Thanks Thanks:  0
Likes Likes:  0
Page 7 of 7 FirstFirst 1234567
Results 61 to 66 of 66

Thread: Help Modifying OMC 31.8 cid

  1. #61
    Team Member JohnsonM50's Avatar
    Join Date
    Apr 2005
    Location
    Jersey
    Posts
    602
    Post Thanks / Like

    Default

    Yes & no, I never saw one but took the low plug out, pipe threaded it for the smallest compression fitting home depo had, epoxied it in then threaded the inside of it to align with the fixed jet at the low speed needle. size I blunted a needle & ran it, It works & I have 3 but I can't call it better or at least have no way of gaining precise performance data if it is.

  2. #62
    Team Member JohnsonM50's Avatar
    Join Date
    Apr 2005
    Location
    Jersey
    Posts
    602
    Post Thanks / Like

    Default

    Quote Originally Posted by Tim Weber View Post
    You guys seem to be expert on this motor.

    I have a 1993, 25 hp on my Lund fishing boat. To bump it up to 30 or 35 I've read to change the carb and the intake manifold. Is that it?

    Tim
    It might be, check the timing spec too. It might have a different crank but that shouldn't hold it back if aspirated as a 30. Also look at the power pack, might be rev limited. I'll look & get back If I see or think of anything else.

  3. #63
    Team Member JohnsonM50's Avatar
    Join Date
    Apr 2005
    Location
    Jersey
    Posts
    602
    Post Thanks / Like

    Default

    Quote Originally Posted by JohnsonM50 View Post
    It might be, check the timing spec too. It might have a different crank but that shouldn't hold it back if aspirated as a 30. Also look at the power pack, might be rev limited. I'll look & get back If I see or think of anything else.
    I just spent a few on BRP parts cat. it does appear to have a different block-crank assembly as well as a different intake. Theres alot of choices that year..20, 25, 28, 30 & 35 all of the 31.8ci. Even tho most look alike on the outside there's quite a few internal variations thru the years & models. Id look at the things I mentioned before, be sure its not restricted, rev limited & timed as a 35, might help some.

  4. #64
    Team Member Chairman's Avatar
    Join Date
    Nov 2005
    Location
    Hackensack, MN
    Posts
    22
    Post Thanks / Like

    Default

    For GT Pro we simply upgrade the carb and don't bother with the intake (unless you are upgrading a 20hp). One racer did compare intakes and got slightly better results with the 25hp intake.

    One internal difference you will find over the years -- some (most) powerheads have drilled ports, but some have oval ports. The oval port motors seem to have come on some 30s and 35s. I've raced both types and have found no difference between them.

    Tunnelboat and V-bottom Plans for 15-35 hp, 40-60 hp, 60-80 hp

    Dillon-Racing.com also on YouTube, on Facebook, on Twitter

  5. #65
    Team Member JohnsonM50's Avatar
    Join Date
    Apr 2005
    Location
    Jersey
    Posts
    602
    Post Thanks / Like

    Default

    Quote Originally Posted by Chairman View Post
    For GT Pro we simply upgrade the carb and don't bother with the intake (unless you are upgrading a 20hp). One racer did compare intakes and got slightly better results with the 25hp intake.

    One internal difference you will find over the years -- some (most) powerheads have drilled ports, but some have oval ports. The oval port motors seem to have come on some 30s and 35s. I've raced both types and have found no difference between them.
    I agree that most intakes are the same but since 18's & 20's are made of this motor, a few have a markedly restricted hole where the carb mounts &/or some carbs have a plastic insert venturi that resticts some.

  6. #66
    Team Member Chairman's Avatar
    Join Date
    Nov 2005
    Location
    Hackensack, MN
    Posts
    22
    Post Thanks / Like

    Default

    That's right, which is why I said "unless you are upgrading a 20hp." The 20 intake is very restricted.
    Attached Images Attached Images  

    Tunnelboat and V-bottom Plans for 15-35 hp, 40-60 hp, 60-80 hp

    Dillon-Racing.com also on YouTube, on Facebook, on Twitter

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •