1990 Mercruiser 350 Magnum Alpha One tune for altitude?

[EagerToLearn]

I have a 1990 Mercruiser 350 Magnum Alpha One (SN# OC757852) that runs very well. I'm located at 3000 feet above sea level where it normally operates and have been told that since its a carbureted engine that I'll want to tune it for a lake we're going to spend a week at in the near future that's located at 6,000 feet.

I've read that it's possible to tweak the air mix to perform better at the altitude and from what I gather it's a screw that should be turned in 1/4 turn increments.

Before I go off and do anything stupid, I want to validate that it is the case and what I've heard is correct? Is this something that can be done by the average Joe? I'm not the most handy man in the world but not completely incompetent either (I've dropped a drive shaft, replaced an alternator, rewired my trailer). I don't want to get in over my head but as my username suggests, I love to learn.

Thoughts on if that's something that can be done or should I just make an appointment with a local shop the first day and get them to tweak it? Thanks for the advice!

 

[dubs283]

I've read that it's possible to tweak the air mix to perform better at the altitude and from what I gather it's a screw that should be turned in 1/4 turn increments.

sounds as if your referring to the idle air mixture screws which could help improve your idle but that's all you can do with that

going up in altitude you'd have to install smaller jets (requiring disassembling the carburetor). you may notice the engine running a little rich as there is less air pressure at higher altitudes but if you are just spending a little time there and will be returning to a lower altitude you're better off leaving it. unless of course you like to tinker and don't mind tearing down your carburetor

a quick fix would be to change the prop but again may not be worth it unless you are trying to win a race or something

 

[nola mike]

I have a 1990 Mercruiser 350 Magnum Alpha One (SN# OC757852) that runs very well. I'm located at 3000 feet above sea level where it normally operates and have been told that since its a carbureted engine that I'll want to tune it for a lake we're going to spend a week at in the near future that's located at 6,000 feet.

I've read that it's possible to tweak the air mix to perform better at the altitude and from what I gather it's a screw that should be turned in 1/4 turn increments.

Before I go off and do anything stupid, I want to validate that it is the case and what I've heard is correct? Is this something that can be done by the average Joe? I'm not the most handy man in the world but not completely incompetent either (I've dropped a drive shaft, replaced an alternator, rewired my trailer). I don't want to get in over my head but as my username suggests, I love to learn.

Thoughts on if that's something that can be done or should I just make an appointment with a local shop the first day and get them to tweak it? Thanks for the advice!

What carb do you have? The Weber/Edelbrock could be adjusted by changing the metering rods, which takes 5-10 minutes.

 

[achris]

Mercury's recommendation for altitude is to just re-prop to get the RPM back into the recommended range. 4400-4800rpm.

 

[EagerToLearn]

sounds as if your referring to the idle air mixture screws which could help improve your idle but that's all you can do with that

going up in altitude you'd have to install smaller jets (requiring disassembling the carburetor). you may notice the engine running a little rich as there is less air pressure at higher altitudes but if you are just spending a little time there and will be returning to a lower altitude you're better off leaving it. unless of course you like to tinker and don't mind tearing down your carburetor

a quick fix would be to change the prop but again may not be worth it unless you are trying to win a race or something

Thanks for the suggestion! I do have a couple of 4-blade prop (steel vs current 3-blade stainless) that I can swap in. I think I'll try that---mostly concerned about pull/start vs top speed.

 

[EagerToLearn]

What carb do you have? The Weber/Edelbrock could be adjusted by changing the metering rods, which takes 5-10 minutes.

No idea unfortunately--I'll have to look and post some follow up pictures. Thanks for the suggestion!

 

[nola mike]

Mercury's recommendation for altitude is to just re-prop to get the RPM back into the recommended range. 4400-4800rpm.

Up to what point? You'll still be running with a less than ideal AF mix

 

[Scott06]

http://www.tiptopgarage.com/pdf_files/merc_weber_4barrel_97_09.pdf

although this is geared to the webber carb it gives you a general idea of high altitude adjustments On pg 6&7 . As chris mentions for intermittent use just go down in pitch . The carb calibrations aka jetting changes are all stock tune up to 5 k ft so you probably dont need to do anything except the prop unless you keep using it at higher altitude.

all this assumes everything is currently in good working order…

 

[80corvette999]

Up to what point? You'll still be running with a less than ideal AF mix

I rejeted mine useing the eddelbrock chart. Runs ok at 5000 ft.