88 90HP Johnson not running more than 3000RPM

[myrion74]

I was out yesterday and the engine was running fine for the last few months and most of yesterday. I was running at 5000 rpm or so and then it made a sputtering noise and then slowly started throttling down till finally it would not go over 3000rpm (7 knots). This happened over a period of less than 5 minutes. At full throttle it will not go any faster than that with a load on it, with no load it runs fine. It is not overheating, I shut it down for over an hour, checked the fuel filter and water seperater, the water seperater had small black flecks in it. I cleaned the filters and the engine still does the same thing. In a nutshell engine was running fine and then in less then 5 minutes it started losing rpms and power even with full throttle until 3000 rpms. No smoke, hot water, extra heat or anything else out of the ordinary, any ideas? I could use all the help I could get especially with the time of year that we are in for fishing. Thank you in advance.

 

[myrion74]

Re: 88 90HP Johnson not running more than 3000RPM

Also, I smelled strong fuel smell after I got to the ramp after getting fuel, there was fuel slightly leaking before the trip into the bilge from the tank area. When I was done ands put it on the trailer 4 hours later there was roughly 1.5 gallons of gas in the bilge. The problem I have is I have a 25 gallon aluminum fuel tank under the deck that I can not get to unless I take the center console and deck up. This probably is a fuel issue but i am not sure which kind. Again, any and all advice will be of great help and taken seriously. The compression was done 4 months ago and checked out fine.

 

[emdsapmgr]

Re: 88 90HP Johnson not running more than 3000RPM

When the engine was down on rpm, did you repeatedly prime the fuel hose bulb? If you have a fuel delivery problem, the carb bowls can starve for fuel and can loose rpm. Repeated priming of the hose bulb will force fuel into the carb bowls to keep them full of fuel. Those engines will seem to run fine when running on the garden hose, even when one spark plug is not firing. You need good spark on all 4 when the engine is under load. Check the spark after the engine has warmed up. Possible you have a fuel leak somewhere between the fuel tank and the engine. Any fuel leak into the bilge is a dangerous problem and requires immediate attention. A fuel leak can starve the carbs for fuel, creating a lean conditon and the running issues you describe. If the boat is a few years older than the engine, the fuel lines may be deteriorating. Older fuel lines are not made to handle the ethanol in today's fuels. The black dots in the water separator could be pcs of the fuel hose which are delaminating-causing a leak. You may want to pull the fuel hose out and inspect it. Perhaps attach a new hose to the end of the old one when you pull it out from under the deck. Also check the fuel hose connections. Many boats which have a belly tank also have an access panel in the floor to service the fuel pickup and float sender. Some have a removable floor section, others have a 5 or 6" round plastic inspection cover which screws in. If you know where the fuel pickup is under the floor, you can cut a round access hole, then cover it with one of the those round waterproof covers.

 

[myrion74]

Re: 88 90HP Johnson not running more than 3000RPM

I did pump the ball repeatedly with no difference. It runs smoothly but will not clear around 3000 rpm. It feels as if the engine is only running on one cylinder, definately not more than one due to the speed. I had all the wiring recently replaced, the carb cleaned out, compression test, and a new power pack. Would one bad power pack possibly cause the engine to run this way? I do have an access round panel on the deck above the float sender that I am going to check out and find the leak and also look in the tank to see if I can find a problem. Also the fuel overflow vent on the side of the boat I believe is clogged. When I filled the tank before my last trip the fuel came out of the top and not out of the vent as it is designed to, could this be the cause? I will check the spark and the fuel lines today also. If anyone has any other ideas I would appreciate hearing them. I will also keep you posted if I find the issue so that I won't waste anyones helpful time and advice, thank you.

 

[ezeke]

Re: 88 90HP Johnson not running more than 3000RPM

It's essential that any gas tank have a properly working vent; the fuel pump cannot overcome a vacuum greater than it's physical limitation.

A bad power pack can cause the engine to run poorly. Use an inductive timing light underway to "read" the spark across the range of RPM and you will be able to tell if ignition is your problem. You will need a helper to do this.

 

[emdsapmgr]

Re: 88 90HP Johnson not running more than 3000RPM

It is not unusual for the fuel vent line to become filled, esp if it a middle section of it is lower than both ends. Fuel can overflow into the vent line and get stuck there. Usually that fuel will not cause any running problems, as air will still usually still pass through the line into the tank. It can cause problems when you try to refil the tank, as you noted. Best to try to have the vent hose positioned so that it does not have a low spot along it's length.

 

[reeldutch]

Re: 88 90HP Johnson not running more than 3000RPM

Before you do anything, i would check the compression.
sounds like you are 1 cylinder down.
could be either fuel, spark or compression.
start with the easy ones first.
compression and spark.

 

[lovemywife]

Re: 88 90HP Johnson not running more than 3000RPM

Before I did anything else I would make sure I found the source of the fuel in the bildge. That is a major problem I think once you find that everything else will work. I would replace all the lines once I got to them.

 

[maj_hazard]

Re: 88 90HP Johnson not running more than 3000RPM

Just my 2 cents...

I agree that finding the source of the fuel leak is your first concern..

Outside of that I had this problem 3 times now with my 86 Johnson 40HP. Lots of RPM until it was in gear and under load and then nothing above 3000 RPM..

The first time the problem ended up being some dirt in the hi speed jet on one of the carbs.

The second time ended up being a fuel pump failure which was resolved with a new VRO..

The third time was the result of a prop strike on a rock. This issue has not yet been resolved.. I suspect I've bent something inside.. and just to ashamed to take it apart.

Hope this helps

Cheers

Hazard

 

[amitorgad]

Re: 88 90HP Johnson not running more than 3000RPM

I have exactly same problem. during cruising in deep water, engine dropped RPM's and would not go over 3000.
Starting, idling, and without load, its perfect.

Changed power pack, plugs (wires are relatively new), tested firing with light device, testing compression, rebuilt carburetors, changed fuel filter and lines, ran the boat on external fuel tank with fresh fuel.... nothing helped.

One thing to note is that after running the engine two plugs are dry and two are wet.
Current assumption is that fuel is not hitting my lower cylinders.
Will try to take apart and rebuild carburetors again...

If anyone has any additional information, please share.
I am trying to deal with this mess for months now.