of my data is with Dynojet and jets are not compatible. Others on here do use factory so they may have some numbers to make sure you are close.
 

·
Registered
Joined
·
2,693 Posts
has it got a fuel flow issue? tank venting issue? pull the snorkles to see if 137 is too rich.
 

·
Registered
Joined
·
143 Posts
Discussion Starter #8 (Edited)
I've always appreciated your thoroughness and detail, @ducvet

I do have smaller pilots and mains on the way. The bike was originally jetted for K&N pods.

The pickup sensors / gap were set exactly to the Hayne's recommended distance. Don't remember exactly what that was...

NGK spark plugs, gapped to 6mm. The size is also the Hayne's recommendation. (For what it's worth, I pulled them as a result of your questions, and the horizontal is as clean as a new plug. The vertical has that sought-after tan appearance. What does that tell anyone?)

Haven't tried switching the Kokusan boxes around...

As I had mentioned in a followup post (way up there :) I took her out with the airbox lid completely removed; just strapped the filter down, and she pulled through to 7K before she hit the wall — but only in 2nd. 3rd was just as lethargic as before.)

She has no vacuum leaks, at least not with the carb cleaner spray while idling method.

I'll be taking the main jet down to 132.5 and maybe even 130, will experiment with the 40 pilot. Adjusting clips is always this vague space for me, to be honest...

I'll report back. Thanks to everyone for their thoughts. It's always helpful.
 

·
Premium Member
Joined
·
4,275 Posts
If you run aftermarket plug wires with aftermarket plug caps and non-resistor plugs you run the risk of electrical noise effecting the ignition boxes much like you are hitting a rev limiter which you do not have. If you have resistor plugs as well as resister caps this should not be the issue.

There are plenty of aftermarket kokosan looking boxes that do not work so if the boxes are oem you should be fine if they are functioning.

be sure to stay with one brand of jets to keep things easily understood while jetting no jets that are sold as "fits mikuni carb" or " made for mikuni carb" or as good as mikuni jets unless you are comparing what you have now stay with one brand only whatever it is.

"The pickup sensors / gap were set exactly to the Hayne's recommended distance. Don't remember exactly what that was... "
okay that is gap what about position of the timing plate? you can advance or retard the timing by rotating the plate the pickups are mounted on. I have not seen this cause your issue but something is off.
 

·
Registered
Joined
·
7,018 Posts
Make sure your fuel flow to the carbs is adequate with the new fuel tap.
You still have the restrictor “pill” in the return line, right ? Without it you may return too much fuel to the tank and starve the carbs.
Make sure your vacuum slides are rising together and completely. The vacuum lines to the diaphragms have little alloy cans in them which can clog. The vacuum lines themselves are thin and prone to kinking and collapse. I have replaced them with fuel line and fuel filters .
40 or 42.5 pilot jet should work, you can adjust the screws to fine tune out to 5-1/2 or 6 turns if necessary.
The worst carb problems usually turn out to be ignition problems. Replace your plug wires and caps. One of these, but not both, must be resistor .
 

·
Registered
Joined
·
143 Posts
Discussion Starter #11
You still have the restrictor “pill” in the return line, right ? Without it you may return too much fuel to the tank and starve the carbs.
I am running the HICAP cable kit from MotoElectric which includes all the necessary restriction that you’ve mentioned, so that’s good.

both of my ignition boxes are stock, and while diagnosing the problem with pick up sensors, they both checked out fine.

I must admit that I am not familiar with the “pill“ that you speak of.

Is there a photo or a diagram you can send to help me understand this?
 

·
Registered
Joined
·
143 Posts
Discussion Starter #13
UPDATE:

With a boatload of new jets, and new exhausts installed, she's running better than she ever has.

The only issue now: Pulling off the line when the light turns green. L e t h a r g I c.

Everything else is smooth and predictable. Good acceleration once in second, nice pull through the gears. Idles like a tiger.

What do you think I should adjust? Here are my "low end" specs:
  • 14mm floats — carbs balanced with my MotionPro tool
  • Pilot jet — 40
  • Fuel screws (here's a hint, perhaps) 6, yes 6, turns out.
And just for good measure, the rest of the setup:
  • Main jet — 132.5
  • Needle — 2nd position
 

·
Registered
Joined
·
143 Posts
Discussion Starter #15
It’s a genuine Mikuni replacement, @ducvet.

Please pardon my newbie stare, but...restriction?

I was planning on putting a 42.5 back in in the morning.

What would you suggest for a next step?
 

·
Premium Member
Joined
·
4,275 Posts
As normally a 40 pilot should be right you have to ask "why " does your bike need more fuel than others . Often it is when a passageway is partially plugged causing a restriction so you need to open the fuel feed (pilot jet/fuel screw) more. Looking at some notes from m750's jetted in the past I have almost always run 40 pilots and am between 3-4 turns on the fuel screws.
 
1 - 16 of 16 Posts
Top