carburater elevation jet question

[checkmate150]

Hi all, I just bought my first boat last week. I appreciate being a member to this forum and all the information you all have provided.

My boat is a 1977 checkmate 16ft with a mercury 150 hp 2 stroke outboard. I got it in michigan and ran this there in dads garage, runs great. I live in wyoming and brought it back with me. The elevation in michigan in 0, sea level basicly. The elevation here is 5000 ft. I had to change the main jet in my dirt bike which is also a 2 stroke to a smaller main jet due to the less air less fuel ratio. My question is what do I have to do to my carbs to get this engine to operate properly. Thanks in advance.

Neil

 

[emckelvy]

Re: carburater elevation jet question

Normally you'd go down in main jet size about .002" and reduce propeller pitch by 2" or so. It does depend on what year/type of motor you have, Inline or V6.

Let us know what you have (Ser # would help greatly) and we should be able to come up with a more specific recommendation for you.

HTH............ed

 

[j_martin]

Re: carburater elevation jet question

On all the V6 powerheads, the recommendation is:
2,500 - 5000 feet above sea level
Main jets -.002
Vent jets -.002
Idle jets +.002

5000 - 7500 feet, it's:
Main jets -.004
Vent jets -.004
Idle jets +.004

If there is no vent jet, do not install one.

If it ran plenty rich at sea level, I'd go .004. If you don't know for sure, go with .002 first.

On an L6, main and vent jets go down .002 and .004 just like the v6's. Idle adjusts.

hope it helps
John

 

[checkmate150]

Re: carburater elevation jet question

Its an inline 6 year 1977, i'll get the serial number later today, thanks guys

 

[checkmate150]

Re: carburater elevation jet question

Serial number is 4750174 , not sure if its an xs or not
Merc 1500
(Inline 6) 150 4602012 thru 4868997 1977

Merc 1500XS
(Inline 6) 155 4567952 thru 4855152
4868648 thru 5168573 1977 1978

it has power trim and electric start

 

[emckelvy]

Re: carburater elevation jet question

Here's the info outa my book:

1500LS, 1975-77, Ser 4121435 & up, 2500 to 5000 Ft, .0785 jet

1500S, 1975-79, Ser 4121435 & up, 2500 to 5000 Ft, .080 jet

I'd venture to guess that the "LS" (Longshaft) motor has more exhaust backpressure and doesn't need as rich of a main jet. The "S" (Shortshaft) motor would have less backpressure thus flowing more air, needing a richer jet.

That being said, there's very little diff between Zero elevation and the 5000 ft level. So your motor might just run OK as is.

If it seems like it's not running real sharp, probably 'cause it's a tad rich at that elevation.

The safest thing to do would be to pop one of the main jet plugs, the brass-colored hexagonal plug at the bottom front of every carb. Then check the main jet # and it should be one size larger (.002") than the jets listed above, if the motor was properly jetted for the Lowlands.

If you find everything as expected, see if you can order up a set of leaner jets. the .0785 jets are kinda scarce but were also used in older 1350's so you might be able to find a set kicking around somewhere.

If you decrease the jet size and the motor is still not pulling rated rpm, that's to be expected; thinner air's gonna make less power no matter what you do with jetting. That's where the recommendation for 2" less pitch on the prop comes from.

Bottom line you'll still want to prop a lightly-loaded (i.e. driver only) boat so the motor spins up to at least 5500rpm. 150's are quite happy to run up to 6000 but no sense in flogging the Ol' Gal! If you can get 5500-5800 as a WOT rpm, she'll be propped perfectly for skiing/watersports, have a good hole shot and cruise at a reasonable rpm.

Goes without saying that the motor should be properly linked n' synched and have max spark advance properly set. The 1500 is quite a highly-strung motor and susceptible to detonation/melted pistons. Mid-grade is probably a good compromise for fuel, unless you can find a non-gasohol premium. It would like that just fine!

Here's some good info on the link 'n synch procedure:

http://forums.iboats.com/showthread.php?t=168855

HTH & let us know what you end up with............ed

p.s. unless you have a "Red Band" louvered cowling and low-water-pickup lower unit, you don't have a 1500XS.

 

[checkmate150]

Re: carburater elevation jet question

Thanks so much for the info, and prompt responses. I'll try it out as is and see and go from there, also I don't think its the xs with the blue color of the cover.