Mercury SportJet 90 Stalling

[wakataka]

Re: Mercury SportJet 90 Stalling

JetDriven - <br />I notice the smart tabs come in two different sizes that might work for our size boats. One lifts 60 to 80 pound and the other lifts 80 to 100. Do you know which version you've got?<br /><br />My boat may be a little easier to get on plane than yours, but I still need to be moving about 20 mph to stay there. And when pulling a wake board, the boat really strains to get on plane. I think these could really help.<br /><br />Thanks for the tip.

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

I have the 80 to 100 pounds of lift model. I think we need that much lift. If you've spent some time in other outboards of our length, you'll have noticed that our boats are much more stern-heavy than your average 14-16 foot runabout. <br />Before I bought these tabs, I was looking for another boat to take my family out in. Now, with the new performance, I am no longer looking for a bigger boat. You will be amazed at the difference in performance. It's like moving the driver up to the front of the boat. Really. If you're going to buy them, save yourself some money and order them directly from the manufacturer at NauticusInc.com<br />It's at least $10 cheaper than Overton's.

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

Stalling Again! <br />The boat runs 38 mph at 5400 rpm, idles great, and will run all day at 4600 rpm or less. <br />The problem is when I open it all the way up WOT, it runs perfectly for about 15 seconds and then it stalls. After stalling, it will start right up and seems to run fine as long as I don't open it up all the way.<br />The tank is venting. It's not the rev limiter. It stalls the same with the rev limiter unplugged. <br />It is not losing spark. I ran it with a timing light hooked up and when it stalls the engine is still getting a spark. I am quite sure that it is fuel starvation.<br />The fuel pick-up in the tank is clean. The filter is clean. There is no check valve in the fuel tank or line. The fuel pump check valves work well. The fuel pump diaphragm seemed OK, although it may have been a little stretched out. I installed a new diaphragm since I had it all apart. The vacuum/pressure line from the engine to the pump is good, too. <br />Does anyone think that a stretched out fuel pump diaphragm could have caused the WOT fuel starvation, but at half throttle or so it can run all day? Anyone else have any ideas? I'm running out of them.

 

[wakataka]

Re: Mercury SportJet 90 Stalling

JetDriven - <br />That's a real bummer.<br />Can you describe how it stalls? Does it seem like the motor is being pulled down under a load, does it just cut out like you turned off the switch, or does it sputter and kick? Does it quit running altogether when it stalls or pick up again when the RPMs drop? I'm hoping it's not seizing again.<br /><br />Do you have a squeeze bulb on it? I mounted one on mine for a time just to do diagnostics. I don't think the Coast Guard would approve the installation because the bulb wouldn't pass the flame test for inboard installations. It could help eliminate the fuel pump / tank pickup as possible problems. If the bulb collapses under load, the fuel pump is working but there is a restriction in the line upstream from the bulb.

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

Great suggestion about the bulb. I do have one, so I'll bring it along next time I take the boat out. I have an in-line electric fuel pump I can try, too. <br /><br />The engine is not siezing. It runs like when it was new except when at WOT. When it stalls, the engine drops off just like you ran out of gas. It still has fire going to it, though. <br /><br />Thanks for your suggestions.

 

[kenimpzoom]

Re: Mercury SportJet 90 Stalling

Jet,<br /><br />Two things:<br /><br />Need to take a look at fuel pressure and see how that holds up.<br /><br />You could try the electric fuel pump to see how that works.<br /><br />Did you run the timing light on all three cylinders? I know its a long shot, but still possible. Doesnt really sound like ignition, but this boat has fooled us many times.<br /><br />Ken

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

I did not use the timing light on all three cylinders. <br />I have a fuel pressure gauge now. I'm going to install it as far down the fuel system line as possible...between the top carb and the middle carb. At least that way I'll know what's going on.

 

[wakataka]

Re: Mercury SportJet 90 Stalling

Jet - <br />Are you sure it isn't seizing? I'm bugged by the fact it stalls completely from WOT. If it was a weak fuel pump, the top carb would tend to run out of fuel first, it being higher. Seems to me the engine would run rough for a few seconds before it stalled. Does it start up immediately after it stalls?<br /><br />What size main jets are you running? I've seen a post on this board somewhere that there was a service bulletin specifying richer jets for the 90hp motors than were originally spec'd in the manual. I run mine at an elevation of about 700 ft with 0.072 in the top and middle carbs and a 0.074 in the bottom. The manual says to use 0.070 and 0.072 at this elevation. If one cylinder is running too lean, it can seize and cause the engine to stall. Check your plugs to see if they show any indication of a lean running condition.

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

When I tore down the engine, I saw no evidence that cylinder #2 was seizing. I think that I have had a combination of a bad piston and a weak fuel pump. There was no aluminum on the cylinder wall when I took out the old piston. I really don't think it is seizing now. It runs too well, when it runs. Starts right off without ever having to use starting fluid. Idles perfectly. <br />I put a pressure gauge in the fuel line. With the new fuel pump diaphragm, it makes 7 psi of pressure when cranking the starter. An hour later it still held 6 psi.<br />I did some searching on this forum for "fuel pump pressure" and I found several matches where people were describing the same type of behavior that I have seen. Their problem was a fuel pump one. I think I may have it licked now. I'll know for sure when I get it in the water again.<br />Thanks for all the suggestions.

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

I don't know the jet size. I forgot to look when I had the carb apart. I will let you know when I can get to it.

 

[jetdriven]

Re: Mercury SportJet 90 Stalling

The problem with my boat was some trash in the gas tank that was intermittently plugging the fuel pickup line while pumping fuel. It looked like a chunk of rubbery gasket sealer. The boat runs really well now. Fuel pressure is 10 psi all the time. Looks like this adventure is over!!!

 

[Jdeagro]

Re: Mercury SportJet 90 Stalling

Jetdriven;<br />It is nice to here your happy with the Smart Tabs. If you have any performance questions please do not hesitate to contact us. In any event as I scaned the postings and did not notice if you have picked up some top speed since you instsalled the Smart Tabs. If yes then they are adjusted correctly (should pick up about 2 to 4 MPH). If not, reduce the pressure at the plate bracket only. Move the Ball stud about 1/2" closer to the plate, then try it again. Should not take more than a few min.