Motors revs up and down on it's own at idle

Bill kubiak

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Jan 31, 2004
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1997 Merc 40 hp 4 cyl 2 stroke
I started it up this morning on muffs, it ran fine, got it to idle and let it run a few minutes and then it revved up a few hundred rpm by itself and then back down, it did this every minute or two, what the heck makes it do that??
 

joeanna

Petty Officer 2nd Class
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Jun 5, 2016
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have you checked it for vacuum leaks, sometimes that happens when cold/hot, as the metal expands and contracts,
 

Texasmark

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How about sticking spark advance linkage. Idle linkage on 2 strokers (my last 90 to name one) runs about TDC to 20-22 at WOT. With the engine just sitting idling, if you had the timing linkage disconnected from the carb linkage so that the butterflies didn't move, you could run the rpms all over the place just by moving the timing around. On that 90, timing is how you set the idle, not carb adjustments. Course up and down with you doing nothing makes you wonder. Lean running problem. When's the last time you did carb kits? Fuel hose inspection for cracks, leaks. Fuel pump replacement.
 

Bill kubiak

Chief Petty Officer
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Jan 31, 2004
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How about sticking spark advance linkage. Idle linkage on 2 strokers (my last 90 to name one) runs about TDC to 20-22 at WOT. With the engine just sitting idling, if you had the timing linkage disconnected from the carb linkage so that the butterflies didn't move, you could run the rpms all over the place just by moving the timing around. On that 90, timing is how you set the idle, not carb adjustments. Course up and down with you doing nothing makes you wonder. Lean running problem. When's the last time you did carb kits? Fuel hose inspection for cracks, leaks. Fuel pump replacement.

I did the carb kits last summer and all the hoses and fuel pump kit all at the same time, I followed the carb timing linkage adjustment according to the manual at that time too.
 

Bill kubiak

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I have section 2C of the merc manual on Timing and Synchronizing and after reading it several times I find it very confusing
First it says remove the plugs and plug wires from the plugs then connect a spark gap tester between the top plug wire and ground and the very next step is to connect a timing light to top number 1 cylinder lead, That's two things to the same lead.
Then engage electric starter, no mention of hooking up ear muffs, we all know that spinning the motor dry will wreck the water impeller, and checking if the specified primary pickup timing mark aligns with V notch in timing pointer window and the picture shows a 0 lining up and it goes on and on and becomes more confusing since it hardly shows which arm is which, I guess it assumes I know which is which and BTW the wide open stop screw is frozen in place and I cannot move it.
it would be nice if there was picture of that whole throttle linkage assembly with arrows pointing to all the parts and ID'ing them so I know what they re talking about.
Does anyone have a better manual them mine which is an actual Merc manual???

I have no idea what a spark gap tester is.
 

Texasmark

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1. The spark gap tester will allow the voltage to rise to a level suitable to fire the timing light but will limit the max voltage which on a '97 Merc can shoot up in microseconds to 40kv if nothing breaks down first....like a spark gap. So 2 on one terminal is ok. Get a standard spark plug of any variety and gap to .040 inches. Put the cap on it and connect the case to the engine block.....do both plugs for the reason stated above. That's your el cheapo spark tester.

2.You shouldn't need ear muffs as the engine is just spinning over..If you personally feel you need them then that's your choice. Wads of Kleenex tissue will suffice if you don't have the foam 30 dB professional muffs.

3. Spinning the engine with the starter for short testing will not kill your impeller The housing is slick stainless steel and the impeller blades have a pretty hefty durometer rating. Heat and friction is what will kill it while moving and you have neither under those conditions.

4. I boats sells subscriptions to Seloc or the actual manuals of which I have purchased after having a Merc OEM manual initially. I did find things in the Seloc missing in the OEM. Main thing were pictures and pictorials. Look at the top of this page under Boat Parts and Accessories.

Pictures on your part of the things you don't understand can help us to help you by seeing what you are talking about. Sitting here and you there does little in the way of helping and stretches the imagination.
 

Bill kubiak

Chief Petty Officer
Joined
Jan 31, 2004
Messages
629
1. The spark gap tester will allow the voltage to rise to a level suitable to fire the timing light but will limit the max voltage which on a '97 Merc can shoot up in microseconds to 40kv if nothing breaks down first....like a spark gap. So 2 on one terminal is ok. Get a standard spark plug of any variety and gap to .040 inches. Put the cap on it and connect the case to the engine block.....do both plugs for the reason stated above. That's your el cheapo spark tester.

2.You shouldn't need ear muffs as the engine is just spinning over..If you personally feel you need them then that's your choice. Wads of Kleenex tissue will suffice if you don't have the foam 30 dB professional muffs.

3. Spinning the engine with the starter for short testing will not kill your impeller The housing is slick stainless steel and the impeller blades have a pretty hefty durometer rating. Heat and friction is what will kill it while moving and you have neither under those conditions.

4. I boats sells subscriptions to Seloc or the actual manuals of which I have purchased after having a Merc OEM manual initially. I did find things in the Seloc missing in the OEM. Main thing were pictures and pictorials. Look at the top of this page under Boat Parts and Accessories.

Pictures on your part of the things you don't understand can help us to help you by seeing what you are talking about. Sitting here and you there does little in the way of helping and stretches the imagination.

Does anyone have a picture of a the spark plug gap tester in question? I have lots of unusual tool for testing spark, one I just accidently broke is a spark tester with a pressure gauge and a clear plastic double tube arraignment where I can watch the spark jump while increasing pressure inside by pushing one tube into the other, sort of a syringe with a pair of electrodes inside, it's cool but the wires just broke off, looks like another solder job, got that at harbor freight years ago.

I really like the idea of adjusting the idle with the boar tied to a dock, I wonder if that would also work if I kept it tied on the trailer and did not let it float off and adjusted things that way?
 

Texasmark

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Dec 20, 2005
Messages
14,544
Does anyone have a picture of a the spark plug gap tester in question? I have lots of unusual tool for testing spark, one I just accidently broke is a spark tester with a pressure gauge and a clear plastic double tube arraignment where I can watch the spark jump while increasing pressure inside by pushing one tube into the other, sort of a syringe with a pair of electrodes inside, it's cool but the wires just broke off, looks like another solder job, got that at harbor freight years ago.

I really like the idea of adjusting the idle with the boar tied to a dock, I wonder if that would also work if I kept it tied on the trailer and did not let it float off and adjusted things that way?

Can't help on the spark tester but I don't see why the trailer test wouldn't work. I think the idea is to have a light load on the engine when you set the idle timing. It's really a seat of the pants thing as I see it. Too slow and the engine dies when shifted into gear and too fast and it makes a big clunk.
 

Bill kubiak

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Can't help on the spark tester but I don't see why the trailer test wouldn't work. I think the idea is to have a light load on the engine when you set the idle timing. It's really a seat of the pants thing as I see it. Too slow and the engine dies when shifted into gear and too fast and it makes a big clunk.

While chatting with my mechanic neighbor that subject of water testing came up, we were discussing what would be better, running the motor in a barrel, or running it while still on the trailer backed into the water or running it while tied securely both front and stern to a dock.

My thoughts here and mine only, I would like to hear what others think
In a barrel, lots of aerated water and froth, better then on the muffs
On the trailer, in very shallow water probably better then in a barrel
Alongside the dock, I think the prop will always be biting into undisturbed water, might be better then on the trailer, I'm not a rocket scientist just a boat owner

What do you all think????
 
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racerone

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When a carburetor fuel level drops to near empty the motor " leans out " and RPM increases.-----Nothing else changes or moves.-------I would suggest you go over your fuel system right from inside the tank to the carburetors.---I am sure your response will be that you have already done all that !
 
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