i have a Yanmar 2gm20f in my 86 hunter 31 that I bought a couple months ago. I was able to get the engine started relatively easy seen how it had sat for 2 years. The problem I have is that at full throttle the tach only reads 2000 rpm. Idle is between 500 and 1000. I have never heard a engine like this but to me it sounds like the engine could not put out another 1000 rpm leading me to believe the tach is incorrect. At first I was thinking it was a exhaust elbow issue but I removed it and it is in very good shape. Does anyone know of a way to test the tach for accuracy? Thank you
Yanmar 2gm20f tach
- Thread starter Dfisher
- Start date
Did you connect the the sensor that you would usually put around the spark plug wire to anything?
Disregard that last comment. I just ordered one of these - https://www.amazon.com/gp/aw/d/B004Q8L894/ref=ya_aw_od_pi?ie=UTF8&psc=1
I will report back on what I find
I will report back on what I find
The tachs from that vintage are notoriously inaccurate now that they are pretty old. I found mine was 800 RPM low at cruise.. 2000 indicated was 2800 by the phototach. The pick-up is on the flywheel housing just to port of centerline. You could try cleaning the connections and that may help a bit but most of the problem is usually in the instrument itself. You can check the clearance of the pickup-to-ring gear teeth too. I have attached a page from the Yanmar shop manual showing the tach and specifications. Usually, if the tach is working at all, the pickup is OK.. There are tachs available (other than the hyper expensive Yanmar one) that can use the pickup and be calibrated for your engine's ring gear teeth number..
+ 1 on that, we had the same tach and same Yanmar on the Islander and the Island Packet, I never knew what the true RPM was on either boat. Most times the tach would read different when I first started compared to running for a little while. We used to cruise at about 5knts, the engine sounded fine at the speed and used so little fuel it didn't really matter to me if it was a couple hundred off one way or the other.
The others are correct: the 2GM20 tach gets its reading from a sensor on the flywheel housing (it basically counts the teeth as they go by). But somewhere in my cluttered old mind I have a feeling that there are two models of tach, and if you get the wrong one, it displays 2/3 (or 3/2) of the correct reading. Might want to check if you have the right tach, or if there's some kind of switch on the back of the tach.
It's a good idea: a flywheel-based tach should work better than an alternator-based tach. But the devil's in the details...
druid
It's a good idea: a flywheel-based tach should work better than an alternator-based tach. But the devil's in the details...
druid
I took the $12 digital tach out to the boat this weekend and the tach is underbidding by about .75. Now that I know that I can just adjust how I throttle accordingly. The cheap digital tach from Amazon was the way to go. Thank you