Dellorto SSI Choke

[JasonB]

I have a Dellorto SSI 27 mm on my 250 N/C and I can't find any info on when the choke is on, or when its is in the off positions.
A mate who has Parilla's reckons when the choke plunger is up the choke is off, hope that makes sense.
Anyone like to share there wealth of knowledge.
Regards
JasonB

[Bevel bob]

Thats correct.

[JasonB]

Thanks for that.I thought it was the case though on or off doesn't seems to alter idle.

Regards

JasonB

[JimF]

I have the SSI29, but I imagine it's the same.

I don't even have my choke connected.

It is weird though - the float bowl has a spring loaded depression post which effectively acts as an enrichener by swamping the carb with fuel. The two ways to aid cold starting are to choke off air or add more fuel, both of which serve to enrichen the fuel-air mixture albeit from directly opposite methods.

Why then does the Dellorto have both methods on the same carb? I have ever only seen one or the other method implemented on other carbs.

[Bevel bob]

Amal Mono's have both.

[DewCatTea-Bob]

By: JasonB...
" I have a Dellorto SSI 27 mm on my 250 N/C and I can't find any info on when the choke is on, or when its is in the off positions.
A mate who has Parilla's reckons when the choke plunger is up the choke is off, "

____ That's of-course correct only for carbs which actually have a 'choke' block/plunger, which tends to block-down air-flow (thus increase the fuel-mix) !
Such carbs as Amal and the 22 & 24mm UBS-Dellorto models have both a REAL 'choke' plus also a float-bowl tickler, (as Jim has made mention of) !
__ However the SSI-27 & 29mm models don't employ such actual choke-setups !
What they do seem to have is a poor attempt at what should be an enrichener-circuit, (like that successfully produced in all later Dell.models), and with such, the cold-starting 'position' is where the tube-plunger/chamber-block is in it's LIFTED-position, cuz when it's lowered (as with a true choke), it then blocks-off the alternate circuit-passage which ought provide a richer fuel-mix (for cold-starting).
__ That anyone has to ask which position (UP or DOWN) is the correct-position for the plunger to be in for supposed aided cold-starting, confirms my long-held conclusion that the SSI's (so-called "choke") setup is of no use to speak of ! ...
Cuz of the well over 100 SSI-model carbs I've ever dealt with, I've never found either position to help any noticeable amount for getting a cold (or warm) engine started !
__ For cold-starting, the 'tickler' (on the float-bowl) is the only thing on SSI-carbs that's of any useful help for getting a cold engine started-up !
____ Good-luck to anyone who tries to get that worthless plunger-setup to help assist with anything at all !
I've heard-of a number of theories as to what it's actually meant to do, such as even help provide a "boost" of power ! - (Yeah, right.)


Dukaddy-DUKEs,
-Bob

[Bevel bob]

My SSI 29D has had the choke assembly removed and the cable entry plugged (a common modification on carbs used on racers) , out of interest I have duct taped the air entry slot to see the effect on cold starting, and as far as i can see the effect is similar to dropping the choke slide on a mono, a partial covering of the slot to enrich the mixture has been usefull in diagnosing mixture problems. I think the reason the choke is pretty inefective as a starting device is because of the poor control of airflow through the carb and the low airspeed with the Hot cams.If the throttle slide is set to close fully down the choke is much more effective, but this means there is no tickover at all, but then this is a RACING carb and thats what its designed for, to get a reliable tickover a road carb is needed. On this RACE carb the choke is really only usefull as an aid to checking if the slide or needle are too weak.I could see the choke being usefull in very cold weather, but who rides a little Duck in those conditions?.

[Jordan]

I agree with the two Bobs. Ducati owners' manuals don't give any guidance on cold starting with SSI's.
The SSI doesn't really have a choke. To help starting, use the tickler to flood the float bowl. The air slide doesn't help starting in my experience.
This carb is Dell'Orto's version of the Amal TT, a racing instrument from before WW2. The TT refers to Tourist Trophy, which is a race on the Isle of Man. The course is a long one, around 40 miles, at changing altitudes as it climbs from sea level to the mountain. The air lever is an aid to keep fuel mixture correct as altitude changes - about 1300ft. I'm not sure how effective this device really was.
It could also help when tuning - adding or reducing air and seeing what the effect is when riding.
For normal road use, I think it may be that whether the lever should be up or down for normal running at a constant altitude depends on how the carb is tuned. I guess the handlebar lever looks neater, when it's in the "closed" position.
Most racers of old bikes on short courses I've seen remove the slide and tune the carb accordingly, as the small amount of extra air can carry a tiny bit more fuel - sort of a slightly bigger carb size.

Jordan

[MotoMike]

In trying to follow the discussion, I was getting lost so pulled up a couple exploded views of the carb in question. It looks like an unduly complicated carburetor from the parts diagram. does the "choke" plunger open or block a passage on the exterior of the carb body that allows air flow from that long rectangular port into the main venturi? Can't find the diagram that shows where the passage enters the air stream. If before the jets it seems that it might give the appearance of a slightly larger venturi size, but if after it seems it would just weaken the mixture when open and enrichen when closed. Apparently to a very minimal degree. It is the first time I really looked at that carburetor. Is the general view that it is a poor design?

I like the theory that it is a in race tuning adjustment for TT type races. even if it isn't true, I want to believe it

[Jordan]

No, the SSI is not a poor design - for racing. It's a proper smoothbore, clearly derived from a successful Amal model.
Amal diverged and developed their racing carbs further though, placing the jet needle outside of the mixing chamber, out of the way of air flow. Dell'Orto might have got into trouble if they'd copied that feature I guess, and there are enough differences to say an SSI isn't a direct copy of a TT. It's clearly and expensive carb to make, with lots of parts (typically Italian?), and suits the sporty image Ducati had established.
For daily road use, an ugly(?) VHB square slide Dell'Orto is much better at starting and idling than an SSI. But who rides them daily now? The racing type carb is more handsome.

Jordan