Small Engines (Lawn Mowers, etc.): Higher operating engine speed, throttle linkage, pitch angle

Question
I am writing with regard to the 20-hp B&S Vanguard Twin V engine fitted in my hovercraft.

I live in Malaysia.

In order to achieve maximum lift capability, the fan must spin at a very high speed. On the outset, the engine power rating of 20-hp @ 3,600 rpm and the lift fan size/pitch angle are as per the build plans. I tried two types of fans, the standard 4-bladed fan and the heavy duty 8-bladed fan, both purchased from the reputable hovercraft designer.

With the hovercraft in full trim (fitted with the skirt) resting on a flat ground surface, I managed only 2,700 rpm with the standard 4-bladed fan. (On the 8-bladed fan, I managed only 2,400 rpm). At this speed both butterflies in the carburettor are already fully opened.

I accept the fact that the fan pitch and the pressure in the skirt would generate a physical resistance to the engine, which may shave off, say 500 rpm. Still that would in fact, leave me with 3,100 rpm.

(Idling speed is rather high, say at 1,200 rpm).

At this point, I am wondering whether
1.   adjusting the speed governor could compensate for the loss of 900 rpm (3,600 rpm - 2,700 rpm), OR
2.   whether it would be too much to ask for the additional ‘900 rpm', OR
3.   whether I was wrong in thinking it could be done, in the first place.

Anyway, based on instructions from my friends, I went for the governor crank and the governor lever to make the necessary adjustments, but the ‘slackness' was too little. Rotating the governor crank (tried rotating both ‘in the same direction' or ‘anti direction' as the throttle) with the throttle linkage held in high speed position, made no difference at all to the maximum engine speed.

Can you please confirm whether it is in fact possible to increase the maximum engine speed to compensate for the load imposed to the engine? If so, how do I do it.


Answer
First of all we need to clear up how a governor works.  The spring works to open the throttle to wide-open.  The governor arm is attached to the centrifugal governor assembly inside the engine to regulate RPM.  When you set the governor speed, it controls the maximum no-load RPM.  Under load, the engine naturally tries to drop RPM, the centrifugal governor assembly "retracts" because the RPM is too low for it to act and the governor spring pulls the throttle to wide-open.  The rating of 20hp @ 3600 rpm means that a load equivalent to 20 hp applied to the engine at full throttle limits the engine RPM to 3600rpm.  If a higher load is placed on the engine at WOT, the RPM would drop.  If less load is applied the RPM would increase.

If the throttle is wide open (maximum available engine power) under full load and you cannot achieve sufficient RPM, the problem is a lack of engine power.  This is also demonstrated by the difference between a 4-blade and 8-blade fan.  The 8-blade takes more power to run and the engine RPM drops accordingly.  If the engine is running properly (sufficiently rich mixture, good compression, proper spark timing, etc.) you may need to investigate a more powerful engine or modifications to this engine to produce more power.  Hope this helps.