250 mark 3 Dellorto SS! tuning Help ....

[mark3kb]

Hello to all and anyone who could give me some input as to the proper method of tuning my SS1 29 D on my 65 Mark 3. I have been struggling for days ,trying to get my fully restored completed bike jetted properly so it will take 1/8 to 1/4 throttle. To start with , the bike is a 65 Mark 3 with 40/36 mm valve head complete with Mach 1 cam and a slight porting job to 30mm from 29.5mm stock.It has a 10 to 1 High comp piston as a Mark 3 would have been delivered in 65.Also is complete with megaphone.
I started with the base carb settings of 125 main, M14 needle, 265 on needle jet , 50 on pilot , needle in 2nd position and the only thing I cannot replicate as Mick walkers book states is the slide at 60.( I have a 70 in there) You just cannot purchase them in 60 size anywhere.
The bike will start and idle,but the idle is high. It did get better with rotating the float chamber to the point of just before flooding and seems to work the best for starting there.Now that the idle is taken care of (at least it works but not perfectly) I went after the next step of choosing the correct slide cutout for the tuning area of 1/8 to 1/4 throttle. The downloaded Dellorto tuning instructions state to go after the slide size second ,then pursue needle and main jet for the other two segments of throttle position. I guess my big question at this point is ,should I consider just finding a 6o slide like the Mach 1 /Mark 3 specs generally show, or continue going bigger on slide to 80 /90 or 100 thinking the bit of porting work my have changed things a bit.The tuning instructions state that heavy running with a bit of black smoke indicates the slide cutout is not large enough. I'm going to convert one of my 80 slides to a 90 and see how that goes, any other suggestions from the pros, believe me its getting frustrating ! thanks mark3kb.

[Moto Chuck]

The Ducati workshop manual states that for 1/8 to 1/4 throttle follow this rule: If the engine spits back flames, the cutaway is too high. If the engine loads up the cutaway is too small.

Your carb specs do match the ones in my repair manuals

Checking my workshop manual it lists both ignition systems with fixed advance and automatic advance. Do you know which your bike has. I know that under load the fixed advance will cough and sputter till the engine reaches about 3,500 and then pulls strongly. A fixed advance bike should still rev willingly without any load.

[Bevel bob]

They were often very fussy as regard to silencer and exhaust joints, Leaving the cone end off the silentium would mess it up. I never ran a 250 with a megaphone but its likely that you can forget about factory settings. What passes for petrol today is no such thing. If you can buy a spare slide 70 or80 I would consider it a good investment.You could cut down your old 70 to a 60 ,this also alters the needle position. Your motor is likely to have an auto advance unit. Around 35 degrees should work. You could try a 260 needle jet ,thats what mine uses . A hot spark will make a big difference and help to reduce plug fouling. A New "repro" silentium will not be the same a the orriginal and will restrict the motor. I run a 110 main but could probably go a bit bigger . A new 65 Mach 1 that I used had no carb issues at all back in the day. The best "silencer" I had was a reduced Gold star pattern similar to that used on the Rocket Gold star. You don't usually get a reliable tickover as the Idle speed device on the top of the carb is unreliable and crude in action. The "choke" is not there for starting and has little effect at kicking speed.

[Vintmoto]

Moto Guzzino sells slides for the 29D that do not have the cutaway. This way you can make it whatever number you need. Very easy to do on a belt sander to fine tune the cutaway. I found the #6 cutaway works great on my Mk3.

[motoital]

I understand your fustraton. I couldn't get my M3 running with a factory megaphone to run satisfactory on anything but racing gas. On pump gas there was always a portion of the Rev range that gave me issues. I gave up and now run a Conti style muffler. Good luck!

[mark3kb]

The Ducati workshop manual states that for 1/8 to 1/4 throttle follow this rule: If the engine spits back flames, the cutaway is too high. If the engine loads up the cutaway is too small.

Your carb specs do match the ones in my repair manuals

Checking my workshop manual it lists both ignition systems with fixed advance and automatic advance. Do you know which your bike has. I know that under load the fixed advance will cough and sputter till the engine reaches about 3,500 and then pulls strongly. A fixed advance bike should still rev willingly without any load.

Hello Moto Chuck and thanks so much for the feedback, my bike has the automatic advance 356 unit I believe the fixed unit was usually found on the Early Diana US model 250s around 62/63 .I am going to start the jetting process again now that I have the float level perfect. Was thinking to fatten up the pilot circuit first to a 60 or 65. I have a 50 in there now and to get an idle anywhere near 2500, I have to screw the idle/fuel mixture screw all the way in which indicates richening is the correct direction to go I hope LOL cheers

[mark3kb]

They were often very fussy as regard to silencer and exhaust joints, Leaving the cone end off the silentium would mess it up. I never ran a 250 with a megaphone but its likely that you can forget about factory settings. What passes for petrol today is no such thing. If you can buy a spare slide 70 or80 I would consider it a good investment.You could cut down your old 70 to a 60 ,this also alters the needle position. Your motor is likely to have an auto advance unit. Around 35 degrees should work. You could try a 260 needle jet ,thats what mine uses . A hot spark will make a big difference and help to reduce plug fouling. A New "repro" silentium will not be the same a the orriginal and will restrict the motor. I run a 110 main but could probably go a bit bigger . A new 65 Mach 1 that I used had no carb issues at all back in the day. The best "silencer" I had was a reduced Gold star pattern similar to that used on the Rocket Gold star. You don't usually get a reliable tickover as the Idle speed device on the top of the carb is unreliable and crude in action. The "choke" is not there for starting and has little effect at kicking speed.

Hi Bob, thanks for all the info, only experience will give a person this kind of knowledge you have on this. To complicate things a bit, The head has a slight port job from the previous owner to 30mm even as opposed to 29.5mm, may also affect things. I'm going to try from the beginning with a fatter pilot than the recommended 50. I have 70 80 90 100 slides to try and would be willing to cut down the 70 as last resort. The auto 356 unit is in place.
When I get further along ,I will try dropping to a smaller main cheers

[mark3kb]

Moto Guzzino sells slides for the 29D that do not have the cutaway. This way you can make it whatever number you need. Very easy to do on a belt sander to fine tune the cutaway. I found the #6 cutaway works great on my Mk3.

Hi Vintmoto,just to let everyone know, Moto Guzzino no longer has availability on the slide blanks, I just checked.Fortunately I have a select few I can work with thanks

[Vintmoto]

I just checked with Paul and Moto Guzzino and he has a few uncut slides left. Just contact him through the site. He is out of the oversized uncut slides though, so if your carb is worn, you may have a problem.

[Vintmoto]

I just switched my Mk3 to the megaphone exhaust and I can report that the factory settings for the carb seem to work very well. Very impressed with the difference in performance with the megaphone. Pulls as strong as my 67 Bonneville. Making sure all the exhaust joints are sealed as mentioned earlier is very important. I will add though that I needed to lean the bike out so the plugs looked better. Ended up lowering the needle to the lowest slot and the plugs look pretty good - light tan color to the electrode. I will try leaning out the enricher and see if I can get them closer to white.