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1999 90 HP 4 stroke

R

rowejd11

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Runs great at idle. Runs good on muffs. Under load cant get above 4000 rpms. Have rebuilt carbs, replaced fuel lines, bulb, changed sparkplugs. Compression test good on all cylinders. Am confused what to do next. I checked with induction pickup timing light and number 2 and 4 cylinder is at 900 rpm +-, and 1 and 3 are at 1800 +-. I've read this narrows down whats wrong but cant' find anything to narrow it down to.... Any ideas on test ect and what parts? Been searching and looking for three days and havent found any help.
Thanks for any help guys,
Jimmy
 
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Not prop. Has run out better with same prop its always had. Took it out today after sync. carbs. Still can only get up to 4500 RPM's. About 22 MPH. Use to run about 27mph at little above 5000rpm. Anyone know about the difference in induction timing rpm on the cylinders?
 
You have found the loss of power but why is the question. Just a WAG but test the trigger and stator w/an ohmmeter to see if they are w/in specs.
 
Have pulled the flywheel and cannot find how to test the stator and trigger or specs for them. Might be in my manual but just havent found them yet.
 
Thanks been looking at that. Stator test out .5 between all 3 wires. No continuity to lamintations. Still searching to see how to check trigger. Trigger under the stator, 2 different ones with 2 wires out each?
 
Might have found my problem. Top ignition coil was mounted upside down. Noticed this yesterday. Didn't hit me until I was studying the wiring diagram. This swapped 1 and 4 cylinder wires up.... Would this cause the rpm to double on 1 and 3. Cylinders 2 and 4 were the right rpm. It will be tomorow after church before I can get everything back together to run it and see.......
 
Got everything back together and now 1, 2, and 4 cylinder show 1900 +- and 3 shows 950 +-. Any ideas? Is the spark on a 4 stroke twice the rpm? What tells the plugs to spark and how often? Know these are dumb questions but i need help...... What could cause 1 cylinder sometime two to do this?
 
I'd like to say I know what the problem is but I don't. Spark is controlled by three things: trigger coil, switch box and CDMs. I would call Mercury headquarters and talk to the engineeers there about the findings of different RPM readings per spark plugs...to me it is either the switch box or the trigger coils causing the problem. Did you get a good look at the trigger coils?
 
CDMs are connected together electronically. Failure of one can affect another one. You could swap the CDMs from #s 2 & 4 to #s 1 & 3 to see if the RPM change follows. If it does then see if CDI will test them for you and buy new parts from them.
 
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