Starting procedure - 250 Monza

[Nick]

Hi Halfpast

As to which carb to use, it kind of depends on how much money you want to spend and what you happen to come across.

I used the Mikuni simply because I had one laying around. If I'd had a 26mm Amal I would've used that. The Mikuni is more work because you have to fab some sort of manifold arrangement (I used PVC pipe, a short rubber hose and hose clamps - basically the whole set-up cost me nothing). The Amal however bolts right on, though you might need to file the carb holes a bit to make it slip over the studs.

Of the two carbs, the Mikuni is much better. If you do use the Amal though, be very careful not to over-tighten it. Use a new O-ring at the flange and tighten the carb only enough to compress the O-ring. Over-tightening will warp the carb and ruin it. Also if you use an Amal you need to use a thin phenolic spacer to isolate the carb from the cylinder head heat. Look closely at the Amals mounted on BSA 650s and the like and you'll see the spacer.

However, any modern 26~27mm carbie should work fine. If you come across something at a junkyard or swap meet that's about the right size, give it a shot.

It's really a shame how many otherwise excellent Ducati singles got parked and forgotten because of that junky stock carb. The dodgy stock electrics didn't help either.

[halfpast]

Thanks all for the replies.
Greg, please keep me posted as how your install goes - the way things are looking you might get things going before I do.

[cooperplace]

I have the Mikuni on a 250 narrowcase. The jets that Chad recommended were a good starting point but I bought others and made improvements.

With the Mikuni the bike starts easy, idles perfectly, accelerates great.
.

Hi Jim, I'd be grateful to know more about how you did this. I've got a VM26 on my n/c 250, jetted by Chad, and it feels too rich. I don't know where to start with tackling this problem and I'd sure appreciate your advice,
thanks
Peter

[JimF]

I will try to make some time soon to take the Mikuni carb apart and record all the jets and settings I ended up with.

When I bought the 250 it had the Mikuni on it. However I quickly discovered the bike would foul with a soaking wet plug at 300O RPMs. I called the previous owner who was a friend of mine and I told him the bike was fouling out at 3K. He admitted it did the same for him, but he never really rode the bike. It was living room art at his house.

He had long ago sold off the bike's original Dell'Orto and bought the Mikuni as new but 'second-hand' from a fellow that said it was the right one for a Ducati single. The seller was not wrong, it was the right Mikuni for a 250 Ducati, but when I called the fellow at Sudco and told him what jets I had he said that the jetting that was in it was just the stock jetting the carb came fitted with when shipped new to the US from Japan, with no predisposition of what bike brand or displacement the carb would eventually be mounted on.

I bought the Sudco 'recommended' jettings for a 250 Ducati, and then made small adjustments from there.

I recall that I approached each circuit independently and sequentially, beginning with the idle circuit and working through progressive throttle openings until I could kick start the bike easily when cold, get it to idle smoothly, and climb through the different circuits associated with incremental throttle openings until I reached wide open throttle (main jet) and had all the circuits working perfectly. The litmus test of each jet was both the engine performance and the plug color as I progressed.

Jim

[narrow_monza]

I recently installed an Amal 626 (26mm) on my narrow case 250.

Will let you know how it runs.

[halfpast]

Not to bring up an old subject but a friend I and I were talking the other night, and we were trying to figure out what about the stock Dellorto carb makes it so that the bike is more difficult to start when it's warm? We're assuming it has to do with vapor-lock or fuel getting to be too warm before going into the motor...?

[Nick]

When I was riding my 250 on a daily basis with the stock carb I usually just push started it. Tickle carb, put in gear, back it up against compression, shift it back into neutral, then run down the street, and pop it into 2nd gear when your butt hits the seat. Don't do a side-saddle bump start - get both feet on the pegs and pop it into 2nd gear without the clutch just as your you sit on the seat. Push starting is easy because the bike is so light, and it saves you the worry of wrecking the somewhat delicate kickstart mechanism, eliminates sore feet from kick-back and is good exercise! I've used this starting method on both my narrow case ducs, 350 and 250 for many years with no ill effects.

As to why the stock carb does or does not work so wonderfully, the simple answer is because it's 60-year-old Italian technology!

Having said that, my Mk III 250 Diana runs the stock carb and actually starts and runs pretty well with it. It also looks the part, so I leave it on there. Too, it's not my daily rider so its minor issues don't bother me.

Proper timing and a clean spark plug are also important.