Mercury Sportjet 90 bogs past 4K

[79Merc80]

Re: Mercury Sportjet 90 bogs past 4K

Hello:

You made a statement that wasn't entirely corect

"I rebuilt a Mercury (built by Force)"

Actually, it's a Force outboard powerhead built by Mercury. It's the old Chrysler block.

 

[CharlieB]

Re: Mercury Sportjet 90 bogs past 4K

Pull the fuel llne from the first carb, point it into a suitable container and crank the motor.

Do you have a STRONG, PULSED flow, that is the diameter of the hose?

Or is it more like a small spurt?

Junk in the dip tube in the tank, crub in the check valve in the fitting on the tank, a restrictive check valve on either in of an aftermarket primer bulb, any or a combination can cause the carbs to run low on fuel and limit high RPM power as you are experiencing.

Have you tried using a small portable fuel tank to test, use a different fuel hose/primer, if all is well then that should prove the problem is in your fuel line/bulb/tank.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Force built by Mercury - OK thanks for the clarification. Craig as far as the plugs they pretty much all look the same after each run - slight carbon on each - none are oiling - none are steam cleaned - none are dry...

I'm attaching a photo of the before and after of the piston cleaning. #1 Top was the worst, but all had a coating of carbon on them. My suspicion is from 40 hours of rich running and not being able to open it up.

Someone suggested to me last night that one of my crankshaft seals is broken causing a drop in crankcase pressure. They said if I was dropping a cylinder electrically I would never make it past 3500...

Charlie - just saw your post. Yup it has a strong, pulsed fuel flow that would quickly fill the pint jar I used. I'm using brand new 3/8 ID hose from tank to fuel pump and pump to bottom carb. 5/16 ID hose between the carbs as 3/8 seems to be too loose.

I have run the engine from a brand new 6 gal test tank with 89 octane fuel directly connected to the fuel pump. It did have a primer bulb and it was not going flat when bogging. Pumping the bulb when bogging was no help. I also removed the shutoff valve on my main tank and cleaned it and pulled the pickup tube (no crud). Also the fuel pumping into the jar was PERFECTLY clean - no sediment settled to the bottom of the jar.

I can switch back to the test tank again since it's been about a month or so since that issue was written off - but since the fuel is good and clean and the flow is strong at cranking from the main tank - is that necessary?

I know once I figure this out all other problems are gonna be easy ones. Thanks again everyone for everyone's thoughts. Promise to post a final resolution one day

George

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

Someone suggested to me last night that one of my crankshaft seals is broken causing a drop in crankcase pressure. They said if I was dropping a cylinder electrically I would never make it past 3500...

If you used RTV on all the gaskets, you may have small leaks everywhere. The head gasket looks suspicious, water may be leaking by, the cleaned right side of pistons 2&3 might be from water. They are all firing.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Thanks again for your replies.

The only place I used RTV that the manual didn't call for is the fuel check valve and carb elbows. The head gasket said to apply RTV to the cylinder head side of the gasket only which is what I did. All of the pistons had carbon deposits and were cleaned with GM Top Engine Cleaner and a soft brass brush.

Another forum member mentioned that the a crankshaft sealer rings might be broken (which I assuming he's referring to the small metal rings - 2 of them) that are kinda like a small piston ring and once the crankshaft is seated in the crankcase half the manual says to rotate them 180 degrees so the open end is on the bottom - which I did). Then mate the two halves together starting with the core plugs.

I guess if one of these seals is broken it doesn't allow the crankcase to hold pressure - which would cause a bog? Won't know till I go back in I guess.

Also, another bit of information that might apply a crankcase seal issue. After each trip to the lake I wipe the powerhead to jet adapter plate bolts in the front of the engine near the hull bottom. A small amount of fuel/oil combination accumulates and has never been a problem. I know though this shouldn't be normal - is this a clue to a leaky crankshaft seal that could be causing the bog? See pics. During the rebuild I did not replace the crankshaft seal at the bottom of the engine. I have a new one - why didn't i?

Also, I tried to understand the blue RTV sealant path between the crankcase halves (about how it should be traced) directly from the Merc Sportjet manual. See pic - does my work look OK to you?

I sure appreciate your advice. Once this is resolved - I promise to post the outcome and help others find solutions and your great site!

George

 

[Dave1027]

Re: Mercury Sportjet 90 bogs past 4K

The only place I used RTV that the manual didn't call for is the fuel check valve and carb elbows.

Why would you do that?

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

Grasping at straws at this point. The clean spots on pistons don't look right, could be loose carbon that washed off from fuel on intake side, could be from water leak. Improper torque on head bolts, warped head could cause the gasket compression ring not to seal. Too much RTV can cause poor bolt torque reading, RTV is never used in a OEM powerhead assembly. The only time it is really needed on a rebuild is as a gasket substitute(not advised) or to fill voids that a gasket wont, like corroded surfaces or poor surface mate.

Looks like you are getting plenty of fuel through transfer ports, doubt low CC pressure is a problem.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

RTV - Could this be the smoking gun of my ignorance????

I can always learn and be grateful if this is it, but the manual - I thought until tonite - see pic highlited in yellow - to apply RTV to the "apply sealant 92-90113-2 to cylinder head cover on cylinder block side of cover". Well in my mind this means between the cylinder head and the gasket on the cylinder head side.

But until tonight - I did not see the exploded drawing (see pic) that shows that the cylinder head seperates into 2 halves and sealant is applied between THEM and not the head gasket.

So this means for the past 6 weeks I have been chasing every possible GREMLIN on an engine that had a poor sealing head gasket due to my ignorance of using RTV on the gasket.

I sure hope this is it! I'll be happy to report that I've solved this problem and will report back later this week. I have a new head gasket in stock, I will pull the engine again (good thing I have a overhead lift in the garage). I'll carefully disassemble/reassemble with new knowledge (thanks to you) and hopefully have a boat running like a scared rabbit.

All for now - and thanks!

George

 

[milehighboater]

Re: Mercury Sportjet 90 bogs past 4K

Not to through anything more at you, but it sounds like you have worked over the motor preaty well. I do hope the gaskets were causing the problems but if it doesn't check you might check the jet housing. I don't recall you saying if the problem existed in neutral and drive or just drive. If it does not occure in neutral it maybe the jet inlet getting clogged or a bearing binding causing momentary load on the motor until it slows....

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Another good suggestion with appreciation. I was beginning to think of the pump being and issue also. I do have a complete spare pump in stock if the new head gasket (WITHOUT RTV) and proper torque doesn't solve it. I did hand turn the pump (easily) before putting the engine back in and it seems perfectly smooth - course I know that wasn't under load.

This engine/jet combination will only fast idle to 2500 rpm. There isn't really a transmission for a Neutral - just a divertor that goes over the jet to allow some water to go forward and backward at the same time to make a "Neutral". It is not possible to go higher than this RPM without leaving the fast idle mode - so it only happens in Drive. I have checked under the boat - and disassembled the back half of the jet to the impeller. All was clean with no clogs.

So if this solves it - can anyone grant me a wish of 8 more weeks of SUMMER here in OHIO?

I'll report back soon....

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

I appreciate everyone's replies. I'm making progress. I pulled the powerhead, thoroughly cleaned and replaced the head gasket with a new one (no RTV - learned the hard way). While I had the powerhead out, I also switched the complete ignition system back the newer one with the red stator, new switchbox, regulator, coils, wires, and rev limiter.

The new head gasket made a dramatic improvement in the engine's power, ease of starting, idle, etc. Also, no fuel/oil/water mix is leaking around the bottom powerhead to jet bolts. No fuel smell in the engine compartment - a first in a while.

So now the bog would be better described as a "surge". I immediately noticed an increase in hole shot power, ease of keeping the boat on plane --- for a few seconds ---- it runs great to 5000, then it's like someone is standing on the fuel source and QUICKLY slows to 4200,QUICKLY jumps to 4600, etc all without me touching the throttle. Once it finds it's happy place if I pull back on the throttle (in the usual spot for about 4000) it will run 4300 and stay on plane - surging only 200rpm +/-. Plenty of power and it runs smooth LIKE NEVER BEFORE THIS YEAR - but I can't get it to hold this RPM at WOT which is where I need it.

• I disconnected the black/yellow wire on the rev limiter just to be sure - no change.

I read something last night to pull the plugs immediately after a high speed shutdown and see what stories they have to tell.

• Top #1 - Wet - light fuel/oil mix - no carbon deposits

• Middle #2 - light carbon deposits - dry

• Bottom #3 - light carbon deposits - dry

I swapped spark plugs and leads - no change - still #1 was the wet cylinder. The coils tested fine last week - but coil #1 could be suspect or is it fuel?

Could my float be sticking in #1? I just had them apart this past weekend, cleaned, checked and reassembled. I did not pull the welch plugs (but did reseal them) so I could not get to the nozzle. I sprayed carb cleaner in everything and blew out with clean compressed air.

The new floats that came with the Merc rebuild kit were not adjustable. Could this be a problem? Should I try the old float? It seemed to be in good shape - just wanted to go for reliability on the rebuild.

Other thoughts. I can go back to the test tank again and be 1000% sure there isn't a fuel delivery problem - but fuel doesn't seem to be the issue - there is fuel to all 3 carbs and even when I pulled the head all 3 cylinders had fuel/oil in the bottom of them.

I'm just figuring there is too much fuel to carb #1 which is causing the smoke after the bog at the ramp when going under full load. (can't see it on the lake at speed). If there is too much fuel dumping would it create a wet cylinder? I'm guessing a loss of spark would also?

Again, everyone's thoughts are appreciated - I feel like I'm fixed a symptom with multiple problems or vv?

George

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

Swap top for middle carb & see if the wet moves with it.

Are you shutting down high speed before pulling back throttle?

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Pullling back throttle and then immediately shutting off.

Carb Swap? - I thought about that - but each carb has its own stock #

820614 - top
820615 - middle
820616 - bottom

So I'm assuming there is something different about each design? The rebuild kits were the same with the exception of the jetting sizes.

0.070 - Top
0.070 - Middle
0.072 - Bottom

Do you still think it's OK to swap?

George

P.S. - If the wet plug did move with the carb, we'd know it was the carb. Could a float (the new ones don't adjust but they were horizontal) cause this overfuel problem. I guessing it, or a sticky needle (the new ones had the red teflon or something on the tips) cause the fuel not to shut off?

George

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

I doubt carbs are any different, other than inlet fittings, but I don't know for sure. Also doubt a flood issue, if inlets are not closing you can pump fuel through with bulb.
Double check main jets for size and debris.
Check for carb base air leaks running with unlit propane torch, revs will increase with leak as propane gets sucked in.
Double check carb sync.
Do high speed shut down with just kill, leave throttle engaged, see if results differ.

Is comp test different with new head gasket?

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

I doubt carbs are any different, other than inlet fittings, but I don't know for sure. Also doubt a flood issue, if inlets are not closing you can pump fuel through with bulb. Double check main jets for size and debris. Will do - they were just completely (other than the welch plugs) dissassembled on Monday and checked - This is the part where I get confused. If a cylinder is getting too much fuel what is happening in a carb to cause this. Is the float too low and not allowing the needle to seat? Or an intermittant elecrical issue would cause a monetary no-spark which would result in excess fuel.
Check for carb base air leaks running with unlit propane torch, revs will increase with leak as propane gets sucked in. I tried this already but will try again - this time I'll remove the airbox.
Double check carb sync. Did check again today before reinstalling powerhead. Butterflies are right on horizontal at WOT
Do high speed shut down with just kill, leave throttle engaged, see if results differ. Will try

Is comp test different with new head gasket? Ran out of time today - but I'd say for sure it has increased. Will test next trip later this week. It was harder to hand turn the engine and it fires immediately and runs like the new motor that it is when it's happy with the fuel mix.

 

[CharlieB]

Re: Mercury Sportjet 90 bogs past 4K

Looking at the picture of the carb floats, I just noticed another difference that may be the cause of your problem.

The new floats do not have the cut out area for the main jet.

Pull the bowl from a carb and see if the float is CONTACTING the jet, if so, it is restricting the needle valve opening and restricting fuel flow.

Reinstall the old floats.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Wow - great eye Charlie. I am being lead to believe that the design of the NEW floats could be the problem and am planning to install the old floats that were in fine shape - I was just being overcautious.

But all my concerns were over the non-adjustable part. I never noticed the indentations. Just looked at the old float, and there may actually be two that are present on the old and not on the new.

I found another great post here

http://forums.iboats.com/showthread.php?t=423505

and I'll be doing these tests as well when I have them off tomorrow.

Thanks again so much. I've got this boat perfect for my needs now - just need to engine to perform like it should and I can live with it for years....

George

 

[gonufer]

Re: Mercury Sportjet 90 bogs past 4K

This thread of posts has been extremely informative, as I have the exact same issue with my 1997 Tracker Fishin Barge with a 1997 90 hp 3 cyl Merc (Tracker) outboard.

I had a no spark issue and replaced plugs, temp sensor, stator and trigger with no results. Took it to repair place (Pete at Outboard Doctor in Lake Havasu City, AZ) and all three CDI's (??) were replaced plus because boat sat for 18 months carbs gunked up. He rebuilt #1 carb and total damage was $600. Ran boat and disappointed with power range. Pete recommended rebuilding #2 & #3 carbs, which I did. Had some trouble figuring out how black barrels on throttle and shift linkage worked (I stupidly inserted plug into front-of-motor cover backwards after it fell out) and am sure we (wife assisted) moved them on the threads.

Motor now starts up and runs fine, but is doing exactly the same thing - revs up well above 4000 rpm, then settles (bogs?) back down to 4000 rpm even though throttle is pushed to wide open. I saw on another thread about doing a link & sync, which will be my next task. My neighbor (who owns 3 boats) recommended installing a basic fuel filter in between the squeeze bulb and the motor gas line attachment to the motor, which I did.

Sounds like carb bowls are not getting enough fuel at WOT (from my reading of earlier posts here) and possibly some kind of restriction. I hope the link & sync helps correct. Worry about running cylinders too lean and damage(?).

We are thrilled to finally have the boat back on the water, and just want to overcome this inconvenient problem.

BTW...I just recently joined and this is my first post. I'm obviously a newbie to this boat and motor stuff, but the info provided here is phenomenal....many thanks to all of you.

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

George-
You can easily tell if you have a flooding issue, if the bulb pumps up hard and no excess fuel pours out of carburetor then inlet needle is sealing to seat.
Flooding occurs when inlet valve fails to seal. The float has to raise high enough to push needle closed, the float needs to drop enough to allow bowl to maintain enough fuel for demand and a small reserve. Fuel is delivered to the intake by air passing through the throat of carburetor that creates a venturi effect which actually sucks fuel out of the bottom of the bowl through the main jet which blends with the air creating the correct air/fuel ratio. The more air that passes, the more fuel gets sucked up, this air is controlled by the throttle plate. When the throttle plate is fully closed, air is still passed through vents in the throttle plate and air vents that can be fixed or adjustable by means of an air mixture screw. This air is required to pick up fuel through the pilot circuit at idle and to maintain combustion. The bowl must remain vented to atmosphere to operate properly, the bowl vent is very important and is often overlooked. A blocked vent can cause a bowl to overflow, air is trapped on top of the float which restricts the float from rising and keeps inlet needle from seating. Fuel never fills bowl because air is in the way and fuel is pushed up the tube that fuel normally gets sucked through. Fuel continues to dribble out once a siphon is established with the fuel line bulb and continues with fuel pump pressure. When the motor is started at this point the over rich mixture is burned up(the excess smoke that smells very rich), then the carburetor starts working the way it is designed to, sucking fuel from a bowl that will never get filled to the proper level. A partially blocked vent can cause problems similar to a blocked main jet. The amount of fuel that can get sucked out is in direct relation to the amount of air allowed back in through the bowl vent.

That's the basics. The actual efficiency is carefully calibrated and adjusted through jet and vent sizes in relation to the venturi.

 

[79Merc80]

Re: Mercury Sportjet 90 bogs past 4K

Craig:

Keep in mind that the Tracker is actually a Mercury outboard, whereas the Sportjet 90 and 120 are a Force powerhead. The engines are nothing alike. Although they both have 3 cylinders, they have different CID.

Craig