83 Mercury 70 Loses power when warm

[Nautique93]

When cold will run up to full speed for about 1 min, then lose power down to about 3000 revs. Let it sit for an hour and it will run flat out again then lose power again.

Things ive done to eliminate:

New fuel pump
New fuel line and bulb
checked fuel tank vent
changed plugs
checked compression- 130psi all 3 cylinders
changed switch box (for a used one but with no change)

tried pumping primer bulb while it is plodding along at 3000 revs- no change
tried pressing choke button on while at 3000 revs and you hear the engine start to choke up.

From what have read am i right in thinking this is an electrical problem from the above paragraph?

Is there any way to test the coils/ switchbox/ trigger/ stator?

Could one of the coils be breaking down under load?

Thanks, peter

 

[walleye1968]

Re: 83 Mercury 70 Loses power when warm

yes all can be tested
need more info on the motor to help you out

 

[Nautique93]

Re: 83 Mercury 70 Loses power when warm

1983 mercury 70 hp serial 9353189. Im fairly handy with a multi meter just need to know what to test and what it should read.

Not sure what other information i can give?

3 cyl, 2 carb.

 

[petryshyn]

Re: 83 Mercury 70 Loses power when warm

Do a spark test on each cylinder when engine acts up. If you localize the offending cylinder, it will make trouble shooting a whole lot easier....

 

[Nautique93]

Re: 83 Mercury 70 Loses power when warm

Thanks for the reply,
Thats the next thing im going to do, have to wait till i get a chance to take it to the lake as it doesnt act up unless its under load.
Just wondering if there was a safer test than hanging off the boat while underway! But i guess its not showing up until its under load.

thanks, peter

 

[emckelvy]

Re: 83 Mercury 70 Loses power when warm

Sure sounds like it could be a heat-related stator problem, here's some info on taking resistance measurements with a meter:

http://www.boatpartstore.com/mercreadings.asp

135-165 Ohms High Speed Winding
5800-7000 Ohms Low Speed Winding

http://outboardparts.com/mercury/troubleshooting/troubleshooting10.htm

CDI brand Replacement Parts:

http://www.boatfix.com/searchcdi.asp

Note that the High Speed winding measurement is made between the Blue stator wire and the Red stator wire.

The Low Speed winding measurement is made between the Red stator wire and Engine Ground.

This is with the stator wires all disconnected from the ign power pack.

It might be very informative to take resistance readings when the motor is cold, then after it gets hot and starts to act up. Any significant change in readings in either High or Low Speed windings means the stator is shot. Most likely your High Speed winding has issues, thus the 3000 rpm limitation.

HTH & let us know what you find............ed

 

[Nautique93]

Re: 83 Mercury 70 Loses power when warm

Thanks, i will try this next time i get it to the lake.

In the meantime can someone explain the difference between the stator and the trigger and how they work. I was under the impression the stator was like an alternator to charge the battery, and the trigger told the coils when to spark. I see the stator also has 4 wires going to the switchbox- i now know theses are the high and low speed windings, but im a little confused on how the trigger, stator, switchbox and coils work together.

thanks, peter

 

[petryshyn]

Re: 83 Mercury 70 Loses power when warm

Stator coil - used to generate a/c for the charging system
Charge Coil - used to charge the "switch box" capacitor to a high level
Trigger coils (3) - used to trigger the "switch box" into dumping the capacitor charge into the appropriate cylinder's high tension coil primary winding when the timing is correct for that cylinder.

 

[Nautique93]

Re: 83 Mercury 70 Loses power when warm

Thanks, i understand now.

Peter

 

[Nautique93]

Re: 83 Mercury 70 Loses power when warm

Update- Thanks everyone for suggestions- turned out the stator high speed winding was tempermental- sometimes checked out within spec and sometimes not. New stator cured the problem.
Hope this helps someone, thanks again, peter