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  1. #1
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    OMC V4 Crossflow Idle Orfice Definition Help Needed - Rich vs Lean Numbering

    Guys, I need some help understanding what those idle orifice (jets) do on the OMC V4 crossflow.

    Do the idle orifices meter Air or Fuel? Is a "30" more or less rich than a "35"? I ask because I'm building a hotrodded 115 to a 140 porting scheme, added the bubbleback, and scalloped pistons yet using the looser heads of the 115. The 1979 140 (carb doner) calls for 4 x 30 idle orifices while the 1978 115 calls for 3 x 33's and 1 x 35 orifices. Are the 30's rich or lean? -It all depends on whether they are metering air or fuel.


    On motorcycles, those idle jets in the fuel bowl or it's path, would meter the fuel volume at idle. A jet above or after the fuel bowl would be metering idle air flow through the air bleeds. I've looked in multiple manuals I have on different crossflows, but I have not found a definition of Air or Fuel for the idle orifices. I know the mains/high speed orifices are Fuel metering where the bigger the number, the more fuel flows (bigger is a larger hole and would be richer).

    Thanks, Gordon

  2. #2
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    I think it’s air but not certain. What reeds are you using on this build? Aside from the piston and port mods are you doing anything else to it? I’m building pretty much the identical motor lol
    Hydrostream dreamin

  3. #3
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    I've got Chris's reeds, his spacers, and I'm using the factory 140 rubber stuffers - following his advice. As you read it, I'm not looking for jetting advice, just the understanding of the numbers for those idle orifices. If the 30's are in fact richer, then that's what I would start with based on those being stock for the big carbs (1 3/8") I'm running. If the 33/35's from the smaller carbs originally on the 115 are richer, I'll run those. What I don't want is lean during the first couple of low-speed hours of break in.

  4. #4
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    Gordon, the idle circuit is an bleed. So yes, its backwards. The 30's are richer than the 35's. i was confused when I first started messing with the idle jets. Going to the 30 chokes down the air, making the mix fatter. The gas side(of the mixture) at idle is that long thin tube that runs in the main tube, down to the float bowl. I was able to clearly see this on the modern carbs with a removeable top plate instead of the lead plugs covering up the channels. Air and gas mix up in there and come out the back behind the butterflies......

    If I havent spoken correctly on this, I hope that someone will please clarify. But I am 100% sure about the air bleeds.

  5. #5
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    I also meant to mention about the possibility of trimming your rubber stuffers. I have done the same thing you are doing on my 175 big bore. I used Chris' stuffers(behind the cages), his 2+2 reeds, and a set of factory rubbers. When I tried to bolt the manifold on it was being held off. I pulled the rubbers, and it fit fine. So, I had to trim the rubber blocks a little. So, dont be surprised if this is a similar scenario with you. I was freaking out, because I dint want to cut up the factory blocks. But, it all went together fine...........when I got over it.

    BTW, the 30's are definitely the best place to start in my opinion.
    Last edited by keefallan; 09-15-2020 at 02:13 PM.

  6. #6
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    Mine bolted up fine with the factory rubber stuffers and his spacers. Now, I did not use the reed block gaskets, so Chris's spacers worked fine with the rubber in stock form. Thanks for the confirmation of the bleeds. Gordon

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