Jet Doctor install on Dellorto DRLA 40

[JezWest]

I have just installed CB's Jet Doctor kit onto my Dells in order to rid myself of plugged idles. I wanted to describe what changes I made post-install and to see what others might have done. Incidentally, I didn't use the lead balls in the kit to block the idle circuit air intake holes - I used my brother's idea of tapping the hole and using a hex socketed grub screw.

Here's my rig: 1776cc, Fumio heads, Engle 120, dual Dellorto DRLA40, 1 5/8 header.
Carbs: 34 vents, 50 idle, 150 main, 200 air, 33 pump jet, 9164.1 emulsion, 1 3/4 turns on the mix screws

I arrived at this setup a couple of years ago after a few days with my brother, an LM-1 and the A130. It's been peachy apart from the idle circuit getting plugged every so often.

Once the jet doctor was installed, I changed none of the jets or settings and went for a drive. Idled about 2000rpm, heads were at about 290, so 40 degrees hotter, transition about 2500rpm was terrible - really fell on it's nose. When I finally got above 3000rpm, ran just fine.

Now I have synchronised the carbs properly, it idles just under 1000rpm again. I adjusted the mixture screws following the 'carbs 103' on aircooled.net and ended up with 2 1/2 turns. Driving it tonight it feels much better - there's still a little stumble around that 2500rpm transition. The head temperature is way down - around 200, which is cooler than it's ever been. Haven't really explored the top end - it's dark and wet here just now!

So i've ended up with an extra 3/4 turn out on the mix screws. What did you do..?

A question then - the install kit says that due to the increased air flow, that I might want to run a smaller air correction jet. This doesn't make sense to me. The jet doctor is on the idle circuit - so if it is shipping more air, then don't I want to run a slightly larger idle jet..? There isn't a way to adjust the airflow through the jet doctor - I can in the main jet stack by way of the air correction jet.

So I wondered about trying a slightly larger set of idle jets to see if that would smooth out the remaining stumble.

If only I had my own LM-1, a big box of jets and more time..!

[ Sorry for rambling on - I just like to know how things work! ]

[JezWest]

Well I made progress. I bought a set of jet gauges and jet reamers and opened up my idle jets to a bit less than 55, maybe 53 (so the 55 pin fitted in the jet by maybe 1mm).

Driving with the 53 idle jets seems to have made the difference - motor seems much more happy at idle speeds and the stumble has all but gone. I might try going to 55 to see if all trace of the stumble can be removed.

Incidentally, the gauges and reamers are not so easy to find in the UK - here's where I got mine:

(http://www.lambrettaspares.com/spares/carburation/carb-jet-gauges,-set-of-20,-0.45mm-*-1.5mm/sa05779.html)

(http://www.ebay.co.uk/itm/Stahlwille-11085-Carburettor-Jet-Reamers-0-33-2-0mm-/310268586291?pt=UK_Hand_Tools_Equipment&hash=item483d72e933#ht_500wt_949)

[ Hopefully replying to my own post doesn't mean i'm going nuts ]

[Bruce]

Carbs: 34 vents, 50 idle, 150 main, 200 air, 33 pump jet, 9164.1 emulsion,

You are using the wrong tubes.  Look for some 9164.2 tubes.

[JezWest]

So I have added a little extra accelerator pump now and that stumble has completely gone.

Why are 9164.1 tubes wrong..?