1999 Sport jet 175 low RPM with WOT

[jarey2002]

Hope i'm posting this in the right forum... it is technically an outboard.

I had problems with my 99 mercury sport jet 175xr2 last year, took it to mercury dealer and had them rebuild all three carburetors. It seemed to run ok after that except RPM at WOT were only about 4500 or so max (service guide says it should be around 5200)

It again ran like that for a few months this season until a few weeks ago. Took her on about a half hour idle cruise after dark and parked it. Went to start it a week or two later, it started ok, but when i took her out on the water and went to WOT I only had about 4000 RPM... after about 4 or 5 seconds it immediately cut down to 3000 RPM and will stay there. Then after you take it down to idle for a few seconds you can go back to WOT and get the 4000 again for a couple seconds then right back to 3000. After doing this a few times it will finally stay at 4000 for a long time. Still definately not the 5200 RPM I should be seeing.

Does this sound like a carburetor problem or is there something else I should be looking at? Sounds to me like the engine is getting too much fuel

Thanks!

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Took the carburetors out last night... is there anything I could look at? I assume the first place to look will be to see if the floats are adjusted correctly

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

You need to determine if this is a restricted fuel problem or a weakening ignition problem. Either will limit power and RPM under load.

Once the carbs are cleaned, rebuilt with new gaskets and reinstalled, leave the fuel line off, pointed into a suitable container, crank the motor and observe the fuel stream. Your should have a strong pulsed flow the diameter of the hose, any less indicates a restriction caused by any of a weak fuel pump, aging hoses, clogged dip tube or fitting.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

That's a good point, I'll try checking the fuel flow after i reinstall the carbs. I did notice the right bank of spark plugs after running a little while were completely dry, but the left sides all had a slight dark oily looking residue (mostly on the threads). Could that point to anything??? The plugs probably only have a couple hours on them.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Well, finally got re-cleaned the carbs and installed them back into the boat. Still have the same problem. I took your advice CharlieB and removed the fuel pump output hose and checked the flow while cranking. It looked like a good strong pulsed flow like you mentioned.

What should I check next???

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

Switch the switchboxes and see if the spark plug appearance also switches sides, if so it may be a weak SCR in the switchbox, if no change it may be a weak high speed winding on that side of the stator which should be verified by DVA testing before ordering any parts.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Oh yes... I forgot one more vital piece of information. After about 2 to 3 minutes of running WOT at 4000 RPM today the warning alarm started (constant) which the operations manual says is an engine overheat condition. Could that be caused by a possible electrical problem as you mentioned???

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

If the water pump and impeller are known good, AND the (I swear I've lost part of my brain as I know what the part is but can't remember the correct name!) by-pass valve is good, then over-loaded cyl's, one side of the motor, is working it's butt off and overheating, trying to accelerate.

FEEL the motor when this happens, I'll bet you fine one side hot to the touch while the other is still relatively cool. If so, swap the switchboxes and retest to see if the condition switches sides.

If it remains the same, DVA test stator output, it is possible that the high speed windings on one side are weak and limiting spark on one side at speed.

There is NO RETURN on electrical parts, you need to prove the fault as parts get spendy fast. You only want to order what you really need.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

I'll bet you're onto something there. And yes I know electrical parts are expensive. I replaced the stator assembly last year so I at least know that it is good. I'll try the running it and feeling both sides and let you know what I observe.

Thanks again.

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

An internal short in a switch box can overload, overheat, and kill a stator.

If the stator has low output on one side it very well may be that switch box is going and taking out the stator again.

Retest the stator output, DVA test the stator connected, then again disconnected. If the connected readings differ from one side to the other, then the switch box with the lower reading is a problem. Original switch-boxes are recommended to be replaced in pairs, CDI boxes are more durable and can be replaced one at a time. If you need to replace boxes, use CDI parts. But DO NOT order anything until you have tested and PROVEN the fault(s) as there is no return on electrical parts, and you already know what a stator costs.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Charlie, I think you are definitely on to something...

I ran it today (it was much cooler ambient temp) after about 3 minutes I did not get the over-temp warning, but ran it back in and felt both heads. The left side (if I was sitting in the boat and facing the engine) was very warm but not enough to burn skin... but the right side was luke warm at best. Knowing this, I think tomorrow I will try changing the switch boxes and see if the warmth follows.

 

[Garrett1992]

Re: 1999 Sport jet 175 low RPM with WOT

might be dropping cylinders. check for spark

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Well, I was able to swap the wires on the switch boxes and give her a water test today... and sure enough, the hot side of the head followed with the switch box swap. So I can say for sure that I have ruled out a mechanical problem with that bank of cylinders... right?

But, I don't think I have ruled out either the stator or the trigger assembly. I think this because I swapped all the wires which included the input wires from the trigger and stator assemblies. Should I swap either the stator or trigger (or both) back to rule either of them out... or will that cause a timing problem with the switch boxes? I noticed on the schematic that the OEM marked one of the switch boxes with a yellow sleeve on each set of wires coming off it so you can tell the difference between the two. The stator has different colors for each side (red, blue, and red/white, blue/white) but the trigger wires are the same for each side (white, violet, and brown). So would that mean that it wouldn't matter which side of the trigger feeds which bank of cylinders?

Thanks!

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

The triggers are fine or you would have a dead cyl on each bank.

Switch the stator leads between the two boxes, if the weakness follows the stator lead swap then you have a weak high speed winding on one side of the stator.

If it remains on the same side you have a weak capacitor in that switchbox.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

I will do that as soon as I get a chance, but in the meantime... quick question. You state that maybe there is a high speed winding in the stator that has gone out, but I have noticed that it won't stay idling like it should when I return to the lift. It holds around 800 RPM or so for a little bit, then finally dies. Would that be any indication that a high speed winding most likely isn't the case?

Thanks again!

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

OK, i swapped the stator wires and the heat stayed on the same side as the previous test. So I guess I'll plan on purchasing a switchbox. Now the manual says they recommend to replace both at the same time... does this need done, or is it just a suggestion?

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

That is a factory 'recommendation', it is not carved in stone. I think their reasoning is a bad Bias on one box blocking spark on the other box, places excessive load on the capacitor if the non-sparking box.

Many have successfully changed out just the one faulty box without a problem.

CDIElectronics build excellent replacement parts at very reasonable prices, AND their boxes are sturdier.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Well, got the new switchbox and installed it. Man what a pain changing those out on my jetboat is... almost no room to get to the screws and such.

Took her for a water test and had much success. I was getting the full 5200 aprox. RPM for about 5 minutes or so. She was running great! After those 5 minutes or so though it started to slightly hesitate. Not as bad as before, but was fluttering more like 4200 RPM for a little while. I decided to park it and I felt both heads. Couldn't feel any difference in temperature between the two sides. They were both somewhat hot, but not enough to burn skin.

I did inspect the switch box I removed and the only physical issue I could see was that on one side, the epoxy "potting" that they install in the plastic box was separated from the plastic box. This could let water in I suppose.

Do you think the hesitation I'm seeing now could be the result of the other switch box going bad? Do the switch boxes fail slowly, or do they typically fail suddenly?

Thanks!

 

[CharlieB]

Re: 1999 Sport jet 175 low RPM with WOT

Was the new box a CDI?

Have you repeated the spark test since it seemed to 'act up'?

It is still possible that the remaining old box is 'on-the-edge' of going bad. You will soon know.

Double check fuel pump output, V-6's are thirsty at full throttle, and like a lot of fuel.

They also like to run nearer the top of the RPM range, like 6000, you might consider moving one of the shims from the nut side to the inside of your pump, giving just a couple thou morre impeller to case clearance, letting the motor spin just a few hundred RPM higher, that reduces a small amount of load.

 

[jarey2002]

Re: 1999 Sport jet 175 low RPM with WOT

Yes, the new switch box was a CDI

I have not checked the spark since changing the box.

I did check the fuel pump flow a month or so ago during my troubleshooting and found that there was a very good pulsing output of fuel both at idle and full throttle.

So I suppose if I see the same symptoms as before show up that it most likely is the other switch box. Well, at least the boat is in semi-working order now. I can pull a skier up if needed and it was going fastest i've seen it go since owning it.

Thanks for all your help Charlie!