Rat motor mod question

[JohnsonM50]

As usual im playin with getting more speed out of a 31.8ci OMC 35hp. Ive got one together thats square ported & another done ready to assemble. The running one has been broken in with the time set at 29 degrees. 30 btdc's book for a 35hp, same basic motor, 25hp time is 35 btdc. In light of the port work done would anyone have a suggested time direction to go for max power? This is CD ignition & is doing 300+ more Rs than before easily but is still well within the red-line zone. :eek:

[OUTBOARDER]

We moded a 31.8 on a 12' or 13' aluminum bass tracker.
used 1.5" 50 hp carb with velocity stack & #70 main jet , opened up intake manifold, bent reed stops about .030" open more, ran megaphone out of mid section. The boat had awsome hole shot:) .......

wanted to make a 2bbl manifold for rope start engine one day.:eek:

Anthony

[JohnsonM50]

We moded a 31.8 on a 12' or 13' aluminum bass tracker.
used 1.5" 50 hp carb with velocity stack & #70 main jet , opened up intake manifold, bent reed stops about .030" open more, ran megaphone out of mid section. The boat had awsome hole shot:) .......

wanted to make a 2bbl manifold for rope start engine one day.:eek:

AnthonyCool, there seems to be 3 venturi sizes on the single barrel OMC carbs, 2 have 1 3/8" behind the venturi & ones 1 1/2". Ive got some of the big carbs, even 1 with an adjustable hi speed. My experiments with these have decreased acceleration with about the same speed. I find the larger of the 1 3/8" to work best all around. Cant go with a megaphone tho. Ive also wondered about a 2 barrel, finding one small enough seems to be the question.
The motor in Q would reach about 5400 tops B4 build & is now doin 5850. Ill go a little more aggressive on set up when I switch back to synthetic oil, lookin to go about 6200.:cool:

[Mark75H]

With the inconsistent crap fuel these days, I would be conservative with timing

[JohnsonM50]

With the inconsistent crap fuel these days, I would be conservative with timing
Thanks Sam, as stock they seem to run smoothest around 29 with no noticable gain from advancing it. The usual compression would be 120-135, this is 150. Ive compared reg-hi test with no change also.

[OUTBOARDER]

Cool, there seems to be 3 venturi sizes on the single barrel OMC carbs, 2 have 1 3/8" behind the venturi & ones 1 1/2". Ive got some of the big carbs, even 1 with an adjustable hi speed. My experiments with these have decreased acceleration with about the same speed. I find the larger of the 1 3/8" to work best all around. Cant go with a megaphone tho. Ive also wondered about a 2 barrel, finding one small enough seems to be the question.
The motor in Q would reach about 5400 tops B4 build & is now doin 5850. Ill go a little more aggressive on set up when I switch back to synthetic oil, lookin to go about 6200.:cool:


1.) Take the 1" venturi , 85hp 2bbl and sleeve it down using 1.375" od x 1.250" id aluminum tubing , chamfer end to mate up with 15 deg venturi taper.
2.) re-drill 1/4" butterfly shaft holes this will retain the sleeves in place.
3.) carefully re-drill idle and transition holes thru sleeves.( or slot sleeve)
4.) hardest part is making butterflies.

Made a couple of these a while back.......

any way why not come out of midsetion with megaphone? we totally guessed during a late night shop session and it worked great!

Anthony.

[JohnsonM50]

1.) Take the 1" venturi , 85hp 2bbl and sleeve it down using 1.375" od x 1.250" id aluminum tubing , chamfer end to mate up with 15 deg venturi taper.
2.) re-drill 1/4" butterfly shaft holes this will retain the sleeves in place.
3.) carefully re-drill idle and transition holes thru sleeves.( or slot sleeve)
4.) hardest part is making butterflies.

Made a couple of these a while back.......

any way why not come out of midsetion with megaphone? we totally guessed during a late night shop session and it worked great!

Anthony.

Thanks Anthony, Ive seen a photo of a mid mounted expansion chamber, 2cyl-1 pipe. Here tho the noise would be a problem. Good idea makin a carb to fit. Ill keep my eye/ear out.

[Mark75H]

Noise is not a problem with an expansion chamber unless you make it so

[JohnsonM50]

Noise is not a problem with an expansion chamber unless you make it so Come to think of it , it would probably quiet it down some :rolleyes::D

[Skoontz]

The 437 cc John/Evinrude snowmobiles had a hop up kit where to added a new intake and a V-4 2 barrel carb. Could be a good solution, but, you might need to tweek the exhaust to get the full gain from more juice.

I had another thought taking from the A/AXS/J racing rule book...So you can't find a small bore two barrel.... Why not make your own choke collar
(restrictor plate) so if you need or have the lattitude for more juice, open up the plate for a very small cost or labor, and proceed.

[JohnsonM50]

The 437 cc John/Evinrude snowmobiles had a hop up kit where to added a new intake and a V-4 2 barrel carb. Could be a good solution, but, you might need to tweek the exhaust to get the full gain from more juice.

I had another thought taking from the A/AXS/J racing rule book...So you can't find a small bore two barrel.... Why not make your own choke collar
(restrictor plate) so if you need or have the lattitude for more juice, open up the plate for a very small cost or labor, and proceed.

I like the 1st idea, the motor in Q is opened up & is about 500cc. I ran it hard yesterday & its the fastest of my 31.8s so far. Im thinkin the next move will be to $pring for Boyesens since experiments have indicated they should work well. Do ya happen to know where I can look up the snowmobile kit?

[Skoontz]

You would need to find a snowmoblie junk yard, or, someone who has one on a shelf. The intake would be worthless to you, it is a 90° curved intake that was designed to biolt on a Johnson Rampage or Evinrude Bobcat snowmobile. The motor was a 180° opposed air cooled twin.

If I am not mistaken, the carb was off a 125HP V-4, I'm not sure of the bore.

[Mike Menter]

I have done numerous 2 cycle engine mods and have set many records. I have noticed that the 2cycles like the ports eyebrowed and raise the intake up a tad bit. Open the exhaust ports and you should see a diffrence :)

...The same applies to the sleds...however they like big exhaust numbers!!

[Sam La Banco]

Hey Johnson mod 50,

you need to check out the 31.8 cuber in the History section on Page 22.

[JohnsonM50]

Hey Johnson mod 50,

you need to check out the 31.8 cuber in the History section on Page 22. :cool: 10-4, Thanks

[OUTBOARDER]

heres an expeirment
.040" washer between reed stop and reed may require longer screw.
then try the 1.5"x1.25 venturi again.

The reed sop adjustment will help gulp down some more air and help acceleration.;)

[JohnsonM50]

:cool: 10-4, Thanks Sam, Ive been lookin at outboard history for awhile-p-22 What titles it under?:confused: Thanks.

[JohnsonM50]

heres an expeirment
.040" washer between reed stop and reed may require longer screw.
then try the 1.5"x1.25 venturi again.

The reed sop adjustment will help gulp down some more air and help acceleration.;)
I did alot of intake exp. work inc. bent reed dampers.That made the biggest difference since a gearfoot till a reed broke. Expensive mistake even tho it could-a done worse :eek: 1 set of Boyesens are pricey but still less.

[OUTBOARDER]

I did alot of intake exp. work inc. bent reed dampers.That made the biggest difference since a gearfoot till a reed broke. Expensive mistake even tho it could-a done worse :eek: 1 set of Boyesens are pricey but still less. ouch!!! one reason why the O.E.s set the reed stops!


If you really want to get crazy try a leaf blower and a shot of Blue Thunder available at your favorit hobby shop. O ya just take the main jets out:eek:

[Mark75H]

Reed stop correct name = reed vibration reducer

[JohnsonM50]

Reed stop correct name = reed vibration reducer My bad -'Stops' are the most popular mis-naming tho :D

[Sam La Banco]

Go to "Outboard History" section then "Name that Outboard" then page 23

[JohnsonM50]

Go to "Outboard History" section then "Name that Outboard" then page 23 Thanks Sam, Somebody really got into that!:cool: