High Altitude Jets

[rod2610d]

1994 Johnson 175 Fast Strike
Elevation 6500 to 8000 ft

Motor has stock jets and is fouling plugs.

1) Called OMC and they would not take responsibility for any jet modifications due to altitude. They did however send me a list of jets and part numbers that I can get.

2) all 6 of my carbs have 3 jets, High Speed, Intermediate, and Idle

Two of the jets appear to be air jets.....IE suck air into carb, as they are on outside of carbs. I believe they are the idle and intermediate jets.

The 3rd jet is behind a brass plug on bottom of carburetors and is large I believe a 72D jet. I think this is the high speed jet.


QUESTIONS:
At high elevation do the idle and intermediate jets need to be bigger to allow in more air?
Am I correct that the 2 external jets are indeed Idle and intermediate?
I pretty sure the large jet in bottom of fuel bowl is the high speed jet and am planning on replacing it with 6 each 70D jets.

Basically the main question is that I have to increase size of idle and intermediate jets and decrease the size of the high speed jets.....RIGHT?

Also if anyone can recommend the correct jets for all 3 I would really appreciate it.
No the manual does not have a altitude correction table in it.
No I do not need a prop change at this point.

 

[Tim Frank]

1) Is there an OMC dealer in your area? Common sense would suggest that as your go to for info.
2) Where did you get the motor. i.e. What altitude was it run at?
3) 72D would certainly be a HS orifice. Two size decrease is not unreasonable in your altitude range.
4)There were a couple of recent posts with a similar situation/questions to yours. Maybe try a search and see if they are any help.
5) What plugs are you running?

 

[rod2610d]

Yes there are at least 3 dealers in Denver, but none will touch it. One even told me they would not work on motor due to age. One gave me the number to OMC and that's as much help as they could give.

I got motor in Missouri, where it ran great. It has stock orifices in it.

I have searched and read several posts, but none are specific on wither the idle and intermediate need to be increased in size while decreasing the high speed orifice.

I running the plugs that the Johnson manual says to use QL77JC4

 

[flyingscott]

You can do the jets yourself. The dealers around you should have a jet chart for the altitude because they sold those motors back then.

 

[rod2610d]

I know I can do them its easy.

They have no jet chart or not willing to help. That's why I'm here hoping for some input.

 

[Tim Frank]

There was a recent post that was about the same elevation parameters as you are listing.
No mention of anything other than the HS jets. Two size drop worked for him.
Maybe fire off a PM to him and ask about the other two jets? Are they not adjustable? That would perhaps explain why they weren't mentioned.

That is appalling that the dealers will not at least share info,. I sort of understand why they do not want to work on older motors, but maybe if they cannot answer such a simple question, better not to have them try to do the work....

 

[flyingscott]

I would physically go to the dealer and order the high altitude jets for your motor. And just tell them that, once you actually want the part they may be more helpful. Let them figure out the part #s it is worth a shot.

 

[Tim Frank]

I guess it depends what you mean by a jet chart, and what you are hoping it will tell you.
In my OEM manual for an 8 HP Evinrude there is a chart for jets that shows size, thread spec, and part number.....for most if not all available jets up to that year....1992....but no link to what engine they fit, or altitude info.

If you just need the part number for a 70D jet, I can post that on Wednesday when I am in the same location as my manual.
Someone else may be able to provide it sooner.

 

[rod2610d]

No they are not adjustable all 3 are orfice type jets....No screws at all
.
Thanks but I have a list of all OMC jets along with their thread sizes, and part numbers (70D is Part # 327769). So I can order the right ones, once I figure out which ones are the right orfice sizes for my elevation.

I need to know if the idle and intermediate jets need to be bigger due to elevation or smaller. Those 2 jets seem to allow outside AIR into carburetor. As opposed to the high speed jet that is in the fuel bowl and submerged in fuel.

According to OMC there are no "high altitude jets" for my motor. I have to figure out which jets work best for my location. The manual does not list anything for high altitudes. Trial and error, but at $15 per jet times 18 jets trial and error seems to be lining the pockets of OMC Dealers. Spend $270 plus shipping and find out its wrong than spend more $ on more jets.

I can figure out which jets I need if I knew if the idle and intermediate jets should be smaller or bigger than factory.

I would go to a dealer but Denver is 100 miles away and my closest dealers are there. I live at 7800 feet with lakes a half mile away.

I can figure out which jets I need if I knew if the idle and intermediate jets should be smaller or bigger than factory.

I can figure out which jets I need if I knew if the idle and intermediate jets should be smaller or bigger than factory.

I can figure out which jets I need if I knew if the idle and intermediate jets should be smaller or bigger than factory.

For High Elevations​

 

[Tim Frank]

https://forums.iboats.com/forum/eng...0hp-johnson-rubber-oring-chunks-in-thermostat

This guy had a similar issue but caused by oversize jets which he replaced rather than elevation, but the effect should be the same.

Two sizes should be a great starting point. The 70D should be OK for HS....try that first and see where you are.

 

[rod2610d]

Interesting…..

So, I can swap out 6 of the HS jets to 70D and leave the idle and intermediate alone?

I guess that makes sense, for one while at idle very little smoke/oil is noticed out of exhaust, but there is a little. At rpms above 3000 the oil/smoke out of exhaust is VERY HEAVY.

I will try this first because I know the HS jets are too large.

Thanks great advise.

 

[Oldnbold]

Less fuel and more air would be my direction as well. 2 sizes change is a reasonable starting point 1 other thing to consider is your cooling system: ie is the thermostat functioning properly? A constantly cold 2 stroke engine is hard to jet.