1996 90HP Johnson SPL intermittent low WOT RPMs

[mr300z87]

Hi All I am long time Iboats forum member, most of my time on the forum is over in the MerCruiser section since that is how my boats are powered. I am posting here to try and help my friend with a problem he is having with his 1996 90HP Johnson SPL. He had an issue with some water in the fuel, so he pumped out his tank, drained the carbs, replaced fuel filter, and added a large water separator. The water separator was changed again after some fresh fuel was run through the engine. Since then then engine has started great, no stalling but at WOT will intermittently only reach 4500 rpms, one day he ran it all day including pulling kid on the tube and said it ran awesome but the RPMs were down. Next day he took her out to do some trouble shooting and WOT is back to the normal 5400. Then the next time out back to 4500 WOT. He brings it to a friend at well known and trusted Marina they do basic trouble shooting and are baffled. This one day running 4500 and then the next 5400 went on for a week and half until last friday when he took the boat out while he returned the real world of work (he is a teacher LOL). The boat in now sitting in my driveway,

One of the suggestions I had made to him was to check for a sticking choke which I am not sure if he did. So the other day I pulled the cowl for a look. My experience with carbureted outboards dates back the the early 80 with a 50 HP Merc and like on the 50 I expected to see a solenoid activated choke plate on the front of the carbs. What I saw was a solenoid with what looks like vacuum lines going to the carb. I did notice while pushing the choke button (key) every few time it would not click on. Would this solenoid sticking cause the lower WOT RPMs? Can some one explain to me how the choke on these foreign to me engines work?

What else should I look at that would cause the engine to behave this way? I also suggested the cleaning all the electrical connections and maybe carb rebuild.

I have not been out with him when the problem has occurred but from a visual inspection the throttle seem to be opening all the way, he check the fuel vent by opening the filler while running. I am supposed to go out on the boat with him this Friday and would like to have a few thing to check.

As always any advice is greatly appreciated

Mike

 

[interalian]

It's a primer that works when the fuel pump has pressure. Solenoid opens and fuel goes to the two small lines then up to a nipple with a small jet on the top of each carb. Double check the lever is inline with the body of the primer solenoid (normal mode). If it doesn't click every time you hit the key, check for corroded wires at the primer body, and its ground. You can simulate an 'open' primer by pressing the key in whilst underway, checking whether it affects WOT operation in this case.

You might try pumping the primer ball when it's running at the lower WOT rpms and see if it picks up. This would indicate a weak fuel pump or loose lines letting air in before the pump.

 

[mr300z87]

So if figured I would do a quick follow up to this. On Friday even though it was only 65 degrees on Long Beach Island, NJ we ventured out anyway in the little 17 foot Aquasport Osprey CC, the bay was nice and flat for our test run. After a little bit of tough start (more on that below) we headed out for some WOT runs. The fist thing I noticed was the tach kinda jumping around. as we cruised a little to warm her up to operating temp. Then at WOT she sounded great, had plenty of power to get up on plane and ran smooth but the tach only showed 4800. We checked the speed using the GPS on my phone and say 35-37 MPH and to my ear she sounded just right. After checking linkage (all good), pushing the choke (RPM drop) and squeezing the fuel bulb (no effect) I gave the tach a couple good hard taps with my finger and it jumped right up to where it needed to be at 5400 RPM :facepalm::facepalm::facepalm:. So for all my friend fretting over this it was just a BAD Tach. Can anybody confirm what the MAX RPM should be on this 1997 Johnson 90 HP SPL as there is not a lot of information found on these engines.

Another issue he is having is water in the fuel. I think due to the type of vent he used to replace the broken one. The new one has 2 hole that face down but since the boat is so low in the water it is getting splashed. We changed out the water separator, which had quite a bit of water in it, (rough start issue explained maybe??) and then cruised for a good hour and a half from Beach Haven to almost through Barnegat Inlet 4000 RPM 26-28 MPH on the phones GPS APP. I told him to fix the vent and pump the tank out to be ready for next year.

 

[interalian]

I've had the same issue with my tach. I guess a 1983 OMC tach can only be expected to work for so long. And you're right close to your max RPMs. These are designed for 5500 max, and make rated power from about 4500-5500.