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1998 Johnson 250 Not getting up to full RPMs

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1998 Johnson 250 Not getting up to full RPMs

Old 05-28-2016, 12:42 PM
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Default 1998 Johnson 250 Not getting up to full RPMs

I have a 1998 Sunbird Neptune 230 wa with a 1998 Johnson 250. It is new to me, so I am not 100% sure if it has the correct prop or not. It will only run about 30mph at full throttle and trimmed up just before it cavitates. I can hear that it is not up to full RPMs. I have determined that it needs a new regulator/rectifier and have ordered one. The tach is not working and the motor is not charging the batteries.

So, my question is, will a non-working regulator/rectifier cause the motor to not reach full RPMs? I also noticed that the starboard side temp switch was disconnected so I assume at some point it was causing a problem. I ordered a new one to replace it. I'm just wondering if maybe it will need a new power pack too. I have not tested the power pack yet because I want to get the regulator issue sorted out first before taking the next step in troubleshooting. I will say that otherwise the motor runs great. Idles perfectly and runs great up to full throttle. The only other thing I have noticed is that it seems to get atrocious fuel mileage. I know it is supposed to be not great but it is worse than I would expect. My last boat was an 18' center console with a 1985 Mercury 150 XR2 and this thing gets MUCH worse fuel mileage than it ever did.

Thanks for any thoughts and input.
Old 05-28-2016, 01:11 PM
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Because the boat is new to you, things will need to be done bit by bit, regulator has nothing to do with max RPM, is the motor original on the boat?
Old 05-28-2016, 01:21 PM
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Originally Posted by noelm View Post
Because the boat is new to you, things will need to be done bit by bit, regulator has nothing to do with max RPM, is the motor original on the boat?
The motor is original. I was planning to only take it one step at a time and that is why I have not done anything with the power pack yet. Need to fix what I know to be wrong first.
Old 05-28-2016, 01:26 PM
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Have you done a compression check?

Thanks,

Caleb
Old 05-28-2016, 01:29 PM
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Don't quite see why you are replacing the power pack...yet! How much are you "trimming it up" before it cavitates?
Old 05-28-2016, 02:44 PM
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I haven't done a compression check yet. The next time I take it out of the water, I will. Previous owner said compression was good and it was just not something I was suspect of yet.

I was not going to replace the power pack but it was my next suspicion is all. I am not sure how to explain how high I trim it up when running. Trim gauge doesn't work and I'm not sure how to give a specific answer on that.
Old 05-28-2016, 07:01 PM
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Originally Posted by DixieMan View Post
I haven't done a compression check yet. The next time I take it out of the water, I will. Previous owner said compression was good and it was just not something I was suspect of yet.

I was not going to replace the power pack but it was my next suspicion is all. I am not sure how to explain how high I trim it up when running. Trim gauge doesn't work and I'm not sure how to give a specific answer on that.
According to the manual, this engine has to have three things to run properly. Compression, fuel, and spark. If compression is not an issue, fuel and ignition system checks are next. Post your compression check numbers and a picture of your plugs and we can be of more help to you then. If those things are good, we can do a spark test to rule out the power pack.

Thanks,

Caleb
Old 05-28-2016, 07:51 PM
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If your not making the proper voltage to send to the spark plugs for proper spark it WILL affect performance. If you don't have enough spark to properly ignite the air/fuel charge then you will not make proper power. With you thinking you have a problem with the regulator it vould actually be a problem with the stator that creates the voltage/current for the batteries and SPARK. Check all the grounds and check the stator/charging system. There could be a loose connection, corroded wires/grounding point or excessive rust on the flywheel or pick up. You should also make sure your fuel does not contain water and make sure its tge proper octane. If the fuel is old and setting then I would hook up a temporary fuel sorce with fresh fuel with the proper octane to see if that is the cause first.

Last edited by TheFishWishperer; 05-28-2016 at 08:48 PM.
Old 05-29-2016, 05:37 AM
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Yes a broken reg/rec will do what you are seeing but it may not be working alone, you may have other problems as well.

Old fuel, blocked fuel filter or air in the fuel line will also do what you are saying.

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