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A few more adjustments
by Anders Dinsen ... October 07, 2008, 01:02:15 pm
I have just finished another adjustment session... there'll be a few of those coming up Wink

This time, focus was on the idle mixture. I raised the idle speed using the throttle stop to about 1000 rpm. I checked whether this had uncovered the first progression hole, but that was not the case, so I'm happy with the idle stop for now, and there is no need to enlarge the holes in the throttle plates.

Idle screws ended at 1 1/4 turns out, which is fine and indicates that the idle jets are correctly sized. Next step was to check whether progression works, and it does: There was no spitting back or hesitation when slowly increasing the throttle. This is another indication that the idle jet is correct.

I also checked the combustion with my Gunson Colortune plug, and while I've heard much good about this from others (particularly Bagheera owners), it didn't help me. It was consistently showing a nice blue flame in the combustion chamber, even when the engine was close to stalling! I need an exhaust gas tester if I want to improve it.

Listening to the balance, I noticed that number three inlet (counting from the right) had a strange sound, so I decided to check valve clearance on that cylinder, but it was fine. Now that I think about it again (that's another good thing about blogging - you get a free review your thoughts), I think it may just be aux venturi not fixed properly in the carburettor body. This must be the one that had a strange retaining spring? A long throated air horn will keep it in place, but I think I'll take an extra look. Fortunately, these carburettors are so easy to service: I only need to take the air filter off, loosen two bolts on a retaining bracket, and the aux and main venturis can be pulled straight out.

Another possibility is that the manifold gasket is not sealing on that cylinder, so I tightened the nuts a bit more. The top nuts on the manifold studs are the one being subject to the most strain, and I expect that they will need regular tightening to keep the paper gasket from breaking and getting sucked in, as it did on my old manifold.

I haven't been on the road today, but the fact that progression is good and idle is too suggests that the emulsion tubes need to be changed for richer ones to fix the flat spot I complained about last time: F2 or F16. F2's are the "richest". I have asked the supplier for advice, and Roy has already suggested F16's, so if I can return the ones I don't need for a refund, I may end up ordering both and trying which works best. Tuning main jets and air correctors is probably best done on the rolling road.

In case you wonder which resources I have read during the course of this project, below is a photo showing my collection:

From left to right:

Dave Andrews: How to choose and tune Weber DCOE carburettors (internet resource, printed)
Des Hammil: How to build and power tune Weber and Dellorto DCOE, DCO/SP and DHLA Carburettors
Roy Gillards article in the september 2007 Matra magazine about jettings for different cams/carburettors on the 2.2
Dave Walker: Engine Management
Webcon: Weber Tuning Manual, and last, but certainly not least:
John Passini: Weber Carburettors, part 2. Tuning and Maintenance

(The hammer does not represent any kind of knowledge!)
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