Ducati 250 Jetting

[alno]

Hello,
I recently acquired a 1966 250 Mach1 replica. It has a Dellorto PHBH30 carburetor. The engine has a 10 to 1 piston, big cam and big valves. The bike starts well and runs well to 4000rpm, after that it runs dangerously bad until 8500 to 9000. Could someone help with a starting point on jetting or what is the best Carburetor and jetting for the bike?
Thank You
Al N

[JimF]

I'm hoping that someone has a link to a manual for that carb - it seems like there should be a PDF of it on-line somewhere.

It sounds to me like you just have one jet that's off. The idle jet is fine given that it idles and rolls on cleanly, and the main jet is fine given that it runs wide--open well, perhaps you have a pilot jet that's wrongly sized, or a needle that's in the wrong position.

Since about two-thirds of your throttle twist is working well, if it were me I would try to learn the carb I have and tweak that mid-range rather than start with a new carb and have to figure out all the jets.

Give it a day or two and somebody should come up with a tuning manual for that model of carb. Once you figure out what jet is in play for the throttle opening that is running poorly, you can order some larger and some smaller jets that the one installed so as to be able to experiment. If you go with a larger jet at the onset to test the results you won't risk a dangerously lean condition. If richening the fuel makes the problem worse you can then move in the leaner direction by installing a smaller jet and working in that direction.

Jim

[motoital]

Hello, Here is a link to a manual for the Dellorto PH models.http://www.thisoldtractor.com/gtbender/ ... manual.pdf
Also have a look at http://www.thisoldtractor.com/gtbender/ ... de_1.1.pdf do you have the two piece welded up manifold? Hopefully it is the needle as that was the easy part for me. Also what muffler are you using as that had a big impact on my tuning.
Good luck!

JD

[alno]

Thanks Jim,
The engine does feel so lean that I can't figure out why it would even run on the top. I will get it apart over the weekend. Again Thanks
Al N

[Eldert]

Hi Al

take the carb apart and write down wat is in there now .
not only the main and pilot jet but also what needle and needljet and post it here

Eldert

[alno]

Hello Eldert,
The Carburetor body PHBH30
Main Jet 145
Slide 30
Needle X15
Needle position 3rd (lean of middle)
Needle & Seat 150
Pilot Jet 100
Jet Needle 45
Emulsion tube 266P
Float 24mm
Air Screw 1 1/3 out
250cc
Big Cam
Big Valves
Standard Muffler Mach 1 Small tip on the end (No Megaphone)
Thank you for any help you can give me.
Al N

[Bevel bob]

I have been told that genuine Mach 1 silencers had a bigger outlet than others, replacements are too small and restrict performance, I use a Norton cigar silencer which looks similar to the silentium but has a better flow. The main jet seems large.