2000 70hp Johnson 2-Stroke low RPM at full throttle

cuweathernerd

Recruit
Joined
Jun 4, 2015
Messages
2
Hi, I'm new here and hoping I'm posting this in the correct thread. I have a 2000 70hp johnson two stroke that's been acting up a bit. The motor was serviced about 2 years ago, when the carburetors were cleaned out, seals replaced, that kind of thing. Most of the time, it runs fine, around 5500 rpm, when the engine is fully open. For a few years, but only very occasionally, the engine will be running smoothly, and then 'cough', and then not get above maybe 2000rpm and a much lower speed. Not fully stall, just not getting above the middle of the engine's ability. Shutting the engine off and letting it sit a minute fixed the issue, and the motor ran as it is supposed to once I restarted it. I never gave it a ton of thought, since it wasn't a persistent issue. It was quirky, but ultimately didn't impact my time on the water.

It has been happening more frequently recently, and a couple times now, restarting the motor hasn't fixed the issue - it just caps out around 2000rpm and sounds audibly different than when it is running correctly. Letting the motor sit for a lot longer fixes the issue (i.e. starting it cold, the engine runs normally). Here is a 5 second youtube clip of the motor when it's not running correctly (this with the throttle lever nearly fully forward) - here's - https://www.youtube.com/watch?v=qToZGcp6s_0&feature=youtu.be - a different time, when the rpms were closer to 3000 at fully open. I didn't take a video of what it sounds like normally.

When it's running like this, with lower maximum rpms, the engine shakes more than normal when it's running. Are there some simple steps I can take to trouble shoot this before going to a marina to get it looked at? Filters to look at, connections to check, that kind of thing? Do you know what would cause something like this? Thank you for your help (and I'd be happy to give any other videos/pictures/etc if you need them or answer questions).
 

mfkadz

Petty Officer 2nd Class
Joined
May 26, 2015
Messages
135
Something is getting hot and recovering slower. A cheap try at a fix is new spark plugs. Especially if a few years old. If that does not fix it you will need to do some troubleshooting on the ignition to see which cylinder appears to be failing.

mike
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
39,301
Step one is to test compression.---Step 2 is to check for spark that will jump a gap of 3/8" or more.----Not just the gap on the plugs !
 

daselbee

Commander
Joined
Jan 20, 2009
Messages
2,765
It is going into SLOW mode due to overheat. Get a temp gun and measure the temps of the head to see if it is actually HOT or if it might be a bad sensor.

It should run about 140-150, and SLOW mode kicks in at 200+. It should be very hot to the touch IF overheating, and when the problem is occurring.

If it is NOT hot, then bad sensor (probably).

Also explains the 2000 rpm limit, and the shaking, and the fact that letting it cool down "fixes it".

Your cooling system needs to be looked at.
 

cuweathernerd

Recruit
Joined
Jun 4, 2015
Messages
2
Hi all,

thank you for your help. I went ahead and ran it again and got the same low rpms, so hit the heads with a temp gun and didn't see actual overheating, so figured it could be a faulty sensor or thermostat. Pulled out the sensor and it ran fine (which limited it down to a safe/limp mode), and did a little research - it's just a bimetallic strip, so mechanical failure didn't make a ton of sense to me. Cleaned up the contacts a little, plugged it in, and so far, running great! Either there was a little oxidation on the connection or it jiggled loose in 15 years, but happy to not take it in and spend the time on the water instead!

thanks again
 

daselbee

Commander
Joined
Jan 20, 2009
Messages
2,765
Hi all,
Pulled out the sensor and it ran fine (which limited it down to a safe/limp mode), and did a little research - it's just a bimetallic strip, so mechanical failure didn't make a ton of sense to me. Cleaned up the contacts a little, plugged it in, and so far, running great! Either there was a little oxidation on the connection or it jiggled loose in 15 years

Your failure symptoms were pretty much cut and dried. But the temp sensor you are describing makes no sense.
Get on the parts pages and give me a part number for this temp sensor you were able to clean the contacts on.

All I have ever seen are sealed.
 
Top