Installing Dual Carburetors on Your Spitfire
SUs and Webbers

from the Spitfire List

Mike Perry begins the discussion:

A general observation on the dual HS4 changeover from my old single ZS....this changed the road personality of the car! The handling is much improved, as now the car can push into a turn, as oposed to "floating" in without the power to help get out. Here's what I ended up with. Perhaps someone can tell me if I have committed some cardinal carb sin that will doom my poor little car a few miles down the road:

1 - Hooked a "vent line" on the first carb's fuel bowl. This prevented gas from pouring out of the carb when the engine was shut down.

2 - Synced the carbs to the point that they idle around 650-700 rpms by ear and unisyn did not arrive in time for the weekend. Adjusted the mixture by ear, using the lifting pin/listen for rpm change method.

3 - Could not get rid of a vacuum leak with the crankcase vent tube from the valve cover hooked to the carbs. Gave up. Plugged both ports and ran a line from the valve cover to the charcoal canister. Discovered positive crancase pressure was pushing oil out of the dipstick tube, so I am on my way to the parts store to fit a crankcase breather on top of my charcoal canister before I discover more new and interesting places to push oil through.

Two of the inlet/outlets on my "triple" lid are on the same level (top). The other, on the bottom, does not supply fuel (i.e., if you try to use this as a "pass through" to the other carb, no fuel is delivered. I also have a cap with two ports, each on a different level, which acts in the same manner (the bottom one will take fuel to the bowl, but the top one will not deliver it to the next carb).

My final set-up is as follows:

First carb is a triple top. Fuel in one side of the lower level ports, and out the other, routed to the second carb. The third, and non-fuel delivering port, now has a line attached to it which I routed away from the headers in case it ever decides to spew fuel instead of air.

The second cap is a double top. Fuel is delivered in one side, and the other side is rather crudely capped with a short piece of fuel line, plugged with a bolt and a hose clamp until I get ahold of a single cap (hey, it's a work in progress!!)

I worked my way through four caps to determine that some have a vent and some don't...both my carbs, in turn, exhibited the dumping fuel upon shut down until I got this right, so I was able to isolate the phenomenon.

I think venting one carb actually vents both, as returned pressure from the second carb vents into the first.

For what it's worth, the vent tube has not yet spewed fuel!

Richard Stieg comments:

Mine has two-I have AUD 603 SU-HS-4's. I "think" I remember Ted Schumacher telling me they were off a MGB. Anyway, there is a single fuel connector on the front carb, and two "connectors" on the rear carb. Therefore there is nowhere for excess fuel to go once the engine is shut down, other than dribble out of the bottom of the the new K&N air filter.

Should I try to find two "replacement" float bowl lids with the fuel overflow connections? The Rimmers catalog has a "lid kit" which might work. I"m leery of running the thing very far until the problem is solved.


Tom Strange wrote:

Regarding the following note, I would be interested in seeing the setup numbers you received from various people for their situations. By the way, I did a study a few months ago on the setup requirements for twin DCOE's on Spitfires and received several inputs on the "correct" sets and other tunable pieces for the Spit engine.

Recommended Tuning for Twin Weber DCOE 40 Carbs On a Triumph Spitfire

Choke (main Venturi)    30
Aux. Venturi 4.5
Emulsion Tube F16
Main Jet 115
Air Corrector (jet) 155
Pump 35
Idler Jet 45F8
Bleed Back 0

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