98 Mercury 150 running issue

[Drew0923]

So the boat has been at the mechanic for a few weeks for loss of power at 1800 rpm and a bad idle. Found chipped reeds, replaced with Carson reeds and added spacer plate. Idles fine now on muffs and in tank but once on the water it still doesn’t want to go after being in gear. All electrical has been checked( stator, switch boxes, rev limiter, trigger, coils , wires etc) . Motor will rev up out of water and seem as if there is no issue. Went through through all the carbs entirely, set floats etc. timing is right on. Fuel pump has been checked. We did find however that the top two plugs on the port side were black.. don’t know where to go from here. Mercury 150 serial number 0g665789

 

[LEY]

You indicate black spark plugs. Possible detonation? The serive manual about detonation:

A few of the most common causes of detonation in a marine 2-cycle application are as follows:
a. Over-advanced ignition timing.
(1.)Use of low octane gasoline.
(2.)Propeller pitch too high (engine RPM below recommended maximum range).
(3.)Lean fuel mixture at or near wide-openthrottle.
(4.)Spark plugs (heat range too hot -- incorrect reach -- cross-firing).
(5.) Inadequate engine cooling (deteriorated cooling system).
(6.)Combustion chamber/piston deposits (result in higher compression ratio).
Detonation usually can be prevented if:
1. The engine is correctly set up.
2. Diligent maintenance is applied to combat the detonation causes.

 

[Drew0923]

You indicate black spark plugs. Possible detonation? The serive manual about detonation:

A few of the most common causes of detonation in a marine 2-cycle application are as follows:
a. Over-advanced ignition timing.
(1.)Use of low octane gasoline.
(2.)Propeller pitch too high (engine RPM below recommended maximum range).
(3.)Lean fuel mixture at or near wide-openthrottle.
(4.)Spark plugs (heat range too hot -- incorrect reach -- cross-firing).
(5.) Inadequate engine cooling (deteriorated cooling system).
(6.)Combustion chamber/piston deposits (result in higher compression ratio).
Detonation usually can be prevented if:
1. The engine is correctly set up.
2. Diligent maintenance is applied to combat the detonation causes.

I will run through all of that to double check. Thank you for the input

 

[Chris1956]

Black spark plugs indicate they are not firing. They either do not have spark, they are getting flooded with fuel or they have water in the cylinders.

You might take a compression test and see if those two are low.

 

[Drew0923]

Black spark plugs indicate they are not firing. They either do not have spark, they are getting flooded with fuel or they have water in the cylinders.

You might take a compression test and see if those two are low.

Compression test last night showed all cylinders between 120-122.. using an spark tester during idle all coils were firing and open air tester showed plugs sparking good

 

[racerone]

Maybe the top carburetor is flooding?----Does manual fuel pump go hard ?----Most motors will rev up on a hose and that test means nothing at all.

 

[Chris1956]

Do you see grey deposits on the black spark plugs, or water droplets?

 

[Drew0923]

None at all

 

[Chris1956]

OK time for SWAG. Are the main jets in the port carb barrels on the top two carbs missing or loose? Did you use new gaskets when rebuilding the carbs?

 

[Drew0923]

OK time for SWAG. Are the main jets in the port carb barrels on the top two carbs missing or loose? Did you use new gaskets when rebuilding the carbs?

Chris, New gaskets when going back on, made sure everything was seating properly. Nothing was loose..

 

[Chris1956]

OK, I am out of SWAGs.

Black plugs mean rich combustion or no fire, in my experience.

Your carbs have a single float chamber, so flooding would affect starboard side of motor.

It affects the top two port cylinders, so potentially two carbs, but only one barrel each.

OK, one more SWAG. This one is definitely Stupid Wild Guess.

Check resistance of spark plug bases to ground. Swap plugs around and see if different plugs get black. Spark plug wires arcing to ground, with cover on?

 

[Drew0923]

OK, I am out of SWAGs.

Black plugs mean rich combustion or no fire, in my experience.

Your carbs have a single float chamber, so flooding would affect starboard side of motor.

It affects the top two port cylinders, so potentially two carbs, but only one barrel each.

OK, one more SWAG. This one is definitely Stupid Wild Guess.

Check resistance of spark plug bases to ground. Swap plugs around and see if different plugs get black. Spark plug wires arcing to ground, with cover on?

So finally got back around to working on the boat. After going through everything again I noticed I wired one switch box incorrectly so firing order was off. This was why I had the 2 black plugs . It idles awesome now and plugs look as they should. Took it out on the water and I’m still stumped. I gave it full throttle , it stumbled for maybe a second or 2 and took off and planed great. Figured maybe it just needed to get all the unburned oil out. Ran it on plane for 5-10 minutes, brought back to idle, idled fine, went to take off again and wouldn’t go past 1800 rpm again. Long story short. I’m tired . I hate this motor

 

[Chris1956]

Your carb idle mixture adjustment is likely too lean. Open the mixture screws 1/8 turn and see if that helps. using the trial and error method, adjust all six mixture screws until she accelerates well, and still idles.

Of course, this supposes that you have mixture adjustment screws. Not all V6 motors do. It also assumes that you do not have accelerator pumps on the carbs. Some V6 motors do.

Any idea why the reeds failed? They usually do not.

 

[Dukedog]

should be WMV's.. has screws.. WMH pumpers only made 1 year ('91), maybe a little of second year before they were dumped....

 

[Drew0923]

Your carb idle mixture adjustment is likely too lean. Open the mixture screws 1/8 turn and see if that helps. using the trial and error method, adjust all six mixture screws until she accelerates well, and still idles.

Of course, this supposes that you have mixture adjustment screws. Not all V6 motors do. It also assumes that you do not have accelerator pumps on the carbs. Some V6 motors do.

Any idea why the reeds failed? They usually do not.

Adjusted mixture a bit to no avail. Went as far as richening up all until motor died and slowly coming back in with all until i had a decent idle again. Nothing improved. Played with them a bit more but to no avail. As far as the reeds go i have no idea. I repowered with this motor a few months ago, ran great for me a few times out and then the first sign of a problem occurred when I started the motor on the water after fishing and it not wanting to go over 1800. We rechecked all new ignition components just to make sure having the switch box wired wrong messed with anything and everything checked out with a dva while running and ohm tests when engine off. We are running out of ideas. Sounds odd but im thinking of random things at this point. Any way the motor could be too low and the upper exhaust is taking in water at low rpm? just throwing things out at this point.

 

[Chris1956]

Is the link and synch proper? That motor should have an idle pickup timing of 0-9* ATDC, so the carbs should be closed until the timing is in that range. Then they should start to open. Max spark advance s/b 21* BTDC. Also, the carbs should open fully with the throttle fully open.
Carb throttle plates should be adjusted to open at the same time, as well.

 

[Drew0923]

Is the link and synch proper? That motor should have an idle pickup timing of 0-9* ATDC, so the carbs should be closed until the timing is in that range. Then they should start to open. Max spark advance s/b 21* BTDC. Also, the carbs should open fully with the throttle fully open.
Carb throttle plates should be adjusted to open at the same time, as well.

Chris , doubled checked timing and made sure carbs were synced correctly. Just took it out and same deal. Not much left on this darn thing. I'm going to go over everything once again later today and if I have time, I'll try to get it back to the lake to get a video. Might give a better idea. PS, the mechanic who has my boat was referred to me from about 6 other marinas near me as the best Mercury outboard guy around. Don't know if that makes me happy or not at this point lol. 4 weeks in. He's around the corner from my body shop so we have become friends and we speak daily on any updates. Hes also had 2 other outboard mechanics come in after hours to try and diagnose with no luck

 

[Chris1956]

OK, what kind of boat is it on? What pitch prop? If the prop pitch is too much, the motors can be finicky.

I doubt water is getting in the exhaust relief. They are pretty high on the engine.

On another note. My '93 Merc V6 has a "Z-shaped" exhaust hose from the block to the cowling. It is for exhaust relief. If it leaks, exhaust will be drawn into the carb, which could affect idle and possibly acceleration.

I had an issue with that hose leaking sometimes. When the engine was cold it didn't leak and the engine ran great. When the engine was hot from a high speed run, the idle was real bad from the exhaust leak. Took me a while to figure it out.

 

[Drew0923]

Prop pitch I’m not 100% sure on. It came off of the exact boat that it is on now, a nitro fish and ski hull 18.5 ft. First 15 times out it performed great so I’m not sure if that’s something that would happen all of a sudden. I have a Z shaped hose running to the bottom of the block, then out the lower cowl, it’s plugged and if I’m correct I thought it was a flush port. May be wrong

 

[Chris1956]

I think your motor does have a flush port. It should also have some kind of exhaust relief. Exhaust relief is a port that is used for exhaust when engine is in the water and in neutral. It is hard for the exhaust to push the water out the prop, hence the exhaust relief port. They are usually high up on the midsection or lower cowl.