Sudden RPM Loss?
 

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?

Re: Sudden RPM Loss?
 

sounds like 1 of 4 things i seen cause this

1: Replace all the plugs may be one bad plug

2: Check for fire on all cylinders could be bad coil pack

3: Check that Magnets are still in place under the flywheel should be able to without removing it

4: Test Compression let us know all ya find

Good Luck Hope it's not an MJ type motor issue ???

Re: Sudden RPM Loss?
 

I won't know anything till this weekend when I get back down to the boat. The mechanic I was planning to use said that I shouldnt expect to get my boat back for 8-10 weeks because he is backed up. 2 1/2 months :o I thought that was crazy. So I just bought a manual off Ebay, full price but its a factory manual and I needed it. Hopefully I will have it by this weekend so I can get to diagnosing. I hope its not an MJ like problem, thats my worst fear.

Re: Sudden RPM Loss?
 

I had a 120 doing the same thing and it was just a bad plug somewhere they all showed fire. I went threw that motor over and over finally put a new set of plugs in the motor out of a known good running motor and that fixed the problem. I'd try a new set of plugs first off. Actually do a compression test if all good go to the plugs. the set i threw away from the 120 were new only had the boat out once and it did that so new is not always good.

Re: Sudden RPM Loss?
 

there is a possibility you could have sheared a key way in the lower unit......that is a common reason for loss of rpm when you have it at full throtle

Re: Sudden RPM Loss?
 

Angler, I can see it being something with the lower unit because it lost rpm when I was going trough a choppy wake. I was racing my brothers boat and should have slowed down but I didnt and when I hit the wake is when I lost power. How would I check for the stripped keyway?

Re: Sudden RPM Loss?
 

fuel pump also?? after installing plugs go in water if still the same try having someone pump the ball, and see if it gets full RPM's?

Re: Sudden RPM Loss?
 

Mulv. Whats the gossip on your motor ?? any luck on finding the problem ?

Re: Sudden RPM Loss?
 

I cleaned the fuel filter, changed the water seperator, drained all six carbs and also messed with something called the vapor seperator. *I dumped her in and hit the magical 5500 rpm's I longed to see. *Thanks for the suggestions boys.

Re: Sudden RPM Loss?
 

Great news!!