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Steve Foss

Sudden power loss on 1988 Merc 90 Hp

31 posts in this topic

So I had the Alumacraft and the tall black Merc out for a workout today. Had last year's gas with Seafoam added last November.

Ran great for awhile. But as I was working up toward peak throttle it suddenly lost power and started missing, and when I maxed out the throttle I could only get about 3/4 the total power I'd been getting, with max RPM of 4400 instead of about 5000.

It idles great and runs smoothly when at highest possible speed, but now it takes several seconds with some stuttering even to build up to the 4400 RPMs I can get out of it. Once there, it runs very smoothly. It's always been fast as lightning out of the hole.

It's a 6-cylinder with two carbs. It has very low hours and has been babied and well serviced its whole life.

Any thoughts?

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bad gass even tho you put sea foam in it, timing off or some gunk worked its way into something..

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Most likely bad gas. I had gas in mine that went over the winter last year and I had the same thing as far as symtoms. I drained some of the gas and it looked like gelatine from the ethenol in it. I replaced all the gas and it was fine after the rest of that gas got worked out of the carbs.

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Steve, I'd start out with checking the gas.

Crank up the tongue jack and let the boat sit long enough for any water to run to the lowest point in the tank. That'll be by the inlet. Depress the pin and hand pump via the primer bulb gas into a clear glass container. If you see swirlies in the gas that be moister. If there are droplets of water in the bottom of the container then I'd siphon at least 5 gallons off, let the water separate and siphone the water out and then burn the gas in your Datsun.

Next check the fuel line for leaks or a swollen condition from within that could be restricting gas flow.

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Sounds about like what my '96 3-cyl 90hp did a couple years ago.

Ended up being gunk in the carbs that required them to be removed and cleaned out. Dunno how the crud got in there because there were filters in the system...but it did.

Kind of worrysome being out a ways on Rainy Lake when the wind starts coming up, and you have several miles to get home, then you fire up the engine to head back to shore, and find the engine will only rev to 3,000 RPM, and you've got to work it just to get that.

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It could very well be a dirty carb(s).

If the above doesn't check out a carb clean and overhaul is a must.

I'll go on to say that the lack off studder at the higher RPM range is worrisome. That is a symptom of a true lean condition from a dirty jet. Continuous use will scorch a cylinder.

This is sounding like "Free Beer" to me. smile

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Thanks, guys.

I've got a fresh 6-gallon can of gas/oil mix from yesterday I haven't used. I'm going to syphon what's left in the boat tank into other cans and replace with fresh gas. Then I'm going to check the carbs, but I looked them over a bit today and they are very clean.

I'm going to try to rule out an electrical problem by running the motor at idle for a few minutes with muffs on, and then pulling the plugs. If there's a cylinder not getting spark, I'll know because the plug will be sopping wet with unburned gas. If it's a spark issue, could be one spark plug, plug wire, etc, or could be more involved.

Another way to rule out gas is to run the motor up and when it starts to stutter, push in the key choke, and if it runs better for a sec or two it's a fuel starvation issue. If pushing in the choke has no impact, it's likely spark.

We'll see.

I'm going to work on it tomorrow. Good news is I can still troll Burntside all I want, because I just got an old late

'60s 9.5 Johnson, the low-slung round-topped one. My grandpa bought it new, and my uncle just had it rebuilt two years ago. Upon my uncle's death, the motor came to me, so now I've got a bulletproof kicker that has become a family heirloom. gringrin

Frank, if I still lived in your neck of the woods I'd drop off the boat/motor/trailer and a couple cases of Leinies and come back a few days later. The Leinies would no doubt be gone, and the motor no doubt fixed. If I can't figure it out on my own it'll be coming your way even though the drive is long. smilesmile

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Steve, instead of siphoning the fresh gas, hook up your fuel line to that tank. Yep you'll have to take the Johnson end off and put on the Merc end. All long as the dia is the same thats all that matters. Just use hose clamps in place of the bands.

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Frank, I wasn't going to syphon the fresh gas, but the old gas in the Alumacraft tank, which is an integral stern tank. Just seems like a good idea to get it all out of there.

Then the fresh gas will go into the Alumacraft tank so that's good gas from now on. I've got the Johnson 6-gallon tank with fresh gas and another 6-gallon can with fresh gas.

I hear you about swapping Merc and Johnson ends and hooking up the Merc to the Johnson tank. But if there's gunk in the carbs/lines, would running fresh gas through the system clean that out? Seems like it wouldn't. But I suppose if it's an issue of water in the gas, that would take care of business right away. What do you think?

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The remote tank would just eliminate the bad gas and swollen gas line. If you drain the onboard tank thats fine and your Datsun wont mind burning it. You still have to eliminate the swollen or air leak in you onboard tank line.

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First thing I will do is run the motor and pull plugs as I mentioned above, to eliminate spark issues. I will drain the onboard tank no matter what, whether it's a spark or a fuel issue, and replace with fresh gas/oil mix.

If it's spark, I'll deal with that. If it's not, then it's a fuel problem, and I will then swap out so the Merc is running off the fresh Johnson tank and then trailer the boat to the lake to see how it runs. If the problem goes away, I'll conclude it probably was water in the onboard tank gas and hope the problem is solved. If the problem is not spark, but fuel, and the problem persists after running on the Johnson tank, then I'll start looking at tearing down the carbs and/or replacing fuel lines.

I don't know that I have a swollen line or air leak, though. It'll be just one more thing to look for, yes?

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That about it. Might add squeezing up the primer bulb when it acts up. If it takes off then your carbs are fine. That would lead to fuel line or fuel pump.

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Steve, I think you are headed in the right direction. I always stressed, start with the easiest to check. Spark is Number 1, compression is #2 and fuel is #3. Unless the problem started right after adding fuel.

On this model Merc, it will have a tough time running if one carb is acting up. The upper carb runs cylinders #1, #2 and #3 and the lower carb takes care of #4, #5 and #6. This engine will not run on 3 cylinders, or at least, I have never seen it.

This engine will loose 1000 plus RPm from a dead cylinder though.

If you find a dead cylinder, let me know. If it is the distributor-less ignition, I have the test equipment to figure out where the problem is. Could be the power pack or the trigger

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Thanks, shamrock. I'll update when I get a few things done tomorrow. Well, if it gets above freezing tomorrow. gringrin

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It does have ADI ignition and I agree with shamrock that it sounds like it lost a cylinder. Check the compression first however. If shamrock has the Merc o Tronic (or comparable) compresion tester, get that too because you wont get an ordinary tester down into #6. IF it is ignition, you will be able to pinpoint the cylinder acting up by using a timing light while running. Once you determined if it is ignition, using the meter shamrock has and the trouble shooting flow chart, you should have no problem finding the problem. If it is dropping a cylinder, expect a bad Switch Box or Coil but dont over look the high speed side of the stator.

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Guess you won't be getting much done then. Ha Ha Give me a call as I don't get on here that much during the busy season.

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Actually, I've been a busy boy this morning. smilesmile

1. Drained old gas.

2. Replaced with fresh gas/oil, including a can of isopropyl and a can of fuel system cleaner.

3. New plugs.

4. All plugs/wires deliver strong spark.

5. Ran motor at idle, pulling and then reattaching one plug wire at a time. With each of the six cylinders, the motor logged down and ran rougher when the wire was pulled, so they are all functioning.

6. Did a thumb compression test, and all felt equally strong.

7. Drained bowls on all three carbs. Could only get a little jar under the top carb, but there was some water in that bowl.

8. Examined the whole fuel system for cracked/leaky lines. None found.

9. Shot carb cleaning spray into the carbs.

The problem still remains.

Randy (Shamrock) and other guys, thanks for the help. I think I'll take it out tommorrow trout fishing on Bside and see if it was a fuel problem that will work itself out.

If not, more work to do.

Further thoughts, guys? smile

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Steve, It still would be a good idea to try a timing light at higher speeds with a load on it, from what it sounds like, it's a higher rpm problem and could still be cutting out at higher rpm's. Interestingly enough I have a Mariner in the back yard waiting on switch box that loses spark on 1 cylinder above 1500 rpm, and it's certainly not the first one to have that happen. I'm not trying to convince you that IS your problem, but more just trying to completly eliminate the ignition system.

Watch the light and listen and feel for the engine losing power. You may have to go through every cylinder. I agree 100% with Randy that spark and compression are the 2 easiest things to eliminate, if all check out then concentrate on fuel and exhaust. Heck, a critter could have even made nest in the exhaust.

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Thanks, Bruce. Actually, it runs rough and accelerates slowly under load at all but the upper end. Once it reaches its new max of 4400 RPM it smooths out.

Also, it doesn't have a distributor, so how would I use a timing light on it? Randy showed me how to check for a gummed up high-speed jet, in case that's what's causing it to smooth out at higher RPM, so I plan on doing that out on the water tomorrow.

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Just attach it to the plug wires as you would any other plug wire. No doubt at all it could be a restricted high speed jet.

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Just attach it to the plug wires as you would any other plug wire.

DOH! gringrin

What should I do to check the exhaust? We get mice in the fall looking to nest in strange places, and wasps, too.

I suppose that even though the spark on all cylinders looks strong to me, and the compression feels consistent and strong in all cylinders to my thumb, an actual ignition eval and compression test using the right equipment might show a problem in either of those directions. A guy can only go so far trying to do things on the cheap. winkwink

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5. Ran motor at idle, pulling and then reattaching one plug wire at a time.

Dunno if it's an urban legend or not, but I've always been under the impression pulling plug wires can cause ignition modules to blow if the spark doesn't have anywhere to go. I.E, any disconnected plug wire should be grounded or put on a spark gap before starting the engine.

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Pulling plug wires is a very common diagnostic tool. Granted, there are some manufacturers that do recommend that you ground the plug wires before cranking the engine. No outboard manufacturer that I am aware of though.

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I had it out today. Top end now is 3600 RPM. When I punch it, it immediately gets to 2800, then gradually reaches plane and, once planed, adds RPMs to get to about 3600. Idles and runs unevenly until it's punched, and at idle or lower RPMs I get a cough/pause/spit now and then. Once punched, I can't hear a miss, but at higher RPMs I can only detect strong misses anyway, so maybe it's missing and I just can't hear it.

I have one simple experiment to try tomorrow (thanks Frank!). I'm going to put the muffs back on, start her up, warm her up, rev her to about 2000 RPMs and shoot gas/oil mix from a spray bottle into each of the three carbs, one at a time. The butterfly valves at those RPMs should be open enough to allow the spray to enter.

If shooting that mix into the carbs evens out or revs the motor, that will tell me there are some plugged jets/lines in that carb and it's time to remove and clean out all three carbs. If it has no impact at all, I think it's time for a full-blown compression and ignition/spark test.

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