full throttle to half throttle without throttling down

[Patricia K. Green]

I have a 78 Johnson 70 hp.that will go from full throttle to roughly half throttle without throttling down. There have been days where this does not happen. When it does happen I throttle down and sit there for a few seconds and then throttle up. The boat starts to come out of the water only for a second then back to half throttle. I have replaced the spark plugs (3 times). I have replaced the fuel tank and lines. I have rebuilt the carbs and fuel pump. When this happens I can choke it and nothing happens. I can throttle down and choke it and the motor starts to flood, which is normal. Vent is open. The boat did sit up for two years.

 

[JB]

Re: full throttle to half throttle without throttling down

Howdy, Patricia.<br /><br />Welcome to iboats. <br /><br />What you have is an engine trouble. I will move this to the Johnson Trouobles Forum.

 

[JB]

Re: full throttle to half throttle without throttling down

Sounds like you are losing a cylinder, Pat. My most likely suspect is a heat related failure in the ignition that "cures" itself when the engine cools a bit.<br /><br />The first thing I would do is run it with the bonnet off. If that prevents the trouble try this:<br /><br />Put the muffs on the engine, fire her up and let her idle while you put heat from a hair dryer on the ignition system parts. Do not use a heat gun. . .too hot.<br /><br />If heating up the ignition parts, one at a time, gets it to drop a cylinder (idle speed drops) you have a heat failure and you have nailed which part it is.<br /><br />Good luck, and let us know what you find.

 

[Patricia K. Green]

Re: full throttle to half throttle without throttling down

I have run it with the bonnet off. Didn't help. It acts up cold or hot. I don't think my idle speed drops. Any more suggestions? Thank you for the help.

 

[JB]

Re: full throttle to half throttle without throttling down

Hmmmmm. I would try the hair dryer anyway.<br /><br />Perhaps someone will come along with other suggestions.

 

[ezeke]

Re: full throttle to half throttle without throttling down

There are some similarities in this to another thread. Maybe reading it would help you. Click here<br /><br />Most important is the next to the last entry by Joe Reeves.