'98 mercury 3cyl.40hp mysterious problem

[JP]

Hey crom or anyone that can help, I'm a first time user ;however, I have been reading iboat questions and mostly crom's answers with great interest for some time now. I find myself needing help with a motor problem that I know I can fix once a diagnosis is made. <br />Problem: motor runs great at various throttle positions including full speed (5000)rpm for 10 - 20 minutes or so after initial startup but then suddenly cuts back to about 3400 rpm at full throttle. Once the motor starts this behavior, it will not run at full rpm (5000)without coaxing. I did discover that the engine will run at 5000 rpm if I manipulate the electronic push button choke intermittently. <br />Facts !!:<br />1. After extensive mason jar test, I can find not a trace of water in the gas tank. 2. I did replace the entire gas supply line including the pressure bulb anyway. <br />3.I have throughly cleaned all three carbs twice using OMC's top engine cleaner and that helps for a short while. <br />4. I am finding a white substance in the air box and around the carb. throats each time the engine is run at the lake.I know that two cycle oil turns white when mixed with H2O so this has to be water getting into the motor some how.<br />5. The motor is not running hot.<br />6. The power packs have tested good.<br />7. I can find no evidence of water in the oil tank that supplies the injectors.<br />8. I found that a cleaning spray of gas into the carb. throats at the dock will fix the problem temporarily.<br />9. This motor is on a 21 ft. Tracker pontoon.<br />10.Pulled the flywheel-keyway in good shape.<br />11.My summer has been ruined because I can't figure this problem out.<br />12. If I had the money, I would pay to have this problem corrected. This is really frustrating me because I feel that this has to be something simple. <br /> Please Help !!!<br /> Thanks, JP in Ky. on Caverun

 

[corm]

Re: '98 mercury 3cyl.40hp mysterious problem

Hi JP,<br />I'll come down and tell you how to fix it<br />while I'm fishing off the boat.It may take<br />2 weeks or so.

<br />At first I thought you had carb iceing, but the motor has a 110 degree thermostat so that<br />should not be the problem. I would rebuild the fuel pump with the heavy duty diaphram kit.Didn't have time to get a part no. from the service-aid today. Have your dealer look it up or I'll post it when I get home from work thur about the same time as tonight 11:30. Have you checked for va***e leaks. (crankcase,reed plates) The water in the air box is from the hummidy condensing as it cools comming in the box through the carbs.<br />Does it run if you squeeze the primer bulb<br />repeatedly while the motor is running WOT?<br />Hope this helps.<br />...have fun...corm

 

[JP]

Re: '98 mercury 3cyl.40hp mysterious problem

Thanks crom for your suggestions. No, squezing the pressure bulb at WOT does not have any effect on the motors' preformance. In my initial posting I was afraid of being to long with the stated facts so I kept it as short as possible. Here are some additional facts I did leave out. <br /><br />Additional facts:<br />1a. This motor was bought new as a '98 leftover in '99 and has less than 50 hours total running time at present.<br />2a. Replaced the fuel filter.<br />3a. Tried squezing bulb at WOT-didn't help.<br />4a. Checked fuel pump pressure with motor running. Demonstrated sufficient pressure.<br />5a. Checked cylinder compression with all plugs removed. Results are as follows: <br />Cyls.<br /># 1 (top)----120 lbs/sq. in.<br /># 2 (center)-120 lbs/sq. in.<br /># 3 (bottom)-120 lbs/sq. in.<br /> I was wondering if the motor could have a blown head gasket since the cylinders all show the same pressures, or if the water jacket portion of the head gasket may be blown causing a leak of H2O into the cylinders, but the motor has tremendous power when I can manage to coax it into running at WOT for a while. It seems that if the head gasket where blown, the motor would lack power but thats the one thing the motor has plenty of when it desides to run at WOT. <br />I'm pretty sure that water somehow getting into the carbs. is the #1 problem. What I need to do is narrow the possible modes of travel the water can take into the carbs. and start a process of elimination. <br /> Crom, thanks for your suggestions. I will double check the fuel pump and in the meantime I've decided to try a new fuel tank(6 gal size) in order to completely remove any doubts about the fuel source being contaminated. Oh yeah, I'd wait 'til Sep./Oct for the musky to start if I were you. Thanks Crom.<br /> Man, this site is too cool. JP

 

[corm]

Re: '98 mercury 3cyl.40hp mysterious problem

Hi JP,<br />Muskies,I thought you only had Bass down there!<br />You already have the heavy duty fuel pump diaphram in that motor.<br />To find out if water is getting in the cyls<br />run it at WOT with hood off shut the key off<br />and take the plugs out as fast as you can(ah<br />after the toon stops so you don't fall in and scare someone to death and wear a life vest)If water is in there it will show as little beads on the plugs. I don't think that the head gasket is blown,1 cyl would be at 20-40 lbs of compression. Also run it with the hood and air box off to see if there<br />is a spray from somewhere.You can also look<br />down the carbs and see if the gas fog looks the same in each carb. If one carb has a largegas mist in front of in(out side the front of thr carb throat) you may have a bad reed valve.<br />Water use to get into motors by comming up the shift shaft,spraying on the bottom cowl<br />an then getting into the carbs. You will see this with the hood off,have some one you trust drive the boat when looking. Swimming off the back of the boat while moving is grounds for a ticket if lucky,worse if your not.<br />Use plastic pliers to pull off a plug wire<br />at a time when motor is acting up to thr to isolate which cyl it is. pull a bad one and motor will run the same,a good one it will cut down more.Make sure your hanging on when you do this,no swimming on this test either.<br />enough for tonight. need sleep.<br />...have fun...corm<br /><br />------------------<br />cubic inches = power<br />power = speed<br />speed = FUN

 

[JP]

Re: '98 mercury 3cyl.40hp mysterious problem

Corm, Thanks again for the info. We're going to the lake tomorrow to give it another try. I'll let you know what happens. By the way, Caverun Lake is the Musky capital of the south. Thanks JP

 

[JP]

Re: '98 mercury 3cyl.40hp mysterious problem

Hey Corm, when to the lake this past Sat. (8/12)and came home a in a better mood than usual after our typical trip to the lake. I also Found myself a little more confused about my boat problems. Here's the facts:<br /><br />1b. Started out and the motor ran perfect on the new 6 gal. tank for about two minutes.<br /><br />2b. Ran the motor at WOT and immediately pulled the plugs to check for H2O beads. Found NONE.<br /><br />3b. Pulled the air box and looked inside while my son Jesse(great guy) drove the boat. The motor ran great at all throttle positions including WOT with the air box off.<br /><br />4b. Looked inside the carb. throats while running at WOT. Observed a slight sprinkle of liquid hovering in front of carb. #3(bottom carb.). Couldn't determine if the liquid was fuel from the carb. or H2O from the lake splash because the position I was in would not allow me to let go of the boat. I think it was H2O because the liquid was hitting me in the face and did not cause my eyes to burn.<br /><br />5b. Replaced the cowling and ran the motor without the airbox at WOT for about 5 minutes and the motor went back to its' old habits of bogging down to about 3400 RPM.<br /><br />6b. Removed cowling and ran the motor with each of the plug wires disconnected in order 1 -3 to observe the effects on the motor. I found that removing each plug wire ,one at a time, made the motor run noticeably different so it seems that all cylinders are functioning at some level. <br /><br />7b. After stopping an hour and a half or so for a late lunch, started the motor(airbox still removed, cowling on) and headed towards the dock. The motor ran perfect all the way back to the dock which was about a 4-5 mile trip.<br /><br />8b. Came home wishing that I had found something wrong so I could fix it. But no, I'm right back where I started...lost.<br /><br />PS - I can understand why some problems just have to be turned over to professional mechanics who deal with these problems every week. Being a shop teacher and a decent machanic, this has almost become a "mission impossible" for me. I haven't given up yet. Thanks JP

 

[corm]

Re: '98 mercury 3cyl.40hp mysterious problem

Hi JP,<br />I've got it! I'll mix some bat wings,eye of newt,frog feet,ground turtle claws,with a few secret ingredients and send them down.<br />I think a little voodoo is in order. We have hung bats over motors and invoked incantations before. Mostly on old Scott-Attwaters however.

With some success

<br /><br />For a moment I thought you had an exhaust leak inside the hood till your 7b.statement.<br /><br />Hope you had a nice lunch any way. I'am also glad you hung on tight,those tricks are not<br />recomended but necessary sometimes.hehe<br />I'll page through the service manual to see if something slaps me on monday.<br />...have fun...corm

 

[Mav23rick]

Re: '98 mercury 3cyl.40hp mysterious problem

I have the exact same problem with the same motor???
I've been pulling my hair out trying to figure out whats wrong! I just read a posting about a simular problem that was caused by heat. It turned out one of this guy's trigger swiches was bad and only acted up when it got hot!
You said: "7b. After stopping an hour and a half or so for a late lunch, started the motor(airbox still removed, cowling on) and headed towards the dock. The motor ran perfect all the way back to the dock which was about a 4-5 mile trip." Anything you may have removed may have cause a better cooling effect on this part. Tell me if this helps... Mav, huntgide@charter.net
If I find something I'll tell you...