90HP Merc, Occassionally "drops out"

[John In Wisconsin]

I have a 1979 (I think..may be slightly older) , 90 HP Mercury outboard Model 900 that will run very smooth, then occasionally when running at higher speeds, all of a sudden something happens and engine RPM and speed drop significantly, then sometimes after a second or two, the motor takes off again to orignal speed. The motor does not completely stop and does not sputter or miss, It's like someone takes the throttle and pushes it back half way real fast, then goes back to full throttle. Seems to happen more after I boat for a while and engine is warmer. Replaced fuel lines, plugs, distributor cap last year trying to fix it, no luck. Compression is good, and it does run and sound great, except for this problem. Thinking it may be an electrical problem or the fuel pump (never had worked on yet) ? I also had fuel problems around the same time, bulb woudl go flat after I stopped engine for a while, replaced fuel lines at the end of the season, dont know if bulb problem is completely fixed yet, still "drops out".

 

[waynebo]

Re: 90HP Merc, Occassionally "drops out"

Is it like u shut it of or is it a missing/sputtering type of pulldown in rpm's? If its sputtering im thinking trash in in the main jets.might want to instaall a spin-on type fuel filter.You may have to also remove carbs and screw out the jet in the high speed jet well and blow the trash out.waynebo

 

[John In Wisconsin]

Re: 90HP Merc, Occassionally "drops out"

No "sputtering" or missing occurs, its like someone took the throttle and pushed it back to 1/2 throttle real fast, then after a second , or two (sometimes longer) pulled it back to full throttle and it runs fine again. I can sometimes boat for 1/2 hour or more going full throttle on and off when skiiing with no problems at all, then it starts acting up., Sometimes it acts up sooner. I was thinking it was electrical since its more pronounced after the engine is hot..like a couple cylinders drop out or something. (Its a straight 6 cylinder)

 

[Gold Bear]

Re: 90HP Merc, Occassionally "drops out"

John In Wisconsin <br /> <br />Something to try.... I had a inline 6 Merc act the same way. I found that over time the fuel pump diaphram got stretched (probably from squeezing on the primer bulb to hard!!!).<br /><br />Rebuilding the fuel pump worked for me. <br /><br />Good Luck,<br /><br />Gold Bear

 

[KIWI MERCMAN]

Re: 90HP Merc, Occassionally "drops out"

John<br />distributor cap? 79 model had ADI ign with six coils and 2 switch boxes.<br /><br />But your problem does sound like a ign problem, for it to turn on an off so fast it has to be droping spark on 2 or so cly's.<br /><br />Let us know about the cap.<br /><br />Kiwi

 

[John In Wisconsin]

Re: 90HP Merc, Occassionally "drops out"

It does have a distributor cap, was told it was a 79, it is on a 76 boat though. So maybe I was told wrong and its actually slightly older.<br /><br />I did have a problem with the primer bulb going flat then losing its prime when I stopped for a while, couldnt pump it back up, Had to disconnect at engine and press in the check balls and then pump it to get gas back full up in the bulb. Struggled with this all last year, got fuel line replaced from tank to motor, they found two leaks. This was way at the end of the year when I finally got it fixed...and I'm not sure if the gas problem is really fixed or not yet.<br /><br />I didnt have them rebuild the fuel pump, since the prohblem was intermittent they didnt think that was it. Seems the "dropping out" problem and gas problem could both be attributed to the fuel pump diaphragm as in the above reply. Does seem strange (to me anyway) it could be the fuel pump since it is intermittent and seems to happen more when engine is hot.<br /><br />Thanks for the input so far, let me know what you think about the fuel pump being my problem.

 

[John In Wisconsin]

Re: 90HP Merc, Occassionally "drops out"

Edited original Topic to clarify some points, please comment. Thanks