KAD 42 Compressor problem

I have twin Kad 42's and the starboard has a compressor problem.

revving the engine in neutral, the compressor cuts in at around 1800rpm but the engine revs immediately drop 300 rpm and the compressor 'whine' drops down too. Opening the throttle more does bring the revs up a little, but it 'dithers'. The belts are fine and the 'spinning' plate at the front of the pulley, can be freely turned by hand. On test runs, with throttles wide open, the port climbs swiftly to around 4200rpm, but the starboard climbs slowly and struggles to reach 3700rpm - can anyone help please!

Seems like a strange one. At 3700 the compressor should be well off and the turbo is providing the boost. If that engine is not revving then you may have a boost problem like a leaky hose or manifold gasket etc. My first test would be a boost gauge on the engine and run a series of tests comparing boost between engines from 1800 to WOT. At a guess you are losing boost pressure somewhere.

Thanks Aliboy

I'm sure the compressor is cutting off where it should, it's the fact that the engine revs drop along with the compressor spin speed when I open the throttle to 1800rpm, that's got me puzzled. I could understand it if the compressor couldn't be turned by hand and had seized somewhere, but as it spins freely, what would make the engine revs drop 300rpm as soon as it kicks in? I read a similar thread that suggested that the boost compensation pod on top of the pump could be the problem by not allowing more fuel to be added as boost increases - so I'm wondering whether it's fuel flow?

One possible theory - if you have a boost leak somewhere, when the compressor cuts in it will load the engine and drop the rpm's, but normally the boost will compensate for this. If you are leaking off the boost you get the extra 'drag' of the compressor without any compensating boost, so lower rpms. Just a possible theory, but for $25 you can get a boost gauge at an auto store. Add in 10 minutes of your time plus a $5 fitting and you can see what is happening with your boost. Compare the engines and you should know a lot more pretty quickly. The fuel thing could well be the issue, but that is much harder to check than your boost.

Hi aliboy
That makes perfect sense, yes! it would definitely explain why the revs drop, I'll get myself a boost guage and also try covering the pipes and joints in soap suds to see if it blows bubbles

Thanks for your help Aliboy-I'll let you know how I get on

Cheers, Brian