Low engine RPM

Oct 22, 2014
352
Pearson P303 #221 RockPort Maine
I do have an optical reader on order and it should be here later this week,I was just calling it a tach because I was thinking in terms of a electronic tachometer.James
Kirby, forgot one thing until you get the raw water situation clear I would recommend the following steps before starting the engine in your normal way.
1. SHUT-OFF your seacock for raw water intake.
2. Start your engine
3. OPEN the seacock and check for water going out the exhaust.
this will prevent water from backing up into the exhaust system and causing big damage.
I would also recommend a new raw water impeller. It's a blue one that's called a RUN-DRY! it allows you to run up to 15-20 minutes of running with no or little water flow.
I have been using the Globe Blue Run-Dry™ Impellers for years and I have been very happy with them. I usually check them at the end of the season and they look almost new with a little discoloration.
 
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Apr 12, 2019
13
Hunter 31 Demopolis,AL
Many thanks to all of the good help replies,I got my optical rpm reader a couple of days ago and I fired the engine up and slowly ran it up to the top speed and when I used the optical reader it showed 3555 rpm and the tach on the control panel was still showing 2600.I thought the engine sounded about right for crusing at 6 or sometimes 7 knots and the reader verified that I was running close to normal even though I usually run about 2800 or sometimes 3000 rpm.Now I will be looking for the answer on my next trip down by checking for loose connections and check the tach sender at 68 degrees F.I did read the sender with my ohm meter and I got a reading of 1.218 ohms instead of 1.6 ohms but the engine was probably about 100 degrees or so.I will check it cold next time and I really appreciate all of the good ideas that I received and my engine is probably in great condition because I checked just about everything suggested.Again thanks to all, James Kirby
 
May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
If your tach is driven off the alternator, there will be a method to calibrate it. Probably a small hole in the back of the tach - on microprocessor driven buttons and/or switches.
 
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
These GM series Yanmars use a two-terminal magnetic unit that screws into the bell housing and counts the flywheel rotations (actually the ring gear teeth.) This sender outputs 1.0v or better at 20C. The resistance is 1.5 kohms across the terminals.

If the sender measures up - they usually do - the culprit ends up being one or more of the (hyper-mysterious) electronical components on the circuit board located inside the tach unit at the instrument panel.

Wish I could find a person who knew the first thing about how to fix these units. Some folks say these people actually exist. Meantime no need to fear - I saw a replacement available right here in the SBO store a while back at a very fair price.

Charles
 
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Apr 12, 2019
13
Hunter 31 Demopolis,AL
I did check the bell housing mounted sender and I only got 1.2 K ohms,I forgot to put the K on my post.so will reading 300 ohms low affect the tachometer in the control panel?Didn't check the voltage but will tomorrow when I go back to the boat.Thanks very much for the info Charles.
 
May 17, 2010
25
Hunter 1983 - 34' Port of Everett
I had a fellow sailor at home port that had similar problem with a sudden sluggish RPM. Turn out that his exhaust muffler internal baffles had collapsed due to corrosion and was reducing exhaust output. Maybe another area to be investigated that didn't seem to be mentioned in this thread (or I missed it).
 
Apr 12, 2019
13
Hunter 31 Demopolis,AL
I had a fellow sailor at home port that had similar problem with a sudden sluggish RPM. Turn out that his exhaust muffler internal baffles had collapsed due to corrosion and was reducing exhaust output. Maybe another area to be investigated that didn't seem to be mentioned in this thread (or I missed it).
My exhaust muffler is only 3 years old and only been in salt for a few hours.I did buy an electronic tach and checked the rpm and it said 3555 instead of the 2600 the onboard tach was showing and I am in the process of isolating which component is causing the trouble the sender or the tach itself and as luck has it I have the repair manual and it shows step by step instructions for checking the tach and sender.Thank you very much for your reply and your time in reading about my problem.This is the most helpful site that I have ever joined and to anyone I missed thank you also.James
 
May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
If your sensor is the alternator as it is on many diesels, it’s easy to get way off when changing alternators.

Les
 
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Apr 12, 2019
13
Hunter 31 Demopolis,AL
Les my sensor is mounted in the bell housing,I wish it was in the alternator it would be easy to get to.I have to lay on the starter battery and with my hands bent backwards you have to be double jointed to get to it.But I guess you know all about that.Thanks for the heads up.James
 
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Oct 24, 2010
2,405
Hunter 30 Everett, WA
If your sensor is the alternator as it is on many diesels, it’s easy to get way off when changing alternators.

Les
As someone else said the 2GM20F uses a magnetic sensor on the flywheel, not the alternator.

Ken