Evinrude 40HP Lurching after 5 mins of WOT

[cenkaetaya]

Hi, I am having a bit of a mystery problem.

So we have dual Evinrude 1996 40HP on a 20FT aquasport, 15 and 17 inch pitch props.

randomly last summer they just started lurching at WOT, after about 30 seconds, so this year i cleaned all the carbs, replaced all hoses, tested different tanks, and the carbs were dirty at one point.

However the issue is BACK but after 5 mins, the boat cruises 25mph perfectly, and after 5 mins the motors just start going voom, voom, voom, you turn them off, wait a min and they work perfectly again.

What could be causing this? they are working great for a "long time" 5 mins and then suddenly it just happens again. I have literally tried everything, they got new fuel pumps, NON-VRO, and all seems good, everything is hooked up carbs cleaned, more then once, all hoses extra clamped. They start up in half a click, go for 5 mins and dead.

It seems like 1 of the fuel tanks once it falls below 40% gas, it just starts doing this, and adding more gas ... seems to fix it.. but I dont think thats the issue, its just a coincidence.

They both have a steady stream of water, both sound great, and then LURCH.

https://www.youtube.com/watch?v=fpJ1i9GA7DU

Here is a video of them cruising. If you watch the last 30 seconds you will see the RPM gauge suddenly go crazy and die.

I am really struggling to find the answer to this problem.

 

[jimmbo]

The fuel pick up tube might have leak, or the tank isn't letting air in fast enough to replace the fuel as it is consumed

 

[racerone]

Test run with " known to be good " portable tanks and hoses.

 

[F_R]

Odd that they are both doing it at the exact same time. Must be something common to both engines. That lets out the carburetors and fuel pumps. And the tanks too, if you are running separate portable tanks. I don't know, that's a real mystery.

BTW, those things are kicking up an awful lot of spray. What's with that?

 

[F_R]

Just wondering, why is the starboard tach reading zero?

 

[racerone]

When motors are moved way to the side they may be sticking down too far.----Look at raising these motors up.---Are you saying one motor uses a 15 " and the other has a 17" installed ?

 

[F_R]

They certainly do sound like they are running out of gas. So what is the common denominator? Both on a common filter or something like that?

 

[Joe Reeves]

In agreement with F_R... Both engines having the same problem at the same exact time? Sounds like fuel supply to me. Running with separate portable tanks would tell the tale.

Does the fuel primer bulb(s) have a tendency to collapse? (restriction?)

What is the fuel supply setup on this rig... separate built in tanks, separate fuel line supply, one fuel line supply graduated to a dual line setup, what?

 

[oldboat1]

I think I would check for water in the tanks, to rule that out. If in fact you have some contaminated gas, it might be from the source (change sources). Fuel will "float" on top of a layer of gas in the tanks, and when tank volume reaches a critical level (40% ?) at a particular angle while underway, maybe the pickup starts pulling water. A couple of water-separating filters might be helpful.

 

[oldboat1]

i.e., fuel floats on a layer of water, when water is present (water at the bottom in reach of the pickup).

 

[racerone]

Does each motor have its own fuel supply from ----TANK PICK UP TO MOTOR----Yes or no ?

 

[oldboat1]

Sure would want to run with separate tanks.

(have assumed you've avoided all of the crab pots.)

 

[cenkaetaya]

1 - Attached photo - you can kinda see how they are positioned in relation to the V - they are kicking up so much water because they could be a bit deeper, they are on each side of the DEEP V, they are good but, could be a tiny bit lower. Its mainly that the boat is supposed to have a motor that is longer shaft.

2 - Tach on the right is reading zero because its currently having a wiring issue

3 - each motor has its own tank, the one on the right (when looking back) goes through the fuel water separator, and the one on the left which is having more issues (well 30 seconds sooner) is currently direct bypassed for testing.

4 - all gas is brand new

My suspicion is this after thinking alot last night. Both motors used to have VRO / bypass pumps, So they started having issues and i purchased the standard fuel pump that fits on the crank case suction hole. I closed up both pulse fittings, and everything seems good.

So now my theory is that these pumps are supplying 90-95% of the needed fuel but after 5 or so minutes that 5-10% kicks in and they simply cannot keep up the fuel demand. This is why if you stop and run at idle for a minute or two, they work fine once again for 5 mins.

My next 2 options:
1 - i have 1 other regular fuel pump with BOTH the pulse and crank hole, so I am thinking of wiring both the pulse connector and the crank, to see if that delivers more fuel.

2 - put back the vro with the bypass and see if its really just that these fuel pumps can ALMOST keep up, but not.

What do you guys think about the regular fuel pump with both the pulse + crank ? will it make a difference over the one with just the crank hole?

 

[racerone]

??----How do you plan to handle the fact that one source will be a positive pressure while the other will be slightly negative ???

 

[cenkaetaya]

??----How do you plan to handle the fact that one source will be a positive pressure while the other will be slightly negative ???

by not going as fast and being a budget build. It works really well we go 25mph, it could be better but these motors are like 400-500$ a piece.

We also get fantastic gas mileage going no-wake speed as we only need to run 1 motor; we can also change between motors to give them a break if we need. We can do 360's and turn around in place as any two motor boat can, which is great.. Overall this is working very very well, I am thinking about upgrading to 2x 90HP.

 

[jimmbo]

Being Polite, that is a less than useless photo when it comes to showing how deep the motors are. How about a few taken level at the height of the gearcases, behind the engines? Proper exposure would help too,

My guess is the engines are 20" models and so is the Transom. which places the engines several inches Too deep.

 

[cenkaetaya]

Being Polite, that is a less than useless photo when it comes to showing how deep the motors are. How about a few taken level at the height of the gearcases, behind the engines? Proper exposure would help too,

My guess is the engines are 20" models and so is the Transom. which places the engines several inches Too deep.

Thanks, I will try to add some (in the photo on the left motor) you can see the top cavitation plate is perfectly aligned with the bottom of the hull. I put them as far apart as possible, any closer to the center and the hull will get in the way.

This is the primary reason of us having issues in wavy conditions. In good water, the motors are fine but in large waves they can struggle due to being high up. When we bounce, they can really loose lots of thrust.

I am still considering adding a 3rd one in the center and making a bracket for all 3, that lowers and brings all 3 to the center a bit more so they are correct height.

Not sure anymore, we are considering doing 2x 90HP instead as it just seems more practical esp in rough water.

 

[jimmbo]

"Perfectly aligned with the bottom of the hull", as in where? Not the keel, I hope

 

[cenkaetaya]

"Perfectly aligned with the bottom of the hull", as in where? Not the keel, I hope

Like in the photo, the metal plate, i believe is called the cavitation plate, is as high up as the V shape of the hull. They are same height. If they were closer together that plate would be above the lower back part of the hull.

 

[racerone]

You mis understood post # 14-----How do you plan to operate one of these pumps with one hose providing pressure to the diaphragm and the other hose providing suction at the same time !!!------Do you understand the purpose of the " pulse limiter " and the basic operation of these fuel pumps ??