carb tuning for 1974 Ducati GT750

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happyhaya
Cucciolo - the Lil Pup
Posts: 2
Joined: Sun May 23, 2010 5:10 pm

carb tuning for 1974 Ducati GT750

Post by happyhaya » Sun May 23, 2010 6:21 pm

Hi
Recently obtained 3 Ducs basically barn finds. 2 1974 GT750 & one 1973 GT250.

I am needing carb settings & carb balancing procedure for the 750's.

I also have an oil leak on the lower part of the bevel tube rear cylinder. Can I change oring or gasket without too much trouble.

The alternator/generator light is not going off when engine is running, where do I start checking for faults

Thanx for the great help guys

Craig

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Craig in France
Cagiva Gran Canyon
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Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: carb tuning for 1974 Ducati GT750

Post by Craig in France » Mon May 24, 2010 6:18 am

happyhaya wrote: <snip>
I am needing carb settings & carb balancing procedure for the 750s.
Hi and welcome,

From Ian Falloon's Restoration Guide (and for PHF 30s, btw):
Needle: K3, middle groove
Main: 110 (I believe some people run bigger than this)
Idle: 65
Slide: 70 (no spec. given for the cut-away, I'm afraid)
Needle: 256AB.

Carb balancing procedure: Steve Allen (the owner of this site) has written it up somewhere. Can't now find it, but ask him, and I'm sure Steve will point you in the right direction. Basically, start with each carb with:
1. The mixture screws 1½ - 2 turns out
2. The throttle stop screws 2 turns out
3. The cables adjusters set so that the slides are dropping at the same time.

And away you go ...
happyhaya wrote: I also have an oil leak on the lower part of the bevel tube rear cylinder. Can I change O ring or gasket without too much trouble.
Think you'll find it's head off, which means engine out of frame. So best try a smear of RTV first ...
happyhaya wrote: The alternator/generator light is not going off when engine is running, where do I start checking for faults
I'm not familiar with the 750 charging system, except to say I get the impression it wasn't too great when new .. and of course it's pretty long in the tooth now. Start with checking the voltage at the battery: should be 14.5 v +/- with the engine revved up to 3,000. If not, suspect reg/rec and/or alternator windings (for which, check the resistances).

HTH

Another Craig

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 8:11 am

Post by Ray O'Donohue » Mon May 24, 2010 8:36 am

The slide that came with the bike is a # 70-_,and the bike will NEVER run properly with that slide. (This is the sort of thing people had to work through in the Bad Old Days.) Replace with # 60-1 slides,and set the pump volume delivery for 5cc per 20 squirts.This is Gospel for every Dellorto application .That bike also came stock with less than 8-1 compression and the weakest ignition ever made.

wdietz186
Cagiva Alazzura
Posts: 703
Joined: Sun Sep 10, 2006 5:40 pm

Post by wdietz186 » Mon May 24, 2010 7:25 pm

The alternator light really isn't an alternator light in that it doesn't go off when the engine is running. It just indicates that there is voltage available to run the engine.I.E. there is more that 10v to the ign. coils.Below that it will quickly foul plugs and not run. The stock system is limp at best and if you have to run with the lights on at around town speeds the battery will not stay charged.The system reaches equilibrium at around 4000rpm with the lights on,below that the battery is providing most of the electricicty to illuminate and spark.Most run a good battery and charge it often.An alt. rotor from an ST2 will help the situation due to stronger magnets and there is an aftermarket rotor from AUS. available but I can't remember who has it.A search on this forum should dig it up.Ray's suggestion on the slide really does make a big difference in tractability and you could try twisting the bevel tube back and forth a bit as it will sometimes allow the o-ring to seat itself and stop the oil seepage otherwise it requires the head lifted and the o-ring replaced.Cool bikes and a lucky find as the round case bikes can do nothing but appreciate in the future.

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Steve Foster
Parallel Twin
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Joined: Thu Dec 07, 2006 2:43 am
Location: Sydney, Australia.

Post by Steve Foster » Wed May 26, 2010 2:10 am

" .. and there is an aftermarket rotor from AUS. available but I can't remember who has it."

Try Phil Hitchcock at Road and Race. See http://www.roadandrace.com.au/alternato ... rsions.htm

If you can get your hands on a copy of "Ducati Tuning - V-Twins with Bevel Drive Camshafts" by Stephen Eke (1986) you'll find excellent info on carb tuning in Chapter 1.

Cheers,
Steve.
1974 Ducati 750 GT

stian buksa
Cucciolo - the Lil Pup
Posts: 20
Joined: Mon Oct 12, 2009 11:50 am

Post by stian buksa » Thu Jun 24, 2010 1:53 am

Mick Walkers "Ducati Twins Restoration (Motorbooks Workshop)" has a table of the settings of all the carbs of the ducatis, and Ian Falloons "Ducati bible" also has some tables
If you don't have access to the books, you probably can view them on google books (I did)

My bike (71/72 roundcase gt) was fitted with PHF32AS/PHF32AD, so I guess somebody switched carbs from a 74-76 750 Sport.
But the carbs are jetted differently from eachother. I read somewhere that they came individually jetted from the factory because the rear cylinder was running warmer because it didn't get as much aircooling. But I see no mentioning of this in any of the of the jetting tables I've found

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