25 hp johnson seems to hit a rev limiter

[Tjazz]

Good morning. I have a 91 Johnson 25 hp tiller that seems to go into some type of limp mode after warmed up. Plugs are like new, brand new fuel line, primer bulb, fuel tank, and fuel. It starts right up and will rev right up until it reaches operating temp, then begins to surge almost like a car reaching it's rev limiter. If I back off the throttle it will idle very rough and almost want to stall. I've read about possible over temp issue causing things like this. Does my engine have these sensors? Is there anything that sounds common to this problem other than normal carb issues? Thanks in advance for your help!

 

[ondarvr]

Check for spark on each cylinder, sounds more like a coil failing when it warms up.

 

[jasper60103]

Tjazz, welcome aboard!
I've only hit a rev limiter when running over 5900 RPMs on a boat.
Is this a new to you motor? Start with the basics (compression, spark, carb) as recommended above.

 

[Tjazz]

I've had this setup for around 12 years now. Always ran like a top. At the end of last season this started to happen. I winterized it and chalked it up to a bad primer ball, but like I said I replaced that and same thing. I have an automotive background, and it feels the same as a fuel cutoff in a fuel injected vehicle. A bad coil makes perfect sense, however I've never been a fan of Champion spark plugs. I may swap those out for Ngk's and see if that makes a difference. Compression is fine. I saw there are three different choices of Ngk plugs for it. Does anyone have a prefernece, hotter plug etc?

 

[ondarvr]

Changing sparkplug brands will make no difference, nor will a bad primer bulb cause this type of problem. Check spark on each cylinder when it acts up.

 

[juno pierrat]

looking at parts list for ignitions for 91' 25's , you power pack is under the flywheel, yes heard same thing about P/P units may be a little heat sensitive,in your case age, there is an after market kit that brings the P/P outside from CDI, still would start with testing spark and compression

 

[Tjazz]

So, I think i'm getting somewhere... I went down the garage yesterday and put two new NGK's in it. (I've seen too many issues with Champion plugs in the Automotive end,so as crazy as it sounds, it made me feel better to get those out of there.) Now she wont fire at all. Pulled the plugs back out and are as dry as could be. Will start with a little spray in the carb, and then die out. Fuel pump and carb coming apart tonight or tomorrow for a thorough cleaning. Really weird how it would only act up when hot which sounds electrical, and now no start at all with no fuel to plugs. We'll see how it goes...

 

[Tjazz]

Update on my Johnson 25 hp. Took the carb off last night and soaked it overnight. Took fuel pump apart and cleaned. Fired right up and now idles beautiful. Plenty of power, but once heated up would start to go into "limp mode" again until it cooled down. I then realized there was no water circulation through the hose that comes off the cylinder head. Found it to be blocked solid! Cleaned out and runs beautiful. I'm guessing the poor circulation or lack there of, caused the sensor to put the engine in limp mode to prevent damage. Now I can take my daughter fishing this weekend. Thanks for everyone's input along the way.

 

[oldboat1]

think I would replace the impeller if it's been a while. Given the clog, think you might want to pull the t.stat as well and at least clean it up and test -- housing and bypass as well. Glad you found the issue.