Hints and Tips - Setting Up SUs

On Mon, 20 Apr 1998, David Laver wrote:

An alternate to a rolling road is a straight section of level, rural road and a stop watch.

Alas in London its a choice of potholed, traffic clogged, heavily policed roads.

I'm sure I'll find a spot on an industrial estate. I'll also look for an uphill section to reduce the scatter in times. It will give a longer period of time to accelerate without over reving. Same also applies for testing in the highest gear compatable with the test track and performance of the car which - being an Austin 7 - isn't so much of a problem. Another advantage of an Austin 7 for such tests is that speed reducing width restrictions are no problem.


Date: Mon, 20 Apr 1998
From: Dr G W Owen
To: David Laver
Subject: Re: Setting an SU

Don't bother messing about with stop watch road tests for setting up an SU unless you can think of a way of overcoming the following:

1. Is the piston lifting at full revs - solution may include strapping a friend to the running board. This is how you determine if you have the right spring.

2. Is the mixture correct throught the rev range with the correct spring fitted? You could attach a gas analyser and then ask your friend on the running board to notice where it is lean, then stop and do some filing of the needle.

Charlotte's brother has just taken his Chummy to Sigma Engineering in Gillingham Dorset. He ended up with 32bhp at the flywheel. Standard engine with decent exhaust, Morris 100 carb, skimmed head and slghtly flattened followers. The man there will know what spring and needle you need. He might even tell you what they are if you use his rollers on the way to Wiscombe!!!!!


Date: Tue, 21 Apr 1998
To: british-cars-pre-war
From: Derek Dawkins
Subject: Re SU setup

IMHO single or twin SU's are dead easy to set up when they are in good condition, and totaly impossible to optimise when they are worn. So if "someting is sticking" (needle not centered, dirty or worn piston, bell chamber) or "you need to blip the throttle" ( worn linkages or butterfly bushes) fix these things first, then tune as per the book.

Of course a chassis dyno is a great idea, money well spent etc, but I don't believe it is necessary for an engine in standard tune. (And I can't afford it :-)

PS I love colour tune plugs, I use one on my Morris and Jag with good results.

regards Derek.

From: John Hardy
Subject: the last word in SU tuning
Date: Tue, 21 Apr 1998

Dear all,

Following the recent thread on tuning SU carbs I feel that I should share the following

Run down a deserted road with the bonnet open. Be sure to secure it with a suitable length of string. Get a friend to ride on the running board bent double with a torch to check the opening of the carb pistons. As he'll be using both hands on the carb and the torch clenched between his teeth tie him on with the bonnet safety string. A loop around the neck should be good. Try several runs timing each whilst driving. With practice it should be possible to hold the stop watch and steer with the knees. Alternatively get another friend to drive thus leaving both hands free.

Be sure to have a selection of needles to hand to try alternatives. Needle profiles can be altered quite easily by spinning it in a drill chuck and removing metal by means of a file or against a piece of sandpaper of, say, 80 grade. Alternatively the brazing torch might be usefully employed to add metal should the need arise or if brazing facilities are not available, body filler would suffice.

If the needle does not seat properly an adjustment may be made by tapping it home with a drift and hammer. A suitable drift may be made by grinding the point off a 4" nail. It is important to remove the point otherwise damage may occur.

If the tickover is irregular, air may be leaking through a worn spindle housing. An effective repair might be brought about by judicious shimming with cigarette packet card wrapped around the spindle. More horsepower can be gained by grinding a radius on the carb intake. Improvements are better noted with the engine running but not in gear as the drill will snatch when the power cord runs out.

In this, as in all things, be sure to wear goggles and ear defenders. Safety first. And beware advice from people who appear in the press driving DIY euthanasia devices. Did anyone else see the insane smile???

Blessings be upon you and yours


Editor's Note: for the hard of understanding the last contribution is not to be taken seriously!

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