Merc 3.0L High Altitude Jets

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Sparks75

Cadet
Joined
Jun 1, 2010
Messages
6
All,
I posted this over the weekend and did not get any response, any help is greatly appreciated.
I am currently running the stock 1.45 jets in my 1989 3.0L merc 15P 4 blade prop, at high altitude, there is a little bogging...
I'd like to change the 1.45s out for the Merc recommended 1.40's for altitude between 5000'-9000', which will cover most of my needs, chatfield ~6000, Steamboat lake ~7000, green mountain res ~80000., Dillon 9000, etc etc
But what about when I go to Lake McConaughy which is I believe about 3250'
Will I need to change the jets back in order to keep from running too lean and burning a piston? or is that not enough altitude change create a problem?
I hope to put the 1.40's in leave them alone.
Any other altitude tweaks?
I hear advancing the timing also helps...
Not sure if it is relavant, but this week I am installing the following ignition components:
PerTronix Ignitor II
Flamethrower II Coil
So spark should be strong?

Thanks for the help
 
45Auto

45Auto

Commander
Joined
May 31, 2002
Messages
2,842
Re: Merc 3.0L High Altitude Jets

I believe the reason you didn't get any responses to your original thread is because your situation is too close to the edge for anyone to specifically state that you would be OK with the leaner jets at the lower altitude. I know I wouldn't guess at something like that, and I wouldn't believe anyone who did. Won't hurt it any to run a LITTLE rich, but let it run just a LITTLE lean and there's a good chance you'll be in the market for a new engine.

Just the production tolerances in identical engines could make enough difference for it to be OK in one, but destroy another one through detonation. Remember, you're putting a lot of load on the engine, not like a car motor where you seldom use more than 10% to 25% of the available power.

Only thing I can suggest is that you try it, but keep a VERY close eye on the plugs until you're sure it's OK under all conditions.
 
S

Sparks75

Cadet
Joined
Jun 1, 2010
Messages
6
Re: Merc 3.0L High Altitude Jets

thanks, I got specific on the lakes, just hoping someone else has run the same area on the sames jets,
If I do it, I'll keep my ears open and check the plugs.
thanks
 
F

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Re: Merc 3.0L High Altitude Jets

I run a 140HP Mercruiser (3.0L) at your altitudes and basically use 1 # lower jet for every 2000' of elevation. I never found specific jetting info for my old engine so I just went with the old rule of thumb for high altitude jetting. My stock sea level jets are 60's and I run those up to 5000', above 5000 I run 57's and at 9000' I run 55's. I have a Rochester carb, and assume you have a Mercarb since the jet sizes are so different.

I wouldn't run down at 3000' jetted 5 numbers leaner as I think that would be asking for problems.

If you play around with the timing, reply back on how it runs with an extra degree or 2 thrown in. I've thought about it, but didn't want to get into detonation and toast the engine.

What prop (manufacturer , diameter and pitch) are you running and how do you like it? I have a cheap second hand 15P Black Max I run ap at 9000' and it's OK, but I like to hear what you have to say about your prop.

Oh yeah, what problems are you dealing with? When you say bogging, is that coming off idle or while the boat is trying to come up on plane at around 2000RPM. I've found coming up on plane is my boat's biggest hurdle to overcome at altitude,,, as the boat struggles to break over and just hangs at a fixed RPM. I don't ever remember having problems coming off idle and have never messed with my idle adjustments, just swapped the jets.
 
jaxnjil

jaxnjil

Lieutenant
Joined
Aug 3, 2007
Messages
1,368
Re: Merc 3.0L High Altitude Jets

i run from steamboat lake to lake mead.

if you go by mercruiser advice on altitude they recommend not doing any thing to motor and change prop pitch to compensate for air lose at higher elevation.
when you run out of prop pitch it becomes necessary to change final drive ratio to gain back prop pitch.
wouldn't mess with timing. your motor is a lot more like a stationary motor and not a car motor. operating conditions are not close to the same. what works for a auto motor isnt necessarily good for a boat motor

even if you re-jet carb you cant make up for lost power and will be lugging your motor at higher elevation unless you gear up or prop down
 
jaxnjil

jaxnjil

Lieutenant
Joined
Aug 3, 2007
Messages
1,368
Re: Merc 3.0L High Altitude Jets

by the way the steam boat lake we go to in routt county is 8100 ft in
elevation


the last alpha drive we ran on the 230 weekender was 1:65 to 1. props were 17 pitch at steam boat lake to 21 pitch at mead(around 1000 ft)

was a little over pitched at steamboat and a little under at mead.
 
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