Carburettor adjustment

With three types of carb fitted it’s important to know the differences.

Over the years Stuart Turner fitted three different makes of carburettor - Solex, Amal, and their own Stuart design. Adjustment to the carburettor settings should only be made as a last resort - though they are one of the few things you can tinker with on these engines. Let’s look at the carbs individually.

SOLEX
This was one of the two alternatives fitted to the twin cylinder Stuart Turner engine and was a specially designed for marine use. The idling speed with this carb is 700 to 800 rpm.

Should the correct settings have been completely lost, this carb can be re-tuned to set the tickover as follows:

Gently screw in the volume-control screw as far as it will go. Then undo it one complete turn. Slacken the slow running adjustment screw until it’s clear, then screw it in until it just begins to move the abutment plate and open the butterfly. Put the starting cam ‘on’ and start in the normal way. When the Stuart Turner engine has warmed up, turn the starting cam ‘off’ and make the final adjustments. If the engine runs too slowly, open the butterfly a fraction more. If it’s too fast, the volume control screw may need very slight adjustment, not more than one quarter of a turn at a time. If it’s too far in, the Stuart Turner engine won’t run slowly: if too far out, the spark plugs will oil up.

The stop screw prevents the the butterfly moving beyond ‘full open’ i.e. parallel with the choke tube.

(NOTE: No volume control screw is fitted to early Solex MHD bronze carburettors - those with a central bolt holding the float chamber to the body). If the float valve dribbles, it must be replaced with a new one.

 

AMAL Fitted to late P66 engines - also some older Stuart Turner engines as a retro-fit. The float chamber attachment screws must not be over tightened, otherwise the float chamber will be distorted. Adjustment of the Amal carburettor is complicated and time consuming but, if the Stuart Turner engine is not running well, could offer a big improvements.

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STUART Fitted only on the single cylinder Stuart Turner engine. Apart from cleaning the float chamber and jet, there’s little maintenance required on this carburettor. Ensure that all moving parts are lightly lubricated to allow proper operation. The main cause of damage to this carb is due to incorrect re-assembly after the jet is removed for cleaning. The throttle must be fully opened prior to inserting the jet back into the barrel, otherwise it may come into contact with the by-pass tube and distort it. With the throttle fully open, the jet enters a slot in the barrel and is thus prevented from contacting the by-pass tube.