2000 Johnson 150 Sudden Rpm Loss

[mulv80]

My 2000 Johnson 150 carbed model developed a sudden loss of rpm this weekend. I was running the motor at full throttle when it dropped from 5400 down to 4500. When I floor the motor it sounds like for a split second that it may achieve full rpm's but never exceeds 4500. The motor is also stalling occasionally at idle. It had been stalling at idle before the sudden loss of rpm's. I have also noticed that the throtle advance plate under the flywheel seems to be sticky. I plan on checking compression and spark and I have elimintated a water in the fuel tank problem, by running off a portabe tank and also changing the fuel water seperator. Any suggestions?

 

[Jquest]

Re: 2000 Johnson 150 Sudden Rpm Loss

Get yourself a sparkplug tester at autozone or Discount autoparts and fire it up. Go spark plug cable by sparkplug cable and see if you have spark. You might of dropped a cylinder.<br />You should also get yourself an owner's Manual for that motor. They have great trouble shooting tips and help you diagnose and find alot of things. Not to mention they are cheaper than a wrench.

 

[DHPMARINE]

Re: 2000 Johnson 150 Sudden Rpm Loss

You won't know until you do the testing,but I think you're loosing spark at one or more cylinders.<br /><br />But also be aware,if you had water in the gas,it won't run right until you get fresh gas AND take the carbs apart for cleaning.Gas in the carbs will not burn away or dissappear on it's own.<br /><br />DHP

 

[mulv80]

Re: 2000 Johnson 150 Sudden Rpm Loss

I will buy a spark plug tester, I didnt realize they were so cheap or I would have had bought one already. I won't be able to test anything till the weekend. My biggest fear is that I may have blown a piston. Is it possible with the symptoms I'm describing that I may have blown a piston?

 

[Jquest]

Re: 2000 Johnson 150 Sudden Rpm Loss

If the spark plug, spark plug wire or coil cut out... nothing probably happened. Gas and oil would of kept going into the cylinder and kept it lubed. All you were doing was dumping unburned fuel into the exhaust. <br />Also, since we're on the subject...you could get a cheap compression tester. That also helps with problems. Here is a nice one I use. Can't beat the price.<br /> http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92697

 

[mulv80]

Re: 2000 Johnson 150 Sudden Rpm Loss

I checked the plugs after the rpm drop and they all looked the same. The plugs have only a couple of hours on them and all look clean. I thought if a cylinder wasnt getting spark the plug would foul from the unburnt fuel oil mix?

 

[Seasport]

Re: 2000 Johnson 150 Sudden Rpm Loss

Can you clarify what you mean by the throttle advance plate sticking. On this motor you have an optical ignition sensor mounted on a rotating plate which is under a plastic cover on *top* of the flywheel. These can stick which will cause stalling problems.<br /><br />Is this what you mean?

 

[mulv80]

Re: 2000 Johnson 150 Sudden Rpm Loss

Seasport, When I rev the motor from the control box I can see the mechanism that has the adjustments for the idle speed seem to stick as it moves, this is what I was refering to as the "throttle advance plate" I'm sure I'm not calling it by the proper name. Anyway, the mechanism doesnt seem to be smooth as I rev the motor. I have advanced the controls to the full throttle position with the motor off and it seems that the final position of "throttle plate" is correct from what I can tell. Could this sticking also cause a loss of rpm a WOT.

 

[Walker]

Re: 2000 Johnson 150 Sudden Rpm Loss

In addition to the tests already mentioned you will probably need to do a load spark test while running on the lake. An automotive timing light is good for that test.

 

[Seasport]

Re: 2000 Johnson 150 Sudden Rpm Loss

Remove the plastic cover on top of the flywheel and you will see the plastic timing wheel (has lots of slots in it) which rotates with the flywheel. Under this is the optical timing sensor (eye) which is mounted on a sliding plate which rotates as the throttle is advanced. This is what advances the ignition timing as the throttle is opened.<br /><br />Check to see that this rotates smoothly as the throttle is opened (don't need to have the motor going - just operate the throttle lever) and importantly that it returns smoothly under spring tension when the throttle is closed. These can stick which will affect the relationship between ignition timing and throttle position.<br /><br />Maybe something esle is sticking/binding there but if you remove the cover you should be able to see the linkages.<br /><br />Your loss of rpms could be caused by incorrect WOT timing but more likely you are dropping cylinders, which as Walker says can be verified with a timing light while running on the lake. The timing wheel has marks on it corresponding to each cylinder so as you connect the timing light to each spark plug lead you will see if that cylinder is getting a spark.<br /><br />Suggest if you haven't already got one that you get a servcie manual as this has all the details in it.