Adjustments for higher elevations 69 85hp

[GLT]

I recently moved to Colorado from Idaho and a change of elevation from about 2800 to elevations between 5000-8000 depending on what lake I go to. Anyhow there's a very significant drop in performance of my ob since I moved. Are there any recommendations for high altitude adjustments I can make.

 

[tashasdaddy]

Re: Adjustments for higher elevations 69 85hp

yes the jets have to be changed in the carb. get the advice of a local dealer, they will have the data that is necessary. you are dealing with thinner air, and the engine is running rich.

 

[GLT]

Re: Adjustments for higher elevations 69 85hp

I believe the jets are factory sealed.
I did talk to one local dealer who maintains that the only thing I need to do is change the prop from a 17 to 15. I don't believe that's the whole fix though, since the rpm's are slugish when I throttle up in neutral.

 

[tashasdaddy]

Re: Adjustments for higher elevations 69 85hp

the motor has fixed jets, but are replaceable with a simple special tool. the dealer didn't want to work on a 69. they are electric shift and most of the new guys don't want to be bothered.

 

[tashasdaddy]

Re: Adjustments for higher elevations 69 85hp

http://i155.photobucket.com/albums/s311/frankr_photos/FixedJetScrewdriver2.jpg?t=1177353186

 

[bds85466]

Re: Adjustments for higher elevations 69 85hp

http://i155.photobucket.com/albums/s311/frankr_photos/FixedJetScrewdriver2.jpg?t=1177353186

Did you get those in prison?

 

[GLT]

Re: Adjustments for higher elevations 69 85hp

Okay, where and what size jet do I look for. I would need the tool too, but I could probably create that if I had to.
Thanks

 

[GLT]

Re: Adjustments for higher elevations 69 85hp

Anybody have any ideas on where and what to buy for the jets on these carbs.

 

[Texasmark]

Re: Adjustments for higher elevations 69 85hp

No, but my '05 pickup truck which is used to 800 ft fell on it's arse crossing the 8500' mountains around Alamogordo, NM.

So if they aren't expensive and you have the time.....since so far you don't seem to have any other options, you might purchase several sizes and experiment. The tool doesn't look too difficult to make from a screwdriver.

Then put on that 15" prop the dealer suggested.

I'd start with half the area.

Here's an example.

The late '90's 100 and 125hp Merc engines are being compared (from my service manual). We have a 25 hp increase with the same number of carbs/cylinders...course other things may change too, like maybe a different carb to change the venturi air flow volume, but.........

The main jets are .046 and .066 inches in diameter respectively (these engines happen to be running 4 carbs on 4 cyls. Calculating area, the area of the 125 is twice the area of the 100 even though the hp only went up 25%.

You can't just halve the diameter since area is what lets fuel thru the hole and area is squared in the calculation so it goes up or down much faster than the diameter change....area = 3.14 x [(dia/2) x (dia/2)]. [3.14 is usually symbolized by the (greek letter pi)].....so for the 100 hp jets @ .046 diam, area is calculated: 3.14 x [.046/2 x .046/2] = .00167 sq in. The 125 hp @ .066 have an area of .0034 which, is as I said, about double.

That should give you an idea.

Hopefully an altitude guy will jump in here and give you some feelers.....

Hello QC, BruceB, where are you?

Mark

 

[GLT]

Re: Adjustments for higher elevations 69 85hp

Wow, thanks Mark good information, I'm still not sure where to buy the jets, so hopefulley someone jumps in with some suggestions.
Gary

 

[tashasdaddy]

Re: Adjustments for higher elevations 69 85hp

about 1/2 way down this link it is mentioned. call Jeff at laingsoutboards.com he's very knowledgeable about the older motors, there specialty. he may be able to get you what you need.
http://www.brokeboats.com/stuff.html

http://www.laingsoutboards.com/newp...ncludes Gale, Sea King up to 1963, and others

 

[GLT]

Re: Adjustments for higher elevations 69 85hp

I found a local dealer that said he has the jets in stock, but he didn't recommend changing then, said it can be bad for the motor. He said I should clean and make the adjustments on the carbs and the best thing is to just change the prop pitch to about a 15 and 13 if I go to the mountains. I'm not sure why it could be bad for the motor especially if it makes it run better, I would take them back out again if it ran worse. However he's the second local person that's told me to only to change the prop.
Any other thoughts on this.

 

[tashasdaddy]

Re: Adjustments for higher elevations 69 85hp

the reason they say that, is, that it is changing the recommeded, MFGR fuel/oil/air mix the engine is running on, thus they are covering their fannies. i doing this it is changing the amount of lubricant that gets to the engine. i the higher elevations there is less oxygen in the air, so it takes more air and less fuel to fire the engine, in other words then engine runs rich on fuel/oil. changing the jets cuts down the fuel/oil and adds air.

Basicly, the engine is flooding on gas, without enough oxygen to burn it up.
totally confused now?

you get a better running engine, with the worry of not enough oil. but they did say they could run on 100/1 vs 50/1, but took that back.

 

[GLT]

Re: Adjustments for higher elevations 69 85hp

I do understand actually. I've been kind of backyard mechanic since I started to drive, 30+ years ago. Before that I had a couple of 2 stoke dirt bikes, that the neighbor worked on mostly, and I no he did some rejetting.
So maybe a throw a little caution to the wind, change the jets and put a little more oil into the mix, possibly a hotter plug. What do ya think?