Please Help with Johnson Problem

[PanamaRanger]

I have a 70 HP Johnson about 1992, 3 cylinder with carbs. Had it gone through last year and carbs rebuilt. It has run great since then. Was running great first of summer. We were gone for a couple of months and it rained a lot. It is on a pontoon boat and rained where the gas tank is a lot.

When we got back we took it out. Now it runs really great at about 2000 to 3000 RPM for as long as you keep it there. If you give it wide open throttle it picks up to about 5100 RPM and runs just great for about 20-30 seconds then starts sputtering and missing. I thought it might be water in the gas. I have put Heet and also dry out stuff in the gas and run it for a couple of hours and it still does it. Back off the throttle and it runs fine again If you immediately give it more gas it starts sputtering again but if you wait a little while at a low throttle and then give it full throttle - it runs great for another 20-30 seconds and then starts sputtering again...idles great and runs at low throttle great. THANK YOU!!!

 

[Joe Reeves]

Re: Please Help with Johnson Problem

If you have someone pump the fuel primer bulb, acting as a manual fuel pump, does that make a difference?

 

[PanamaRanger]

Re: Please Help with Johnson Problem

Yes and thank you for answering. My wife drove the pontoon and I pumped the bulb and it firmed up nicely and still missed and coughed and sputtered....I keep thinking it will die but it does not. If I back off to a low to med throttle - it runs fine....If I immediately increase throttle it misses...If I run at low speed for a little while and THEN increase throttle --- it runs great for about 20 seconds or so and then starts missing....

Thank you for the help..I am at a loss and my expertise is not great on carbs but I am trying to save some money --- I don't think it is the carb because it runs great for a little while.....It is on a pontoon in the water and if I have to get a tech out here to fix it - I bet it will cost 2 or 3 hundred or more and this is a bad time for the expense for me. I thought it might be the fuel filter in the engine but a guy I taked to said it hardly ever gives problem....I thought it might be the hose because it is the original one.....Another thing - when we had the engine done last year I had the auto oiler removed and I mix the oil --- It is a VRO motor and I did niot trust the thing so I mix oil and it ran great last year.

 

[Joe Reeves]

Re: Please Help with Johnson Problem

Possibly the high speed jets, located in the bottom center portion of the float chambers are somewhat fouled (in back of that screw at the bottom front of the float chambers). You might try cleaning them with a piece of single strand steel wire. Fouled jets are a common occurrence.

Also, inspect the spark plugs for traces of water. Water would indicate a failing head gasket (unless water is within the fuel system).

With all spark plugs removed, take a compression and spark test. Compression should be approximately 100+ psi and even on all cylinders. Spark should jump a 7/16" gap with a strong blue lightning like flame.... a real SNAP!

Let us know what you find.

 

[PanamaRanger]

Re: Please Help with Johnson Problem

Thanks again for the answer and tips....My question now is this ---

1. If it was the high speed jet, would it run great when I first open the throttle wide open for maybe 20 seconds or so or would it start acting up right away?

2. Is there any snake oil type stuff that really works that might clean it out? Some additive or something....I have used it and gone slow until I now have the gas tank almost empty....I have some Johnson 2+4 I bought (but not put in yet) and I have put Heet and Merc Dry out already in the existing tank which is almost empty now.

3. Could some water in the gas tank cause what I am experiencing....Since it is almost empty I thought about swabbing with a towel thru the fill cap hose and cleaning it out and going back with 100% fresh gas but did not want to go to all this unless it might be the problem...Thanks.