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Thread: intake mod

  1. #1
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    intake mod

    so i came across a coversation on another site about removing the divider between the openings on a 200hp crossflow to make an open plenum style intake.any of you guys care to comment on this?i have a 150 jonny crossflow on my 18.5ft skeeter and am in the process of up-grading the powerhead for more power.i have a spare intake and was pondering this mod along with composite reeds and intake stuffers,maybe a little more timing and a upping the compression a bit(at 90psi on a gauge now).

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    I like the reeds and intake stuffers for a crossflow. I did that on my GT175. I also ran tighter heads, which really woke it up. I don't really like to up compression to much now, with gas where it is. You have to run super unleaded. I had to back the timing down and rejet the carbs, and still ended up cooking a piston. I sure liked how it ran and sounded though.

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    thanks for the reply lockjaw,i have read a number of your threads and have found them quite helpful.have you ever heard of the intake mod i mentioned?what do you mean by "tighter"heads?i only put 93 octane ethenol-free fuel in my boat as i have a local station that has it,besides,i really &^%$# hate that corn-mix gas!!

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    Sounds like your heads on that engine are the low compression bathtub heads. I'd be looking for another set of heads from an older model, such as a 1978/9. Two sets of good heads from that era: 323456/457 used on the 150-175-200 that year. Solid compression around 110 lbs. Even better: 323872/873 used on the early small bore 235 only. Maybe another 10 lbs-up to around 120. You'll need to check the carb throat size on your engine. You really need to be running the 1 5/16" carbs. If yours are smaller (1 3/16") I'd hold off on the heads till you locate some larger carbs. Ideal carb setup would be those from a 1979 200, (with the pullover jets.) Agree with Lockjaw on the reeds and intake stuffers (they are worth 5-8 hp.)

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    I believe you lose bottom end w/opening up the intake as the cylinders now pull from 6 carbs instead of just one. Light hull, high rpm maybe, but doubtful on your application.
    Stuffing the cases as suggested on the other hand...... ; )
    I'd rather be competitive w/junk I built in my garage than win w/stuff I bought.


    I refuse to allow common sense to interfere w/my boat buying decisions.


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    If memory serves me right skeeter91 this mod is a headache till you get it right. I beleive it was racer that told me that its a pain in the arse tuning after doing this mod. Check with him so I am not giving you the wrong info.
    Laser 1550 ...under construction with a serious weight loss
    Motor 2.4....undecided on hp yet

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    i am going to leave the intake mod. alone,as it is not worth the hassle for my application.the engine is a 1987 with .030 overbore in all 6 cyl.the carbs are 1 3/8 with 61c high speed jets.i think i need a 26p prop(running a 24p raker now)as i am running against the rev limiter now at w.o.t,but the boat is a dog out of the hole now as it is,so maybe a 4blade?i will check the heads when i get home but i think they are 456/457.i have new reeds and gaskets on the way,any recommendations on modding the reed cages?i would like to make my own intake stuffers from delrin (plastic) maybe.i run a high precision machine shop so standard machining stuff(mill,grinder,lathe) is no limit!also wondering if the bubble back conversion is worth it for this motor?

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    Quote Originally Posted by skeeter91 View Post
    thanks for the reply lockjaw,i have read a number of your threads and have found them quite helpful.have you ever heard of the intake mod i mentioned?what do you mean by "tighter"heads?i only put 93 octane ethenol-free fuel in my boat as i have a local station that has it,besides,i really &^%$# hate that corn-mix gas!!
    I ran a set of the heads off an earlier model 235, they raised the compression. That adds alot of punch, especially with the reeds and intake stuffers, but you have to run super unleaded, you have to richen up the carbs, and back down the timing. With super at 4 bucks a gallon, I just don't think its worth it. I will say those mod's really woke up my motor though. Those heads are hard to find now, and command a nice fee if you are selling. The GT175's were always regarded as powerful motors for their horsepower rating. I had mine on a boat that had some wet foam in it, and had it sitting back on a jack about 12 inches, and it was strong. When it broke, I hung a Venom 200 off the back, and it was stouter, but most folks said the venom was really about 220 hp, I'd guess my GT175 at maybe 200, with the mods.

    I have never seen or heard about the mod to the intake you are talking about. I am sure some of the guys who really get into the cross flows can help. I'd ask racer if no one else knows.

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    I have built a 235 with this kind of intake mod and if done correctly will make more power on the top end but will take away a little bottom end.
    If done incorrectly by hogging out too much will burn down your engine in the mid range.
    I would only consider this mod on a very light boat.

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    have an offer on a set of XP big bore heads and stuffers,what can you tell me about these?i also have a lead on a complete early model 235hp but the seller is not sure of the year,is there a way to tell if it is a small or big bore motor by looking at pictures of it?

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    Be careful with this one I could not even get my seebold to plane with that mod. Yes it ran on top but no bottom end. It was very lazy so I went back to the stock intake. Get the advice from someone like racer who has a lot of experience with these old motors. Could have been a jetting issue?

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    Quote Originally Posted by skeeter91 View Post
    have an offer on a set of XP big bore heads and stuffers,what can you tell me about these?i also have a lead on a complete early model 235hp but the seller is not sure of the year,is there a way to tell if it is a small or big bore motor by looking at pictures of it?
    I'd get the stuffers for sure. There are differences between the flat back blocks and bubble back (235's and GT175's) that you need specific info on. If the 235 is a whole motor then swapping would be easier, just swap the whole motor. But I think putting a 235 block on a 150 mid takes some parts swapping.

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    so the 235 was a bust,was missing parts and the guy just wanted too much$.been online this morning looking for stuff,found a 1979 175 but the fella's asking $1500 for it!seems a little high to me?anyway,i am looking for good crossflow heads and carbs w\pullover circut,anybody out there want to sell?

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    tried finding something there but those guys dont seem very responsive to performace or mechanical questions.
    Last edited by skeeter91; 03-02-2013 at 10:03 AM.

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    Quote Originally Posted by skeeter91 View Post
    tried finding something there but those guys dont seem very responsive to performace or mechanical questions.
    They are not responsive to OMC modding at all. I'd hold out for a big bore 235, or look for a GT175, which is a low port 235 rated at the prop. It does better then 175 at the prop, and comes with big bore carbs. Add reeds and stuffers, and they run pretty good.

    You may need to send a prop off to say Steves Custom props for him to drill holes in it so you can run PVS plugs. Then you can adjust it to slip more, helping you get out of the hole better.

    Put a post down in parts wanted for what you need and go from there. I got something, took me over a year, but someone hunted me down and said they had it.

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