NGC New Dellorto style carb idle screws

[Canuck750]

I bought a new pair of Dellorto PHF36 carburators to replace the badly worn originals on my 1975 Ducati 860GT. The new carbs are slightly different in a couple details, the location and design of the idle adjuster screws have been updated.



The new Dellorto idle screws are big fat things now and in two pieces, on the outside are large diameter screws with an outer compression spring, the outer screw is hollow inside and a solid tube slides inside the outer tube, the slide has a beveled edge that engages the underside of the slide. This inner solid tube has a fine spring around its length and at the inner end sits a washer and an O ring. The idle adjuster screw does not raise the slide very high, total lift is about 2mm opening.

I can not get my 860 to idle now, I have to hold the throttle slightly open to keep the bike running. I have tried a whole range of idle jets from 48 to 58 and no difference.

Anyone had any experience with these new Dellorto carbs on old bikes?

I am trying to figure out how to revise the idle screw inner tube so that it will lift the slide higher. If I could build up the ramp on the underside of the slide that may be one soloution, I don't think welding up the ramp is a possibility, Lab Metal may work, I hear it can be machined once it cures....

Open to any ideas....

Thanks
Jim

[tetarabra]

Have a look on the mix circuit. They changed the shape of the mixing screw and install an extra jet inside (CD/CS type for low emission).
I faced idling problem when fitting phf32 old new stock on my bike (oversized valves). No way to have correct idle and popping a lot on overun. Problem fixed after fitting old fashion mixing screw and richer needle.

[Groover]

Does the barrel lift if you remove the spring (as a test) so in can screw in more? I'm trying to understand the new design.

[Canuck750]

I think I have it sorted, I compared the extent of lift between the old style and new Dellorto carbs, very similar.

Checked the pilot circuit, all clean, my largest pilot jet was a 64, I bought a set of tiny jeweler drill bits and ran them through an assortment of bigger and smaller jets to extrapolate an approximate drill size to a Dellorto jet, I have a bunch of spare 50 jets and drilled two pairs out to what is close to 75 and 90. I installed the 90 pilots first and the bike fired right up on choke, after a minute or two I was able to close off the chokes and it continued to run, the idle mixture screws even responded to turns in and out and I was able to set the mixture and idle as per the manual then sync the carbs. Plugs are a wee bit rich on the idle but I am going to leave it as is for now.

The stock pilot jets are supposed to be between 50 and 60, I think these new carbs have slightly different design parameters so its not really a big surprise that the pilot jets needed to be so much larger to get a nice idle.

Just glad its sorted.

Jim

[tetarabra]

Good news you get sorted. Would be interested to see the pattern of your mixture screw.
Do you have an extra jet (bf2 or bf3 ) inside downstream the mixture screw ?

Those later carbs are running on the poor side on idle circuit.

[Canuck750]

Good news you get sorted. Would be interested to see the pattern of your mixture screw.
Do you have an extra jet (bf2 or bf3 ) inside downstream the mixture screw ?

Those later carbs are running on the poor side on idle circuit.

I will take a look and let you know.

I took the bike for an hours run, up to highway speeds, it was running far too rich, plugs quite sooty, I am guessing I need to drop the needle down one notch. I also swapped out the pilot jet for jets that I drilled close to 75~80, still started and idled and was able to set the mixture screws with the slightly smaller idle jet.

I have to keep working on the jetting, getting closer.

[Groover]

I'd go to the smaller, but open up the low end mixture valve. Seems pretty shut from the looks of it. I think a #60 jet on low end then unscrewing this about 3 turns (from the looks of it) might get you there. Throttle response should also improve.


https://ibb.co/19SS3wj

[Canuck750]

I have the rear carb running a 64 pilot, about 1 turn out, front is a 70 about 1 1/2 turns out on mixture screw, so far this seems to get me the best carb sync but I think I could take the rear down to a 62 and the front down to a 66
Plugs running cleaner now. I am pretty sure the main is fine at 130, the needle clip position is probably ok as is. Getting much closer. I take it for a good 30 minute blast at city and highway to feel how it’s responding. It’s a slow process but I am seeing some daylight