Mercury Sportjet 90 bogs past 4K

[SeaRayder4]

Hi guys, here's one that's been driving me crazy this year. I rebuilt a Mercury (built by Force) Sportjet 90 (the same powerhead as a 90hp 3 cylinder outboard) completely this past winter.

New pistons, seals, bearings, connecting rods, gaskets, etc. Newer electricals, new fuel pump. Breaking-in went fine and for about 20 hours perfect except a SLIGHT hesitation between 4-5K rpms. Would get through it and then not reoccur for several hours.

Now EVERY time I try to go into this range (4-5K) to get on plane, after a few seconds it hesitates and returns to 3500 or so. I get the same symptons on a 6gal test tank so I know its not the fuel supply the fuel bulb remains hard even when its bogging. New plugs (no help), all fuel lines were new at rebuild, all new check valves in the recirculation system, new BOYSEN reeds, new rev limiter...I'm stumped . I can run all day at below 4K with no problems, but with this jet boat I need the 4-5K range to get on plane.

I have great compression 125+ on all 3 holes.

Any and all thoughts are welcome. After much searching and another 3 hours of trying new things today with no help, I thinking...

Rectifier or Stator? Both have less than 50 hours on them. Someone said interference from a failing one can cause the rev limiter to trigger?

Any help is appreciated. Below is a pic link if you're not familiar with this engine.

http://i894.photobucket.com/albums/a...talled-Web.jpg

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Bump - no one out there ever have a similiar problem? I changed the NEW fuel pump just to be sure. Same symptoms. Motor runs great - just need the last 1K RPM to hit the 4-5K range and stay there.

Checked coils - all fine - spark hot. Someone mentioned the stator going bad will produce the same symptoms? Thoughts anyone???

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Some updates with no solution:

1) Installed new switchbox, rectifier, stator and flywheel - all from stock that I had so no real cost to me. NO CHANGE. All coils have tested spec resistance and output voltage.

2) Tested for vacuum leaks with un-lit propane torch - NO DIFFERENCE

3) Have tested spec voltage at all critical measurement points with a $100 Merc DVA meter - all specs are fine. Hot spark that jumps 7/16" per spec

4) As mentioned above - fuel pump has also been changed, run from a portable 6 gal tank with fresh gas - carbs were totally (new floats, jets, etc) rebuilt less than 40 hours ago (as well as the complete powerhead).

5) WOT timing is dead on at 30 degrees per manual with timing light. Tested at dock on trailer running WOT.

I'm stumped - maybe have to pull the engine and look for more clues somewhere - but I've sure learned alot

Thoughts anyone? BTW - is there a way to measure crankcase pressure? The only seal I did not change (oh no....) was the bottom crankcase seal. If it's leaking will this cause a drop in WOT? As I said before, it runs to 5000 just fine, but drops off quickly after about 10 seconds to 3500, then I could run at 4000 all day if I wanted to but the throttle handle must be in the full on position.


Thanks for anyone's thoughts and I promise to post a followup when it's fixed. Google is full of forums without answers...


George

 

[CharlieB]

Re: Mercury Sportjet 90 bogs past 4K

A restriction in a fuel line after the primer bulb will alow the fuel level in the carbs to drop once load exceeds fuel flow and reduce RPM to that limited fuel flow.

Finding the problem will require you to disconnect the fuel line at each point between the primer bulb and carbs till you find the restriction.

It could be as simple as a piece of old hose stuck in a fitting, or a split/flap in a hose folding over and restricting flow when the fuel flow increases with throttle and load.

I found one once by taking off each line one at a time and reversing it, testing between each change till finally the problem quit, then replaced that hose.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

I appreciate the feedback Charlie, but all lines were replaced (all fuel lines were new at rebuild, all new check valves in the recirculation system, all new recirc hose as well) at the rebuild less than 40 hours ago. I'm considering pulling the carbs and having a look see - but they too were totally rebuilt using a Factory Service manual for specs. All jets were confirmed correct.

 

[CharlieB]

Re: Mercury Sportjet 90 bogs past 4K

Still, I would disconnect the fuel line at the carbs, fire it up and CHECK fuel flow.

Just because hoses are new does not ensure that the liner didn't split when it was shoved over a fitting, causing a restriction in flow.

Simple to check flow and it can help isolate a problem.

Many times we get stumped by defective new parts, thinking that 'It CAN'T be....'

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

Sounds like the bowls are running dry at WOT.
Does it do it repeatably easing it back up?

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Yup - it's repeatedly. The strongest time is the first in a series, but after the first bog if you wait about 10 seconds it will rev back up again (but bogs sooner). If I wait about 30 seconds - it will run back all the way to 5K and hold for about 10 seconds and then adjust down to 3500-4000. This boat needs 4K and above to stay on plane - that's the frustration. It will run at 4K all day long with the throttle fully forward - but has no power or if it falls off plane due to a wave - I don't have any reserve power left. Compression is 125+ in all 3 cylinders.

Thanks again for everyone's thoughts. I'll be pulling the lines again and tracing everything related to fuel. I did notice that the new carb kits do not feature adjustable floats any more (and indicated as such in the kit - but they were in alignment when installed). See pic of before (on left) and after (on right).

Anyone have any experience with lack of crankcase pressure causing a bog? I did the crankcase sealant per the Factory Service Manual but it still seemed to be a little inadequate IMO.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

The other thing that is interesting is that there is lots of blue smoke AFTER the bog. I can see this when I'm testing with the boat attached to the trailer at the dock running full throttle. It's a sight to see for sure! But that kinda seems to tell me that it is not the absence of fuel? Does that make sense? Seems like the fuel is still there?

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

The other thing that is interesting is that there is lots of blue smoke AFTER the bog. I can see this when I'm testing with the boat attached to the trailer at the dock running full throttle. It's a sight to see for sure! But that kinda seems to tell me that it is not the absence of fuel? Does that make sense? Seems like the fuel is still there?

That sounds like ignition dropping out at WOT and relighting the residual when revs drop and fires again. Trade out the box and try it.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Thanks for the suggestion Craig - but here's the frustratiing part (s). I changed the switchbox, rectifier, stator and flywheel - all new from stock that I had so no real cost to me. NO CHANGE. All plug wires are brand new at rebuild. I do have an entire ignition system from a working engine that I'm thinking of moving over to it (3 hour job or so) - but it is the older ring connector ignition with a black stator - so I know it's not a long term fix.

All coils have tested spec resistance and output voltage. I changed the flywheel since a "rare" ignition problem is when a magnet in the flywheel loses its magnetism. This symptom has been going on since early summer and here in Ohio only a few good weeks are left so I've pretty much written this season off for tubing with the kids .

Anyway, I even used an inductive tachometer on each plug wire to see if I'm losing ignition to a cylinder just before the bog happens - no change. I checked voltage out of the pack (switchbox) to each coil while the engine is under full load and just before the bog - no drop in voltage.

These suggestions are wonderful everyone - I appreciate them. I've learned so much...but apparently not enough yet.

No one seems to be thinking I'm losing compression through a leaking crankcase though - and it seems to be my only real thought at this point. But, I am going to pull the carbs and fuel lines and look again before I pull the engine. I keep telling myself this engine has to run (and run great ) with all the care (and $$$) I put into this rebuild. I'm even beginning to wonder if a piston is heating up (they're Wiseco) and causing extra mechanical resistance.

I'm also thinking a bad ground or wiring harness problem - but the symptom is very predictable and I'd think this gremlin would be more random?

Once this is resolved I PROMISE to post a followup. After many hours of reading dialog on forums - too many never say something like "problem solved it was the ...." Sure would help.

I'm also going to take a video so you can hear the bog and how it sounds at launch. I wear sound muffling headphones when I running full throttle on the trailer with the boat hooked to the van. I've had more than one person smartly tell me "boats are more fun if you un-hook them from the trailer...

Still listening to suggestions everyone...

George

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

Be sure you aren't pulling stray signals from other wires with the inductive gauge. If a leaking crankcase is causing the problem, I think it would show up throughout the rpm range.

 

[CharlieB]

Re: Mercury Sportjet 90 bogs past 4K

Check fuel flow at the carbs.

It runs O.K. up to speed and then the carbs run low on fuel, you slow it to a point the bowls can refill then you can run it up again.

Classic symptoms of a restricted fuel supply.

Just because the hoses are new does not prove that one does not have a problem, or a restricted fitting somewhere in the system, even a defective outlet check valve in the new primer.

Careful testing of flow starting at the carbs and working back thru the system til you find it.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Charlie - I'm going to do this test early next week. Forgive me for asking (but still learning) - I've tested the fuel pump for output (while cranking and disconnecting the line from the bottom carb - see pic) and also a pressure meter in line from the pump to the bottom carb. Besides removing the carbs and checking each hose for restriction, etc - are you saying to disconnect the hose to a carb in the chain while it is running? The fuel pump feeds the bottom carb (see green arrow in pic), then passes to middle, then top.

I also installed clear red fuel recirculation lines (see pic) that allow me to see the fuel flowing through that system while the engine is running. All check valves were new and tested when installed as working. No fuel accumulates at the bottom of the airbox like it used to when I had a restricted valve.

The fuel line that flows from carb to carb (see pic) is thick walled (since the sportjet is an outboard head mounting inboard - Coast Guard regs) and is very difficult to remove without seperating the carbs. I guess I could change it to a thinner more flexible tube for testing.

Can you elaborate more on the test procedure? I appreciate it. If it is a running test, does it need to be done at full throttle - or just looking for some fuel output to each (and is this done by simple pulling the fuel hose at the carb?).

Might be my imagination - but I seemed to notice that when I was running the engine without the airbox on that the bottom carb seems to have some more suction into the intake at idle then the middle or top. Is this normal?

Be sure you aren't pulling stray signals from other wires with the inductive gauge. If a leaking crankcase is causing the problem, I think it would show up throughout the rpm range. - Thanks for the reply Craig. I tried the test at multiple pickup points and moved the wires around some. Good thought.

Thanks again for everyone's input!

 

[CharlieB]

Re: Mercury Sportjet 90 bogs past 4K

Fuel flow can be tested cranking. This can be troublesome in the confined spaces of a sport jet, but necessary to ensure that this motor does not self destruct due to a lean running cyl(s).

There could be a restriction between any of the carbs, lowering the fuel level on the remaining carbs in the line that will cause a serious loss of power once the throttle/load meets then exceeds the fuel flow to that carb.

This is a dangerous situation at that cyl(s) are now beginning to run LEAN, driving the combustion temps high enough to do serious damage, like melting the piston.

Substituting longer fuels hoses between the carbs (adding an extension for testing flow only) is an option, but permanently using a longer hose can cause a pressure drop and a reduction in flow which could possibly cause a lean condition in the last carb, which would NOT be good.

Next question, you stated that it seemed as if the lower carb passed more air than the uppers, double check your link and synch, if that lower carb is not fully closed at an idle, then that cyl is what we call 'leading' the motor, in effect it is trying to run faster than the others and this will also cause cyl heating because that cyl is working harder than the others. Again, this is a recipe for disaster and can melt a piston.

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

You may have some stray RTV floating around in the carb bowls. Most 2 stroke gaskets need to swell with oil and don't require any type of sealant. From your pictures it looks like you used some on the fuel fittings which may have squished through?

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

Update after a LONG WEEKEND in the GARAGE

Well, once again I appreciate everyone's thoughts and advice. After more deliberation I decide to pull the engine and have a THOROUGH going over to see if I could find the SMOKING GUN that has caused me to lose a summer of fun tubing with the kids.

Well here's what I did and found in this order after pulling the engine and working on the bench where it's easy to see everything.

1) Changed / Cleaned every fuel and air hose, fuel elbows, ball check valves on the entire engine. Verified clean and like new or new.

2) Changed ENTIRE electrical system (Flywheel, stator, harness, rectifier, switchbox) from a working engine that has poor compression and needs rebuilt. NO CHANGE -

3) New Fuel Pump from Stock - NO CHANGE

4) Removed Carburetors and thoroughly cleaned and inspected them for sticking floats, clogged jets, removed blue sealant in pics, looked for broken parts, idle tubes, gaskets, etc. All seems well. See pic

Here's some GREMLINS that I found that were:

A) A small stray nut in the intake manifold for bottom carb laying on the reeds. The nut is about the size of a pencil eraser and fits the linkage on the timing - but none were missing so somehow I missed this on engine assembly.

B) Thought for sure I found it here. When I assembled the crankcase, the previous builder used an improper stud and I replaced it as shown in the pics. But after installing the timing linkage I missed that the stud was too long and restricted the timing from fully advancing at full throttle. I THOUGHT FOR SURE THIS WAS THE SMOKING GUN. I removed the flywheel and stator, cut the bolt and reinstalled the linkage and it now moves freely throughout the entire range. The bad news ? NO CHANGE

C) Cylinders (especially top #1) had excessive (I feel) carbon on them for an engine with maybe 40 hours, but it has never successfully been run in WOT to burn it off for any length of time. So could be perfectly normal, but after a THOROUGH cleaning with GM Top Engine Cleaner and a soft brass brush, there was no melting and they seem fine. The cylinder walls in all 3 holes still have great cross-hatching in them. Though maybe this could be the culprit ? NO CHANGE

D) When setting the cranking timing to 32 degrees and inspecting the carb butterfly sync I also noticed that the bottom carb was way past horizontal. OH ? I?VE FOUND THE PROBLEM NOW I THOUGHT ? fixed problem was due to a nut slipping on the sync bar ? replaced entire bar with perfect bar from stock ? NO CHANGE.

When I lake tested last night ? the boat did all of the usual things. Started easily, warmed up just fine. I even installed a head temp gauge that allows me to easily tell the temp of the engine and it warmed up as normal. I tried multiple attempts for the boat to hold full throttle (getting about 4950 on my digital Actron timing light) and dash tach. Stays there about 5-7 seconds running perfect?.then the bog to 3500-4000, lots of smoke and it I slow it down a little, it will adjust and run fine all day below 4000. If I let the ?whatever ? fuel I?m guessing? clear, then I can run up to WOT for a few seconds before it bogs.

Last night I even ran it on plane with just me in the boat at 4000 for about 2 minutes (before I backed off) but I had to keep the throttle in WOT position. It would just hold plane with me in the boat.

I?m running out of ideas but learning a lot about this engine. If I finally fix it and find the problem I feel I can keep it running for years as all other problems it should have in the future should be easy to find with the skills I?ve learned.

Because of the excess smoke AFTER the bog, it seems to be like a carb is dumping too much fuel. See my FLOAT and REED questions below. I thought I was losing a spark to a cylinder under load, but after many hours of testing and the complete change of the electrical system as mentioned above it doesn?t seem to be electrical related.

The (2) ideas I have left are:

1) CARBS - The carb rebuild kits I got from Merc had a different type of non-adjustable float that they said in the instructions replaced the old float design. They do not have a metal tab to adjust the float level, but were perfectly horizontal when installed. See pic of before/after in carb photo. Maybe I should try to old plastic adjustable floats again. They seemed to be in good shape, but my rebuild of this engine wanted it to be reliable, and I didn?t spare coin in replacing stuff
2) REEDS ? The reeds in this engine are Boysen Reeds which are supposed to be better than stock. When I pulled the intake manifold to remove the stray nut they appeared to be in perfect shape. I lightly lifted up on each reed from the inside to be sure none were stuck or damaged. Should I change back to stock reeds? Could this cause I have some in stock from a working engine that needs the block rebuilt.

Other than buy a completely new fully dressed powerhead with warranty for $4000, I?m out of ideas. Sure hope I can help many in the future with what I?ve learned.

Thanks again for everyone's input. I'm in Columbus Ohio if anyone is close enough to see the boat in action (or should I say non-action) it would be appreciated.

Happy Labor Day to all you folks on the lake with your WORKING BOATS!

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

A few more pics as mentioned above showing stray nut in manifold and timing linkage crankcase stud problem.

 

[SeaRayder4]

Re: Mercury Sportjet 90 bogs past 4K

BTW - Charlie - before doing any of this mentioned above, I did pull all the carb hoses and verified flow while cranking as you mentioned. I pumped the fuel into a clear jar, fuel flow seemed consistent and was perfectly clean and green - easy to see though with no contaminents.

 

[Craig-]

Re: Mercury Sportjet 90 bogs past 4K

What does spark plug from bad cylinder look like, picture?