To jet or not to jet?... that is the question.

[jbigtrout]

Hello, so I purchased my '78 mercury 115 about a month ago in Phoenix moved it to Albuquerque. 1,000 ft to 5,000 ft. It runs but does not idle properly and when you hit it it bogs down 'til it hits a certain rpm then it takes off like a bat-out-of-hell. Half the folks tell me I need to re-jet for altitude and half the folks say no. I am baffled. Has anyone re-jetted for altitude and has it made a difference?

Thanks for your input.

 

[Chris1956]

Re: To jet or not to jet?... that is the question.

The jets you would change are the main and backdrag jets, not the idle jets.

Since the motor runs good at high speed (main jets in use), why not adjust the idle mixture jets to be a bit leaner? i would leave the mains alone until you have spark plug evidence that it is running too rich.

 

[jbigtrout]

Re: To jet or not to jet?... that is the question.

Thanks Chris,

I did notice what seemed to be more blueish smoke bubbling up when running it especially when it seemed to be bogging down so to me it seems like it is running too rich but nor sure.

 

[Coloradolakeboy]

Re: To jet or not to jet?... that is the question.

I changed to high altitude jets and the idle improved at 5000 ft. But I want to run at the higher elevation lakes here so that is why I changed them. Most shops around here do not recommend it. Did you change your prop? I would go down on prop size or two first and see if the bog goes away.

 

[jbigtrout]

Re: To jet or not to jet?... that is the question.

Yes, I dropped the prop two sizes and it did help but am still having bogging issues. But it definitely ran much better.

 

[j_martin]

Re: To jet or not to jet?... that is the question.

1000 feet would have the stock jetting in it. 5000 feet would be 2 stages leaner to run right. If you plan to run it at 5000 feet, you should change to the recommended fuel jets. It will also need a lower pitch prop. There just isn't as much air there as there is at sea level, so your fuel requirements and horsepower production are both reduced.

The jetting charts are in the factory service manual.

 

[jbigtrout]

Re: To jet or not to jet?... that is the question.

Thank J!