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1GM10 yanmar diesel

Feb 29, 2020
2
O'Day O'Day 26 Baltimore
I am looking for some advice for an 1986 O'Day 26 with a Yanmar 1GM10 (C) which I bought last Fall. The engine (2008) ran away ( not over 3500 rpm) and we think the block may well have a crack. After careful inspection wear limits etc. , we rebuilt the engine and it still ran away. 1. Is it possible that the breather is causing this? It seems a very simple mechanism. 2. If not, are new blocks or remanufactured Yanmar 1GM 10 engines available? 3. Has anyone repowered an O'Day 26 with a new diesel? I am 76 now and have dreamed all my life of having a sailboat like this. I have spent the last year fixing the trailer and boat and do really appreciate your advice. I am naming the boat Solar Harvest.
 
Nov 22, 2011
844
Ericson 26-2 San Pedro, CA
I am looking for some advice for an 1986 O'Day 26 with a Yanmar 1GM10 (C) which I bought last Fall. The engine (2008) ran away ( not over 3500 rpm) and we think the block may well have a crack. After careful inspection wear limits etc. , we rebuilt the engine and it still ran away. 1. Is it possible that the breather is causing this? It seems a very simple mechanism. 2. If not, are new blocks or remanufactured Yanmar 1GM 10 engines available? 3. Has anyone repowered an O'Day 26 with a new diesel? I am 76 now and have dreamed all my life of having a sailboat like this. I have spent the last year fixing the trailer and boat and do really appreciate your advice. I am naming the boat Solar Harvest.
Not sure I have advice on the cause of the runaway, but if you did not run it over 3500 rpm that is certainly within spec for that engine and ought not to have damaged it.

I am wondering if you have an issue with the governor or the fuel rack or something of that sort? I will say that the 1GM10 is a very simple and reliable engine.
 
Last edited:
Nov 6, 2006
8,812
Hunter 34 Mandeville Louisiana
I agree with Alan, Carefully check the linkage, fuel rack and governor..
 
Nov 22, 2011
844
Ericson 26-2 San Pedro, CA
I agree with Alan, Carefully check the linkage, fuel rack and governor..
If the issue does or might relate to the governor, I think I'd suggest calling in a knowledgeable mechanic, preferably someone with specific experience on engines such as yours. While the mechanical governor in the 1GM is not exactly insanely complicated, I would say that it's not the simplest system on that engine, either.

I wouldn't be too quick to give up on your little Yanmar. As I said, it's a good engine. I think a good mechanic should be able to get to the bottom of this runaway issue, and once he does I'll bet you'll be in fine shape.
 
Last edited:
Nov 22, 2011
844
Ericson 26-2 San Pedro, CA
By the way: here is a quick overview of how the governor works on a 1GM10, as best I understand it. This may be more detail than you want, but I offer it for your interest. Do with it what you will. :)

The injection fuel pump has a "rack," which is like a slider/throttle that allows more or less flow of fuel depending on how far it is slid in the open direction. The position of this fuel rack depends on opposing spring pressures. On the one hand, there is a throttle spring that wants to open the fuel rack, while the governor spring opposes that and wants to close the rack.

The governor, for its part, provides the spring pressure that closes the rack as follows. When the engine starts and spins up, two weights fly outward through centrifugal force. These weights in turn are linked to a cone-like device that shoves forward on a spline, which in turn provides the spring pressure to close the rack.

When the engine is off and the throttle is in neutral, the rack is actually all the way opened and therefore prepared to output maximum fuel. However, in neutral there is also very little throttle spring pressure holding that rack opened. Of course, very little spring pressure is needed to do so because there is no pressure countering it. When the engine starts and begins its rotation, the governor kicks in and opposes the small amount of throttle spring pressure holding the rack open with a relatively larger amount of spring pressure from the governor via the engine rotation, with the net result that the rack slides in a direction to reduce the amount of fuel and lessening the engine speed.

When the throttle lever is slid forward, this tightens the throttle spring which opens the rack, giving the injector more fuel. But as the injector gets more fuel, the engine runs faster, which throws the governor weights out more because of the increased centrifugal force, which adds to the spring pressure on the part of the governor that slides the fuel rack in the direction of less fuel. It is the balancing act or stasis between the spring pressure increasing the fuel (via the throttle spring) vs. the spring pressure decreasing the amount of fuel (via the governor spring) that maintains a particular rpm.

The injector control shaft adjustment puts a hard limit on the travel of the rack, such that it can only open so far and thus only deliver so much fuel. Screwing out the injector control shaft increases the range of travel on the rack, thus allowing more fuel on the top end, while screwing it in restricts the range of travel, limiting the maximum amount of fuel the injector pump can provide.

I think from this that you could imagine that an issue with one of the springs might result in a runaway condition. Anyway, just more grist for your mill.
 
Jul 30, 2019
173
Seaward 25 777 Fort St. James
If it stops increasing its revs at some point (3500 rpm you seem to say), then it is not a runaway as I understand them. In a true runaway the crescendo continues to the point of destruction unless completely starved of oxygen. Much more likely to be a linkage or spring problem. Get a diesel mechanic to look at it, where it is, in the boat.