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Could it be the tach?

B

bluesflyer

Member
Engine recently started showing only 2200 on the tach at full power. Starts fine. Plugs not fouled. Mag drop is between 50-75, both mags. Carb heat shows minor drop and then back up again when closed. SOUNDS like it's making full power, but tach doesn't show it as such. I tried to use a digital tach that uses a laser, but I cannot seem to get a reading. I probably have too much light. Might have to do it at night. Lycoming O-360 180 hp. Approx 450 SMOH. Mechanical tach. I have heard that where the square cable enters the back, the area can become worn and the cable starts to slip. Any thoughts? This is in a 50 year old Beechcraft.
Any help is appreciated.
 
bluesflyer said:
Engine recently started showing only 2200 on the tach at full power. Starts fine. Plugs not fouled. Mag drop is between 50-75, both mags. Carb heat shows minor drop and then back up again when closed. SOUNDS like it's making full power, but tach doesn't show it as such. I tried to use a digital tach that uses a laser, but I cannot seem to get a reading. I probably have too much light. Might have to do it at night. Lycoming O-360 180 hp. Approx 450 SMOH. Mechanical tach. I have heard that where the square cable enters the back, the area can become worn and the cable starts to slip. Any thoughts? This is in a 50 year old Beechcraft.
Any help is appreciated.
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Also, it never showed above 2500-2550 since I bought it in 2024.
 
Find a good optical tach to see what RPM you really have. Then you'll know if you need to troubleshoot or not.
 
bluesflyer said:
Can you provide some more info on an optical tach, please. Sad to say, not sure what it is. Similar to a timing light? Thank you.
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I think that's the laser light device I have. Just need some better light conditions. Right now I've been trying to do that in daylight. Probably not the best condition for that.
 
The least expensive types will likely have a piece of reflective tape you put on one blade and read via laser. There are other types that you can put inside the aircraft and they'll read the RPM of a 2 or 3 blade prop. That type may be more expensive, though.
 
D-Dubya said:
The least expensive types will likely have a piece of reflective tape you put on one blade and read via laser. There are other types that you can put inside the aircraft and they'll read the RPM of a 2 or 3 blade prop. That type may be more expensive, though.
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I had an old optical tach that I used for model RC airplanes. I had someone point it at the prop while I was inside my plane, with the engine running. The optical tach reading matched the tach display of the engine monitor.

My $.02 this afternoon.
Steve
 
Okay, I just re-read your original post, and it's on a 50 year old Beechcraft. Anything we speculate here is somewhat of a moot point, as you're going to need an A&P to make a repair anyway. But your recent post said the hand-held tach correlates with the instrument panel tach reading. So you've got something else going on that's keeping the engine from reaching full RPM. We're back to the A&P...
 
I bought a little $8 tach on amazon for adjusting the governor on my old generator. It has a wire that you wrap around a spark plug wire. Very easy to set up and accurate. Way easier than an optical sensor for your situation. Can run the wire out through the cowl bottom.
 
Larco said:
View attachment 121829Maybe someone has one of these you can borrow?
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My buddy has one of these (TruTach II) and just for grins I borrowed it a couple of weeks ago. I took it with me on a flight and it verified my Dynon SkyView Touch reported RPM at a few different RPM settings. I placed it on the glareshield and it worked well.
 
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D-Dubya said:
Okay, I just re-read your original post, and it's on a 50 year old Beechcraft. Anything we speculate here is somewhat of a moot point, as you're going to need an A&P to make a repair anyway. But your recent post said the hand-held tach correlates with the instrument panel tach reading. So you've got something else going on that's keeping the engine from reaching full RPM. We're back to the A&P...
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I think that was someone else's response. I have not been able to get mine to read yet. I will try again next week. Out of town till then.
 
lr172 said:
I bought a little $8 tach on amazon for adjusting the governor on my old generator. It has a wire that you wrap around a spark plug wire. Very easy to set up and accurate. Way easier than an optical sensor for your situation. Can run the wire out through the cowl bottom.
Click to expand...
Do you have a link to that, or even just the name? Thanks.
 
 
Fixed pitch or constant speed? If your setup is fixed pitch that doesn't sound far from being on the mark. most fixed pitch props get 2200 to 2300 rpm static and then spool up as you go down the runway. if you have a super climb prop you might see 2400 static. If you have a constant speed setup the governor has died or the prop is stuck at high pitch or something possibly leaking in the control system. All this assuming your tach is accurate . Good Luck, Mahlon
 
Fixed pitch. It's just that it seemed like it had a higher static rpm. Our weather is starting to play nice again, so next week I'll get her airborne and see what numbers I get. Thanks for the advice.
 
a super quick way to evaluate this without an optical tach is to just video it with your phone. Go into the camera settings on your phone and see what your frame rate is. If it's 30 fps, thats the same thing as 1800 rpm so a video taken at 1800 rpms should make the prop blades appear more or less stationary. Other frame rate & rpm combos are easy to calculate with a little basic math
 
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