Dellorto SSI Choke

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Rick
Posts: 340
Joined: Thu Nov 11, 2010 1:12 am
Location: Northern Plains, USA

Re: Dellorto SSI Choke

Postby Rick » Thu Aug 18, 2011 9:04 pm

I'd like to use an SSI, and get it to idle- reading through the SSI manual in the tech section of this site I found this:

"Idle Device: in these carburetors, two are the idle different systems:
with Interchangeable jet and air adjusting screw fitted on carburetors where the engine needs a high operation sensibility at low speed;
with variable pilot jet by screw and air fixed, fitted on carburetors for racing engines, where these, according to their use, do not require a meticulous idle."


and this:

"IDLE ADJUSTMENT
In the first idle system,, where is subsisting the replacing jet and the air adjusting screw, you must operate on this air screw in order to obtain a correct carburation, bearing in mind that by tightening it the mixture strength is enriching, while by unscrewing it the mixture strength is weakening.
In the second system, where is subsisting the variable screwed pilot jet and the fixed air, the wished engine running will be obtained by operating only on the screw which adjusts the fuel passage; the mixture strength will weaken by tightening this screw and will enrich by unscrewing it.
This second idle system, although it is not exact and sensitive like the first one (suitable to particular uses and engines as told in the general features) allows a better and quicker mixture strength change; it favours therefore easy corrections of carburation also in the passage runnings and necessary easy enrichments in case of alcohol feed."


Maybe I'm the last to know about this, but there appears to be 2 styles of SSI carbs, possibly the 'SSI-A' and the 'SSI-B', that have much different idle circuits- one designed for racing and another designed to behave better for street use.
I can't find a good exploded diagram to show the different parts involved, but the main castings are probably the same- has anyone ever converted the 'race' style idle or pilot circuit into the 'street' style'?
I'll keep looking for good diagrams.
Rick

Rick
Posts: 340
Joined: Thu Nov 11, 2010 1:12 am
Location: Northern Plains, USA

Re: Dellorto SSI Choke

Postby Rick » Thu Aug 18, 2011 9:19 pm

OK, it's not the SSI-B, it has a larger bore through the body.

Bevel bob
Posts: 1042
Joined: Thu Apr 15, 2010 8:01 am
Location: Bromley Kent UK.

Re: Dellorto SSI Choke

Postby Bevel bob » Fri Aug 19, 2011 6:42 am

Hi Rick, I've read that spiel plenty of times and non the wiser,the description of the first system sounds like the SSi with a changeable fuel jet, however mine behaves like the second system !!, the worst aspect of the SSI is the throttle stop on top of the carb, which is not accessable on a Mach1 or consistent, I have built up the bottom of my slide with hard solder at two points to give a reliable base setting ,I'm also experimenting with another slide with a hole drilled at the back ,this slide sits right down on the bridge and the hole provides airflow at a higher speed (we hope). similar modifications are made by Goldstar owners to the Amal GP.

vintagetour
Posts: 32
Joined: Tue Dec 20, 2011 3:06 am

Re: Dellorto SSI Choke

Postby vintagetour » Wed Dec 21, 2011 1:47 am

Just joined and found this thread. I have a 67 250 Scrambler. Without my choke lever fully pulled I can not get more than 2000 rpms and it will soot and foul the plug within 5 miles or so. It starts, idles and revs well off bottom end but just won't get any rpms. Pull the lever completely and it will get maximum rpms and the plug is grey (good). I have checked float level (good) and have double checked points, timing valves etc. It just seems a little strange that the lever can make such a difference especially since reading this thread :o . Any thoughts from anyone? :o

JimF
Site Admin
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Re: Dellorto SSI Choke

Postby JimF » Wed Dec 21, 2011 3:36 pm

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
.


Someone replied that this is correct and it seems to align with what you are seeing, that is to say when the cable is pulled on and the plunger rises the choke is off making the mixture leaner and more what the hot engine needs as opposed to being too rich when the choke lever is supplying no tension and the plunger drops.

I rely on the float bowl tickler for cold engine starts and as such the choke is not a functional item to my engine starting routine. Perhaps it should be.

I am unsure where I have my lever. Maybe I tuned the carb jets around a choked carb. I am not above making dumb mistakes.

Welcome to the forum.

Jim

vintagetour
Posts: 32
Joined: Tue Dec 20, 2011 3:06 am

Re: Dellorto SSI Choke

Postby vintagetour » Wed Dec 21, 2011 7:38 pm

OK. So it sounds like running with the lever pulled would not be out of the ordinary. My next question would be jetting. As I have said before the bike runs OK and the plug check is good (if you call a 10 mile shake down run sufficient) but I started with a 110 main and a 50 low. It now has a 108 main and the same 50 low with the needle dropped to the 5th notch all at 500' above sea level. Does that sound about right ? I know if it runs good I shouldn't worry about it but dropping 2 sizes on the main makes me suspicious that something else is going on in the carburetor. By the way it requires a tickle to start it when cold like you said Jim.
Tom aka Thomas the Doubter

Bevel bob
Posts: 1042
Joined: Thu Apr 15, 2010 8:01 am
Location: Bromley Kent UK.

Re: Dellorto SSI Choke

Postby Bevel bob » Thu Dec 22, 2011 5:21 pm

The difference between a 110 and a108 would not come into play unless you are at nearly full throttle. The wear on the needle jet makes a big difference and is likely why you need to drop the needle position to where you are at. I'm still wondering why my needle jet (a 260) has no cross drillings , a dodgy pattern part? .I think it makes it run weaker but I'm only guessing as most of my symptons are rich!! .

Vintmoto
Posts: 43
Joined: Sat Apr 11, 2015 2:54 am

Re: Dellorto SSI Choke

Postby Vintmoto » Fri Dec 01, 2017 8:24 pm

My experience with the "choke" lever is that my bike runs rich with it off and just about right when it is pulled. So an enricher as a description is accurate in my opinion. I to run my bike with the needle in the lowest position and seem to get the best-looking plugs.

Bevel bob
Posts: 1042
Joined: Thu Apr 15, 2010 8:01 am
Location: Bromley Kent UK.

Re: Dellorto SSI Choke

Postby Bevel bob » Mon Dec 04, 2017 12:25 pm

On the ssi29D (which is a common carb on the narrow case sport bikes) the "choke" is better described as an enrichment device and has very little effect at kickstart engine speed (unlike normal choke systems). Its there to allow you to richen the mixture during normal running if that was what the motor needed. Real racers used to set up the mixture with the right jets and block off the "choke" cable entry after removing the plunger as it was always likely to close itself at the wrong moment and cost you the race!.I think in use its action was to increase the "pull" on the main jet and so has little or no effect whatsoever at starting when the main jet is not in use.


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