Jetting and needle for PHF32s on stock 750 GT

Post your 'Carburettor & Fuel System' FAQs, comments & questions regarding Ducati motorcycles here.
Post Reply
jamesonthemove
Cucciolo - the Lil Pup
Posts: 3
Joined: Mon Jul 07, 2014 11:29 pm

Jetting and needle for PHF32s on stock 750 GT

Post by jamesonthemove » Tue Nov 25, 2014 9:54 pm

I have a set of Dellorto 32mm PHF carbs (from an 860GT) that I will put on my 1975 750 GT. I’m running stock 750 GT heads (pistons, etc), Conti pipes, and K&N cone air filters.

What, in your experience, should my needle and jet setup be?
I understand that this will be a starting point for me to dial in from.

Ducati Twins Restoration by Mick Walker (page 228) is helpful. I won’t expect to change the needle jet (265AB).
I suspect I require a 65 Pilot Jet.
A new set of needles are not cheap, so I’d like to get that right first attempt.
I will stick with the 70/2 slides unless convinced that the 60/1 has proven to perform for this setup.

Thanks for your insight.
Current:
1975 Ducati 750 GT

Retired:
1973 Ducati 750 GT
1975 Ducati 860 GT
1978 Ducati Darmah 900 Super Sport
1974 Moto Guzzi 850 Eldorado
1973 Norton Commando 850cc
1989 Honda Bros 650cc (aka Hawk, GT)
1989 Honda Dream 100cc
1979 Can-Am Bombardier 250cc

User avatar
geodoc
750 GT
Posts: 177
Joined: Thu Jul 18, 2013 3:57 pm

Re: Jetting and needle for PHF32s on stock 750 GT

Post by geodoc » Wed Nov 26, 2014 12:39 pm

Heh James,

I've got a 750 GT that was recently brought back from a basket case that has Sport pistons and PHF 32's, but otherwise stock for mods that would effect jetting. I started out with the jetting specified for the 750 Sport:

MJ 125
NJ: 265AB
PJ: 65
Needle: K5
Slide: 70/2

Variables are - Chevron 94 no ethanol
Sea level
~70 deg. F

I took it to the dyno and established that the 125 MJ produced best power. Didn't spend much more time with the dyno beyond that since I've found with other bikes set up with different PHF's (Guzzis) that I ended up tweaking by "ride-feel" anyway.

Interestingly, I ended up leaner than the settings above after a lot of fooling around including trying 60/1 slides that I happened to have from a Guzzi.

So the current set-up is:

MJ 125
NJ: 265AB
PJ: 62
Needle: K16 (bottom / richest notch)
Slide: 70/2

I had a hard time finding the K16's, but did locate a pair from Herdan in the E USA.

And of course the caveat - my settings might not translate to the best for your particular bike (density altitude makes a big difference and, to a lesser extent, so does the fuel that you're using).

George

jamesonthemove
Cucciolo - the Lil Pup
Posts: 3
Joined: Mon Jul 07, 2014 11:29 pm

Re: Jetting and needle for PHF32s on stock 750 GT

Post by jamesonthemove » Thu Nov 27, 2014 9:04 am

Thanks, George! That's really helpful!
Congratulations on your 750 GT basket rebuild! Your posts offer superb documentation and guidance.
viewtopic.php?f=24&t=4543 (page 5 for those searching for carb tuning details)

Fortunately, I think I might be able to source some of those other needles for trial. I'll source the jets from Steve at BH.

Other links:
I've found this PDF of the Haynes Motorcycle Carburettor Manual helpful:
http://www.lunadesign.org/images/gixxer ... Manual.pdf

And this is helping me get my head around the differences between the Dellorto needles:
http://www.ducatimeccanica.com/needle_chart2.jpg

Carburettor settings for Ducati twins from Mick Walker's book:
http://books.google.nl/books?id=PTdUqLE ... &q&f=false

All the best,
James

User avatar
geodoc
750 GT
Posts: 177
Joined: Thu Jul 18, 2013 3:57 pm

Re: Jetting and needle for PHF32s on stock 750 GT

Post by geodoc » Thu Nov 27, 2014 11:55 am

jamesonthemove wrote:Thanks, George! That's really helpful!
Congratulations on your 750 GT basket rebuild! Your posts offer superb documentation and guidance.
viewtopic.php?f=24&t=4543 (page 5 for those searching for carb tuning details)

Fortunately, I think I might be able to source some of those other needles for trial. I'll source the jets from Steve at BH.

Other links:
I've found this PDF of the Haynes Motorcycle Carburettor Manual helpful:
http://www.lunadesign.org/images/gixxer ... Manual.pdf

And this is helping me get my head around the differences between the Dellorto needles:
http://www.ducatimeccanica.com/needle_chart2.jpg

Carburettor settings for Ducati twins from Mick Walker's book:
http://books.google.nl/books?id=PTdUqLE ... &q&f=false

All the best,
James
Be sure to keep us posted on how it goes.

JasonBav
Cucciolo - the Lil Pup
Posts: 6
Joined: Sun Jul 22, 2012 11:29 am

Re: Jetting and needle for PHF32s on stock 750 GT

Post by JasonBav » Sat Feb 07, 2015 12:18 pm

Hi James, just wondering how your bike is going, I have a GT with stock pistons etc but with a Dyna and better charging system. I am running 32's and below is what I have. Use mainly 95 octane fuel down in Australia.

Slide = 70/2
Needle= K3 middle notch
MJ=115
Ij=60
AB 265

User avatar
geodoc
750 GT
Posts: 177
Joined: Thu Jul 18, 2013 3:57 pm

Re: Jetting and needle for PHF32s on stock 750 GT

Post by geodoc » Sat Feb 07, 2015 7:54 pm

What altitude? Curious since you're running 115 MJ's.
JasonBav wrote:Hi James, just wondering how your bike is going, I have a GT with stock pistons etc but with a Dyna and better charging system. I am running 32's and below is what I have. Use mainly 95 octane fuel down in Australia.

Slide = 70/2
Needle= K3 middle notch
MJ=115
Ij=60
AB 265

JasonBav
Cucciolo - the Lil Pup
Posts: 6
Joined: Sun Jul 22, 2012 11:29 am

Re: Jetting and needle for PHF32s on stock 750 GT

Post by JasonBav » Sun Feb 08, 2015 4:58 pm

I say almost sea level (Sydney Australia) , I didn't put these jets in, they were in there when I got the bike.

User avatar
geodoc
750 GT
Posts: 177
Joined: Thu Jul 18, 2013 3:57 pm

Re: Jetting and needle for PHF32s on stock 750 GT

Post by geodoc » Tue Feb 10, 2015 2:49 pm

JasonBav wrote:I say almost sea level (Sydney Australia) , I didn't put these jets in, they were in there when I got the bike.
Hmmmm .................... seems a smidgen on the lean side for the MJ. If you can get a pair of 125's I'd give them a try and see how it effects the upper end. Also, to a certain extent, the MJ will effect the rest of the range too.

Here in Canada, I'm running Chevron 94 octane, no ethanol fuel. It's amazing how much the particular fuel blend seems to effect jetting.

G

jamesonthemove
Cucciolo - the Lil Pup
Posts: 3
Joined: Mon Jul 07, 2014 11:29 pm

Re: Jetting and needle for PHF32s on stock 750 GT

Post by jamesonthemove » Fri Feb 13, 2015 5:18 pm

Hi Jason,
The weather turned cold and wet here before I had rebuilt the carbs. I figured on waiting till the weather is better before running them.
I'll be trying this setup:

MJ 125
NJ: 265AB
PJ: 65
Needle: K6 (middle notch)
Slide: 70/2

That's a stock setup for the 32PHF for the 750 Sport, apart from the K6 needle. The K6 has a profile that is not too far off George's K16 (as can be seen by the spec here: http://www.ducatimeccanica.com/needle_chart2.jpg). I'm looking forwarding to taking the 750 out for a spin and see how the setup works under a load before more changes. I'll be sure to put an update here.

Good luck. Please let us know how you get on.
Current:
1975 Ducati 750 GT

Retired:
1973 Ducati 750 GT
1975 Ducati 860 GT
1978 Ducati Darmah 900 Super Sport
1974 Moto Guzzi 850 Eldorado
1973 Norton Commando 850cc
1989 Honda Bros 650cc (aka Hawk, GT)
1989 Honda Dream 100cc
1979 Can-Am Bombardier 250cc

Post Reply

Return to “Carburettor & Fuel System Shop”