Mercury 90 hp misfiring above idle

Dinokal

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Hi guys,
I have 2002 mercury 90 2 stroke s/n: OT443297. I have rebuilt the fuel pump, cleaned the carbs, replaced 1 cdm, trigger, stator, and the darn thing is still misfiring above idle, first it was above 2,000 rpm, now at 1,200 to 1,300 it will run for a few seconds with fluctuations and then misfires with a puff of smoke out of the exhaust. I have run all the electrical tests, regulator is fine, kill circuit is fine, flywheel magnets appear fine. It will not hold DVA voltage above 180 but the battery is charging. I disconnected the yellow wires to the regulator, no change. Any help in the right direction would be appreciated.
Thanks,
Dino.
 

racerone

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First thing is a compression test.----Then a spark check.----Does spark jump a gap of 7/16" on all leads , yes or no ?
 

Dinokal

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Thanks for your prompt reply. Sorry I should have mentioned it. Compression is exactly 90 psi on all thee cylinders, spark jumps 7/16" on all three. The problem is on cyl. #1, I pull the spark plug wire of and ground it, there is spark but the engine does not sound any different or decrease rpm's. with number 2 it will run lower and number 3 kills the engine 3-4 seconds after I pull the wire off.
 
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ondarvr

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Compression should be up in the 120 PSI range, but gauges can read differently.

When you adjusted the low speed screw on the top carb did it affect how it runs?
 

Dinokal

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No, I even screwed it all the way in and kept running almost the same until I backed it out. On the other two carbs it's business as usual a slight turn in or out and you notice the difference right away. What puzzles me is why the DVA voltage is below spec with pretty much all new ignition components.
 

ondarvr

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With the carb cover off put your hand over the top carb to restrict the flow slightly, if it improves you have a carb issue. Or squirt some premix into the carb.
 

racerone

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Perhaps a reed valve issue.------Perhaps the cylinder sleeve has rotated ??
 

Dinokal

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Water pump is working properly, I will try restricting air flow on top carb in the morning, it was 27 degrees here this evening, I still don't think a fuel issue with one carb would cause an ignition voltage drop. Will keep you posted.
 

ondarvr

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The reason to check the carb is because you need to eliminate possible problems, and it?s easy and free. Plus you said that cylinder had spark, doesn't mean it's firing correctly though, if you said it had no spark you'd already know where to look.
 

Dinokal

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I restricted air flow to top carb and there is no difference. I will keep digging, weather permitting, thanks everyone for all your input and keep posting your suggestions. I will update you on any new developments.
 

racerone

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Your trouble shooting leads me to believe you should pull the exhaust cover off.----Inspect pistons and rings.
 

Texasmark

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In addition to what's said, and yes 120 IS the number the service manual for that engine says "If below 120 expect problems", I liked to use a timing light with the pickup close to the wire but not on the wire to check the spark under load which can be different from plug in air or gap readings. I used a magnetic clip and observed the stamping on the side of the pickup that says plug on this side since the spark is direction sensitive. If I could move the clip approximately the same distance away from the wire and still get it to fire I figured all was good to go. Otherwise went after the slacker.

On the DVA reading you may have a bad DVA. The ignition spikes are microseconds (e exponent -6 plural) wide and occur at the running rpms. A regular meter can't detect something that narrow occurring at such a slow rate....when hits per minute (rpms) are compared to microseconds per hit......so you use a sample and hold circuit.....a diode rectifies the signal to pulsed DC. That information is applied to a shunt capacitor which charges up to the peaks of the input pulses after it has received enough of them....takes a second or two.....then there is a very large bleeder resistor across the capacitor to ensure that the capacitor can't charge up indefinitely and miss subsequent potentially bad pulses.....it has to recharge ever so slightly on each pulse. If the capacitor is leaking it can't hold the charge long enough and the average value of the voltage it's holding will be lower even though the input signal was at the proper amplitude.

Having had that exact engine for some 12 years I know that it can be tricky troubleshooting. Listening to it idle and thinking you are catching or missing this or that is a crap shoot. If you run the low speed jet all the way in on a carb and idle the engine, you are starving that cylinder for not only fuel but also lubricating oil. The residual will only last so long.

If you pull the spark plug wire off and ground it while the engine is turning over, you are dumping your charged capacitor in your CDI module in a dead short in series with the coil (contained therein) which can over heat the coil and burn off the wire to wire insulation and wind up in a shorted turn which will seriously affect the performance of that CDI, if not kill it completely. If that doesn't kill it and you run with the cap off the plugs, the opposite occurs. The spark jumps up to 40 kv because of no load and all the parts in the CDI can be damaged by over voltage punch through.

You're not going to have a puff of smoke with fuel problems. A puff of smoke is a load up of one or more cylinders with unburned fuel and after a number of revolutions of that, if the ignition is returned to that cylinder, it and the exhaust path out of it go up in one big bang....aka smoke and that explosion screws up the looped feedback exhaust gasses for the cylinders that are trying to run properly and all that results in a big disruption of the whole engine's performance.
--------------------------

1. Pull all your plugs and clean them up with chem tool aerosol, something to get all the fuel related stuff off the (surface) gap.
2. Set your low speed jets to 1.5 turns after a soft snug (don't want to cut a ring on the jets conical tip which makes low speed control next to impossible).
3. Get yourself a battery operated timing light and connect to the engines battery at the ⅜" input terminal to the starting solenoid and ground at where the battery cable grounds at the engine block.
4. Get your engine running preferably in the water with the cowling off and somebody driving for you...obviously being careful. Get in F gear on the water, engine idling.
5. First, clip the timing light on each spark plug wire and check for consistent firing on all 3 cylinders. Then unclipping the sensor and just moving it close to the plug wires, check for equal distance from each plug. If all that checks out, accelerate to your trouble range and check again. If the spark holds there I DON'T KNOW WHAT TO TELL YOU. If it doesn't, you may have found your smoking gun. Since all 3 CDIs get triggers routed through them and the only one that fires and makes a bang is the one on the compression stroke if you find a bad one, you can physically and electrically swap it with another and see if the problem follows the CDI or remains with the cylinder. If it remains and is spark related, check the plug or the magnet in the flywheel. If it follows the CDI, get a new one or two if you have one bad one and one half bad one.....it can happen.

You still have low compression and that needs to be corrected before you get any kind of real performance out of that engine.
 

Dinokal

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Thanks Texasmark, cdm's are new and I got a spare that is also good. Compression has been the same every year since I bought the boat and have been getting 5200 to 5300 rpm's and was running good prior to this. Maybe my compression gauge reads different. DVA adaptor is fairly new but maybe my multimeter is running wild. As soon as I get someone to help me I will test it in the water with the sensor unclipped, I have been running all the tests with a big tub and was able to run it in gear. When I removed the flywheel, I noticed a light scratch (not a crack) on the trigger magnet, running horizontally for about 1.5 inches, Would that throw the trigger off? Also, would a trigger malfunction cause stator voltage to drop? Thanks again for all your suggestions.
 

Dinokal

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Oh one other thing, when I was checking each wire for spark, I was holding it close to one of the cover bolts and not actually grounding them, as I mentioned earlier.
 

Texasmark

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Oh one other thing, when I was checking each wire for spark, I was holding it close to one of the cover bolts and not actually grounding them, as I mentioned earlier.

Phew. Does/can make a difference.

Scratch I can't see as a problem.

Crack is overheating due to a mechanical short internally on encapsulated coils and transformers. Overheating can change the permeability of the core which can reduce the BH curve (gain of the core vs frequency), or melt the transformer varnish used to insulate the wires from oneanother. Any shorted wire making a closed loop around the core will suck most if not all of the power out of a magnetic wire wound device, especially iron core.
 

Dinokal

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Update: Fixed the spark problem, it was the first CDM I bought to replace the bad one, turns out it was defective too, returned it to CDI and got a good one. I got good spark on all three cylinders now but the problem remains, I rev it up to about 1,600 rpms, it runs ok for about a minute and then a puff of smoke and misfire. I shut it down immediately, take the spark plugs out and they are wet. I don't think my oil supply is messed up which leads me to believe my compression is the cause of it. I am getting about 85 to 87, manual says minimum 120. So unless you have any other suggestions, I think I am going to check into having the power head rebuilt. BPS in Tulsa does it for $1,500 shipping included both ways, using the existing block unless you guys know of a more economical option.
Thanks again for all your help.
 
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