KAD 32 acceleration hesitation.

[malcolm2]

On acceleration under normal load, when the Revs reach around 2200rpm there is a hesitation, and the revs drop back by around 200rpm.
After this, pushing the throttles further fwd the revs pickup again and build to around 3700.

No squeaky belts, no black smoke, or any other sign tell tale signs.
My first thought was fuel filters, or drive belts slipping. Accordingly I changed them all, but the problem still exists.

Slightly surprised that the point of hesitation/revs drop when under no load (ie in neutral) is then at 1700rpm. (not at 2200 when under load) and then from 2200rpm onwards the tacho very quickly moves around to 3000+. In other words the increase from 2200 to 3000+ does not appear to be "linear". e.g. small throttle movement, but big increase in revs.

Any ideas as to cause of hesitation/ 200rpm rev drop. ?

 

[j24jam]

No expert, but.. is this not when the supercharger kicks in/ out?

It comes on at 1500rpm, so perhaps turns off at 2200?

 

[malcolm2]

Yes I assume so.
I suppose my question is. What would cause the gap between the compressor and the turbo taking over.
If that is in fact the problem.

 

[Momac]

Try giving it more throttle to get it going up to 3000rpm or so and then throttle back when the desired speed is achieved.

 

[BruceK]

Dont dally when coming onto the plane. Gently come on to the throttles till you are balls to the wall and then back off once you start planing. I am guessing that you are not far enough into throttle to initiate kickdown which holds the SC's in past the programmed 2200 or 2400 mark which ever it is on the black box of a 32. If you are at full throttle and the SC's are dropping off at preset speeds then look at replacing the kickdown sensor (magnetic hall effect type sensor) that is situated where the throttle bellcrank terminated at max throw. Check with a piece or ferrous iron while the engine is idling. If you place it on the sensor the SC should kick in. If not. That's where I'd start.

Part 42. Observe the feeler gap in the manual

 

[BruceK]

Also note if you are coming too slowly on the throttle and the engines drop the SC then when you reach WoT and kick the sensor off the initial parasitic drag of the SC's will cause a significant and obvious drop in revs and power until their boost effect comes into play in which yes the revs will rapidly climb.

i.e. on 42's the SCis programmed to kick in at ~1800rpm and drop off at ~2400 rpm. If WOT is sensed then the SC's will stay in to about ~2800 to 3000 rpm. Any WoT engaged at rev speeds between 2400 and 3000 rpm will cause the SC to re-engage

 

[malcolm2]

Thank you for your input BruceK.
I have been able to download a VP workshop manual, and now have a better understanding of what controls the SC.

Still puzzled by the point of hesitation/revs drop when under no load (ie in neutral) being at 1700rpm, and at 2200 when under load.

Other engine runs fine so will try swopping sensors over from one engine to the other. Cannot help but wonder if the problem lies with speed sensing, either the sensor itself or the signal it sends to rpm relay.

 

[BruceK]

On my 42's
@ ~ 1700 no load your SC will kick in regardless. Initial parasitic drag will cause a momentary rev drop
@ ~ 2400 rpm under load but no WOT, the SC's will drop out and cause a momentary rev drop if the throttles are not open wide enough.

In the SC controller black box there is a pot you can adjust for when it drops out. Check your manual as 2200 may be set too low. IIRC although the SC on mine will drop out at 2400 (without kickdown engaged) the SC will only re-engage when the revs drop to 2200 or coming off the throttle at 2200. This is to stop it hunting as it comes off boost.

 

[BruceK]

Note when setting anything use a hand held tacho. Your rev counter may not be calibrated correctly.