Evinrude V8 runs badly with airbox installed

[fdahl_009]

Hi all!

I have an Evinrude V8, been told that it should be a '90 300hp, but the identification plate is missing so I do not know for sure.

It is setup with a Holley Blue fuel pump and regulator @ 5psi, feeding directly to the carbs (no overflow issues) and running premix.

The carbs are plastic and have the number 398743 on them, and are jetted like this; upper jet on the left side 42, idle air 48 and high speed 71d.


The problem I have is that when I install the airbox (the previous owner did not have it installed) the engine looses a lot of power. It idles well in neutral, slightly lower rpm in gear compared to without box, but you can hear something is not right as soon as you start to put load on it, and it takes full throttle to put the boat on plane (without box installed I can easily pull three skiers out of the water with far from full throttle).

To me this feels like it is running way to rich with the airbox on, but why?
The electric pump will not effect things as long as the carbs do not overflow?

I have checked the timing and done a link and sync, and can't find anything wrong there..

Where would you start on this one?



Fredrik,

[pyro]

Maybe he re-jetted the carbs to run it without the air box!

[sharpeye Mike]

I was going to second that but I checked the parts for that year and model, turns out those are the correct jets.

[racer]

Does the air box have a baffel or not?

[fdahl_009]

Am I thinking correct that 4-5psi delivered from a electric pump will not overfuel the engine as long as the needle can do its job and the carb is not flooding?

I do have two original vro fuel pumps from another v8 I have, but this is a lot of job to install, since many hoses and fittings are the wrong size and needs to be replaced / rerouted.

I tested the original vs electric setup on the other v8 as well. As far as I can remember, on that engine I had problems with the needles not being able to stop the fuel coming in as it should.
Because I am missing the original electric primer pump, and there are no primer bulbs available in the right size for these engines, starting them for the first time for the day needs a lot of cranking since they can not be primed as they should. The electric setup fixes this, and they start right away on the first try.

I use 1/2" ID pickup and hose from the tank, to a filter, then the holley pump with the regulator on the outlet and 1/2" ID hose to a y-fitting at the engine, splitting into 2x 3/8" hose feeding into the two original fittings that splits into 2x sets of supply to 4 carbs.

The airbox consists of 3 pieces. The "big" one that you see as the airbox. Inside that one, two plastic pieces are fastened, those have the holes of the carbs and a gasket that touches the front of the carbs to seal.
Those two pieces have some holes on the sides, which the engine would get its air from when the box is beeing used, the holes are facing backwards, I guess this is what you mean by baffle?

One thing I noticed is that the air supply holes do not line up to anything similar in shape when bolted to the engine, some of the holes are kind of blocked, but I think this is the way it should be.

Anyways, there are no doubt the airbox assembled and installed will restrict the airflow to the engine if the engine are able to swallow the extra air it gets with the box off. It should be running lean with the box off I know, but to me it seems like it is running way too rich with the box on, it is not running correctly on all cylinders. Take the box off and the engine feels a lot more "alive" and needs very little throttle to go into plane.

I can not think of anything else causing these problems, than the airbox restricting the flow, making the engine run too rich, but if it is correctly jetted this should not be happening.

[Jason Joe]

Am I thinking correct that 4-5psi delivered from a electric pump will not overfuel the engine as long as the needle can do its job and the carb is not flooding?

I do have two original vro fuel pumps from another v8 I have, but this is a lot of job to install, since many hoses and fittings are the wrong size and needs to be replaced / rerouted.

I tested the original vs electric setup on the other v8 as well. As far as I can remember, on that engine I had problems with the needles not being able to stop the fuel coming in as it should.
Because I am missing the original electric primer pump, and there are no primer bulbs available in the right size for these engines, starting them for the first time for the day needs a lot of cranking since they can not be primed as they should. The electric setup fixes this, and they start right away on the first try.

I use 1/2" ID pickup and hose from the tank, to a filter, then the holley pump with the regulator on the outlet and 1/2" ID hose to a y-fitting at the engine, splitting into 2x 3/8" hose feeding into the two original fittings that splits into 2x sets of supply to 4 carbs.

The airbox consists of 3 pieces. The "big" one that you see as the airbox. Inside that one, two plastic pieces are fastened, those have the holes of the carbs and a gasket that touches the front of the carbs to seal.
Those two pieces have some holes on the sides, which the engine would get its air from when the box is beeing used, the holes are facing backwards, I guess this is what you mean by baffle?

One thing I noticed is that the air supply holes do not line up to anything similar in shape when bolted to the engine, some of the holes are kind of blocked, but I think this is the way it should be.

Anyways, there are no doubt the airbox assembled and installed will restrict the airflow to the engine if the engine are able to swallow the extra air it gets with the box off. It should be running lean with the box off I know, but to me it seems like it is running way too rich with the box on, it is not running correctly on all cylinders. Take the box off and the engine feels a lot more "alive" and needs very little throttle to go into plane.

I can not think of anything else causing these problems, than the airbox restricting the flow, making the engine run too rich, but if it is correctly jetted this should not be happening.

You just solved your own problem, that model engine should have the air box gutted to run properly as jetted as you describe. It shouldn't have the inner baffles and foam gasket. The stock air box was just the outer shell with some filler pieces around the edge where it bolts up to fill the void. You have an older model air box which can be used if gutted.

Jason

[fdahl_009]

You just solved your own problem, that model engine should have the air box gutted to run properly as jetted as you describe. It shouldn't have the inner baffles and foam gasket. The stock air box was just the outer shell with some filler pieces around the edge where it bolts up to fill the void. You have an older model air box which can be used if gutted.

Jason

Thank you very much for the info. I will try again with the inner parts of the airbox removed.

Because the previous owner have run it without the box, all the sound deadening material in the cowling is removed. So I would like to mount the airbox again, and add some material to the cowling again.

[racer]

Exactly why I asked if it had guts in it, the 300 of that vintage does not use them.

[fdahl_009]

Thanks again for your input! I might have tried this myself, but then I would not have know if this was the right solution.
I have also checked some split-drawings today, and from 1990+ they are as you said, no inner baffles, just the "outer" airbox.

The engine have been running happy nearly all day, swallowing somewhere around 25 gallons of fuel.

It is running a bit rough, rich I suspect, at idle in gear and up to about 1600rpms, from there it "sings" very happily.
I can feel that the engine is a bit more sluggish at first when pulling a wakeboard or a skier out of the water, but I have the exact same top speed and rpms so this seems to be the right solution!