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Thread: 31.8 CI OMC 25-30hp carb question- indepth topic

  1. #1
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    Default 31.8 CI OMC 25-30hp carbs- Updated!

    Hello,

    I have read everything that I can on this topic and I am grateful for any insight that I can get.

    I am currently running a 2000 rope tiller Johnson 25hp 2 cylinder on a heavy 16ft. I am spinning a 10.5 x 11 SST prop. Last year I had a 1989 30 Evinrude 30hp on the boat. It died a slow death due to electrical issues.

    The new engine is about 4mph slower and a slug out of the whole compared to the older engine.

    The Johnson is stock. I have four different carbs for it currently.

    Two 25hp carbs late model.
    One 30hp carb from the 1989 evinrude
    One late model 30hp carb with a plastic top that looks different.
    All have primer tube.

    Is the 30hp plastic top carb better then the old style 30HP carb? Would an older 35 carb with the choke plate be any better? What is the best carb I can run? I am looking for low end grunt. Top speed is nice but not goal. Any 50hp or 70 HP carbs work? Anything you suggest?

    Thank you for our insight.

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    Quote Originally Posted by captbone View Post
    Hello,

    I have read everything that I can on this topic and I am grateful for any insight that I can get.

    I am currently running a 2000 rope tiller Johnson 25hp 2 cylinder on a heavy 16ft. I am spinning a 10.5 x 11 SST prop. Last year I had a 1989 30 Evinrude 30hp on the boat. It died a slow death due to electrical issues.

    The new engine is about 4mph slower and a slug out of the whole compared to the older engine.

    The Johnson is stock. I have four different carbs for it currently.

    Two 25hp carbs late model.
    One 30hp carb from the 1989 evinrude
    One late model 30hp carb with a plastic top that looks different.
    All have primer tube.

    Is the 30hp plastic top carb better then the old style 30HP carb? Would an older 35 carb with the choke plate be any better? What is the best carb I can run? I am looking for low end grunt. Top speed is nice but not goal. Any 50hp or 70 HP carbs work? Anything you suggest?

    Thank you for our insight.
    It depends on the displacement of each 30hp motor compared to yours. If either is the same, it should work. However, from what I am reading, it sounds like you really need a lower pitch prop more than anything. I would try that first.

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    Thanks for the insight. I have been eyeing a 11 x 9 SST prop but the old 30hp did much better with the same prop. All the engines and carbs are from the same 31.8 CI two cylinder OMC block.

    I ended up getting a electronic tach and I will install that to see what I am turning for RPM in the next week.

    The amazing threads on this topic here are really helpful.

    I also got a lot out of this thread as well.

    http://forum.tinboats.net/viewtopic.php?f=4&t=23898

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    Try the bigger carb first since you already have one. I would use which ever carb that looks the best.

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    Well, the bigger carb can be put on to start anyway, but if the engine is loaded on the heavy side (not able to reach max RPM with current load), then a bigger carb likely wont help and can make acceleration worse. Bigger carbs work best in racing or high performance set ups. For a bigger carb to do its job, the engine has to breath (that means to allow rpms). That wont happen if its loaded heavy. You've got to lighten the load if the hole shot is weak. I suggest checking your rpms at WOT. if it is a little below max factory spec rpm, or barely touches it after a long period of time, then calculate for lower pitch. Every inch of pitch means 190-210 (200 average) rpm, plus or minus. Many general purpose outboards available in different power using the same displacement are rated at higher rpm for higher hp versions. It could be as much as 200-500 rpm for every 5 hp for engines in this range depending on cubic inches and design.

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    That makes sense. I need to get it into the upper RPM range to allow it to make the power. I will try the 30hp carb and check RPM at WOT.

    I was also considering adding a three vent holes to the prop to allow it to spin up faster to the rpm range. I have used them with success on larger V6s but never on anything this small.

    Any insight on the performance difference between the flat top plastic 30hp carb vs the all metal 30hp carb?

    Thank you for your time and insight. I am grateful.

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    The carbs that are not restricted are close enough that any would work. They are easy enough to change so testing is the best bet. I'd think the 35 carb best for overall performance but the 30 is possibly the same. The twin 31.8s over the years have subtle differences but remain mostly the same. What to look for: some have restricted intake manifolds, the feed hole should be at least as big as the carb throat where it meets. Some carbs have a plastic venturi insert, it means 'slow'. Look at book spec's, the same year model might have a different time setting for a 35 than an 18, 20 or 25. As for 'other' carbs it's a shot in the dark, the ones that come with them are pretty good & real simple.
    It might be also that the 30 was a fresher power head with better compression, if so the 25 would be hard pressed to catch up.

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    Quote Originally Posted by captbone View Post
    Hello,

    I have read everything that I can on this topic and I am grateful for any insight that I can get.

    I am currently running a 2000 rope tiller Johnson 25hp 2 cylinder on a heavy 16ft. I am spinning a 10.5 x 11 SST prop. Last year I had a 1989 30 Evinrude 30hp on the boat. It died a slow death due to electrical issues.

    The new engine is about 4mph slower and a slug out of the whole compared to the older engine.

    The Johnson is stock. I have four different carbs for it currently.

    Two 25hp carbs late model.
    One 30hp carb from the 1989 evinrude
    One late model 30hp carb with a plastic top that looks different.
    All have primer tube.

    Is the 30hp plastic top carb better then the old style 30HP carb? Would an older 35 carb with the choke plate be any better? What is the best carb I can run? I am looking for low end grunt. Top speed is nice but not goal. Any 50hp or 70 HP carbs work? Anything you suggest?

    Thank you for our insight.
    With low power to be more the goal than speed the best way to achieve that is a prop change. 10 is pretty low but 9 to 9-1/2 P might do it. You might not be sacrificing much speed either because alot has to do with how much the load will allow the motor to gain RPM's. The motor works best in the upper area of it's range & is really hammered when pushed too hard at lower R's.
    Being stock you want to check your syncronizing of timing to throttle, it's an easy adjustment. Also look at the economizer adj, [be sure the carb actually gets to WOT] As for carb size I've tried many OMC single barrels, 2 barrel from a 90 & 85, for all the 2 barrel effort & got 1 MPH under the best single barrel. It's no surprise that the ones that worked best for me are the ones made for the motor. I've also modified several singles, added hi speed jets, removed the choke plates & plugged the shaft holes. I've taken a router & carbide bits to the throat, beveled 1 to act as a velocity stack & round overed several. For working with what you have, the plate-less 30 carb interests me, that's what I'd try 1st ,then the older plate style.
    Is the 25 on the same transom setting as the 30 was? Do you have actual RPM & speed #'s ? It's hard to tell without altho the grunt difference would be more noticeable than speed.

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    Quote Originally Posted by JohnsonM50 View Post
    With low power to be more the goal than speed the best way to achieve that is a prop change. 10 is pretty low but 9 to 9-1/2 P might do it. You might not be sacrificing much speed either because alot has to do with how much the load will allow the motor to gain RPM's. The motor works best in the upper area of it's range & is really hammered when pushed too hard at lower R's.
    Being stock you want to check your syncronizing of timing to throttle, it's an easy adjustment. Also look at the economizer adj, [be sure the carb actually gets to WOT] As for carb size I've tried many OMC single barrels, 2 barrel from a 90 & 85, for all the 2 barrel effort & got 1 MPH under the best single barrel. It's no surprise that the ones that worked best for me are the ones made for the motor. I've also modified several singles, added hi speed jets, removed the choke plates & plugged the shaft holes. I've taken a router & carbide bits to the throat, beveled 1 to act as a velocity stack & round overed several. For working with what you have, the plate-less 30 carb interests me, that's what I'd try 1st ,then the older plate style.
    Is the 25 on the same transom setting as the 30 was? Do you have actual RPM & speed #'s ? It's hard to tell without altho the grunt difference would be more noticeable than speed.

    Thank you for your time.

    I ordered the ting tach and will have the wot rpm in the next couple of days. What you are saying makes perfect sense. The 25hp is rated at 5000 and the 30hp rated at 5500. I will take a base line reading with the stock engine then swap out to the all metal 30hp carb and calculate the actually difference. From there I will determine if the 11 pitch will work or if I should drop down to a 10 pitch or 9 pitch depending on how the wot rpms come up.

    The 25hp rpm range is 4500-5500 while the 30hp is 5200-5800. After reading about the Pro GTracing class I have come to the conclusion that the carb swap and proper SS prop is all that need.

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    Installed tach and ran the 25hp carb back to back against the 30hp carb with an hour of each other with same load.

    25hp engine stock ran 5150 WOT rpm.
    25hp with just 30hp carb ran 5600 WOT rpm.

    400 rpm increase just by swapping carbs. The engine was much more powerful coming out of the hole and felt like a different boat. Engine sounded better and did not feel like it was lugging vs the stock carb. Huge difference.

    I am currently running a 10.5 x 11 SST prop and need to get my slip numbers down. I am dropping it one tilt number and will retest. I think the 10.75 x 10 Solas Saturn maybe the perfect prop. I wish I time to run each with different loads to compare. 30 hp carb is a huge improvement.

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