Not quite making full RPM?

DesertNomad

DesertNomad

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DesertNomad
I have a Dakota which has an O-540 and is limited to 2400 RPM. I have both an Electronics Intl tach as well as a JPI 830. The JPI usually reads about 20 RPM higher than the EI Tach, so I tend to use the EI one as it is the certified unit. Normally on initial climb from Reno (4415') on a cool morning I get about 2360 on the EI tach. Today however, taking off at about 95-100F and DA of about 8000', I only got 2340. Is this normal on a hot day? I think I was correctly leaned, but have not noticed this before.

I wonder if my prop cable to the lever needs a minor adjustment?
 
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Are you making full static power at sea level? Meaning, if you are firewall forward at sea level does the prop limit to 2400 RPM?

Have a look at the Lycoming O-540 operating manual.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiXgryK_pDrAhWP9Z4KHZKbA4QQFjAAegQIBhAB&url=https://www.lycoming.com/sites/default/files/O%20%26%20IO-540%20Oper%20Manual%2060297-10.pdf&usg=AOvVaw3rpnwYIH3-BXhTzkdIiNDn

See page 3-16 for the start of the ops range section. If your particular engine is O-540J look at section 3-24.

Tell us what manifold pressure you see full forward at this altitude.
 
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DesertNomad said:
I wonder if my prop cable to the lever needs a minor adjustment?
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The max RPM is set by a stop screw on the governor. It limits the range of motion of the lever.

If the prop control knob is bottoming out against the panel, then yes, it's out of whack. There should be about an eighth of an inch of "bounce" there. Same with the throttle and mixture controls. That ensures that the engine end of things is getting full travel.
 
Dan Thomas said:
The max RPM is set by a stop screw on the governor. It limits the range of motion of the lever.

If the prop control knob is bottoming out against the panel, then yes, it's out of whack. There should be about an eighth of an inch of "bounce" there. Same with the throttle and mixture controls. That ensures that the engine end of things is getting full travel.
Click to expand...

Shocking how many A&P's don't put 1/8" cushion or gap between the panel and the control at full forward. That cushion is required and essential as one tool to know proper control operation.
 
I think there is still some movement available at the panel when it is full forward, but will double check. I live at 4415' and have not been to sea level since February so there is no easy way to check that aspect of it.
 
DesertNomad said:
I think there is still some movement available at the panel when it is full forward, but will double check. I live at 4415' and have not been to sea level since February so there is no easy way to check that aspect of it.
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What MP were you seeing with the RPM maxed at 2340 and throttle maxed?
 
Most of us without a digital tach would never see a difference. MP would be key in this situation.
 
NordicDave said:
What MP were you seeing with the RPM maxed at 2340 and throttle maxed?
Click to expand...

I'll have to look at that tomorrow - it certainly did not feel like there was any limitation in power. I'm flying in the morning when it's cool and the DA will be about 7000'.
 
So. Based on an optical test, the JPI is spot on and the EI is reading low by 20 rpm at low rpms and 50 rpm at high rpms.

5,000' MSL, 7,300' DA, 79F, 30.10", initial climb out of Reno (4415').

JPI: MP 24.9, 2380 RPM
EI: MP 24.9, 2330 RPM
 
Looks close to normal. I agree with Doug, that an analog gauge the error could not be easily witnessed.

Were you able to check the prop control cushion? Were you at the firewall or have an 1/8" gap?

Looking at the O-540-J engine operation manual, the engine is supposed to make about 2400 RPM at 24.9" MP for pressure altitude 4400'. The drop in MP from sea level is about right as well. Pretending to be an armchair mechanic, does not seem off based on data provided. I'd probably have the RPM professionally calibrated and determine if the primary RPM gauge is accurate.
 
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I think everything is ok except the EI tach needs to be looked at by EI. I was going to upgrade to the larger LCD anyway so it will go back to them at annual in a. couple months. The control cushion looks good.
 
DesertNomad said:
I think everything is ok except the EI tach needs to be looked at by EI. I was going to upgrade to the larger LCD anyway so it will go back to them at annual in a. couple months. The control cushion looks good.
Click to expand...

I'd be surprised if you hadn't thought of this already. It it were me, consider having an IA measure RPM with a calibrated RPM counter. Easy for him to adjust RPM during annual.
 
NordicDave said:
I'd be surprised if you hadn't thought of this already. It it were me, consider having an IA measure RPM with a calibrated RPM counter. Easy for him to adjust RPM during annual.
Click to expand...

I did that. The real measured prop rpm matches the JPI 830 and thus, the EI is reading low by 20-50 rpm depending on prop speed.
 
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