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Carb Re-jetting

Started by dp351, February 13, 2024, 08:46:09 AM

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dp351

February 13, 2024, 08:46:09 AM
The results from installing a Factory Pro jet kit in my 1996 M900 are exactly what I was hoping for. New main and pilot jets, needles and emulsion tubes. The transition from the idle circuit to the needle jet is smooth as butter now. Still need to iron out a slightly rough off idle situation but the bike is much more rideable in stop and go traffic.

However, I took the bike into the canyon twisties and after a while, I started getting a lean condition. The roads I was on were very tight corners with lots of back and forth and mostly 2nd gear, winding it up until the next corner. As I was going down the mountain on decel I started to get some popping and eventually loss of power. I pulled over to see if my fuel line popped off (it's happened a couple of times on this bike) but all was fine.

My thoughts are that my float levels need to be adjusted and that I was sucking all the fuel out of the bowls quicker than it could fill back up. On the highways and around town it's doing great, but extended high RPMs at 3/4 throttle to WFO, is it draining the bowls?

What are your thoughts?

ducpainter

#1
February 13, 2024, 09:40:09 AM
Two things I'd check.

First I'd check to be sure the fuel pump is pumping. There are options if yours isn't.

If it is pumping, check your float levels. IIRC they should be at 14mm. The directions for the jet kit should have a spec.

If those two guesses don't solve the issue, there's a small filter at the fuel inlet to the carbs that could possibly be clogged.

"Once you accept that a child on the autistic spectrum experiences the world in
 a completely different way than you, you will be open to understand how that
 perspective
    is even more amazing than yours."
    To realize the value of nine  months:
    Ask a mother who gave birth to a stillborn.
"Don't piss off old people The older we get, the less 'Life in Prison' is a deterrent."


koko64

#2
February 13, 2024, 11:33:04 AM
÷1.
Yes float is 14mm.
Then add to your checklist to methodically go through:

Does your bike have a manual or vacuum fuel tap? If it's vacuum, then replace with a 5/16 minimum bore manual fuel tap.
What do the spark plug colours say after the ride? That informs the next point.
What jetting option you settle with from the kit?
Standard airbox?
Check for inlet manifold leaks too.
2015 Scrambler 800

koko64

#3
February 13, 2024, 02:27:21 PM
Oh forgot to ask, V or W heads?
2015 Scrambler 800

ducpainter

#4
February 13, 2024, 02:48:20 PM
My 96 M900 has V heads. I think they all did...probly wrong. ;D
"Once you accept that a child on the autistic spectrum experiences the world in
 a completely different way than you, you will be open to understand how that
 perspective
    is even more amazing than yours."
    To realize the value of nine  months:
    Ask a mother who gave birth to a stillborn.
"Don't piss off old people The older we get, the less 'Life in Prison' is a deterrent."


koko64

#5
February 13, 2024, 04:35:19 PM
I think the Ws started appearing late 96, but it could have been 97. Glad you got the good ones ;D
2015 Scrambler 800

koko64

#6
February 13, 2024, 04:37:52 PM
I actually like the Factory Pro needle and spring better than the DJ ones. I just think FP are a little conservative with their main jets.
2015 Scrambler 800

Frank C

#7
March 24, 2024, 05:54:31 AM
Sorry for the late reply for an older thread...but just to clarify...according to Ducati the W heads were for the 97 model year.  But the W heads on my 97 were stamped 96.  So you are both right.

As for the issue...why ask V vs W?  Just curious.
Sounds like the diaphragm in the fuel pump is failing.  Should check and see if the oil level is suddenly increasing.  Ask me how I know :)
With a failing pump, the fuel is now gravity fed.  And with the tank outlet at the back of the tank, going down hill with a partially tank of fuel could starve the carbs. 

koko64

#8
March 24, 2024, 08:19:10 AM
V and W heads have different jetting requirements and the jet kits are different. That's reflected in the DJ catalogue.

Gravity feed allows about half a tank use before fuel starvation as the carb inlet is higher than the tank. Suspect a fuel pump failure without a blockage under those conditions,  but suspect a blockage if fuel starvation occurs with a full tank. It can occur in a few places, tank outlet, tap, hose collapse, pump, carb inlet mesh, float valve, blocked jets. A blocked tank breather strangles fuel too which can be crud or a pinched hose, stressing fuel pumps in general.

2015 Scrambler 800

Frank C

#9
March 25, 2024, 05:07:49 AM
Didn't realize.  Makes sense, especially since the cams are also different. Its been so long, I forgot which kit I used.  Couldn't stand the needle jets becoming ovals.  So I solved the problem by going with V heads with IE cams and flat slides :)

As always, thank you for your insight.

koko64

#10
March 25, 2024, 11:40:57 AM
Nice motor recipe. High comp pistons would complete that nicely (with heavy duty engine studs, altered ignition timing and higher octane fuel).
2015 Scrambler 800

Frank C

#11
March 27, 2024, 06:51:40 PM
Funny...that was the last thing left to do on the monster.
Focused on everything else first...suspension, wheels, brakes, lighting, ignition, exhaust, and lightweight rotating stuff on the engine.
Then I picked up an ST4s just a mile down the road and never got around to the big bore kit.

Son turns 16 next year.  Might just wait for him to blow it up first before putting new pistons and bores in.  Same reason why I have a big block sitting in the basement for the old F-150 :)
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