new guy

[Magman]

I received my main and choke jets from Paul at Guzzino.com. The parts are very high quality. The undrilled mains are counterbored so all I need to do is is drill the jet size. I am wondering if I need to ream the hole or if drilling is adequate? Perhaps I could polish the hole? thoughts? Anyway, I have 4 blank main jets to play with and would sure appreciate any advise on what size drill to start with and what procedure to use to dial in the carb for this bike. For now I am going to block the high speed air circuit in the closed position. The main jet that came out was stamped 120mm but one nice fellow stated that was probably not a factory size. Anyway,any help would be greatly appreciated. Maybe I will get the bike running again this weekend

thanks
Magman

[Magman]

I installed the new jets for both the SSI and VBH from Guzzino onto my 69 Desmo 350. Both carbs showed no improvement in solving the problem with the bike constantly jerking running under low engine loading while cruising down the road. I found a new round slide Mikuni 28 mm from Alibaba.com for only $65 and decided to purchase to see if there would be any improvement. I installed the Mikuni last night with a piece of flexible vacuum cleaner rubber hose and 2 Adel clamps. I had to drill the throttle cable ferrul holder on the top of the carb slightly larger and file the cable barrel slightly smaller. I hooked up the gas and the bike started after about a dozen kicks. It runs great down the road now but only one short ride up to 50 mph for 5 miles to work this morning.. right out of the box with no adjustments. Also, the inside and outside diameters of the Mikuni are very close to the inside and outside diameters of the stock intake manifold....
Now I can ride
Magman

[Nick]

Good work on getting your bike running well and using the Mikuni. One can only wonder how many perfectly good Ducati went to the junkyard due to that goofy stock carburetor (probably about as many as due to that goofy electrical system). Such a shame, because the engine is otherwise a work of art.

Regarding the manifold, I used PVC (plastic water pipe). If you have access to a lathe, turn down the OD to match the OD of the Mikuni. Then bore the ID to 28mm. Then bore part of the ID out to allow the pipe to slide snugly over the stock Ducati manifold. Mount using automotive radiator hose and hose clamps. The whole setup costs less than $10.00.

Anyone can make a bike run well by throwing money at it — the art lies in putting together a well-running machine without spending much money on it.

[Daniil]

Steve, what size of the jet is that? Is there a number? Dan

[Magman]

I never took the new Mikuni apart Dan if you are asking about the Mikuni main jet. If you are talking about the Delorto SSI 29D I am using a 120mm but the carb is missing the choke parts and surges under low load cruise conditions. I have a Delorto VBH 29mm also that I put in a choke jet and have the same surging problem. let me know what specifically you need.

The temp this morning was 48F and the Mikuni fired on the first kick with a short squirt of gas from the throttle and full choke. The bike is running great except there is a flat spot around 2000-4000 rpm (guessing) but if I twist the throttle open it lags but does accelarate. The Mikuni does not accelerate like the SSI but I guess it just needs some tweeking. I am going to run this Mikuni for awhile and put some hours on the bike before winter.

This Mikuni likes to run best with full choke? Does that mean I need to richen up the jets?

thanks
Steve

[MK III]

" If I twist the throttle open it lags but does not accelerate"

This is a common trait of four stroke singles. Modern singles have somewhat solved - well alleviated - the problem with pumper carbs which squirt a small stream of fuel into the airstream when cracking the throttle wide open. So, to somewhat alleviate that initial lean condition you can try running a richer pilot jet, then try raising the needle a notch (don't do both at once). You'll probably need to go considerably richer on the pilot, not just one or two numbers

[Nick]

No way should it run with full choke. The jetting is clearly too lean.

Likewise, if your bike starts up well when cold without a choke, it's too rich.

You might try contacting Sudco, the Mikuni distributor, and ask for some recommended base settings for your bike. They probably don't have specific settings for the Ducati, but jetting for any 250 should be a good starting point.

[MK III]

For a ballpark idea on a main jet to start with, I pulled the 30 Mikuni out which I had on the bike pictured and it's a #220. Someone (?) removed the pilot so I can't give you that number. That particular engine & setup was probably the smoothest, most reliable and easy starting single of any that I've built. Engine here is actuallly a 350 with a green & white "scrambler" cam (lots of overlap) big valves and an 11:1piston. Also, the bike and I lived at 8300ft elevation so at sea level the main would have been more like a 240 or 250. Your results may vary!

[Nick]

That's a mighty cool dirt bike you built there.

[Magman]

Ok, I pulled the plug and checked the plug gap and it was too tight. The plug was running rich. I opened the gap to .026". This morning it was 48F and I started the bike with no choke with 2 kicks and it ran great for the 5 miles to work. Yesterday I rode the bike for about 1/2 hour but not above 45 mph. I pulled the plug and it is a rust color and dry. I think I need to run the bike on the highway for about an hour and see what the plug looks like. I need to put a new tire on the back first and I need to add fluid to the front end. I appreciate all the help. Many thanks!

What modern spark plug should I be running? re:1968 350D for street use. The plug in the bike is an NGK so not original.

thanks
Steve