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Triumph TR3 - Adjusting the Fuel Mixture

I have a simple (dumb) question. I am led to believe that the mixture on my TR3 is too rich currently. I wish to lean the mixture a flat or two, and I am obviously no mechanic or I wouldn't be asking these questions. The manuals tell you how to re-build a carb, how to do appropriate tuning, but not the simple stuff, like which way, when I'm leaning over the fender with my nose above the carbs, do I turn the mixture nuts to make it leaner. Clockwise (while looking down) or counter clockwise?
Dennis Nelson

Counter-clockwise while looking down. I think of it the other way. My explanation is the following.

I imagine that I'm looking up from the bottom. Inside the carb is a needle protruding down from the upper piston in the dashpot. This needle is tapered (smaller OD near the bottom tip) and around it is a brass thingy with a central hole in it that forms an annulus around the needle. When you step on the gas to accelerate, the piston rises in the dashpot and the tapered needle with it. Since the needle is tapered, the annulus gets larger and this lets in more gas (petrol), so you go faster.

To make the carbs leaner at idle, you screw vertically upwards to make the annulus smaller at idle because the hole in the thingy will be higher around the tapered needle. Since threads are right handed, to screw it upwards from the bottom, I screw it like any RH thread ( clockwise) when "imagined" from below.

When I drove to Loveland Colorado, (5000 ft altitude), I screwed the mixture upwards 2 flats when I got there to make it leaner for less Oxygen when I would get to the top of Estes Pass (12090 ft) and spend 10 days around Breckenridge (9800 ft) and more. If I hadn't done this the engine would have been running rich with less air at that altitude. It ran fine and when I got back to Boulder CO, I screwed them down again 2 flats.

Try it as per the book, 1 flat at a time. Before you start and after running at each new setting, check all 4 sparkplugs till you get the grey colour you want.

Don Elliott, 1958 TR3A
Don Elliott

This thread was discussed on 28/03/2004

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