125 elevation adjustments

[danny9696]

I have just bought a boat with an 89 force 125hp, the boat has spent its life running at elevations from 1500ft to 3000ft, i will be running it around 7000ft is there any adjustments that i need to make to get it to run right, my carbs in my 200hp merc were a fixed jet set up with no adjustments and had to be rejetted can anyone give me any ideas or should it be ok as is

thank you all for your help

Danny

 

[Frank Acampora]

Re: 125 elevation adjustments

Well, FIRST, you need to know if it was re-jetted for 1500-3000 feet. If not, that is, if it has the stock sea level jets, then you need to step down to a leaner jet. If it already has leaner jets then you may be able to run it but certainly you would not re-jet say, for example, four steps if it is already two steps leaner.

It will run, but because of the thinner air at 7000, with standard jets it will run too rich and will lose power. BUT, you need to be careful: Too lean and the engine will be damaged. Mercury SHOULD have some technical help to tell you what size jet to use. Even though the 89 was not made by Merc, they still have a database and should be able to help.

You are going to lose power anyway, because of the less fuel and air burned so you will also need to step down in prop pitch, probably at least 2 pitches.

If all else fails, try The Chrysler Crew There's a couple of guys there that should know.

 

[danny9696]

Re: 125 elevation adjustments

Frank thankyou for all your input, I called everyone I could but no one could tell me what jets I should use, the stock jets are odd, top carb 0.101, bottom carb 0.0937, the options to by are 0.092 1500'-3000', 0.090 3000'-4500', 0.088 4500'-6000' and 0.086 6000' and above.

Because of the difference in top and bottom carb jets I am confused as to what I should put in my carbs and none of the phone calls I have made have gotten me any closer to knowing

Can anyone give me advice on this I am more lost now than when I started

Thank you

Danny

 

[Frank Acampora]

Re: 125 elevation adjustments

before 1989, Force engines (and Chrysler even earlier) used the same size jet in both carbs. I suspect that because the top two cylinders run a little hotter, AMF or Merc tried to compensate and cool the burn by running the top carb a little richer.

Prior to that, only the Chrysler 150 used jets as large as .100. Everything else used jets of .096 or less.

I would try the .088 jets (4500-6000) and run a tad richer on the low speed needles OR raise the float level a smidge to let the carbs run richer at high speeds. (Remember: Too rich on the carbs and you can readjust. Too lean and you will be rebuilding. It's much easier to change fouled plugs.)

That said, on my Chrysler 140, the one in the avatar, since I had ported it, I experimented with jetting. You would normally expect port work and other mods for greater horsepower to need bigger jets. I drilled out my .096 jets to .098 and then .100. I did not notice any difference in power or spark plug life.