Only goes to 4000rpm

[skunked56]

Found nothing in the archives so am posing the question here. Had issues with my 1984 70 horse Johnson and cleaned and renewed the carbs, drained and cleaned the tank, two new coils(the third one is good), and new fuel lines and filter. The engine runs wonderful but can only get 4000rpm at full throttle and advance. I am puzzled.
Thanks in advance for any help/suggestions.

 

[gm280]

There are few things you can do to see what's happening. First, are you positive your tach is reading correctly? I ask that because some tachs have selector switches and they are selectable on the back per the engine and pulse readings. So if that is your type tach. Make certain you have it set correctly. AND even if it is, switch it back and forth a few time to clean any possible corrosion the switch could have gotten... Next, when running can you remove anyone of the spark plug wires and the engine doesn't change speed? If you can remove any plug wire and the engine doesn't change speed, that cylinder is not firing and that can account for the lower RPMs. Verify each one individually and make sure you are running on all cylinders at WOT. And lastly, what are your compression reading in all cylinders? Remove all plugs and do a compression test to verify each cylinder is doing its job. If you find one (or more) that are not within 10% to 15% of all the others, you have a bad cylinder...probably rings... JMHO!

 

[1983 ercoa 21']

Is this boat or motor new to you?

when was the last time it achieved full rpm ?

have you changed props ?

 

[skunked56]

The tach is good and removing the plugs netted me two new coils. It has new plugs and wires and the fuel pump will be rebuilt Wednesday.
I have owned it for a few years and has run great til recently and has the same prop.
I took it out yesterday and it ran up to 500 rpm for a time then back to 4000. I will see how it runs after the fuel pump rebuild. I suspect a fuel issue or it needs a link n sink.

 

[boobie]

What RPM did it used to run ??

 

[skunked56]

It used to run 5400 wot.

 

[Silvertip]

Is it dropping a cylinder after warming up? Do a spark check when it drops rpm.

 

[skunked56]

I will do that tomorrow.

 

[rothfm]

After verifying spark when issue happens, or drop test, look for link/sync if it is running smooth on all cyls but missing some rpm, May be dropping due to timing/throttle advance adjustments.

Had one recently missing 700 rpm from that adjustment, after verifying fuel and spark stuff.

 

[F_R]

Link & sync & timing stuff should not change unless somebody has monkeyed with it. Your motor probably is not running on all 3 cylinders. OR it is ingesting water into a cylinder (s).

 

[skunked56]

Link n sync needs to be done but I found the problem,just a tiny something in the fuel pump blocking a tiny hole. Back to normal great running. Thanks for the help.

 

[fireman57]

Amazing what looks like a speck of junk can do to these engines depending on where it is. Glad you are ready for the weekend.