Evinrude 31.8ci Mod Advice

[JohnsonM50]

The Mikuni slide carbs will probably perform better to start with. I made up a manifold to accept a 2 barrel carb from an OMC 85hp. Each barrel the same size as the stock single. Each cylinder draws from one barrel so it won't double the fuel requirement. It draws the same amount of air thru each barrel at half the pulse rate per barrel out of a common float chamber. No need to syncronize. This gave more mid range power but was not faster on top.

I also see in the pic that you set this up as a 31.8 manifold. [clever]

[RaisedByWolves]

OK, I got my 1 3/8" carb on and it starts and runs but the idle jet is off.


Iw will idle ok with the primer out on the first notch but when I take the primer off of the cold setting (First notch) it slowly revs faster and faster until it sneezes and quits.

I know I need to go down on the idle jet, but how much or how many sizes?

Im currently running a 40 idle jet.

Do I go 39? 38?

It doesnt run terrible but it certainly needs improving.

Quoting myself here to keep things coherent.


Took the boat out and saw no improvement in speed. I was able to trick it into idling with the primer set on cold start and that worked OK, but the mid range was a total **** show.


I took a pic of the plug but my phone is dead right now, it was a tan chocolaty color. I couldn't run it wide open for long as the boat was hopping too much and actually only had full throttle for 20-30 seconds.

The motor acting strange in the midrange is what has me baffled. It would stumble and or be unresponsive to throttle input, seemingly have no change in power with throttle input or it would lag then seemingly catch up with a decent burst of speed.

Could this be due to the idle circuit running lean?

Could it be a linc/sync issue?

All I did when I swapped the carb was to adjust the two plates on the side of the carb to get the carb to open between the timing marks. Is there more that needs to be done here?(Timing)

Any input greatly appreciated.

[JohnsonM50]

[QUOTE=RaisedByWolves;157157]Quoting myself here to keep things coherent.


Took the boat out and saw no improvement in speed. I was able to trick it into idling with the primer set on cold start and that worked OK, but the mid range was a total **** show.


I took a pic of the plug but my phone is dead right now, it was a tan chocolaty color. I couldn't run it wide open for long as the boat was hopping too much and actually only had full throttle for 20-30 seconds.

The motor acting strange in the midrange is what has me baffled. It would stumble and or be unresponsive to throttle input, seemingly have no change in power with throttle input or it would lag then seemingly catch up with a decent burst of speed.

Could this be due to the idle circuit running lean?

Could it be a linc/sync issue?

All I did when I swapped the carb was to adjust the two plates on the side of the carb to get the carb to open between the timing marks. Is there more that needs to be done here?(Timing)

Any input greatly appreciated.
Hard to put an answer on what you're getting but a couple Ideas: Throttle/timing syncronizing is for acceleration mostly off low throttle, after about a third it goes to full advance and runs the rest of the throttle range there. It could be lacking some but I doubt it's making the motor hesitate or stumble. Secondary air in the float bowl? Needle packing or bowl gasket leak? [be sure the center doughnut gasket is on the pickup stem] Surging is the usual symptom. Be sure the carburator is seated and the mount gasket not leaking. Make sure also that your fuel lines are sealing well and not drawing air. Lack of fuel could be it as far as hesitating goes. With a normal stock motor the speed won't be improved much on engine enhancements beyond proper tuning. The reason is the gearing, if you gain 100 rpm's on the power head, you're subtracting the gear reduction from that. With the non thru hub [older versions] that 100r's would end up 57.1 r's at the prop, less on he thru hubs. To see a real speed increase you'd need alot more. Time invested in getting the boat optimal and a well suited prop in place would go much farther for the effort.

[PPRINT]

These are pictures of the Link&Sync instructions in one of my service manuals. Maybe it will help.

One picture is a factory jet chart.

Rob

P.S.
Carburetors are very sensitive to float level. A low float level will make a motor lean. Too high will make it too rich.

[PPRINT]

Jet chart picture rotated correctly.

Rob

[RaisedByWolves]

Thanks pprint!

Especially for the jet chart, that will give me a better starting point than trying to suss out the info from different threads.

As far as speed, yeah I need to move up to a 10x15 prop, and was hoping the carb swap would give me a better hole shot with the higher pitch prop.

I’m trying to do this on the cheap so I’m just buying what I can find at a good price now in hopes of having it all come together at some point next spring.

Would the 30/35hp intake be of any benefit?

I’m also wondering if I should work on motor height and or tilt before having the correct prop on hand as I really can’t hang on to it going 28mph as it is.

With just me in it(310lbs) it was proposing so bad the prop sucked air and I had to get out of it.

So I guess my question here is get the motor/hull to run correctly now or wait for the right prop?

If I do that work now (have to make/modify things) will I have to again make major changes with a new prop?

[JohnsonM50]

Props are only as good as test results, different per boat or motor Genrelly if you do change a prop it only makes sense to try different settings. The manifolds are not that significant as long as you dont have one of the restricted ones. The port should be no smaller than the carb. You should try to get that porpoising under control. Tucking the motor in and maybe raising it a little would help. Not gonna do much good going faster if it's not controllable. The deeper the motor, the more leverage it has against the bow. Tilted out too much will make that even worse.

[RaisedByWolves]

I’m working up a tapered spacer to tuck the motor in but I had it out for a couple weeks to modify it as it had too much taper originally. This is with the motor on the pin closest to the transom.

I have the motor raised up 3.5” and I’m making a new bracket to go up to 4” currently. It is honestly worse now that it was before I raised it due to the speed increase I got from that and a better prop of the same pitch.

[JohnsonM50]

Sometimes its tuff to tame them and settling for it to smooth down at top speed is the best it will get. Outboard race runabouts are prone to porpoising [not all] but are faster smooth. Seen it many times where a boat starts hopping and it falls right back against others holding smooth. The drivers have to 'work' these boats to keep control. A stationary drive position in a regular boat, that's less of an option. Can you post pics?

[RaisedByWolves]

Boat is currently under construction so pics wouldn't help much.

Its your basic 14' aluminum vee with two pedestal seats and a front deck with trolling motor and battery. I removed the stock bench seats to put in a floor and the pedestals, and both seat positions are forward of where they were stock.

The description I gave of it porpoising was with me in the back and about 150lbs of gear within 2' of the bow.

With the wife in the front (150lbs) it rides fair, but with just me it is a rodeo.