1986 merc 90 six cyl troubles....please help

[divo]

I just purchased a used boat. It's a 1986 16' runabout with a 1986 mercury 90 inline 6. the sn is...a149755. Both are in very good contition and appear to have been taken care of very well. I put boat in the water and it ran good for about 5 minutes (at low rpm...less than 3000). then back down on throttle and try to accelerate and it wont go past 2000. it just loads up...drops some rpm's and jumps back up to 2000....but never over. I take it home...with service manual, 3 carb kits, fuel pump kit and plugs. Do the carbs (looked clean), do the fuel pump ( looked ok), changed plugs. I take boat back out and now it wont go past 3000 rpm. (sometimes 2500). it idles perfectly. if i rev it up a little to about 4000) in nuetral it sounds perfect. do carbs agin....all new gas, replace all fuel lines, install inline fuel filter,.When put in the new gas ( super premium) it increased the max rpm to 3000-3100. it seems like it is loading up. tank vent is ok. spark on all cylinders (at least when on muffs) primer bulb on gas line stays firm. The engine looks brand new under the cowl and the original owner installed a mercury service meter which reads about 80 hours. I dont know what to try next? Could it be electrical? Any help or ideas would be appreciated. Thanks

 

[emckelvy]

Re: 1986 merc 90 six cyl troubles....please help

Get a manual and do resistance checks on the high-speed and low-speed charge windings in the stator. Very common to see stator problems on this vintage of motor.<br /><br />The red and red/white stripe, and blue/blue-stripe wires are the pairs of the 2 sets of windings. The manual will give the specific testing procedure for your motor.<br /><br />One other thing to look at, on your motor you have (6) separate coils, it's not uncommon for these to develop cracks and cause problems. Visibly check for obvious arcing spots. You can also run it at night and check all the electrical components for cracks, and signs of arcing. Sometimes cracks will develop on the back of the coils and you'll have to remove them from their mounting brackets in order to do a complete inspection.<br /><br />I'd do the stator checks first, to rule that out as a problem and then work out from there.<br /><br />Really doubt it's a fuel issue as you've gone thru everything several times and the Inline fuel system ain't exactly Rocket Science!!!<br /><br />HTH and G'luck.......ed<br /><br />HTH.........ed

 

[divo]

Re: 1986 merc 90 six cyl troubles....please help

thanks for the reply. last night i fired it up in the dark an saw arcing in some of the plug wires. today i replaced all plug wires then took it out to test....no change. still cuts out 2500-3100 rpms. took coils off and looked at them....all looked good. did res checks on all coils...all within specs of manual. checked stators....within specs. tested the trigger....all readings were the same... but didnt match the specs. all three read 2.9 . Why am I getting that reading? could the trigger be the cause? tonight i will fire it up again and look for more arcing. I thought for sure the plug wires were it.....thanks

 

[achris]

Re: 1986 merc 90 six cyl troubles....please help

Divo,<br /><br />Have you done a compression test? Have you checked the timing and synch adjustments?<br /><br />It sounds like either a bad cylinder or the timing isn't advancing.<br /><br />Chris..............

 

[divo]

Re: 1986 merc 90 six cyl troubles....please help

ok.....after a couple nights of testing and tuning...today i took it out on the lake and the problem still existed. I took the cowl off and started messing with stuff while we were moving slow. when i unhooked the center yellow wire on the rectifier....it ran like a champ. hook it back up...prob comes back. I go get new rectifier...install it....still have prob. unhook center yellow wire....prob gone. there are three terminals on the rectifier. a red +, the center yellow, and on the left one is another yellow and a grey wire. the diagram i have shows the two yellows going to the stator. what should i do? what is the purpose of the yellow on the center terminal? it runs perfect with it unhooked. Thanks

 

[Clams Canino]

Re: 1986 merc 90 six cyl troubles....please help

Yellow outta be on the two sides with RED to the center. Look for a + mark - I think it's mis-wired.<br />(grey to either yellow - no matter)<br />-W

 

[dakk1]

Re: 1986 merc 90 six cyl troubles....please help

divo<br /><br />I have read several posts on this site with that symptom. Consensus suggests stator might be going bad. Do a search on "rectifier". You'll need a DVA to do a thorough test. About $35.<br /><br />Dave ..

 

[divo]

Re: 1986 merc 90 six cyl troubles....please help

Thanks for the replys. It is wired correctly. I was describing it off my diagram. when i when out and looked at it wires are hooked to correct terminals. I tested the stator resistance per the manual with a Fluke digital meter and it tested within specs. I cant find anywhere in my manual (seloc) another test procedure. What is a DVA? What does that terminal do? Can I just leave it unhooked? Thanks

 

[johnwilli]

Re: 1986 merc 90 six cyl troubles....please help

divo, did you solve your problem?? This is john 80 with the same problem. I am pulling my hair out after spending 3 weeks on this issue. I refuse to put any more money in this engine. I've been through the carbs, fuel pump, bulbs, plugs, plug wires, timing & syn too many times. The engine idles so well I sometimes can't believe it is a 2 cycle - but hesitates and stumbles on the way up to 3500rpm. Then dies, but starts right up.How do these reed valves work in a situation like this-do you know? Can it be fuel starvation in the carbs????

 

[Clams Canino]

Re: 1986 merc 90 six cyl troubles....please help

If it was the reeds it would show up at idle and low speed stuff. <br /><br />High speed stator winding.... or fuel delivery.<br /><br />-W

 

[divo]

Re: 1986 merc 90 six cyl troubles....please help

John80...I have it running perfectly but didnt really fix the problem. All i did was disconnect one of the yellow wires from the rectifier. I describe above what wire it is. I believe its for charging the battery. <br /><br /> Thanks

 

[johnwilli]

Re: 1986 merc 90 six cyl troubles....please help

Thanks for the tip. I went to CDI electronics which makes the switch box and trigger and they have a downloadable manual. In the manual. they say the voltage must stay within 12-16 volts. I've seen my meter read 20 volts. Pulled the darn thing and it did not check out to the spec. Motor ran great. Thanks.

 

[Clams Canino]

Re: 1986 merc 90 six cyl troubles....please help

Did you guys ohm out the rectifiers also? Are we sure they are good? <br /><br />I'm thining a shorted one might somehow interact with the stator depending on how the stator is wound. <br /><br />-W

 

[divo]

Re: 1986 merc 90 six cyl troubles....please help

my manual had no test for the rectifier so i never tested the old one. I did replace it with a new one from the merc dealer which didnt fix the prob. so now it runs perfect with the new rectifier ($40.00) unhooked. Thanks for all replies.

 

[Clams Canino]

Re: 1986 merc 90 six cyl troubles....please help

Prolly a stator issue then....<br /><br />-W

 

[SaltyDawg69]

Re: 1986 merc 90 six cyl troubles....please help

I seem to have the exact same problem. Do any of you guys have a pciture of the wires you are talkign about. I have a 1983 Mercury 90 Hp inline six and it does the same thing.. runs fine for the first 30 min of an outing, and then will not go above 2700 rpm while in gear. Out of gear i can runt eh rpm's as high as I want. Water pump's good and i think it has to be electrical.

 

[johnwilli]

Re: 1986 merc 90 six cyl troubles....please help

My 1971 80 hp would idle fine but had a slight almost unnoticable miss up through the rpms and then cut out at 3500rpm. Went through complete fuel system, rectifier, trigger, timing and syn, compression and spark. Finally realized the bottom spark plug was unusually clean. Looks like water in the bottom cylinder which lead to the misfire of no4 at higher rpms.

 

[johnwilli]

Re: 1986 merc 90 six cyl troubles....please help

My 1971 80 hp would idle fine but had a slight almost unnoticable miss up through the rpms and then cut out at 3500rpm. Went through complete fuel system, rectifier, trigger, timing and syn, compression and spark. Finally realized the bottom spark plug was unusually clean. Looks like water in the bottom cylinder which lead to the misfire of no4 at higher rpms.

 

[Hoserpoppasmurf]

Re: 1986 merc 90 six cyl troubles....please help

If you go to outboard parts dot com (http://www.outboardparts.com/), they have the complete procedure for checking out the electrical system on these motors. I got too late smart, after ordering in a complete set of ignition for mine, with no change, I discovered my $40 rectifier was gone. Give it a try.

 

[Clams Canino]

Re: 1986 merc 90 six cyl troubles....please help

Originally posted by SaltyDawg69:<br /> Out of gear i can run the rpm's as high as I want.

No you can't. Operating an inline with no load (neutral) above 2000 RPM is a great way to blow it up.<br /><br />Assuming the carbs are clean, the fuel tank is vented properly, and the boat is not overpropped (common error) - then some kind of electrical thermal intermittant is suspect. Starting with the stator.<br /><br />-W