1987 175HP Merc Troubleshooting

[selfel]

I'm at wits end trying to get my motor running right again. It idles fine, runs reasonably well to about 4000 rpm but won't pull beyond that. At 1/2 to WOT you can hear/feel what seems to be a cylinder or cylinders hitting then dropping off again. Here's what I've done to date:

1. Compression 120-125 psig on all cylinders
2. Visual inspection of stator and trigger looks fine (nothing oozing out, no cracks)
3. Timing light on each cylinder at WOT (on trailer, in water) shows fire. Can only see the timing marks on flywheel on 2 cylinders (I think this is normal)
4. Tried inline spark tester between plug cap and spark plug on each cylinder at idle and 4000 RPM on hose, showed spark in each case.
5. At idle, squeezed primer bulb till rock hard, no affect on how engine ran.
6. All 6 plugs appear the same, black, a little oily. No obvious lean conditions or water.
7. Ohm'ed stator, Blue and Blue/white to ground in spec, 6300 ohms
Red and Red/white to ground in spec, 157 ohms
8. Checked trigger resistance, brown to white, white to purple, and purple to brown all in spec, about 1300 ohms.
9. Stator was replaced several years ago, switch boxes are original.

The motor sounds great on a hose, revs freely to 4000 rpm, could probably go higher, just don't want to break something. I'm thinking switch boxes but I would really like to determine for sure, plus narrow down to which one before buying.

Any help would really be appreciated.

 

[achris]

Re: 1987 175HP Merc Troubleshooting

When was the last time the carbs and fuel pump were removed, cleaned and had new diaphragms installed.?

Chris........

 

[selfel]

Re: 1987 175HP Merc Troubleshooting

I pulled the carbs not too long ago, they looked spotless. I was surprised, no gunk or varnish. I really didn't do much to them other than shoot carb cleaner through all passages I could find.

The fuel pump/diaphragm is original equipment.

 

[achris]

Re: 1987 175HP Merc Troubleshooting

....The fuel pump/diaphragm is original equipment.

They are suppose to be changed annually!!

Might be the problem????

Chris..........

 

[j_martin]

Re: 1987 175HP Merc Troubleshooting

Check the wires going to the trigger as you move the throttle through it's range. They can rub and short, dropping 2 cylinders at a time.

The wires can also break under the insulation, usually near the trigger itself

hope it helps
John

 

[selfel]

Re: 1987 175HP Merc Troubleshooting

Wow! I had no idea the fuel pump needed that kind of attention. I think I'll try that next.

I just had the flywheel off but I'll pull it again to check the trigger wires.

Thanks so much.

 

[Paul DuLaney]

Re: 1987 175HP Merc Troubleshooting

While your at it when was the last time your water pump was rebuilt? I do my fuel and water pumps every two years and if need be I can do the fuel pump on the water.Keep a new on with me when I travel any distance.As I was in Tennessee and got a hold of some bad fuel and the next morning had to go back to the dock on idle pumping the primer bulb.luckily I was only a couple hundred yards out and not windy.In two hours and less than thirty bucks at the time I was up and running.

 

[j_martin]

Re: 1987 175HP Merc Troubleshooting

Fuel troubles can be hard to track down on these beasts. They are quite hungry at WOT.

The carbs are plumbed with a fuel rail through the flange from side to side. At least one of the threaded holes (Usually bottom port) has a pipe plug in it. You can take that out and put in a hose barb, hose, and a 0-10 or 15 lb pressure gauge that can now lay in your lap when you're testing it.

Mine flamed out at WOT and it was fuel pressure. I found
1. Floats out of adjustment.
2. Fuel pump had warped valve leafs in it.
3. Fuel filter plugged.
4. Tank had chit in it, plugging the filter
(fixed 3 & 4 by cleaning out and installing a racor fuel/water seperator and chit canning the stock filter. Also I run fuel with no alcohol in it.)
5. Attwood fuel line and bulb, wouldn't flow enough.
6. Aftermarket plastic fuel hose bayonet fitting, wouldn't flow enough. (bypassed and went direct to the oil metering T)

Each and every item fixed improved the fuel pressure. The last one let it stay at 5 psi at WOT, instead of 1 psi.

You should be able to see any damage to the trigger wire without pulling the flywheel. They come out under the flywheel toward the back and take a hard turn to starboard to go to the switchboxes. That's where the trouble usually is, either interfering with the flywheel, or just cracking up at the bend. Another item is if the trigger harness is strapped down real tight, it won't allow the trigger to rotate (spring on the throttle lever takes up the slack.)
Would be pretty doggy, but shouldn't miss.

hope it helps
John

 

[selfel]

Re: 1987 175HP Merc Troubleshooting

j_martin, how was you motor acting at WOT when you had fuel problems? From an idle, if I go to WO the motor builds RPMS fine, boat comes out of the hole O.K, struggles a little but then runs fine. Just never gets over 4100 rpm. It doesn't bog or try to die at any point. Just runs at 4100 like it's governed. I was under the impression that when one ran out of fuel on top end it would really bog until rpm's dropped to a point when there was enough fuel to run again.

 

[j_martin]

Re: 1987 175HP Merc Troubleshooting

j_martin, how was you motor acting at WOT when you had fuel problems? From an idle, if I go to WO the motor builds RPMS fine, boat comes out of the hole O.K, struggles a little but then runs fine. Just never gets over 4100 rpm. It doesn't bog or try to die at any point. Just runs at 4100 like it's governed. I was under the impression that when one ran out of fuel on top end it would really bog until rpm's dropped to a point when there was enough fuel to run again.

You're right, that's how it acted.

Here's another test. With clean (maybe new) plugs in it, go for a ride, get it to WOT quickly and hold it there for about a mile, then kill it as quickly as you can safely. (I shut the switch off, and turn on the bilge pump to deal with my back wave.) Pull and inspect the plugs.

If they are wet and/or dark you have an ignition problem.
If they are squeaky clean or appear to have run hot, you have a fuel problem.
If they are normal, you have a mystery.

If it is leaning out, you are in danger of overheating a piston with catastrophic results. (You are not supposed to be able to inspect the wrist pin from the top.)

If it were mine, the next thing I would do is visually inspect the trigger harness, and also observe the trigger rotation, being sure the timing arm goes fully advanced far before WOT.

Another item to check is the idle stabilizer/advance module(s). Sometimes they are combined, some engines have one or both on them. They all connect to the bias circuit on the switchboxes and mess with the timing by pulling up or down on the bias. Just completely disconnect them. If it solves the problem mostly, then retime WOT to 23 degrees and discard the module(s)

When they work they slightly improve driveability. When they fail, they are usually just discarded. Mercury doesn't even bother to stock them anymore.

hope it helps
John

 

[selfel]

Re: 1987 175HP Merc Troubleshooting

"You're right, that's how it acted" - So yours would bog at WOT?

It's interesting that you mention the idle stabilizer/advance module. Early on in my troubleshooting I noticed that the wire from this module (I only have one) was corroded in half right before it connected to the switch box. I thought this might be my problem. I cut it back a bit and re-connected it to the switch box, but no change in how it ran. Sounds like I should disconnect it and re-time.

 

[j_martin]

Re: 1987 175HP Merc Troubleshooting

It might have fuddled up the switchboxes, but then again maybe not.

I sure would throw that one away and retime. If there wasn't a change when you connected it, it's probably functionally equivalent to a blown fuse anyway.

The advance modules mess with the bias, which is, roughly speaking, the trigger voltage that fires the spark off. As engine rpm's increase, the speed, but also the amplitude of the trigger signal increases. Bias is supposed to key off the peak voltage and set a trigger point part way up the sine wave coming from the trigger coil. A variation in bias changes timing by varying that trigger point. As it's pulled toward ground, it advances the timing. That's why a catastrophic failure in an advance module can grenade your power head if it occurs at speed.

You can put an oscilloscope on a trigger signal and see a blip where the spark plug fires. That's the bias point. You can also measure the bias at the bias terminals with any DC voltmeter. I forgot what range it runs, but I've done it and played with bias a bit. I put in a toggle switch and a resistor to ground to advance the timing 3 degrees at will. Gave it a slight kick at 6000 rpmm maybe 2 mph. Not worth it, at that speed there's lots of other things that need full attention, like steering and butt pucker.

Well, you have some fodder to work through this weekend. Hope you get it.

John

 

[selfel]

Re: 1987 175HP Merc Troubleshooting

John,
First, I really appreciate your help here. I've checked the trigger wires and freedom of movement, all O.K. Timing is at full advance well before WOT. Butterflies wide open at full throttle travel.

Going through my manual, I decided to do a rectifier test for kicks. I followed the test steps and don't get continuity in any combination. I seem to recall reading somewhere that a bad rectifier can have some impact on the ignition on these motors, or I dreamed it.

Next step is plumbing in a fuel pressure gauge. Should have results of that test by end of weekend.

Also, disconnected the advance module but haven't lake tested yet.