Inconsistent performance 84' blackmax 150

[karnage]

Hello! It has been over 2.5 years since I've made a performance related post on my outboard, thankfully.

When I bought my boat with this outboard I had a huge learning curve, got an OEM service manual and I set my primary pickup, spark advance, and idle within spec. Pulled apart and cleaned my carbs, adjusted my throttle cable, and some other odds and ends. Soon after all that the boat went 58mph and climbing with my laser 2 prop.

I only take the boat out every so often, but I have a desire to have this thing running good, every time. And it doesn't. I'll get to the point, and describe my last 3 outings.

1. Boat top speed with just me (120lb) was 43mph fully trimmed. I don't remember rpm.
2. The next day, it ran great. With me 120lb, and another 280lb person equaling 400, it shot on plane very fast and went about 52mph. Did this twice.
3. A few weeks later, yesterday, the boat struggled awfully to get on plane with me and another 180lb person, 300lb. Had to have them get up front and it crawled on plane with a top speed of 36mph and only 4500 RPM. If I remember correctly, it should be about 5400rpm full trim WOT.

The engine starts, idles, revs just fine.

I'm gonna do some maintenance and testing. Pull and clean/rebuild carbs. Clean/replace plugs. Inspect/replace pump diaphragm. Locate a filter (I've never located one). What else!? Any hunches? thanks.

Side notes: Engine is not overheating. Oil pump was deleted by previous owner, 2 stroke mix. Throttle cable and carbs are still properly synched.

 

[Mc Tool]

Im thinking that your not always running on all cylinders (can be hard to tell sometimes ). I would check the spark plugs (remove and short to block to see if they are all sparking ), and drain float bowls to check for water.

 

[karnage]

Im thinking that your not always running on all cylinders (can be hard to tell sometimes ). I would check the spark plugs (remove and short to block to see if they are all sparking ), and drain float bowls to check for water.

I guess it could be hard to tell, they say to do the usual maintenance of plugs etc. annually, which I haven't. Ill look through my carbs and plugs before my next trip.

Thanks for the reply.

 

[airshot]

Check for water in fuel or fuel system.....drain and clean tank, check for spark with a gap jumping gage. Gonna bet on bad fuel....

 

[jimmbo]

Again. it sounds like a Cylinder dropped out. Hopefully it is an Ignition problem, cause even though the Engine is strained by the higher load on the remain Cylinders, it still getting Oil. Fuel Starvation also starves that Cylinder of Oil. Of course a Fuel Problem could be Too Much Fuel, and the Cylinder misfires
What do the Plugs look like?
Heat can wreck Havoc on the Ignition Switch Boxes
In addition, that is a 40+ yr old motor, and Mercury has a Reputation for Wire Insulation Problems. Check all the Primary(low Voltage) Ignition Wires for cracks, Breaks, chaffing. The Wires from the Stator/Trigger are subject to movement as the Spark Advances/Retards, and sometimes the Conductor breaks inside the Insulation

 

[Dukedog]

tha only fuel filter on it will be a screen on tha fuel pump (under tha single screw on tha pump) unless one has been added inline somewhere.. one thing ta check is tha idle stabilizer.. get rid of it completely if its still being used.. it can and will cause timing problems at any rpm if it goes south.. you didn't say what age tha 'lectrics are?.. start tha motor after dark without tha cowls off.. take a screwdriver and go around tha motor checkin' for any wires (mainly around coils and plug wires) for any leakage.. check values of stator, both switch boxes, rectifier... if it still has tha ngk's they are probably still jus fine.. they usually last forever unless ya developed a major problem....

 

[karnage]

Great info you guys have given me. I got to mess with the boat today.

Pulled my plugs and they were all pretty uniform, none seemed overly wet or burnt. I found one with a cracked porcelain, replaced that one and cleaned the rest. NGK BU8H.

When I bought the boat he said he just replaced the stator, and it had an old switch box with the old parts he gave me. So, those are at least newer.

I drained my bowls, and tested flow a little with the primer bulb and it seemed to fill all the carbs very easily so hopefully the engine is not leaning out like you said jimmbo.

I know my wires look a little rough, so I'm looking forward to testing that aspect for leakage.

I drained my fuel cell with an external fuel pump, and inspected my tank, not bad. Also cleaned my screen filter on the engine near the carbs, a little debris.

I will do more testing next week. Testing spark, locating my Idle stabilizer box, values of stator, testing temperatures...

I gave you this update now so I can possibly get a recommendation of an inline filter or water fuel separator? I'd like to get one on order. How many microns should I be looking for? Thanks.

 

[Chris1956]

Most any water separator for a boat will work. Most are 20GPH, which is a lot more than you need.

Water is separated by gravity, and the gasoline above it is filtered, much more than the dinky screens or inline filter that came with the motor.

 

[jimmbo]

Having a Bowl full of Fuel, doesn't prove a Cylinder isn't being Fuel A dirty/clogged Jet or Fuel Circuit inside the Carb can cause a Fuel Starvation.

 

[Dukedog]

Great info you guys have given me. I got to mess with the boat today.

Pulled my plugs and they were all pretty uniform, none seemed overly wet or burnt. I found one with a cracked porcelain, replaced that one and cleaned the rest. NGK BU8H.

Thanks.

this by itself can cause a multitude of inconsistent problems........

 

[karnage]

Avoiding pulling the carbs apart to save gaskets, but better safe than sorry is true. Was hoping reading Temps and taking the boat out one more time and testing would possibly safice. We will see next week.

Having a Bowl full of Fuel, doesn't prove a Cylinder isn't being Fuel A dirty/clogged Jet or Fuel Circuit inside the Carb can cause a Fuel Starvation