Is this Vapor Lock on my 1983 25HP Mercury?

[mcdgary]

After running my 25 HP Merc about 2-3 miles, shutting it off and attempting to restart here is my problem usually. When I pull the cord I get a hard negative resistance (like a kick.) Yesterday (hot day btw) after I realized the problem was present I took the cover off, removed the upper plus, pulled the cord and the pull seemed normal. Put plug back in, pulled and it cranked easily. Shut it off and the resistance returned (would not crank.) Removed bottom plug, pulled cord and this too seemed and felt normal. Put plug back in and cranked it easily. Shut it off and tried again - resistance back and it would not crank. Incidentally the motor got very warm on my 2-3 mile test run. I then left the boat while a put a walk board on my trailer, which took about 20-25 minutes. Returned to the boat and it now cranked easily - no resistance; motor had cooled down. Sometimes on a test run this problem does not happen, but it seems to occur if I really put it to an extreme test.

I have had this problem for over 3 years. I did replace the fuel pump (kit) 3 years ago due to a bad diaphram, and I lined the inside of the cover with styrofaom to reduce engine noise.

 

[mcdanida]

Re: Is this Vapor Lock on my 1983 25HP Mercury?

It's possible you have a vapor lock, but you'll need to find out what's allowing the pressure build-up, bad plugs not firing, etc?

Hopefully, somebody in here can be more helpful.

 

[Silvertip]

Re: Is this Vapor Lock on my 1983 25HP Mercury?

Vapor lock is the inability of the fuel pump to pump fuel because of a build-up of vapors in the system. It has absolutely nothing to do with the ability to spin the engine over. I will assume by "not cranking" it means you cannot turn the engine by pulling the cord. Not starting means you can turn the engine but it simply won't fire and start. The fact that you said the engine appeared very warm is an indication that it indeed may be running hot due to a bad water pump impeller (ever been replaced????), stuck thermostat and/or bypass valve (if it has one), leaky head gasket, or simply timing that is advanced too far. Some engines when hot, will attempt to fire while the piston well before top dead center which gives the feeling that the cord is being pulled out of your hand. That is pre-ignitiion or early timing -- not vapor lock. If the engine is overheating it may be on the verge of seizing up. Once cooled down it starts normally. However, repeated overheating can destroy the engine. The fact that you see a good stream from the tell tale does not mean the engine is not overheating. It simply means the water pump is working.

 

[mcdgary]

Re: Is this Vapor Lock on my 1983 25HP Mercury?

Can pull cord -- You are right it feels like the cord is being pulled from my hand. Why is this as I posted...Removed top or bottom plug, pulled cord and this too seemed and felt normal. Put plug back in and cranked it easily. Shut it off and tried again - resistance back and it would not crank.....

I always have to put it to a real open water test at near full throttle. In a "yard tank" cannot get into the situation. Also, in around a no wake zone running well below full throttle it does not happen although I run maybe 2-3 miles.

 

[Silvertip]

Re: Is this Vapor Lock on my 1983 25HP Mercury?

Please explain what you mean by "would not crank". Can't make the engine turn with the rope or it just that it won't fire and start. Pulling a spark plug and spinning the engine clears hot gases and helps cool the engine down so that's probably why it seems to solve the problem. But I get the feeling you are ignoring the "gets very hot" situation. That's not good and plays into the pre-ignition situation. A very hot engine can ignite a fuel/air mixture without spark. Hence it tends to fire too soon and actually tries to make the engine rotate backwards which would tend to pull the cord out of your hand. Old lawn mowers used to do that alot and Model-T's used to break lots of arms when too much timing was used when hand cranking. An old Harley with too much timing could nearly put you over the handlebars when kick starting it. You really need to have this engine checked out or continuing to run it hot will destroy it.

 

[mcdgary]

Re: Is this Vapor Lock on my 1983 25HP Mercury?

What I mean by "would not crank." First the cold engine cranks normally, i.e., choked with fast idle lever up and maybe a pull or two. Then if I did not have my current problem and I stopped and shut it off. A single pull, no choke and usually no fast idle lever up. If I say fished a while I might need a little fast idle and a couple of pulls; if I fished for two hours it would be choke, fast idle and 1-2 pulls again.

I took it out to the Poquoson River on the Chesapeake Bay this morning. I wanted to test the results of removing the styrofoam under the cover. Off the ramp used the above cold start procedure and made a roundtrip of 3-4 miles at almost full throttle. Tied back up at the ramp and shut it off. With no fast idle and no choke I knew all was correct and it cranked easily. I took the cover off and block/components felt normal wrt heat. I had checked pee hole on the ride and it felt warm, but not scalding hot. Thought maybe lining the inside of the cover with 1/2" styrofoam was bad and maybe I had found my problem.

I then took it for about a 6 mile roundtrip. Tied back up at the ramp and knew as soon as I felt the initial pull my problem had not been solved. The pull as was you describe. Incidentally, I used to have to handcrank cars and tractors were I came from years ago. This is why I know what a "kick" is. When I pull it feels like everything has gone backwards! [THIS IS WHAT I MEAN IT WOULD NOT CRANK.] I took one plug out and it cranked easily and sputtered along on one cylinder. Put plug back in and there was the negative effects.

I've tried setting the timing, but it requires full throttle and I'm not about to aim timing light out on the water at full throttle. In my yard tank it blows all the water out in seconds. I even made me a prop with "zero" pitch to get some load in the tank - ma little help but not much. Again it cranks easily when cold so I think the timing is close. The hot engine is problem fooling me.