1988 Merc 90 stalls at idle/won't run at low rpm"s

While on the water started engine and wouldn't stay running at idle. Hitting the enricher didn't help. I had to adjust the idle up with the throttle cable barrel to over 2000 rpm's and slam it into gear to get back to the dock. Ran mint at higher rpm on the way back.
1988 Merc90 3 cylinder- can get the idle down to about 1000rpm's before it dies.
Dva tests on muffs:1000-1200rpm's
Stator blue(low)178-184v
stator red(high)138
coils(green wire)80-104v
stop ckt 184v
switch box bias 5v
Bias to ground 14,400ohms
Trigger
1 3.1v
2 3.1v
3 4.0v
I then grounded the sparkplug wires and cranked engine-149v on the blue stator wire
I disconnected the blue wire from the switch box, plug wires grounded, crancked it and got 170v.
The manual says I should have 200 volts minimum on blue wire and 4volts or more on Trigger.
Does this look like stator and trigger failing? (All Voltage tests are with dva).
Thanks.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Bump

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

My response will "bump" your question of it serves no other purpose.

You said you have the manual and in there should be trouble shooting diagrams to aid you in your search. My manual is for late '90's engines and the wire colors and all don't match what you posted.

All I can glean out of your problem is that you possibly could have a fuel problem in your idle jet circuits; i.e. dirty carbs. I would think that if you were to have an ignition failure it would be more like high speed problems rather than low speed being the nature of magnetic and solid state capacitor discharge circuits being what they are.

I see that your voltages don't match what your manual says they should be (minimum) but that could be an instrumentation error. You are close enough in my opinion, to the minimum value to have combustion.

A real quick way to check your problem is to get a battery powered timing light like we used to use when ignitions had points and all. Also, stop by an auto parts store and pick up an aerosol can of Sea Foam and bring your tools (#2 Phillips screwdriver at least).

Take your boat to the water on a slack day and back it into the water leaving the boat on the trailer and backing down to get ample water over the water pump...like half the transom is submerged. Pop your cowl and connect your timing light; or do that first.

Run the engine in N at fast idle and get a reference...I get on the starboard side and have a clip on the pickup which allows for monitoring with the engine wiring intact. I shine the light at the side of the oil tank and watch the reflection which gives me a very good view of the firing and walk through the 3 circuits and take a reference. Once happy with that, reduce rpm's and see what happens. If all is good, back to normal idle (if you can) and put it in F gear and see what the spark(s) are doing.

If that test passes, take the front cover of the airbox off so you can access the carb throats. Spray Sea Foam aerosol into the carb throats and check the rpm response. If it improves I'd say you need to be chasing an idle fuel circuit problem; i.e. rebuild the carbs.

Best I can do,

Mark

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

I put the timing light on it and is is firing regularly. The engine ran good before this happened. When it started happening I bumped the enricher valve and it would stall. I can't get the warm up lever down enough to shift into gear, it stalls.
The voltage on the green wires to the coils is about 100v. The chart says it should be 150-250volts. I'm not even close to that.
One thing I forgot to mention was I had to run it back with the cowl off. It wouldn't stay running at the lowest idle I could get it to with the cowl on.
I'm wondering if the spark is just enough to run it but not under any low rpm load because of the low speed stator voltage.
Thanks.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

If you test spark at cranking speed and get a nice hot blue arc across a 7/16 inch gap, then you do NOT have an ignition problem. Many DVA voltage tests are done with components DIS-connected as they will read lower while connected. The proof is in the spark test.

Carb idle mixture is critical to a good idle, as mineral deposits form and restrict idle fuel you loose idle quality, choking or enrichening is still far too much fuel.

Order up carb kits as you do not want to re-use any gaskets. Clean the idles passages with a quality spray solvent followed with compressed air to ensure all mineral deposits are blown out.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

CharlieB said:

If you test spark at cranking speed and get a nice hot blue arc across a 7/16 inch gap, then you do NOT have an ignition problem. Many DVA voltage tests are done with components DIS-connected as they will read lower while connected. The proof is in the spark test.

Carb idle mixture is critical to a good idle, as mineral deposits form and restrict idle fuel you loose idle quality, choking or enrichening is still far too much fuel.

Order up carb kits as you do not want to re-use any gaskets. Clean the idles passages with a quality spray solvent followed with compressed air to ensure all mineral deposits are blown out.

Click to expand...

Listen to what Charlie says. I just had a horrendous problem that I swore was ignition and the problem was that I had diddled my idle mixture screws last summer when it was hot and I had idle problems and I opened the screw like 1/4 turn when I should have only moved it the width of the screwdriver blade. They are that critical.

HTH,

Mark

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Thanks guys, I'll double check the spark jump then I'll redo the carbs.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Should I remove the idle air screws, not the idle mixture screw, the ones with the red sealant that your not suppose to touch when I clean the carbs?

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Sunfish12 said:

Should I remove the idle air screws, not the idle mixture screw, the ones with the red sealant that your not suppose to touch when I clean the carbs?

Click to expand...

What/where are those? You talking about high speed jets? The manual shows the main fuel jet under a screw and possibly that screw is sealed....I could go out and check. One would assume that removing that would allow for a more thorough cleaning ensuring that you put it back just like it was....it is not intended to be adjustable, just a specific diameter orfice.

Mark

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

It's not the high speed jet. It is located on top of the carb on the starboard and aft side. The thing with this idle air screw is it has a spring under it and it is set and sealed at the factory. Each carb has one.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Okaye then I would speculate that back then they had an adjustable high speed and low speed whereas nowadays (EPA) they are fixed for clean air reasons. Unless you suspect high speed problems, I'd leave it be or if you do remove it, carefully measure the number of turns/partial turns and put it back just like you found it.

HTH,

Mark

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Update, I noticed a miss on the trigger when I worked the throttle during an ohm"s test. The flywheel trigger magnet has a crack in it. I'm picking up a used flywheel tomorrow.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Repeat the trigger test on each trigger lead, a 'crack' in the flywheel trigger magnet should show the same on each trigger.

A blip, droop, or failure of a single trigger at any point in the movement of the trigger leads, as when the throttle/timing lever is moved, would indicate a broken trigger wire with the ends abutted, making contact possibly most of the time, yet failing at some position of the wire.

This is most probably NOT a flywheel problem, but a broken trigger wire.

Repeat trigger testing, twice, to prove the fault. Some have had varied success in replacing trigger leads. If it is proven to be the trigger, and you may be ordering a new one anyway, it may be worthwhile to carefully strip the wiring and see if you can replace it.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

When I turn the flywheel 1/3 the failure moves to a different trigger wire. Every 1/3 turn off the flywheel moves the failure to the next wire and is clear on the other 2 trigger wires.

Re: 1988 Merc 90 stalls at idle/won't run at low rpm"s

Update:Trigger ended up being fine, rebuilding the carbs and the fuel pump didn't fix the problem. I bought a CDI stator, that brought up my low low speed coil voltage. I still had low voltage on the ignition coils. I bought a CDI switchbox, that brought up the ignition coil voltages on the green wires. Took the boat out this weekend and adjusted carbs one screwdriver blade width(counterclockwise) for each hole shot until nice and smooth performance. Starts better than ever.
At idle:
red stator wire 154v
blue stator wire 286v
Green ignition coil wires
cyl 1 160v
cyl 2 160v
cyl 3 164v

CDI switchbox Review
The threaded terminals are shorter than the original box and the box is made out of plastic. You have to run a new ground wire to the switchbox and attach the ignition coil grounds to the new ground wire.