Fixed orfice idle jet- Small orfice = leaner mixture??

[logged in]

Ok, I am rejetting my 1986 40hp to run a 7200 ft elevation.
Dealer said to install 36M idel jet. OK, jets arrived today and on inspection
the hole in the 36M idle jet is smaller hole than the standard #40

So how does the smaller hole in the 36M idle jet make the idle mixture leaner?

I would think that reducing the amountof air flowing in to idle mixture would make it richer not leaner.

What am I missing here.

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture??

it lets less fuel though it ?? smaller hole smaller volume of fuel think of it like a blowing though a coffee straw and then a mcdonalds drinking straw which one flows more air? same with the fuel through a smaller jet

 

[Joe Reeves]

Re: Fixed orfice idle jet- Small orfice = leaner mixture??

The 1986 40hp Evinrude/Johnson idle jet is a "Air Bleed" jet.... measures air, not fuel.

You are correct is assuming that reducing the air flow richens the mixture.

A smaller size air bleed jet results in a richer mixture (less air, more fuel).

A larger size air bleed jet results in a leaner mixture (more air, less fuel).

The imprinted numbers on the jets simply indicate the ID size (36M = .036..... 40 = .040).

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture??

alright joe my bad i was thinking in terms of a carb fuel jet .... and yes it would run richer if its a smaller air bleed

 

[Joe Reeves]

Re: Fixed orfice idle jet- Small orfice = leaner mixture??

Levi... I wasn't correcting you. You simply typed faster than I did this time

 

[logged in]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Thanks, for the feedback

When I talked to my local dealer he said the elevation chart said to re-jet
to 36M idle and 32D on the high speed But. the 32D high speed jet is no longer available.

I told him the main reason I wanted to rejet was to troll and would it be OK to just change the idle jets. but after reading you comments I think the both jet play a part in the idle fuel mixture.

Here is my reasoning

Sea level idle (air) jet #40M=.040ID High speed #38D=.038ID Idle jet .002 larger than the High speed (fuel) jet

6000 to 10000 ft setting
idle (air) jet #36M=.036ID High speed #32D=.032ID Idle jet .004 larger than the High speed (fuel) jet

Wtih this reasoning and the notion that the 32D high speed jets are no longer available, would the fix for smooth idling at 7200 ft be drilling out a spare set of idle jets to .042 ID to make a .04 differental? AKA leaner

I am just a rookie at outboards and two cycles so please let me know what you think.

Or does anyone know of a source for the 32D high speed jets?

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

youve gotta look at it in terms of volume for each and change in volume for each so....
im doing brain numbing math get back to you tomorrow

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

screw the math i think i found the part 320016 marineengine.com

 

[daselbee]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

32D = OMC P/N 333184. Any OMC parts dealer.

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

and thats why they pay daselbee the big bux ^ marineengine.com has them but theyre out of stock

 

[Joe Reeves]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Daselbee.... You're getting the big bucks? Zooowie! Andrew said he'd pay me what I was worth. I told him I couldn't live on that.

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

lmfao joe i figured you were getting em too??

 

[logged in]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Daselbee, thank you for finding the part number.

I have order a set but they wont be in intime to install for my trip.

I sacraficed a spare set of #40s and drilled them out to .043

Here is my math

Sea level
idle jet #40 area =0.0012566370614359174
high speed jet #38 area=0.0011341149479459152
idle is roughly 10% larger by area

high elevation setting
Idle jet #36 area=0.0010178760197630928
High speed jet #32 area=0.0008042477193189871
idle is roughly 26% larger by area

solve for high elevation based on stock high speed Jet
stock high speed jet #38 area=0.0011341149479459152 x1.26 =0.0014289848344118
Area for a .043 drill=0.0014522012041218814 close enough I hope....

I will let you know how it goes.

Again thank for all of your assistance.

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

ok you kinda got what i was getting at but its not a relation between the high and the low but a ratio between the low and the low and the high and the high dig? they work semi independant of each other and therefore need to be calculated that way daselbee found the part number before i finished my math so i stopped figurementating (yes thats a real word gosh!)

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

BUT who knows maybe itll act right i figure it was worth a shot .... i dont think you can hurt anything ...

 

[nophun]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

I'm curious to see how this works out. Old auto carbs used an air jet to make a foam (can't think of proper term) of the gasoline because of difficulty metering against the low air flow of the idle state. This "foam" was the F in the A/F ratio; the smaller the jet, the leaner the mixture because the A came from elsewhere.

I have no idea if this is the case with the marine carb but it would explain how smaller = leaner.

 

[daselbee]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Daselbee.... You're getting the big bucks? Zooowie! Andrew said he'd pay me what I was worth. I told him I couldn't live on that.

I am getting zero bucks, and I expect a 10% raise this year!

Who's Andrew?

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

i think hes talking about one of the ceo's of iboats lol

 

[levi_tsk]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Part two

I'm curious to see how this works out. Old auto carbs used an air jet to make a foam (can't think of proper term) of the gasoline because of difficulty metering against the low air flow of the idle state. This "foam" was the F in the A/F ratio; the smaller the jet, the leaner the mixture because the A came from elsewhere.

I have no idea if this is the case with the marine carb but it would explain how smaller = leaner.

what you speak of is an emulsion
and yes in most cases the smaller the jet the leaner it would be but this particular jet only controls the amount of air going INTO the fuel and there by if it lets less air in the engine will pull more fuel and thus runs richer

 

[logged in]

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Update

Re: Fixed orfice idle jet- Small orfice = leaner mixture?? Update

Hello all from June lake loop Eastern Seirras, elev 7200 ft. Well so much for the math.....

Installed the drilled out to .043 idle jets awill not stay running the motor won't idle.

So I installed the #36M with the very small .032 ID with the sealevel main jet #38

and the motor idles just fine. go figure.....

Time to go fishing.......