Just curious about the decompression lever

[stevec22206]

I've never had to use the decompression lever on the M-18 but following an earlier question regarding hand cranking a diesel engine I am curious as to the procedure for starting a diesel with low batteries.

Do you simply use the decompression lever to get the diesel spinning over and then release it to engage and attempt to start it? Is there any risk or potential damage to the motor?

Steve

 

[Johnb]

With my Yanmar 3QM30 I have done exactly that to start the engine with low batteries and it worked to get me out of a jam.

1 Be careful you don't flood the exhaust manifold with raw water from cranking an engine that won't start.

2 You can also use it to manually turn the engine for whatever purpose, e.g. checking valve clearances

3 In theory you can hand start the engine by decompressing, spinning it up by hand and then applying compression. I have experimented and found no way does it work - it is like hitting a brick wall. Since then I have been told it is a matter of technique and it can be done, but I have not tried again. Maybe others can comment.

 

[Ralph Johnstone]

Torn Right From the Pages .................

I

Do you simply use the decompression lever to get the diesel spinning over and then release it to engage and attempt to start it? Is there any risk or potential damage to the motor?

Steve

................... of a Yanmar operation manual, this agrees with your impression. See page 28. The factory shop manual says the same thing but is too large to post here.

 

[Chris Patterson]

It may come in handy with a weak battery. Open the levers, get it spinning over with the weak battery, (sometimes a battery will have enough fire in it to turn an engine with no compression), and when it spools up, close the releases. You can get lucky that way sometimes, but hand cranking like that, I don't know. I would hate to try it..

 

[Skipper]

I have the M18. I've never had to do it, but the theory is that if an engine is very cold, or if the batts are weak\cold, you can crank the decompressed engine to get the oil in the surfaces which would make it easier to crank compressed. It may even fire while decompressed if the plugs are hot, and then you release the lever and let it start. Never use when the engine is running or to stop the engine, as it may damage the valves.

With the M18 and I think most if not all Universals, it was never intended for hand cranking, and there is no facility for this IE: crank handle or receptacle, like some smaller Yanmars.

 

[Charlie Jones s/v Tehani]

My old 2QM20 would hand crank exactly like that. No ,glow plugs in the engine. Decompress, spin it up with the throttle advanced, and release the decompression lever-

First time I ever tried it the brand new engine was still in it's crate, and cranked (to my huge surprise) on what fuel was still in the injectors

I immediately decompressed again to STOP it!

 

[stevec22206]

Great info. I hope I never have to use it but it is good to know it can be used just like that to start the engine on weak batteries. Had heard it could be done to start the motor but I never asked anyone who's done it. I have only used it one time to bleed the fuel system at the injectors after having air in the fuel system.

Steve

 

[Apex]

I found the technique this summer, using it many times to perfect for when the need arose. Get the crank spinning as fast as you can, release the compression while continuing to crank and she will fire up.

 

[NoAgenda]

I had to do this last spring when I accidentally left the battery switch set to the starting battery. After 5 hours the battery was dead. At the time the house bank wouldn't turn over the engine. I flipped the decompression lever and the engine turned over and when I released it the engine fired. I redid the wiring to the house bank so I only have to use the starting battery if I draw the house bank to low.

 

[sailorhokie]

How does one do this without an assistant? I can't imagine it will be spinning long enough to start by the time I make it down the companion way and to the lever...

Josh

 

[Skipper]

How does one do this without an assistant? I can't imagine it will be spinning long enough to start by the time I make it down the companion way and to the lever...

Josh

You have to arc the solenoid with a screwdriver or spare wire. If you've never done this, look up on the internet, and study your schematic. However, without an assistant the plugs won't be hot, so you'll have to depend on the engine firing without them.

 

[Stu Jackson]

How does one do this without an assistant? I can't imagine it will be spinning long enough to start by the time I make it down the companion way and to the lever...

Josh

Some skippers have installed remote start buttons/momentary switches in their engine compartments.

Over the years, in reading Catalina's Mainsheet magazine, I have noted that the C30 group was very "heavily into" the decompression lever for use on their M25 series engines. In reading their electrical material, I soon found out why!

The VERY BEST WAY to NEVER have to use that silly thing is to have a reliable electrical system!

 

[stevec22206]

Solo sailing has it's issues and a dead battery doesn't care. Probably why a lot of people will add a bump switch near the motor. I've considered it but just one of those things that is on that list. You all know that one, The 2 do list. I will have to try starting the motor that way this season just to familiarize myself.

Steve

 

[NoAgenda]

I have a remote switch with clamps that attaches to the starter solenoid. I also carry a jump starter box now just in case.

 

[Skipper]

Josh

Some skippers have installed remote start buttons/momentary switches in their engine compartments.

This is a great idea. Better and safer than my method. Could also be used for many other things regarding the motor.