85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replaced?

I have an '85 v4 140 horse vro, oil injection already discounted and been mixing 50:1 for 2 years without any problems until recently. Power at WOT as well as hole shot isn't what used to be just a few days ago. When problems first started it was losing power completely and quitting at WOT - changed fuel filter, checked hoses, connections and then it would stay running at WOT, but still only hit 3/4 of usually top speed and only rev to about 4000 rpm vs usual of 5500.
Next at home, checked fuel flow (just visually by pulling fuel line off fuel pump output) from onboard tank vs portable tank. Portable tank seemed better, so figured out it was a bad anti-siphon or pick up tube in onboard tank and changed both - next visual check had better results for the onboard tank, putting out just a bit more fuel in 15 seconds of cranking vs the portable tank. Wanted to take it a step further and pressure/vacuum test the fuel pump, which brings me to where I am now - the fuel pump puts out 5 psi at 5000 rpm (on land with garden hose attached) with either fuel tank and about 3.5 psi at idle - which according to the motor repair manual or both good, BUT every 3-4 seconds the pressure dips quickly to 1 psi and then immediately jumps back to 5 and this goes on and on. I know this is pulse pump, but my question is still...

Is it supposed to drop like that and bounce right back or is that drop unusual/too significant suggesting the pump is bad?
Please Help! And Thank You!

Re: 85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replace

Don't rev to 5500 on hose!

-pump sounds normal
-look for a non contributing cylinder.....ignition, carburation or compression issues.

Re: 85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replace

schematic said:

Don't rev to 5500 on hose!

-pump sounds normal
-look for a non contributing cylinder.....ignition, carburation or compression issues.

Click to expand...

Thanks!

I've checked and all 4 cylinders are firing and with good spark and did replace the plugs too when the problems started.
Carbs are clean and adjusted similarly.

Since discovering and fixing the lack of fuel flow from onboard tank compared to portable tank, I haven't had it back in the water - just decided to pressure test fuel pump first - so will try it on the water this Sunday and post again after that.

Re: 85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replace

double up your pumps

the drop in pressure is odd. engine misfire or bad diaphram.

Re: 85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replace

pn said:

View attachment 203075double up your pumpsView attachment 203074

the drop in pressure is odd. engine misfire or bad diaphram.

Click to expand...

the drop in pressure is normal for VRO and OMS systems

Re: 85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replace

Schematic, are you sure the drop in pressure is normal for VRO systems? - That's ultimately what I need to know at this point.
pn, why would I double up my fuel pumps as opposed to just replacing it with one good one, if in fact it is bad? I know the v6's had double in series pumps, but not the v4's to my knowledge.

Thanks again for any help anyone can give!

Re: 85 v4 140 hp vro fuel pump pressure drops to1 psi every 3-4 seconds- need replace

If you are unsure.....have someone squeeze the primer ball continuously while it is bogging. If an improvement is felt, its fuel supply related.