86 Mercury 90HP Timing?

[TimKirk]

I'm really hoping that someone can help me. Situation is this: I've got a professionally rebuilt powerhead (86 90HP Straight 6) that I can't get started. I've had to rebuild the starter twice because it overheats and burns a brush. With the plugs out, the starter spins the motor like a champ. I've got good spark, and I've got wet plugs when I pull them so I think I have good fuel flow. With the plugs in, the first spin is great then it really slows and stays slow until I wait 20 minutes and try again. Think this is a timing issue?

 

[mcginleyc]

Re: 86 Mercury 90HP Timing?

hey tim, I think the first thing to tackle is that starter burning out. The starter will burn out if there is not enough voltage going to it. If its turning over slowly there is heat buildup and the things hate that. You probably have corrosion in the connections, loose connections, or too long or too small gauge battery cables. This creates resistnce and reduces the flow of electriciy to the starter. Go through makes sure everything is in good condition and not corroded. I doubt its a timing issue. Also the carbs should have been rebuilt along with the powerhead. Good luck

 

[TimKirk]

Re: 86 Mercury 90HP Timing?

Thanks for the info but I really believe that the starter is good. It spins the motor great without the spark plugs in and it spins equally as well for the first 5 seconds after they are re-installed. I've replaced all of the cables, the battery and had the starter rebuilt professionally. The carbs were also rebuilt with the powerhead. This weekend, I shot a thimble full of gas directly into cylinders 1 and 4 via the plug holes and quickly put the plugs back in and tried to turn it over...not even a chug-chug. Does anyone have any other ideas? I'm dying to get out on the water.

 

[Chris1956]

Re: 86 Mercury 90HP Timing?

Tim, I had pretty much the same problem. Fully charged battery would spin the starter fast for only about 5 seconds, then it would slow way down. It turned out to be a bad positive battery cable. The cable got a nick in the insulation, almost up at the gunwale (where it was not easy to see), salt water got into the cable and rotted all but 3 strands. Since the insluation was intact, I only found it by spotting a "deformity" in the cable. I spliced in a new cable and the starter spins very well for minutes at a time!

 

[TimKirk]

Re: 86 Mercury 90HP Timing?

Thanks Chris. I too found a bad cable two weeks ago and I've already replaced it. In fact, the list of new parts is growing...<br />Rebuilt powerhead with good compression, rebuilt carbs, rebuilt starter, new plugs, new cables, new waterpump. I tried to see if I could measure where the coil was firing this weekend by turning the key to the "run" position then rotating the flywheel by hand; I never saw the spark! When I crank it over with the key in the "start" position, the spark is strong and constant. The flywheel seems to line up at zero when cylinder 1 (first in sequence) is at TDC. Could the trigger be out of relationship to the flywheel?

 

[TimKirk]

Re: 86 Mercury 90HP Timing?

CORRECTION...The motor is an 84 and not an 86. I doubt this helps with anybody's diagnosis, but it's worth noting.

 

[Chris1956]

Re: 86 Mercury 90HP Timing?

Tim, I am not an expert on Stator-driven ignition (ADC), but I think you need a timing light to see when the plugs are firing. Your manual should advise on this.<br /><br />I will bet that the timing on your motor is way off.

 

[Yepblaze]

Re: 86 Mercury 90HP Timing?

I second the motion that the starter should not slow.<br /><br />I think it needs to spin it fast enough to energize the ignition.<br /><br />Maybe you just have a deep cycle battery or one with a dead cell. Try a jump from your car and see how it spins.<br /><br />If the starter has been abused it may have shorted the armature causing itself to draw excessive amperage, along with perhaps a marginal battery only to exacerbate the issue.

 

[mcginleyc]

Re: 86 Mercury 90HP Timing?

Tim, the reason the starter will turn over the engine without spark plugs installed is that there is no compression being made in the combustion chamber. An engine is harder to turn when plugs are in because there is compression in the secondary area of the cobustion chamber. I would suggest you get a multimeter and do a voltage drop test. The manual should show you how to. If you don't know tim I can give you some instructions. Good luck tim. Also you can't adequately check timing with out the engine turning over at a decent speed.

 

[TimKirk]

Re: 86 Mercury 90HP Timing?

IT'S RUNNING!!!!<br />Many thanks for all of the replies. We went out last night and pulled three plugs to get some speed to the cranking. I put a timing light on it and saw that we were 3 degrees ATDC. After some timing adjustments and moving a spline on the timing linkage arm, we were able to get it set to 21 BTDC. Then we fooled around with the carbs and opened them up a lot. Put the plugs in and it tried but at this point, the battery was cooked. I broke the conventional rules and put two batteries in series to supply 24 volts and bam, cough, chug-chug, she fired right up. We tuned the carbs back into spec and got her purring wonderfully. After it ran for a while on the hose, I went back to 12 volts and she started right up. Then we took her out for the first time this year!!! Moral of the story, timing was off (my first guess), carbs needed opening, battery is shot (replaced today), and I need a new starter for a comfort factor. Anyone know where I could find a starter for an 84 Inline 6, 90 HP Merc?<br /><br />Thanks again everyone!<br /><br />Tim

 

[mcginleyc]

Re: 86 Mercury 90HP Timing?

Hey tim, glad to hear its running. I guess I was off on that diagnosis. But nice job. Something to keep in mind. There are 2 timing spec. One is full throttle and the other is idle cranking. They should be in the manual. Good luck