Evinrude V8 will not run 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 box 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,

 

[DECK SWABBER 58]

"To me this feels like it is running way to rich with the box on, but why?"

If it runs ok without the box then it's running lean not rich. Not sure what the fuel pressure should be, but those 8 carbs have to all be perfect for it run right.
Hopefully an expert on this "unique" motor will chime in here.

 

[Fed]

Where would you start on this one?

I'd start by putting the right fuel pump on it.

 

[fdahl_009]

I have installed the engine on my boat with the setup that I bought it with.
With the same fuel pump (electric), regulator and without the airbox installed - now I know why the airbox were not on..

When it has trouble running with the box on, but running great as far as I can understand with the box off, the engine have to be running too rich with the box on, thinking that the airbox is restricting airflow to the engine.

I have an 275 v8 that is running with the original pumps, though oil disconnected and premix, but pumps work. I can try to install those pumps today.

If the original pumps helps to run with the airbox on, that have to mean that the electric fuelpump is overfueling the engine, even though it is not flooding the carbs, is that possible?

 

[Bosunsmate]

.
Maybe its the wrong air box,. You could put the airbox on then run it until you get this struggling, cut the motor and immediately pull the sparkplugs and look for signs of flooding

 

[Bosunsmate]

If the original pumps helps to run with the airbox on, that have to mean that the electric fuelpump is overfueling the engine, even though it is not flooding the carbs, is that possible?

Are you saying this as a fact or something you are going to try?

 

[Tim Frank]

What you have posted is that you aren't sure what model Evinrude you have, that it has a non OEM fuel pump....(where did you get the figure of 5 PSI as being correct fuel pressure) ....and thus the carbs and jets obviously may or not be correct.
These customisations often present new owners with a real challenge to sort out the source of performance issues that might have been tolerated by the "Modifier" but not so much by the new owner....you.
I'd suggest that step 1 would be to figure out what you really have and what the OEM configuration should be and then work forward.

 

[fdahl_009]

Problem found!

Turns out there are different airboxes for these engines.
And apparently I have got the wrong one, mine is for an older engine.

Someone knowing these engines told me about the differences, and my box have some baffles inside which looks very correct, it fits perfectly, but it have some small holes on the sides (facing backwards) that the engine needs to suck the air through. The newer style airboxes are just an empty "shell", and the holes backwards are missing, but present in the aluminum casting on the engine where the box is mounted to. That casting have a lot bigger holes than the older assembled box have.
In mine the baffles can be removed - and with them removed and the airbox installed it now runs as it should!



Thanks for all the input!

I am not sure 100% of my model engine since the little round metal plate containing the identification numbers are missing.
The psi pressure number I have setup at (more like 3.5 - 4 psi I saw when testing today) I got from forums using these engines (and v6 / v4s) for racing.

Usually I too would have tried to turn everything back to normal, but installing the vro pumps from the other engine, after checking turned out to be a very big job. Would require nearly all the fuel hoses at the engine changed, since they are setup differently for the carbs etc. And the original y- and t-fittings are of smaller diameter into the pumps, so I would have needed to change or step down somehow the hose from the tank.

Now I use a bit bigger than 1/2" ID pickup in the tank, a 1/2" ID hose to a filter, then to the Holley Blue pump, a regulator on the outlet, then 1/2" ID to the engine where it splits into a y-fitting into 2x 3/8" ID to 2x sets of 4 hoses to the carbs and a small hose to the primer solenoid.

If I use the vro pumps, as I have done on the other V8, the big problem is starting the engine for the first time of the day. Since there are no primer bulb available with enough flow, and I do not have the original electric one, the pumps have to suck the fuel from the tank under cranking = unnecessary long cranking time and stress on the starter motor. With the electric setup I have now, the engine starts every time on the first try, just let the pump run for a second or two first to fill everything up.

 

[DECK SWABBER 58]

"To me this feels like it is running way to rich with the box on, but why?"

If it runs ok without the box then it's running lean not rich. Not sure what the fuel pressure should be, but those 8 carbs have to all be perfect for it run right.
Hopefully an expert on this "unique" motor will chime in here.

Not much input from me, had it backwards. :facepalm:

Congrats on fixing your problem.