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When converting from Strombergs to HS6 SUs, you might want to know of a pitfall I came across. HS6 SUs were already installed on my car by a previous owner, but because of various poor running conditions, a fuel leak and some sloppy throttle shafts, I decided to rebuild them.
My HS6s originally came from a TR4A. The former owner, who performed the installation, chose to stick with the Zenith-Stromberg fuel blocks (plastic insulator between the intake manifold and the carb). Bad choice!
Here's the problem. The intake manifold, the fuel blocks and the carbs have a roughly square shape with a 4 bolt pattern and a 1-3/4" bore right down the center. But in addition, the Stromberg fuel blocks have a machined slot breaking out from the main bore, which I think is meant for an enrichment circuit to dump extra fuel when the choke is applied.
Also, the SUs flanges are not really square, in that the top and bottom sides are flat, but the left and right sides are cut inward in a concave shape.
When fully mounted, the slot in the Stromberg fuel block 'can be' uncovered by the concave edge on the right side of the carburetor, creating a nice vacuum leak. I say, "can be", because there's just enough slop in the bolt patterns to either have no vacuum leak or a pretty healthy vacuum leak depending where the various parts are when they're tightened.
I ended up rotating the gaskets between the fuel blocks and the carbs so there was some extra gasket material covering the leak area, but the right way to fix this is to use the SU fuel blocks, not the Stromberg ones. If you stick with the Stromberg fuel blocks, pay special attention to this possible leak path.
I've read in forums around the net about folks failing to succeed with this conversion, some of who have complained about high idle speed and other symptoms of a vacuum leak. In some cases this may be the cause.
Gregg
'74 TVR2500M
My HS6s originally came from a TR4A. The former owner, who performed the installation, chose to stick with the Zenith-Stromberg fuel blocks (plastic insulator between the intake manifold and the carb). Bad choice!
Here's the problem. The intake manifold, the fuel blocks and the carbs have a roughly square shape with a 4 bolt pattern and a 1-3/4" bore right down the center. But in addition, the Stromberg fuel blocks have a machined slot breaking out from the main bore, which I think is meant for an enrichment circuit to dump extra fuel when the choke is applied.
Also, the SUs flanges are not really square, in that the top and bottom sides are flat, but the left and right sides are cut inward in a concave shape.
When fully mounted, the slot in the Stromberg fuel block 'can be' uncovered by the concave edge on the right side of the carburetor, creating a nice vacuum leak. I say, "can be", because there's just enough slop in the bolt patterns to either have no vacuum leak or a pretty healthy vacuum leak depending where the various parts are when they're tightened.
I ended up rotating the gaskets between the fuel blocks and the carbs so there was some extra gasket material covering the leak area, but the right way to fix this is to use the SU fuel blocks, not the Stromberg ones. If you stick with the Stromberg fuel blocks, pay special attention to this possible leak path.
I've read in forums around the net about folks failing to succeed with this conversion, some of who have complained about high idle speed and other symptoms of a vacuum leak. In some cases this may be the cause.
Gregg
'74 TVR2500M
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