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  1. #1
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    Jetting a cross-drilled 2.5 intake

    Just looking for opinions on these intakes and jetting from anyone who has played with these. 2.5 245 carb with cross-drilled intake running WH20 carbs. With a standard intake usually 2 and 5 are the hot ones that need one size bigger jet. I have not seen 2 any different from 1 on the plug or piston top, but 5 and 6 are hotter as you can see from the plugs and piston top. I jetted 5 and 6 one size bigger than 1-4. What do you all think about having 1-4 being all the same size jets and then 5 and 6 one size bigger? Seems to be running better this way than the same size jet in all 6. Again looking for opinions and knowledge from guys who have ran this type of setup.

    thanks.
    RJ

    1973 Viper
    1978 Viking

  2. #2
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    If it were me. I would jet all holes the Same. I had good luck.

  3. #3
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    I got lots of bigger jets. Let me know what ya end up needn

  4. #4
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    I've been told its usually 20 size bigger on the jetting when running a cross drilled.
    We ran 96's on our roundy rounds and 98's to 100's during the Enduros with modified 5 petal and 145psi, Wh46's.
    Are you running 5 or 7 petal.
    Have you tested Wh22 or 46's to compare with your big bores.
    We will be testing the 20's this year.
    Keep checking pistons and plugs and feed it what it wants per cylinder. I think a little on the fat side is good

  5. #5
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    Quote Originally Posted by Baker343 View Post
    I've been told its usually 20 size bigger on the jetting when running a cross drilled.
    We ran 96's on our roundy rounds and 98's to 100's during the Enduros with modified 5 petal and 145psi, Wh46's.
    Are you running 5 or 7 petal.
    Have you tested Wh22 or 46's to compare with your big bores.
    We will be testing the 20's this year.
    Keep checking pistons and plugs and feed it what it wants per cylinder. I think a little on the fat side is good
    I am running the stock 7 petal. I have some WH31 and WH 46 that I may try this fall. From what I have been told the WH20 give up some low end but gain on the top VS a WH22 or 46.

    But your saying jet it just like a non cross-drilled intake? jet each side of the same carb to what each cylinder wants? I am wondering how much does the cross-drilling even out the two cylinders fed from the same carb? Does it even them out, or just get them closer to being the same but still need to jet to each cylinder?

    1973 Viper
    1978 Viking

  6. #6
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    I've never messed with cross drilled , but my modified 200 reg intake runs 84s on all 6 tad rich (safe) and all 6 plugs look exactly same. I've read lots horror stories on here bout tryin tune x drilled.

  7. #7
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    I don't exactly know how the cross drilled intakes confuses carbs into pulling fuel into the Mighty Merc.
    If I searched this site I would most likely find several good responses and I would probably still be confused.
    I know the cross drill intakes work for me which is usually 4000 to 7000 rpm operating range.

    I believe the 2.4 Modvp oval port motors came with a cross drilled "I think" 5 petal not 7, with wh46's.
    I saw a jetting chart that mercury recommended the Modvp motor run 94 jets and the Bridgeport's run 86's. Don't know the stock compression numbers of each motor.

    If we saw any pistons that looked close to being lean we would jet up all the carbs the same size. It's hard to keep a motor alive for 4hrs trying to run it wide open.
    I reread your post and I apologize If I miss led you in any way.
    I don't totally understand the effects of exhaust port timing, intake timing, exhaust chest work and compression but I do know if I don't keep it near the fat side they will eat themselves.

    That learning curve is still kicking my butt.

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  9. #8
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    Thank you. I am just not wanting to go too lean and have an issue river running. Will be testing for 800 foot soon and that will let me get down.on mains further

    1973 Viper
    1978 Viking

  10. #9
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    I have sold every cross drilled intake I have had....or gave them away.

  11. #10
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    Quote Originally Posted by Capt.Insane-o View Post
    I have sold every cross drilled intake I have had....or gave them away.
    Well, that about sums it up I think. General concensus is to get a standard intake for the thing.

    Who has a 7 petal small 2.5 intake on the shelf?

    1973 Viper
    1978 Viking

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  13. #11
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    I think in 800'. EFI .

  14. #12
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    Quote Originally Posted by Mrzip View Post
    I think in 800'. EFI .
    Next year hopefully and this carb stuff can go bye bye.

    1973 Viper
    1978 Viking

  15. #13
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    I chose carbs for the simplicity and budget racing.
    I've been told a well setup carb motor does not give up too much to a EFI system at least running races like Enduros.
    You do need to run slosh tube carbs to keep things happy.
    I have wh20's and will be testing for the Enduro in Oct, if they work better than my 46's I'll be looking for a set with slosh tubes if anyone wants to give up a set or I can borrow or rent.

    Does anyone know if merc still sells baffles and the slosh tube kits for wh's or can the newer carbs handle sloppy water better.

    I even pulled the EFI and all the boxes off of my family ski boat with a 91 2.5 200 because of fuel injectors issues and unnecessary electrical. Trying to diagnose a EFI problem gives me a headache. For me carbs are simple.

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  17. #14
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    Quote Originally Posted by rjdubiel View Post
    Well, that about sums it up I think. General concensus is to get a standard intake for the thing.

    Who has a 7 petal small 2.5 intake on the shelf?
    Fill the holes with Marine Tex gray. Use a large drill bit to make a small chamfer on both sides of the hole if you can. Pinch the epoxy from both sides and It will be trapped in the middle when it cures.

  18. #15
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    Quote Originally Posted by nitro_rat View Post
    Fill the holes with Marine Tex gray. Use a large drill bit to make a small chamfer on both sides of the hole if you can. Pinch the epoxy from both sides and It will be trapped in the middle when it cures.
    I have access to borrow an intake that had the crossdrill welded shut to try. I just want to keep working this one till I think I am at the end of my testing and then try the standard intake and go from there. I think I am honestly going to go to a horn next year but all depends on the deals I find on a 260 over the winter.

    1973 Viper
    1978 Viking

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