1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

[sjj623]

Hello,

This past weekend my 1979 1/2 (it actually says 1979 1/2 on a white sticker along with timing specs) Mercury I6 90hp serial 52999xx lost 600 rpm and 5 mph like someone flipped a switch. The motor is attached to a 1983 17 3/4' Seasprite 889 Sandpiper fiberglass runabout. Two people on board with 15 gallons of fresh gas in the boat.

We were headed back to the launch ramps at full throttle when the engine dropped 600 rpm and about 5mph. I throttled back and stopped the motor to check the brand new prop (13x17) since I had a similar problem last year after catching 100 yards of braided line on the prop, the prop and intake were clean. I started the motor and idle seemed smooth. The fuel bulb was hard. I throttled back up with my passenger in the back next to the motor. We got back on plane pretty normally and trimmed out but max rpm was only 4000 and boat speed was 30-31 mph on gps. Normally we are 4600 rpm and 35-36 mph. My passenger tried pumping the fuel bulb, it was hard. I throttled back to idle and the engine idled normally around 600 rpm. Not wanting to run cylinder(s) lean we slowly headed back to the dock and trailered the boat back home.

I checked the fuel screen at the fuel pump and it was clean. The fuel lines are less than 2 years old and in good condition. All three carbs were rebuilt last year due to a fuel soaked float flooding the engine. I pulled the plugs and they all looked normal. A compression test has cylinders 1,2,5 at 99 psi 3,4 at 100 psi and 6 at 95 psi, not perfect but within tolerances, yes? I have a fixture that I can mount all 6 plugs in and watch them all spark or not spark. They all had spark at cranking speed. I put new plugs in anyway.

The motor was timed last year when the carbs were done but I pulled out the dial indicator and checked it anyway. Timing pointer, timing adjustments, and carb linkage are right on per the manual. Full throttle at the helm opens the carbs fully.

Next I tested the stator per my Clymer manual. Blue to red should be 5400-6200 it is 6380, out of spec. Blue/white to red/white should be 5400-6200 it is 6410, also out of spec. Red to ground 149, in spec; red/white to ground 149, in spec. According to this I should replace the stator.

Next I tested the trigger assy. Brown to white with yellow sleeve should be 1100-1400 it was 1407. White to violet with yellow sleeve should be 1100-1400 it was 1346. Violet to brown with yellow sleeve should be 1100-1400 it was 1428. According to this I should replace the trigger assy.

All six blue colored coils passed the resistance test.

At this point per my manual I should replace the stator and trigger for being out of spec. I have ordered new replacements especially since they look original and those resistance values will increase as the wire windings heat up while running.

My first question is this, will the out of spec stator and barely out of spec trigger assy cause the problem I am having?

I have a fluke 77 series 2 meter and will be making a dva per the drawing on the forum.

While I am waiting for the parts to arrive I am beating myself up by finding websites that that show my stator values can go as high as 7000 ohms which would mean my stator is ok. Additionally since my trigger is only slightly out of spec I am further questioning my ordering of parts prior to checking peak voltages.

I am assuming my next step should be testing peak voltages at high rpm.

Is it possible to lose spark at one cylinder at high rpm and have it return at lo rpm?

Suggestions and experiences are welcome.

Can someone help me out with the peak voltage values?

Thank You.

 

[Chris1956]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

You will want to reprop that motor to get it to 5200-5500 RPM, after you fix your problem. Check the plugs for color. If one is real clean (steam cleaned), you may have a water intrusion issue. You have good blue spark at all plugs? None of the plug wires are arcing to ground?

 

[ZigZaggin]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

Sounds like you are dropping a cylinder. Possibly running lean? I just went through a very similar behavior with my 1985 90HP inline 6. My issue ended up being a couple of problems. One was the result of my own mistake. Check out my post at http://forums.iboats.com/showthread.php?t=379740.

I would first check to see if you have spark to all of the cylinders at WOT. There are low and high speed windings on the stator and although I've read that you can have spark at idle and not at WOT my friend that's a marine mechanic told me that when a stator goes it usually just goes and will be dead. You can check WOT spark with an inductive timing light and someone else driving the boat. Or leave your boat strapped to the trailer and do it at the dock. Much easier with 2 people. In my case the #4 cylinder had no spark due to a faulty coil and then shortly after the coil was replaced I had intermittent spark to the same #4 cylinder and it was due to a faulty switchbox. I replaced my switchboxes (there are 2) with CDI and they recommend you replace them in pairs and to NOT mixmatch a CDI switchbox with a Mercury one.

Also, double check all of your ground wires. One could've vibrated loose. Happened to me. Lastly, I have heard of some rare issues where magnets on the flywheel can come loose and have to be re-epoxied. However, this is a very rare problem.

If all that checks out you could have a fuel restriction. I know you said you had just rebuilt them but with the cowl off and someone else driving the boat CAREFULLTY try "hand choking" the carbs and seeing if the performance changes. That's what diagnosed the final problem on my motor and I was working with a friend/mechanic. On my boat one of the carbs had noticeably less suction.

Also, I agree with the recommendation to re-prop. If you run these motors WOT below 5K RPM you run the risk of detonating pistons.

-zig

 

[sjj623]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

You will want to reprop that motor to get it to 5200-5500 RPM, after you fix your problem. Check the plugs for color. If one is real clean (steam cleaned), you may have a water intrusion issue. You have good blue spark at all plugs? None of the plug wires are arcing to ground?

Thanks for the reply.

The Clymer manual I have been using up to this point recommends 4500-5000rpm for this motor. I am losing faith in the Clymer manual since heading over to cdi electronics to use their troubleshooter. The first item to check in their "will not accelerate beyond 3000-4000 rpm is to disconnect yellow wires from the stator and crank to check the rectifier. Since the top of the motor is apart waiting for the new stator and trigger to arrive, and since I did not want to wake the neighbors last night cranking the motor, I probed the rectifier with my fluke and found one of the diodes from ac to ground shorted. Since the yellow leads are the charging leads for the stator, would a short to ground affect the ignition?

Unfortunately all the plugs looked new and uniform since I replaced them last year and this was the first time out this season. They only had about 10 minutes run time total. All six plugs had spark at cranking speed. The spark might have been a little weak, it was hard to tell in the bright sunlight where I was working.

I will install the new rectifier, stator, and trigger when they arrive and update.

Thanks again.

 

[Droll]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

Check out plug #3... look for aluminum transfer .

#3 are first cylinder to got when you're running to big a prop, WOT rpm should be as close as possible to 5500rpm ( light load in the boat ( you + fuel )), a WOT at 4600 are way to low... 4000 are BAD... 5100 are accepted .



Arne Kjetil

 

[ZigZaggin]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

Actually the yellow wires "feed" AC voltage to the rectifier and the rectifier converts it to DC voltage for your boat and charges the battery. Evidently the rectifier can affect the ignition system... Im not going to argue with the electrical engineers at CDI anyway. Hopefully you're right on the stator and trigger. Whole new CDI ignition system (minus coils) for that motor is about $560 through boatfix.com (when I priced it a couple of weeks ago). Please do post your results. Too many people never post back their final resolution to a problem.

 

[sjj623]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

UPDATE

First, thanks for the replies and info.

I finally received and installed all the parts yesterday, and ran the boat tonight.

To recap: the motor dropped 600 rpm and 5mph at wot, from 4600 to 4000 from 35mph to 30mph.

Tests: stator resistance out of spec, stator leaking. Trigger resistance just barely out of spec. Rectifier had one shorted diode.

Items replaced: Stator, Trigger, spark plugs, and rectifier.

Results: The motor turns about 4800 rpm wot, gps speed is 37-38mph.

The motor seems to be running well, the boat snapped up on plane pretty quickly.

A couple replies stated my wot rpm should be higher. If I assume my motor is now running correctly I should reprop my motor down a little in pitch. I will ask that question in the prop forum. Thanks again for the replies.

 

[cannonford57]

Re: 1979 1/2 Mercury I6 90hp 4600 wot to 4000 loss of power

someone correct me if i'm wrong but a rule of thumb generally is if you change 2 # on the pitch it will get you roughly 300 rpm of change.for an everyday pleasure boat application.