I have replaced the tachometer on my 250 with three different units. Each of them read pretty close at the start of a flight. However, after a few hours, they all started drooping in RPM. The other day, after a two hour leg, the tach would only indicate about 2500 on takeoff. I have checked and calibrated these with other devices, and cross referenced the performance of the aircraft in flight, and so forth, and the tachs are reading low, some worse than others, and all are worse after the aircraft gets hot.

These are the mechanical style tachs, btw.

Given that all three have performed this way, is there any possibility that something else could be causing this? How could anything in the engine be causing the tachometer to read low?

William,

While the controls on my Twin are different, I have seen rpm variance in the past when cables got worn and control quadrant was dirty. The other area to check might be any attachment in the engine that may be worn and subject to slipping. Other with more experience here may chime in.

The other thing you might try is having a prop shop check the prop rpms on the ground and aloft. If we were closer, I'd loan you my gauge. This would tell you conclusively whether it is a gauge or other problem.

William,
I am having a similar problem with mine although I have a twin. I replaced my right tac 100 hrs ago and are still using the old tac on the left engine. The new tac registers 2700 on take off but the older left tac registers about 2550. I know the engines are running at the same RPM because there is no out of sync sound coming from the props. Both engines are generating the same power as well. I just figured it's the old tac that might not be working as good as it should. If there are any ideas out there that might help me diagnose this, I am all ears. I am thinking that I might have to change out the tac for a new one.

Jim

I am getting rather frustrated. I have gone flying several times with various digital instruments (audio, light flicker, strobe) and they all agree that the mechanical tach is wrong. Then it will appear to clear up for a while. Then go wrong again.

I'll fly for a two hour leg with the engine making a constant note the entire time and watch the tach needle read 50 rpm low for half an hour, and then take another half hour to drift down to 120 low. And then come back up to dead accurate over the next hour.

When cold the last two mechanical tachs are right on the money. After they are warm and operating for a few hours, they read low on the next takeoff once the aircraft is hot.

Drives me nuts. I am at a loss to understand why three different mechanical tachs all read low after a few hours, but exact during a cold start and for about half an hour after. Is there anything at the engine attachment that could be slipping?

I am just letting it rot for a while - no more money thrown at this. But an instrument that important ought to be right, or at least consistently wrong...

Went for a flight with.a digital tach and payed careful attention. The mechanical tach is within 30 rpm. No droop at all or change in rpm during a 1.7 hour leg. However, rpm would only go to 2450 rpm on the subsequent takeoff!

I suspect the vernier control on the prop control. I've had grief with this before. Does it make sense that it would be able to be set to max when cold, but only 2500 when hot? It does have to be turned a bit to get set to max, and if you turn it too much it pops back a bit. I can control the rpm in flight no problem.

The prop and governor are relatively new (three blade mcauley) and the prop control is original. Sounds like it is time to replace it. Certainly cheaper than messing with the governor and if it doesn't fix it at least that is eliminated?

Perhaps next flight I'll see if I can set a full fine rpm on the stop in flight after it is good and warm in the cabin, and see if I can set 2575?

The problem appears to be solved. Per advice here and elsewhere, I had the attachment on the prop governor cable wound back 3/16" so that the stop on the vernier control in the cockpit was back from the panel. Then went and did a bunch of flying.

When cold the system works perfectly. When hot, however, adjustment of the prop governor is erratic. An adjustment of the vernier control may sometimes have no effect on the rpm, and then it will suddenly start having an effect. Rather like there is a dead band between increasing and decreasing rpm of about 1-1/2 turns. This is all still present, but there also appears to be slack in the cable, or as if the coil spring that houses the cable is worn and it needs to bunch up a bit for it moves.

Anyways, now that I have more movement available the prop control can be set so that max RPM is achieved on takeoff even when hot. And in flight, once the rpm settles down, it is rock steady. Only when I adjust it does it sometimes seem to stick a bit, and then relax to the new rpm setting after a few seconds.

The tach, however, is working fine. The problem is the governor cable and vernier control. New replacements are on the way.

Thanks for posting your progress and glad to hear you are on your way to getting it completely resolved.