#1
IP: 70.249.170.224
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How to measure RPMs?
Hi - I'm trying to adjust my engine to the appropriate idle speed.
I have a 1974 Ranger 29 with a late model Atomic 4 engine. I bought a tachometer today and added reflective tape to the shaft. I took some baseline measurements and was surprised to find the tachometer registered ~100 rpms. The Moyer manual says the idle stop screw should be set to approximately 800 rpm at idle. I turned the screw twice and got the rpms up to about 140 which is still a far cry from the expected 800. I can keep going in turning the idle stop screw but something seems off to me so I wanted to calibrate before proceeding. While the tachometer may not be accurate, I've eyeballed the revolutions for about 15 seconds and extrapolated to ~100 rpm so I don't think it's entirely off. When the Moyer manual talks about RPMs, are they talking about measuring elsewhere (maybe the flywheel? Is there a 1:1 relationship?) or is it the shaft revolutions that's to be measured? In case it makes a difference, I have a folding prop -- I don't know if the OP did something to the RPM speed (assuming I'm not measuring the wrong thing) to accommodate that. Thanks! |
#2
IP: 100.16.207.20
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tachometer
something wrong with Tach. An engine will not run at 100 rpm.
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#3
IP: 100.36.65.17
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Quote:
Quote:
Quote:
Doesn't make a difference.
__________________
@(^.^)@ Ed 1977 Pearson P-323 "Dolce Vita" with rebuilt Atomic-4 Last edited by edwardc; 07-10-2020 at 08:23 AM. |
#4
IP: 70.249.170.224
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Quote:
That's the exact unit I bought... I guess I'll be taking it back today. Thanks for the suggestion on the electronic tach. |
#5
IP: 32.211.28.40
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By any chance, are you measuring shaft RPM with the reversing gear in neutral? Just the oil drag in the clutch pack could be giving you ~100 RPM with the engine turning many times faster.
I use an old dwell/tachometer that clips to coil (-) for RPM - works well. I also have one of the HF reflective things that I've used for other applications. I've had no problems with it. Both devices have their pitfalls and learning curves. |
#6
IP: 70.249.170.224
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Yes, that's what I'm doing - both the gas lever and drive lever are in neutral. I thought that was what we were measuring when measuring idle speed. It sounds like I should have the lever in forward gear and the gas lever in neutral?
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#7
IP: 100.36.65.17
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Quote:
Yes, but only if you are measuring on the shaft. Note that this will be a "loaded" idle, which may be slower than an unloaded idle, which is usually what people are discussing.
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@(^.^)@ Ed 1977 Pearson P-323 "Dolce Vita" with rebuilt Atomic-4 |
#8
IP: 70.249.170.224
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Thank you very much; I'm now calibrated! I appreciate the patience in explaining these things.
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#9
IP: 24.152.132.140
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Good news.
Selfishly I was glad to hear of your success with the Harbor Freight optical tachometer tool that others gave mixed reviews. It's the same one I've had that has been a proven performer for many years.
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Neil 1977 Catalina 30 San Pedro, California prior boats 1987 Westsail 32, 1970 Catalina 22 Had my hands in a few others |
#10
IP: 100.36.65.17
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Guess you won the quality control lottery!
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@(^.^)@ Ed 1977 Pearson P-323 "Dolce Vita" with rebuilt Atomic-4 |
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