CARB MODIFICATIONS

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The following article was submitted by

ALAN OSBORN ; www.aoservices.co.uk

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SU REVISIONS

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Despite the Reliant product being well designed and respected by most of us

when it comes to the SU carb there are several failings

that Reliant overlooked/ignored that are very easily put right.

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HEAT INSULATION ;

PROBLEM ;

To reduce the ‘boiled’ petrol/carb syndrome, where one pops into the paper shop and on returning to the motor find it hard to start and hiccups down the road until the carb gets full of cooler petrol.

The problem being far more prevalent with the Robin-heat under bonnet etc.

SOLUTION ;

Fit a heat insulation spacer { Held on by two longer studs }.

as per Photo A below ;

Click on picture to enlarge

Note that this spacer is the ideal place to fit a properly working take off for a vacuum gauge.

If you are handy you can make this yourself.

Any insulating material (Tufnol) will do with a couple of carb gaskets.

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AIRBOX INLET TO CARB

Again where Reliant ignored the physics of SU carbs is in the inlet from the air box.

Click on picture to enlarge

As found above , Photo B shows a sharp edge where the air box back bolts to the SU.

This edge causes significant turbulence and needs blending to about a 3/8 radius.

Your ‘Black and Decker’ with a metal burr is the tool for the job.

Although cutting the steel while it is bolted to the carb seems odd but it is the easiest way to get a good blend as per Photo C below ;

Click on picture to enlarge

This carb modification assumes the earlier air box.

The same principles apply to the later air box 'leg of mutton' which has to be approached differently but the same principles.

Click on picture to enlarge

Click on picture to enlarge

While the later carb. float top with two pipes might have to be turned around to clear earlier air box.

LATER CARB FLOATBOWL LID WITH THE OVERFLOW PIPE

Click on picture to enlarge

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LIFTING PIN

Another easy mod that again improves the pickup and response to the throttle is the removal of the lifting pin and drilling the resultant hole out to full size (1/4) as per Photo D below ;

Click on picture to enlarge

Some carbs do not have the lifting pin fitted but the boss is still in place

and the above applies as per drilling through .

The following photo shows the undrilled boss ;

Click on picture to enlarge

This modification needs the original feed hole from air box blocked off as well.

Hence is worth doing as part of the above modification.

This hole is there to feed atmospheric pressure to the bottom of the slide and thus to lift it under acceleration .

Feeding it from real outside atmosphere gives much improved throttle response along with less fuss with regard to the dashpot oil.

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While considering SU improvements there is an oft strange fault that takes some finding.

FALSE TICKOVER ;

PROBLEM ;

Engine revs up, then you reduce it by the normal tickover screw, but find that flooring the clutch causes the engine to stall!

No amount of fiddling with mixture or tickover will cure this, as it is carb wear that steadily gets worse.

The fault is that the butterfly is worn at the side where it sits in its shaft and touches the inside of the body.

SOLUTION ;

The cure is a new butterfly disc.

Shaft should be ok but it might be an idea to replace it at the same time. When I have repaired this fault I have never yet found a worn body in this area.

One reason for the wear could be that the throttle closure spring is pulling the butterfly/shaft at an angle hence wear.

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I will be very interested if any one gets to grip with these mods and I am very happy for people to get in touch.

Contact ;

Alan Osborn ; www.aoservices.co.uk

Ps ; The above modified carb is for sale

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