View Full Version : SU Air Filter Mounting

06-11-2011, 11:19 AM
Hey everyone!

Long time lurker, first time poster.

I just landed myself a new (well, new to me) 73 Spitfire 1500 which is in pretty darn good shape, but with a few things that need attention. My goal is to get her solidly road-worthy by early September in time for a leisurely Labor Day trip up the coast to Portland, Oregon.

The kindly previous owner swapped out the ZS carb for a pair of SUs (HS4?) which appears to match the UK setup as far as I can tell. I'm happy with that choice and probably would have done the same.

I've been spending most of my free time going over her making a to-do list, and when I removed the two K&N air filters, I noticed that they were mounted upside down. On my SUs, there are four holes on the intake side - two for the filter mounting bolts, and two others spaced closer together. The air filter mounting plates have corresponding holes, but with the plates upside down, the other two holes are blocked. WIth my somewhat limited knowledge of SU carbs, I'm thinking that those two holes are static air vents to equalize the pressure on the piston. If that's right, it would seem that if they were blocked, it would interfere with the piston's operation.

Is there a reason why someone would want to block them like this?

Thanks! (having lurked here for a while, I'm really impressed with the level of generosity shown by everyone. I know that I'll have more questions and only hope that in time, I will be able to re-pay the community by contributing answers myself.)

06-11-2011, 11:24 AM
Hello and welcome, can't address your specific question, but as you know, on this site it won't be long. Does seem strange that both filters would be flipped inadvertantly. Good Luck

06-11-2011, 03:48 PM

You are correct, those are the vents. beats me as to why they would block them.

What is the commis # of your Spit. I have FM812U

06-11-2011, 04:21 PM

You are correct, those are the vents. beats me as to why they would block them.

What is the commis # of your Spit. I have FM812U

Of course it's easy enough to flip them around and see if runs differently, but I thought it might be some old trick to solve a strange problem.

My Spitfire's commission # is FM4552U, just a bit younger than yours.

06-11-2011, 09:12 PM
It's my understanding the vent holes on the SU's are required for the carbs to operate correctly. My guess is the PO didn't realize what they were and put the air cleaner mounting plates on upside down and covered them by accident not realizing that the holes needed to be open.

06-11-2011, 11:49 PM
Ok, cool.

I flipped them around and noticed that by the dirt/scratch pattern on the carb-side filter covers that it's been backwards for a long time. I also checked the piston action and it's firm and smooth going up, but it drops much faster going down. It's been quite a few years since I had SUs to play with and I don't recall if that's normal or not. On the to-do list is a complete carb tuning exercise, and armed with the Bently's and Haynes Weber books, they should be happy in no time.

Oh, and one more question: I purchased the Bently's book for the Mark IV because it seems to cover the 1973 US 1500s as well as the "normal" Mark IVs. Is there any good reason to also order the book on the 1500s? I also picked up the Haynes, but ugh, it's a bit of a mess.

Don Elliott
06-12-2011, 09:49 AM
When you're pushing the piston up inside the top dome on an SU carb, the viscosity of the damper oil guarantees that the piston will (and should) rise slowly and gradually.
If you remove your finger or screwdrived used to push up the piston, then the piston should almost free-fall. When the botton of the piston hits the venturi throat or bridge at the bottom, you should hear a "click" which tells you that the needle is truly centered as it should be.

All sound fine to me.