OMC Orifice Plugs

[oledawg]

:confused: Can someone explain the difference between a "C" & "D" hi speed jet for OMC motors. There is also a hi speed jet with no letter designation. Those have the jet size running all the way through the jet, whereas the "C" & "D" hi speed jets have the jet size just part way through the jet. There is visually no obvious difference between the "C" & "D" hi speed jets. Help!!!

[Fast Fred]

the letters tell which way the fuel flows through them.

[oledawg]

the letters tell which way the fuel flows through them.

:) Hi Fred, Yes, I know the "C" & "D" has to do with flow direction, but what is a 1/4"-20 hi speed jet with no letter designation? There are three: Ex: 73D, 73C, & 73. To straight 73 has no counter bored hole in the jet. Just the .073" all the way through. I just used the size 73 as an example.

[Fast Fred]

off hand i would say no letter would mean air jet, 73 seems big for air

[Fastjeff57]

How important are these "air jets" on OBs? I accidentally left one of a carb on my Merc triple and it ran fine.

Jeff

[Fast Fred]

not a small merc guy my self, but, air jets for the most part are for low speed, pulling air in to the idle mix wile the throttle plate is closed or almost closed. the idle circuit flows through the full curve of the moda, if it is rich or lean, it will influence the top end burn.

[Per]

Had an air jet coming loose in my OMC 56ci -92, I didn't notice any change in top end but the idle suffered and the boat struggled to get on plane at all. For me those jets made a huge difference.

[Fastjeff57]

Thanks! Very interesting.

I wonder which way they effect: a larger air jet makes for a richer
idle mixture (or visa versa)?

Jeff

[oledawg]

Thanks! Very interesting.

I wonder which way they effect: a larger air jet makes for a richer
idle mixture (or visa versa)?

Jeff

The larger the air jet, the leaner the mixture. You are adding more air to the same fuel delivery.

[Fastjeff57]

Gotcha! Thanks.

Jeff

[Xpress2830]

Hey guys I have a 93 Johnson 40 milled head to 150psi, ported 50 reed cages with Boyesen rees, 50 carbs ( plastic top 2 jet carbs). Hs jet is 55D ls is 45D. My plugs are stayin too wet and motor doent idle as good / crisp as it should. I did have 49D when I ran it last, also tried the 71D but it didn't idle good. My thoughts are I am getting too much air with the ported cages, bigger carbs and the ls 45D. Should I keep goin smaller on the ls air jets until I get the right mix? Any thoughts would be appreciated...

[oledawg]

:)
Hey guys I have a 93 Johnson 40 milled head to 150psi, ported 50 reed cages with Boyesen rees, 50 carbs ( plastic top 2 jet carbs). Hs jet is 55D ls is 45D. My plugs are stayin too wet and motor doent idle as good / crisp as it should. I did have 49D when I ran it last, also tried the 71D but it didn't idle good. My thoughts are I am getting too much air with the ported cages, bigger carbs and the ls 45D. Should I keep goin smaller on the ls air jets until I get the right mix? Any thoughts would be appreciated...

Sounds like your too rich on the IS air jets. Try going to a larger IS to lean down mixture. The 71D's on HS are way too much. If I had to guess, I'd go 55-56 HS and 50-54's IS. How is throttle response off idle thru midrange?

[oledawg]

Also, the reeds can't give too much air. The air is controlled by the carb size. The improved reed flow merely allows what the carb has to offer to enter engine better. The fuel/air ratio is set by the carb bore and jet size before it enters reeds.

[Revengable]

Leaking float valves only messing with low demand ? It would take a lot of air to wet a plug.

[Xpress2830]

Yeah I tried the 71C and too much air. Dropped down to 49D a little better, now I have 45D idles better but still not as good and crisp as it should. Do you think I need to keep goin smaller until I get the correct mix? I am runnin 55D HS...Hey also I may need to run the C style orifice the fuel flow is reversed from the D...Your thoughts. Thanks, Bobby...

[Xpress2830]

I tried 71D and would not idle good at all. I need to go smaller with ls orifice.....Your thoughts, Bobby..