Johnson 15 carb issue - need help diagnosing

[Roj115]

I bought an '80 Johnson 15 that had sat for a couple years. I got it started fairly easily but it had some running problems that were persistent. Sometimes when accelerating toward WOT it would stumble and the revs would drop. A second try would usually get it to go to WOT although I thought the engine was not putting out full power at WOT compared to my '77 Evinrude 9.9 converted to a 15. Also, upon decelerating from WOT, the engine would usually bog and stall requiring a re-start. Once started, there was no problem idling in as the engine idled very well.

I pulled the carb out and found that it was gummed up pretty bad inside indicating that the engine had not been properly run out of gas and fogged and that gas was allowed to sit in the carb for long periods of time and go bad. Besides varnish, there was also some light brown powder in the bowl which I also cleaned out.

I cleaned up the bowl and replaced the needle valve, float and two brass bushings from a used but ultra-clean 9.9 carb that I had sitting around from the conversion I did on the '77 9.9. All the parts I switched into it were compatible and no different from what was in the '80 15 carb. I also pulled the low speed adjustment needle out and replaced the seal/grommet around it with the one from the '77 carb and cleaned out the channel it screws into with a spray of carb cleaner.

After reassembling and re-installing the carb I have a different problem. The engine starts very well, it idles and trolls very well and you can rev it up high in neutral very well but when you put it in gear and try to accelerate to WOT it will start to gain revs and just as the boat is about to plane and the revs really start to pick up, the engine just dies (stalls). Obviously, not enough gas is able to get through to allow the engine push the boat beyond a certain speed and when the timing advances to a certain point, the mixture is leaning out to the point where the engine stalls.

When I worked on the carb, I did not knock out the Welch plug and do any cleaning in there. Could some crap in there be causing my problem? If the used seal/grommet I used for the low speed adjustment needle is not sealing, could that cause the problem I'm now having?

Thanks in advance.

 

[flyingscott]

Clean under the welch club and try pumping the primer ball when that is happening sounds like a fuel pump issue.

 

[Roj115]

I should have mentioned that I can get the engine to run at WOT if I pull and hold the choke out part way when accelerating toward WOT. And again, the engine will rev up high when in neutral (no load) but not when in gear (under load). I think that makes it obvious that the mixture is leaning out too much when the timing is advancing and the revs are increasing under load.

I need to understand more about how that carb operates and what happens when you transition from low speed to the high speed jet to be able to figure out where the choke point might be. Obviously, it's likely to be in an area of the carb that I haven't addressed as of yet.

 

[flyingscott]

Clean under the welch plug and you may need to soak it sounds like your high speed jet may be blocked

 

[tomhath]

The welch plug covers the idle circuit inlet, I doubt it's the problem if the motor idles nicely.

I'd suspect either the fuel pump or the float valve. But since you cleaned the carb already, either replace the pump or take it apart and see if the filter screen is gummed up or the membrane is stiff. Varnish in the carburetor probably means the fuel pump is very suspect.

 

[Roj115]

I think I have it figured out. Actually, I didn't figure it out,; Leeroy of LeeroysRamblings.com figured it out. The clue is where I said "I cleaned up the bowl and replaced the needle valve, float and two brass bushings from a used but ultra-clean 9.9 carb that I had sitting around". Well, one of those brass bushings was the high speed jet. I thought that part was universal to both the 9.9 and 15 but I guess I had that confused with the other brass bushing with the needle valve inside. The high speed jet for the 15 (PN 322293) has a larger hole and transmits more fuel than the needle valve for the 9.9 (PN 322752). I looked at them and thought they were the same but I guess they weren't. I swapped them because the one in the 9.9 donor carb was sparkling clean and looked brand new and the one in the 15 carb was varnished and needed a lot of cleaning.

With that in mind, it's easy to see why the mixture leans out too much and the motor starves for fuel when you open it up and the revs hit the point where the high speed jet engages. It also explains why I could make it run at WOT by choking it partially. It all makes total sense.

Y'know, I studied those carb parts diagrams quite a bit and THOUGHT I knew what all the differences were between the 9.9 and 15 hp carbs. I don't know how I missed the fact that the high speed jets were different on the two versions. A really dumb mistake on my part. :redface-new:

Unfortunately, I have to wait 2 weeks to get back out to my cabin and try the motor out with the correct jet put back in the carb. I'm hoping like crazy that all the symptoms I was experiencing before I cleaned all the crud out of the carb will have gone away and the problem caused by me substituting the incorrect high speed jet will be gone as well. It's going to be hard to wait that long.