2 barrel 2 banger

[JohnsonM50]

From questionable technologies* a subsidiary of Rude RaT motors.................
>>>>>>>>>>>>>>>>>> 2 banger x 2 barrel >>>>>>>>>>>>>>>>>>>>>>>>>>

Heres some pics of the maniflood project, as you can see if there is a function form follows it. Pretty aint in the picture, pun perpetuated. Im hopin to rig up a powerhead & start the process of starting the motor to see what I can see then ponder pond testing. Well, enuff of that- work or not other serious speed considerations are under delerious speed consideration. me, myself & I - Prez, V-Prez & cleanup boy of Rude Rat un-inc.

[Fast Fred]

[JohnsonM50]

[QUOTE=Fast Fred;86867]
Thanks Fred, Just hope I get it goin ok. It'll be a novelty if it goes as good as stock, get-n more from it to any degree will be neat. Ive got a couple OMC 31.8s that are faster than stock but no great improvement yet. With the hefty crossflow pistons & the flat reed plate, limits are in place.
The carb Im working with is from a 90hp, year unknown. I found a "72 OMC 85hp carb & am glad to see the venturi is significantly smaller than the 90 while the throats & bolts are the same. With that Ive got 2 to test & theres others float-n around.
The maniflod is done, mocked up with all fasteners & the carb bolted up. If I learnt anything about fab-n, the parts need to be made to fit as well as bolt patterns worked out so they dont interfere. That determines whether or not its worth doin as much as the parts workin.

[JohnsonM50]

After some leak chasing...

I mounted a spare powerhead on an old tower, roped to a stand [no clamps]

Made up a couple quicky gaskets & assembled it.

It started right up after a few 'choked with a rag pulls', idled sorta high, [the timings full]. It rev's nice..Wicked nice! So on to testing on water. Hose tests always sound good.. I know, but it shows promise as is.

[Fast Fred]

[JohnsonM50]

Before running it again I cleaned the plugs real good, they came out oiled up & were old ones. Then upon re-running I found the mix to be fat at idle, it would load up & puff a cloud when cracked. I re-jetted the carb hi & low to lean it some. Now it will idle consistantly without loading. I clogged the exhaust with a rag for a few to simulate sitting in water & it didnt effect it.
Ive got a modded powerhead on a boat to run & note performance. Then without changing anything [prop/setup] Im going to put a stock powerhead on with this carb to compare to the other & see how it does in general. If I like what it does Ill put the 2 barrel on the modded powerhead [that does like more fuel] & go for it.

[Mark Poole]

Try velocity stacks on your tests. Someone already mentioned that the air speed will be greatly reduced because of such increased throat volume. You might also try spacers of different thickness between the carb and manifold. Normally a short intake track works best but with such a change in parameters with this carb you never know. I know my 30 Johnson with the single big bore carb will almost suck your fingers off on acceleration.

This leads me to a question for a Merc guy....why did the older Merc carbs have that little plastic "mini venturi" in the throat?

[Tim Kurcz]

Answer is to help fuel atomization during part throttle openings for the large throat carbs. At the dealer we took a set out and tank tested to find soggy acceleration. Reinstall and all was good again!


BTW: Nice project with the two bbl on your twin. If it refuses to behave, you might back up to a 1" venturi version. Good luck!

[JohnsonM50]

Try velocity stacks on your tests. Someone already mentioned that the air speed will be greatly reduced because of such increased throat volume. You might also try spacers of different thickness between the carb and manifold. Normally a short intake track works best but with such a change in parameters with this carb you never know. I know my 30 Johnson with the single big bore carb will almost suck your fingers off on acceleration.

This leads me to a question for a Merc guy....why did the older Merc carbs have that little plastic "mini venturi" in the throat?

Actually, each side of the 2 barrel is almost exactly the same as the stock single. The intake in stock form is open to both cyls & the fuel is drawn back & forth so in effect each cyl. draws from a 1-3/8 carb with a 1-3/16 venturi. What I did is fill the middle of the intake so each cyl has its own smaller intake & a carb of the same size effectivly. 2 barrels at once would definitely flood this little motor. My hope is it will have more velocity & since each barrel will pulse half the amount as a single, a more stable mixture [might be reaching here] I havent run it under load yet but am confident that if it wont 'go' at least itll idle back
Tim, I was looking at a smaller ventury carb from an 85, reasonable on E-buy. For now Ive got this one so if it wont work after fair attempt Ill go that direction, Lean wont be a problem at least.

Thanks for all the suggestions, unless Im real lucky & this flies Ill be looking back at them.

[JohnsonM50]

Ready to test


I have a 1 3/8 tube Im going to make velocity stacks out of as it seems weak mid-range but idles good & revs hi well. Soon Ill get it on H2O.