Mercury 402 - 40 HP problem

[2slow2go]

Hey there everyone. Brand new to this site, looks great, thats why I joined.

I have a (not to sure on the year) 1978 Merc 402 on a 14' boat. The motor runs great at idle, starts right up without any problems. How ever... When I give'r WOT it just bogs down. I've gone thru all the normal stuff. Plugs, fuel, carb, etc... I have not checked the timing, but the motor ran perfect oneday and the next it did not. I recently looked over the motor again and noticed a few wires where dry and cracked and I changed them. There is one wire however that is connected to a small blue thing (about 1" x 1/2") on the side of the motor. it has 1 wire going to the front of the motor (thing with the screws that hold the wires) and the other is a short black ground wire that is grounded right beside it. What is this and could this be causing any troubles as I described? The 1 wire is pretty dry and cracked, but I have not idea what it is to order or look into changing it.

motor serial number is 7112698

Thanks in advance.

[9D280]

Hi, Welcome to Ribnet. .

I'll guess that's your Mercury switch. Not a branded component, but exactly what it says on the tin, so if you dispose of it, remember it is full of Mercury and will no doubt contravene a load of eco friendly waste disposal rules!

Theory is you can't start / run the engine when it's tlited up out the water. It acts like a deadman switch, but for tilted engine rather than helmsman going swimming, but electricaly does the same thing - shorts the ignition.

If you have been replacing wires, has it gone back at the angle it came out from? If not, rotate it around, at a guess so that the wires are coming out the top when engine is in the "run" position. If you didn't move it, disconnect the earth wire, and see if your engine runs. If so, that's likely your problem.

Have a play, & let us know what happens....or not!

[Poly]

Originally Posted by 9D280

Hi, Welcome to Ribnet. .

I'll guess that's your Mercury switch. Not a branded component, but exactly what it says on the tin, so if you dispose of it, remember it is full of Mercury and will no doubt contravene a load of eco friendly waste disposal rules!

Theory is you can't start / run the engine when it's tlited up out the water. It acts like a deadman switch, but for tilted engine rather than helmsman going swimming, but electricaly does the same thing - shorts the ignition.

If you have been replacing wires, has it gone back at the angle it came out from? If not, rotate it around, at a guess so that the wires are coming out the top when engine is in the "run" position. If you didn't move it, disconnect the earth wire, and see if your engine runs. If so, that's likely your problem.

Have a play, & let us know what happens....or not!

9d280 - if his engine is starting and running, but "bogging down" at WOT then I think the mercury tilt switch is unrelated - its surely an "all or nothing" situation - like a deadman problem.

The motor runs great at idle, starts right up without any problems. How ever... When I give'r WOT it just bogs down. I've gone thru all the normal stuff. Plugs, fuel, carb, etc... I have not checked the timing, but the motor ran perfect oneday and the next it did not.

definitely sounds like typical fuel delivery problem symptoms... Are you sure you've checked everything - there's not a hidden in-line filter that might be blocked? or a dodgy hand primer on the fuel line (seems to be remarkably common)?

[9D280]

Good call. I did get a bit carried away with the switch, but "worked Ok yesterday / not today" is a step change, which is more often than not electrical...... If it's been unscrewed & put back a few degrees off, the increased vibrations as it opens up could be lightly splashing the contacts......


A good one to see if the idle mix is a touch lean is as it bogs down, poke the choke, if that gets you "through the bog", you need a touch more idle fuel. (blocked jet maybe?)

[2slow2go]

Originally Posted by 9D280

Good call. I did get a bit carried away with the switch, but "worked Ok yesterday / not today" is a step change, which is more often than not electrical...... If it's been unscrewed & put back a few degrees off, the increased vibrations as it opens up could be lightly splashing the contacts......


A good one to see if the idle mix is a touch lean is as it bogs down, poke the choke, if that gets you "through the bog", you need a touch more idle fuel. (blocked jet maybe?)

Tried the choke test, at WOT if i poke the choke the engine bogs down even more or stalls out.
Another thing i just realized is that out of the water on the muffs she will run at WOT perfectly, but when in the water under load, this is when the problem occurs.
As for the tilt switch it could only good back in the original position as there are groves in the side of the motor that it sits in.

Also, I did notice before it did this, there was on time I was out on a ride and it did this in the water and kinda jumped out and bogged and jump up again, about 3 times all at WOT. Then when I stopped it would not reach WOT again since.

Thanks for the replies,

[9D280]

Firewall just killed last atempt at this post, so apologies if this is a bit disjointed


Anything will run on no load. All the firing cylinder is fighting against is a tiny amount of shaft bearing friction and compressing the air in the other cylinder(s)

My next thought for a "high level" test - pull the spark plugs out - is one of them substantially more oiled than the other? If so, clean & swap it to another cyl. Go for another run. Pull plugs again & see if the oiled one is the same cyl as last time if it followed the plug, replace the lot. if not, try the same test with the HT leads (Looks like yours are a similar type to mine - the crimped ends occasionally let go) Remember to check the Coil end as well. Repeat the swapping trick until you get the oiy cyl to move, then you have found the dodgy component.

Alternatively to check the HT side, if you have a spaky friend, ask him or her to megger your plugs & leads (insulation test). High frequency KV AC will break down insulatio nthat a conventional meter thinks is fine.

Another test could be to pull one plug lead off at a time & see if runs similar to when bogged. (not when it's running, obviously! )


Alternatively I managed to "re HT" my entire engine for about £30 in a local motor factors. A certain Shop with a big orange sign will double the cost of your engone just with the leads.......

[jyasaki]

No offense to 9D280, but I'm thinking fuel. The surging thing sounds just like fuel starvation.

Have you tried pumping the priming ball while running WOT? If that fixes things, it's either a fuel restriction or your fuel pump.

Luck;

jky

[9D280]

No offence taken!

I had much the same thing on mine, and found one of the HT leads had decided to disconnect itself form it's crimp connector at the coil end, hence the suggestion.

Surely if it was fuel starvation poking the choke at WOT should bring it back to life, not bog it down further?

[free]

I'd suggest bogging down under load is a classic ignition symptom, check leads as mentioned before, plug condition (you can also compare resistances of the plugs) and coil. You may need reference data to compare to, especially for the coil.

free

[Damien.]

Hi.

Could you run a compression check on the engine ?

I had a customer with the exact same symptoms, he spent hundreds with a previous mech, replacing bits such as stator, trigger, plugs, new fuel tank, new fuel line, filters, coils, I could go on forever.

When I got the boat I run a quick spark check, all fine, then I run a compression check, 2 cylinders were just about hitting 90 and the third was hitting 50.

Sometimes you can tear your hair out replacing everything you think might be causing the issue, when some quick free checks, will tell you if the basics are in tact first.

Let us know how you get on.

[adrian c]

Hi we have a 402 35hp merc that would run fine with muffs on it and run well in the water, but it would not pick up and get the rib on the plane ( sr4 ) . Problem in the end was the switch box only firing on one side, changed this and it has been as good as gold since

[2slow2go]

Originally Posted by 9D280

Firewall just killed last atempt at this post, so apologies if this is a bit disjointed


Anything will run on no load. All the firing cylinder is fighting against is a tiny amount of shaft bearing friction and compressing the air in the other cylinder(s)

My next thought for a "high level" test - pull the spark plugs out - is one of them substantially more oiled than the other? If so, clean & swap it to another cyl. Go for another run. Pull plugs again & see if the oiled one is the same cyl as last time if it followed the plug, replace the lot. if not, try the same test with the HT leads (Looks like yours are a similar type to mine - the crimped ends occasionally let go) Remember to check the Coil end as well. Repeat the swapping trick until you get the oiy cyl to move, then you have found the dodgy component.

Alternatively to check the HT side, if you have a spaky friend, ask him or her to megger your plugs & leads (insulation test). High frequency KV AC will break down insulatio nthat a conventional meter thinks is fine.

Another test could be to pull one plug lead off at a time & see if runs similar to when bogged. (not when it's running, obviously! )


Alternatively I managed to "re HT" my entire engine for about £30 in a local motor factors. A certain Shop with a big orange sign will double the cost of your engone just with the leads.......

Ok, I am now the fastest spark plug changer in Canada
So I pulled both plugs and the top plug was very oily and the bottom plug was like brand new (started test with two brand new plugs). I then rotated the two plugs and the top one is always oily and the bottm is nice and clean.

What do you mean by HT?

I checked the compression on the top cylinder and it holds about 94 to 100 PSI. I was not able to test the lower one, cause I didn't feel like taking the the bottom cover off to get my tester into the cylinder hole (besides I think the problem one is the upper one).

I also checked the spark on both wires and lots of spark on the two of them.

[T 15]

Hi, had a similar problem a few years back with my Merc 500, it was dirt in a high speed jet in a carb, have you cleaned the carb & jets and check the fuel tank, line etc for dirt/water in the fuel ?

[2slow2go]

Okay, so on Thursday the wife and I decided to head up to the cottage for the long week-end. She asked as we left the driveway why I wasn't bringing the boat. Long story short she convinced me to bring the boat and I decided that I would try it at the cottage and if it didn't work that I would put a forsale sign on the boat at the cottage (may have been my frustration talking).
Soooo, got to the cottage, put the boat in the water, and the boat did exactly what it always does (no high RPMS), parked it at the dock, got a forsale sign stuck it to the side of the boat and it was done.... So I thought! First guy stopped and looked at the boat I told hime how much and the problem with the motor. Hewanted to try the boat eitherway, okay with me. We both jump in the boat I start the motor up and slowly putt away. The guy asked me to go WOT to hear the problem. Well guess what...... The eF'ing thing took off like a rocket like a child being chased with a wooden spoon
The boat was no longer forsale and ran like a charm all week-end, pulling skiers, tubes, went out for about 1 hour ride around the lake.

I did nothing to the motor since I rotated the spark plugs. The top spark plug is still oilier then the bottom.

Anyways thanks for all the comments and suggestions, however i guess she just had to be threatenedwith a little goodbye sign

[9D280]

Originally Posted by 2slow2go

What do you mean by HT?

High Tension. AKA High voltage. It's the spark plug leads.

Apologies, I forget acronyms like that vary from place to place.