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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

SU Carb rebuilding

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Old 06-09-23, 02:36 PM
Join Date: Jul 2019
Location: Westward Ho, Devon
Posts: 30
Default SU Carb rebuilding

Probably (hopefully) been covered but I am stripping and cleaning up a set of SU carbs for my 85 engine. It has been a long time since I have worked on SUís and am not familiar with this setup involving the electro mechanical Starter enrichment (?) and on the central carb what appears to be a standard type choke attachment.

The Bristol manual seems a bit vague and I am looking for information or advice from someone who has done this before. I know I could just send them off to be rebuilt but although very grubby they have no wear on the spindles so the plan is to strip, clean and re assemble.

I grew up in Erdington which is where SU were so was able to amble over to the service area and get all the parts and advice available so also appreciate hearing of experience in obtaining gasket sets etc.

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Old 06-09-23, 10:08 PM
Join Date: Jul 2008
Location: Sydney, Australia
Posts: 412

Michael ,
SU carburettors are a simple and effective , if the butterfly shafts and bodies are not worn they probably only need a gasket set and setting/checking the float levels during the tuning process.

The choke device on the front carby bowl is trouble free. Don't touch the adjustment of the needle and make sure the solenoid is operating , it should work perfectly .

There is one problem with that front float bowl, SU and BAC selected the wrong type for the 85A and 85B engines. The one used is LL3258 which is a low level bowl suitable for side draft SU and when you set the fuel level using a 3/8 rod as per the manual the fuel level of #1 carby is way too low(from memory nearly 1/4 inch). To raise the level I suggest you reshape the brass bridging fork making it a bit flatter, this will raise the level close to #2 and #3 carby fuel level .

I recently bought a used high level HL3255 front bowl but have not as yet had it operating on an engine . Reshaping the fork of the LL3258 gets the level very close to #2 and #3 so it should be sufficient to get the engine running correctly.

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