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.

[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.

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.

Anthony

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.

[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.

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

[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.