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Thread: omc jet sizes

  1. #16
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    Great Thread guys....I would be interested in your opinion on jet size in relation to .030 bore increase? I have bored my 70 OMC .030 and Im trying to figure out how much to increase the jet size? its a 1992 model 56 Cid block, the book calls for 50D jets stock.....would 54D be the right Choice? Some say leave the 50D's? Looking for Dependability....
    Thanks Chuck
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

  2. #17
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    try it with the 50d jets, see how she goes. I would leave them in for running in for sure..

    if you want to be reliable, smooth and live long just go with the 50D mains and 32 tops if its the old style carbs.
    if you go to 54D then what size will you got to in the tops ?

    or 50D mains and 55Dtop jets if its the late 1992 black top carbs !!!.

    did you notice a size difference on the back of the carbs and a larger venturi on the 70hp carbs ?.
    Last edited by phillnjack; 09-17-2017 at 04:33 PM.

  3. #18
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    Thanks for the reply , Thats what i have in there now ( 50D and 32 tops) Ive always heard a few of the old racers around here say go up one jet size for every .010 you bore it. I wanted to get another opinion on this to be sure .

    I did notice a big difference is the carbs, 1.5 " front and back with a much bigger Venturi than the 60 hp carbs.

    these are the old style carbs, and NOT the black top carbs.

    Good question on the 54D jets, Im not sure what I would do to the tops if I went to the 54D, I guess just see how it ran...your thoughts ?


    Timing ??? 18 - 19 - 20???
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

  4. #19
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    I have tried to work out the top jets when going up on the mains.
    if you leave them the same it would be richer I think ? but this is where the carb tuning people come into their own.
    Atleast the jets are easy for you to get over there, unlike herewith very limited stuff around.

    personally I would think a jet size of about 55D mains but no idea about the tops.

    on mine I am going to try 55D mains with about 60D top.. at the moment its 50D main and 55D top.. I am thinking going up 5 on both ???
    this should allow more fuel and air but slightly richer ? I am probably way out though .
    I have no idea if this is about right or not, and my carbs are the black tops, so maybe different way of doing them ? I realy dont know.

    in 1992 the jets on mine were 50D main and 55D tops, then in 1993 they went 53D with same 55D top, then in 1995 they went 52D and 60D tops ?

    finding out about the carbs jets on these motors is awkward because so many different carbs and numbers..
    and then like you what happens when going to 30 over etc, do the jets need to be bigger and by what amount ? all confusing ha ha
    .

  5. #20
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    Most of the time the factory jets are a bit on the rich side for reliability, so I do not change jets on a stock rebuild,even .030. Have never had one come back because of burn up from too lean. Now if you start messing with anything on the motor you will need to check piston domes or spark plugs for jetting.

    I think a bigger problem I have seen is guys thinking they can drill out there jets and gain more hp by doing so. This is where a good set of reams come in handy.
    Last edited by perfmarine1; 09-18-2017 at 09:15 AM.

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    I agree , I understand enough about it to be dangerous, but thats where it stops for me. Im just trying to make it dependable and eliminate the issues the 3 cylinders had with burning cylinders. Ive done all the factory service bulletins, like moving the p tube to the top of the block and installing the upgraded bypass valve and spring. Hopefully all this with the correct carb jets will get the engine running like it should.

    I guess I will crank it up with the 50D jets in it and see how it does... I don't think it will burn it up, but going to a 53 or 54 may help in the long term.

    am I correct in thinking that the intermediate jet does nothing at higher speeds, ( when the main jet takes over) ?

    Thanks for the reply
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

  7. #22
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    You are the second one to say that the factory jets were on the rich side and it should be fine, I believe the other guy was FastFred. I have not messed with anything else on the motor, other than swapping the carbs from the 60 to 70 version. Have you had done the service bulletin mods to the 3 cylinder ? I think they were all related to cooling more than fuel issues.... this motor had scuffed cylinders when i got it, but i didn't know why , I eliminated the VRO and went with the normal fuel pump....so hopefully the dependability will be there.

    Thanks for the info....
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

  8. #23
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    and what about the later exhaust cover and gasket ? it is different.
    new style water pump and plate ? more water for the block.
    also do away with the plastic fitting at the tell tale outlet, its a restriction you dont need..
    Making sure the holes are through to the water gallery's from exhaust cooling ?
    also make sure the little hole at the top is nice and clear going into the top cylinder gallery.

    going pre-mix is miles better for reliability and longer life, and the other mods keep everything tickety boo.

    .

  9. #24
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    Well some of that I haven't heard of but will check if I tear back into it at some point.... I plan on building a spare powerhead so I can check the items you mentioned at that point...Ill let you know how it goes when we run it....I see a provision for a temp sending unit in the head, is it worth messing with? how close would the readings be once they reached the gauge ? or would it be to late at that point? Thanks Chuck
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

  10. #25
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    One of those service bulletins was a jet change for the middle carb on certain year blocks

  11. #26
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    the boss in the head for the temp gauge is perfect, being as it is for an electrical sender its instant at the gauge.
    the boss is on the right of the top spark plug. simple every day auto sender goes in nice and just put a single wire from that to
    the gauge and then a positive from your dash or controls etc.
    I find it works great and you can see the temp rising to its normal position from cold, then dropping slightly when on full throttle.
    its much better than relying on the stupid buzzer for over heat, when and if that goes off its far too hot !!!..

    here below is the simple brass threaded fitting and what its like on the head. and a simple cheap temp gauge that works perfect.
    temp when cruising sits around 60, when idling sits around 70 and when full throttle sits between 57 to 60f..
    also a water pressure gauge is a food idea, this can be put on the side of the exhaust cover in the old original place for the tell tale fitting.
    using a T-fitting in the tell tale is not so good as will give false readings.

    do not drill through into the head, the brass fitting will automatically get the heat from the cylinder head due to being a tight fit.
    I have checked the temp at gauge with thermometer at the head and was plus or minus 1 degree at all different ranges of throttle.
    .
    Attached Thumbnails Attached Thumbnails Temp Sender 1.jpg   20140812_144235_1.jpg   good pic.JPG  

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  13. #27
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    Quote Originally Posted by powerabout View Post
    One of those service bulletins was a jet change for the middle carb on certain year blocks
    Early 90's blocks had the cooling problem on the middle cylinder, think they got it under control by 94. One of my blocks I have the middle jet is 3 sizes larger

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  15. #28
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    Finally got to run the 3 cyl

    Well I finally got the 3 cylinder in the water today, it fired right up , set the timing to 17 degrees, adjusted the idle and took off. After a few check and adjustments, It still seemed to surge a little at 4000 rpm , I bumped the choke and it was like hitting another gear. tried it again with the same results. slowed down and eased back to the trailer and as I tried to drive the boat on the trailer it would bog out, at this point I tapped the choke and it jumped on the tailer.I have 50D jets in the mains and 32's in the mid. Im thinking Ill go to 54D jets and try it. What do you guys think?
    Quote Originally Posted by LittleCharger View Post
    Early 90's blocks had the cooling problem on the middle cylinder, think they got it under control by 94. One of my blocks I have the middle jet is 3 sizes larger
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

  16. #29
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    I'd clean the carbs as you have a fuel restriction and see if that fixes it, if not look at fuel lines etc. Once you get it to run properly jet accordingly.

    Quote Originally Posted by 87vantage View Post
    Well I finally got the 3 cylinder in the water today, it fired right up , set the timing to 17 degrees, adjusted the idle and took off. After a few check and adjustments, It still seemed to surge a little at 4000 rpm , I bumped the choke and it was like hitting another gear. tried it again with the same results. slowed down and eased back to the trailer and as I tried to drive the boat on the trailer it would bog out, at this point I tapped the choke and it jumped on the tailer.I have 50D jets in the mains and 32's in the mid. Im thinking Ill go to 54D jets and try it. What do you guys think?

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  18. #30
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    Well a little update on my 3 Cylinder.....I figured out that the jets should be bigger for the carbs Im using , So I went from 50's and 32's to 52s and 34's, and the motor performed perfectly ......But after riding it around at 4000 RPM for a while I went up to 5000 Rpm and is wanted to run hot....The pee tube has a full stream but it definitely ran hot. This is the year model that required the cooling updates, which I have done. I moved the pee tube to the top of the block and replaced the thermostat and bypass spring that the update from OMC specified. Im thinking the thermostat didn't open or I have a blockage is the water passage ? Will it cool properly if I remove the thermostat and leave the bypass and spring in it?
    1987 Vantage XT 200 Pro Max
    1991 25 Liberator 300 xs's
    1967-8-9 Z28 Camaro
    1968 SS/RS Camaro Convertible
    1969 SS 396 L89 Camaro
    1970 440 6 pac GTX
    14' jon boat 15 John-rude!

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