Motor running poorly

[viciousfishes]

Alright,

Well I have a 1968 20 HP Johnson and it just started running pretty bad. It idles well right when I crank it up and will run great at full throttle for about 1 minute. After that it sounds like it wants to go back up to full power and then it just cuts back and goes really slow and runs rough. Here's a list of parts ive replaces/rebuilt:

New coils
Rebuilt carb
Took apart and inspected fuel pump
New under cowl fuel lines
New spark plugs

What could it be?

 

[RDJuhl]

Re: Motor running poorly

Have you made sure your fuel tank is vented? When it is running rough, can you squeeze the fuel bulb, and if so, will it run smoother?

 

[trobinson017]

Re: Motor running poorly

I just posted a similar problem with my '99 Johnson 130 outboard. I've been taking it out to try and troubleshoot it but not having any luck. Im running off of an externa fuel tank to eliminate any internal tank issues. My will hesitate to go WOT for about 10-20 secs, then it'll jump to WOT for maybe 4 secs, then RPMs/speed drops to about half power. It's driving me crazy!

 

[viciousfishes]

Re: Motor running poorly

Yes it is well vented. The bulb isn't too tight while its running but thats how its supposed to be, and trobinson, I feel your pain. I just wanna go fast again!

It used to just bog down every now and then but now it stays bogged down more than it is going fast

 

[Daviet]

Re: Motor running poorly

Start with the basics.
Compression ok, sounds like it might be because it runs properly at times, but it should be checked with a tester to make sure.
Spark, bright blue and has a snap when checked with an air gap tester. You can use an inductive timing light, the clamp on type, to check spark when it acts up.
If spark and compression are ok, it leaves fuel. When you overhauled the carb did you remove the high speed jet and clean it really good?

 

[Monte1961]

Re: Motor running poorly

Always do the basics first before you start to overhaul anything or use it for an anchor! 1. Is it getting fuel? Is it getting spark? Then move on!

 

[viciousfishes]

Re: Motor running poorly

Yeah I cleaned the jets out but they were already really clean. They are sparking well I have already checked that too. Ill test the compression today just to make sure. Ill get back to yall with the results