Logo
FULL SITE MENU

Help me with my mercury 20hp

J

jonny canuck

Member
Where to start? I was out on the water going WOT for about 20 min when I seemed to lose 1/2 my power. At idle it kind of knocks a bit, but at WOT it runs fine (1/2 power). Fuel doesn't seem to problem, and I have spark. Please help!

sorry I forgot, it s a two stroke
 
Last edited:
so i am I little new to this but here is how it went. the first cylinder was about 110. the second seemed to locked up after about two pulls. (the wife was pulling) either way the pull was harder in the second then the first. I was unable to pull even 3 times on the second because i did not want to do any damage
 
not sure about no plugs but turns over with plugs. and fires up just fine when every thing is connected. i can go try if you think it makes a difference
 
water may have come out already with all of that pulling before hand run it on the hose or whatever for a few secs then do the same check for water first (no plugs) then compression on both cylinders (should be about the same)

If thats good then and no water(hold you hand over it and feel for a mist of water)

look at the coils,Swap em if you can,then the power pack and check your fuel for water

take a sample in a glass jar let it sit for 5 mins and see if theres seperation

coils cracked?
 
i will do that. i did do an ohm meter test on the coils but a mechanic said that an resistance test can be misleading. one coil did have way more resistance than the other. i do not think i can swap the coils because one lead is short and the other is longer
 
Last edited:
UPDATE ... just drop the boat in the water to giver a go. As expected nothing changed. Brought it home (river's about 2 mins. away) to check the cylinders and do the compression. No water in the cylinders. I was about to do the compression check when I noticed what I think is a gas line or a vaccum line (was spitting out a little fuel when pull started) had come out of the head area. Hoping this was the problem put the hose back in and back to the river. Motor had a little more pick up but far from what it was before. However the motor was kind of surging a little bit. Felt like it wanted to go. Came back home to check everything out again. No water came out of cylinders that was noticable on my hand. Cylinder One looked dry and cylinder two looked gas oily wet, spark plugs looked the same. Had spark and I did get shocked at couple of times through the back of the boot, not sure if it tracked or that's a problem. Next, compression test. Even worse then before. Cylinder One - 65 psi after 3 pulls then got hard to pull. Cylinder Two - about 35 after about 4 REALLY hard pulls. Mind you this is only the second time I have done a compression test. Ok thats a novel hope that helps. Thank-you for any advice.

Just thought of somthing else. I had the motor in neutral and manualy throttled the carb and the motor sounded to rev up to full power, granted the cowling was off.
 
Last edited:
You never said what year this motor is so one has to guess:

If it's a magneto engine the points need cleaning, new condensers and or a new coil(s).

If it's an electronic ignition motor you need to change the lower coil.

It's also probable that the lower crank seal is shot, so the power head needs to come apart.

Jeff
 
my bad. it is 1990 20hp mercury long leg tiller. it also has an electric start that is not currently connected to a battery but leads are isolated
 
Last edited:
That motor (I believe) has electronic spark advance, and it might not be advancing. If so, either the spark box or charging coil could be shot, but my money's still on a bad coil.

Jeff
 
how could it be a coil if i am getting a spark? Pardon my ignorance I'm new to this. Last night I had the motor running with only one plug at a time.
 
Last edited:
redid a compression check properly this time (throttle open) and it came back the same on both cylinders. while doing i noticed a fuel line had shaken lose out of one of the cylinders. going out on the water with fingers crossed.
 
no difference guess i'll change the coils tomorrow. Went by the shop today and found out my motor was actualy built in 1990. Thanks seller for miss informing me.
 
Last edited:
1990? That's the manual spark advance design.

A bad coil is a sneaky SOB. They will allow you to get the motor started--even idle around normally--then zap you at full throttle. They heat up then and short out, but when you cool them (and the motor) back down they come right back to trick you again.

Like I said, sneaky SOBs!

Jeff
 
UPDATE...I went to the parts guy after work today and pick up a carb kit and plug leads b/c I thought they were suspect. Changed the leads and wouldn't you know it problem gone. When I removed the old leads the coil end was green and ugly. Will do the carb on the weekend b/c well...why not. Just came back from the river and caught 5 sturgeon between the two of us. :) :)
 
well that didn't take long. ran for most of the day then when i pulled up anchor beyond sunset no start. gotter going but again no trottle. guess i know what i'm doing tomorror...coils & carb kit :( :( i can think of a better way to spend a friday night
 
UPDATE!!!

1) Took the carb off and put in a new kit. Carb was clean and so were the gaskets. Fuel pump was too. New parts all around.

2) Changed the two ignition coils. So now new coils and leads.

3) My rectifier was rusty and ugly. So another part added.

4) New fuel filter and gas tank

WOW! Guess I've turned into a parts changer.

So this is the run down of how its performing. Starts fine on the first pull every time. Idles, no problem. Start to throttle...no problem. 1/2 throttle...no problem. About 3/4 or so it starts to shake and sounds ****ty ( missing ). At WOT motor bogs right down and shakes bad and sounds really bad. :( :( :( Turn throttle down everythings normal. Any suggestions would be greatly appreciated.
 
If the compression is good.Both the same(or close).A good tester is needed.Spend the money.
Then check the diaphram in the pump.J
 
Back
Top