Engine won't accelerate under load

I've got a 1988 90hp Johnson that won't accelerate under a load. I can start it and rev the engine fine in neutral but when I try to go anywhere, the engine maxes out at about 2000 rpm. I've replaced the plug wires and plugs and verified that gas is getting to all four carbs. Any ideas?<br /><br />Thanks!

Re: Engine won't accelerate under load

had a similar problem with a merc 115 and it turned out to be the fuel pump diaphrams.<br />of course could be lots of things....<br />good luck,<br />M.Y.

Re: Engine won't accelerate under load

Is there a rubber key in the prop shaft. Maybe it's sheared?? This would prevent acceleration of the boat, but it should not prevent the motor from going over 2,000 rpm.

Re: Engine won't accelerate under load

Thanks for the suggestions so far. The prop is fine, not too sure about the fuel pump diaphragms, I'll have to check that out. Does anybody know whether it could be a problem with the power pack and how I could tell?

Re: Engine won't accelerate under load

I have heard that if you sqeeze the primer bulb and that increases your rpms, you can identify the fuel pump as your culprit.

Re: Engine won't accelerate under load

Hi PWhittier,<br />Just a couple of things that I would check first.<br />1. Is the timmer base moving as the throtles are opening?<br />2. Are the throttle plates opening all the way?<br /> ( you can check this by the roll <br /> pins on the throttle shafts. They should be pointing straight up when the throttle is wide open)<br />3. Are you getting spark on all cyls?<br />4. How is the compression on all cyls.?<br />Good Luck

Re: Engine won't accelerate under load

Thanks almost retired. You've given me a couple of things to check out. A couple of questions though. <br /><br />1. I'm not familiar with a timmer, what is that?<br /><br />Guess that was only one question. The throttles are opening all the way though, I could see that when I had the cover and the air inductor off.<br /><br />Thanks again.

Re: Engine won't accelerate under load

The timer base is under the flywheel connected to your throttle linkage by a rod. When you open the thorttle all the way it should advance to a stop on the intake. Pull all your plugs and check compression on each cylinder, they should be close and around 100 psi. Check it with the kill switch on (no spark) and the fast idle lever open all the way. If they all check even then I would reconnect all and check for good spark on all cylinders. If that is good then try running it and pushing choke in or squirting pre-mixed fuel thru carbs and see if RPMs increase. If they do you are starving for fuel, check for a restricted filter or loose hoses. If none are found remove the carbs and rebuild them. The main thing is to clean the high speed jets and change float seats and needles.

Re: Engine won't accelerate under load

I am by no means an expert here, but I do know my way around engines. I have just been battling the exact same issue with my Johnson 65, and have received several good ideas here on this board. Amoung them, pulling each plug one at a time to detect a dead or non-firing cyl. If there is no drop in RPM, that cyl. is not firing. Also, cleaning the carb of deposits from the oil. <br /><br />Something I found when troubleshooting, and I now think is the root of my engines problem, is a hairline crack in one of the ignition coils. The spark is intermitently jumping to ground on the coil mount, and there is a subsequent miss in the engine. So I think under load, it just cannot go. Mine bogs down at exactly 2000 RPM.<br /><br />One other issue that I had several years ago, and created similar but not exact symptoms, was a fuel line that was clogged, probably by deposits. The engine would idle, but no run under load. I got rid of the entire fuel line and replaced it.<br /><br />Good Luck!<br />TG

Re: Engine won't accelerate under load

Thanks for all the excellent suggestions. I'll be putting them to the test tomorrow and Saturday.

Re: Engine won't accelerate under load

If the fuel line and other suggestions don't resolve the problem, it might be time to clean/rebuild the carbs. You can get a carb kit from any dealer with all the parts you need. <br /><br />Get a manual, if you don't already have one, and jsut pay attention to how things came apart so you can put it back together properly. While you've got it apart, use some aerosol carb cleaner to clean the jets and passageways.<br /><br />Good luck. Let us know how you made out.

Re: Engine won't accelerate under load

what are you pushing with this motor?<br />also when was the last time you used this befor the problem started?<br />the first thing I would look at is fuel system <br />who old is the fuel?<br />pull the carb drain plugs out and fire up engine there should be roughly the same amount of fuel comming out of each carb. <br />at 2000 rpms is where the carb stats its transition from the slow speed curcit to the high speed side if the carb is durty you may just need a rebuild and sinc.<br />next are you over loading the boat?<br />to much prop?<br />Good luck