1974 Johnson 9.9 fine at WOT but won't idle

[ereuter]

I have a '74 Johnson 9.9 that I recently acquired. It works great at full speed, but surges sporatically at low speed or at idle, and eventually dies. If I rev it, it comes back. It seems as if one cylinder is dropping out.

I have done the following, and nothing has improved:

- Replaced coils, plug wires, plugs (have tried both Champion and NGK with no difference)
- Replaced breakers and condensers
- Set timing for both cylinders, confirmed with timing light
- Replaced fuel pump with used/known good one (bulb stays hard)
- Removed carb, cleaned needle valve and main jet

I have messed around with different settings of the rich/lean knob, starting with it about 1.5 turns out from bottomed out.

I'm at my wits' end. Any suggestions are very welcome.

Thanks-
Eric

 

[Will Bark]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Two things come to mind; first, bulb should not stay hard with engine running it just becomes anothe peice of the fuel line and; you didn't mention removing the welch plugs on the carb and cleaning out the idle circuit holes underneath. Those holes are probably what's stopped up and making the rough idle. You need a carb rebuild kit for this. Good luck

 

[fireman57]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

You need to clean all the jets. Sounds like there is some crap in your low speed jet

 

[nwcove]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

ditto on the above two posts

 

[ereuter]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Thanks. I will get a kit and do that.

Would this cause one cylinder to drop sporadically? It feels like this is what's happening, which is what made me tackle the ignition system first.

 

[nwcove]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

since you have done all the ignition work, the feel of dropping a cyl could easily be a carb issue.

 

[fireman57]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

You only have one carb that feeds both cylinders so if one is getting fuel then the other one should be too. If you have multiple cylinders they are fed by multiple carbs. Not necessarily one carb per cylinder.

 

[ereuter]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

You only have one carb that feeds both cylinders so if one is getting fuel then the other one should be too. If you have multiple cylinders they are fed by multiple carbs. Not necessarily one carb per cylinder.

Correct, this motor only has one carb for both cylinders. That's why I don't understand how a carb issue could cause a cylinder to drop out. But I have a carb kit on the way, and will clean the low speed jet as suggested and report back.

Thanks,
Eric

 

[ereuter]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Finally back around to this project after leaving it for the winter. I got a rebuild kit for the carb and rebuilt it - including pulling the welch plugs and cleaning those passages.

No change in the motor's behavior. Starts up, idles, runs strong. Then, after it's run for a while, it starts to sputter (even at WOT), won't idle, very hard to start.

It's as if it only wants to run when it's cold, and then had an issue once it's heated up.

Any thoughts? I'm about to go shopping for another motor. I've replaced everything on this motor. I'm thinking it's something with the block or head at this point.

 

[AlTn]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

any evidence of water intrusion?...water beads on sparkplug?...one plug/cylinder "steam cleaned"?

 

[the machinist]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Possibly a air leak into the fuel intake ssytem behind the carburetor, like a bad manifold gasket or carb to manifold. Or air bypassing the idle needle jet seal.

 

[Daviet]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Use an inductive timing light and check eack on plug wire and see if you are dropping a cylinder when it acts up. That will tell you if it is an ignition or fuel problem.

 

[vikingsun]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Sounds like coil probs to me after its a bit hot looses spark..make sure to check all wiring maybe a bare wire some where?
one motor one time someone changed the condesors and the fly wheel barely scraped one wire,,it would ark out and make the motor run like crap...
Also check the fuel bulb pressure...when it dies..or tank vent...cap???
if nothing pump it up and see if it starts...

Use an inductive timing light and check eack on plug wire and see if you are dropping a cylinder when it acts up. That will tell you if it is an ignition or fuel problem.

 

[oldybutagoody]

Re: 1974 Johnson 9.9 fine at WOT but won't idle

Thermostat?