90HPV4 is cutting out above 3300 RPM as if starved for fuel. Pumping primer fixes it

[toddschubert]

Crossflow V4 J90TSLECM
Ive been having this problem for a while but it used to only occur at 5000RPM so I didn't worry much about it. Now it has gradually started happening at lower RPM and I am now at max of 3300.
I have tried different fuel, eliminated the internal tank,lines and filter. I have even run with carry-on gas can...same problem.
when the problem starts, I can back off the throttle to near idle for about 30 seconds and it will then run at WOT (or over 3500) for about 20 seconds then it starts to die. It doesn't quit if I back off throttle but will if I leave it alone.
While it is running I can keep it running at high RPM by pumping primer bulb. Keep in mind I have eliminated restriction and siphon concept by running on open can.
I feel like it runs fine as long as there is fuel in the carb bowls but when that runs out it starts to die. Backing off throttle allows it to catch up and it returns to running fine. I have ordered a new pump as they are cheap but any suggestions as to what may be the problem are very much appreciated.
Compression test: 1=80psi, 2=90psi, 3=92psi, 4=92psi

 

[GA_Boater]

The fuel pump diaphragm is probably stretched. The kit will most likely cure the problem. If it doesn't look the fuel hoses. They can deteriorate on the inside and restrict fuel flow

Pumping the bulb is the clue for either cause.

 

[toddschubert]

Thanks, Pump comes in wednesday. I also got new fuel hoses while I'm at it.

 

[racerone]

If your gauge is good you need to look into those low compression values !!

 

[toddschubert]

Yeah. I can live with 90psi but the 80 has me concerned. I used a different gauge but that cylinder has always been lower as I recall.

 

[racerone]

Just 6 wee screws to remove the bypass cover to inspect the rings.----Will maybe cost you a $2.00 gasket.

 

[toddschubert]

Ok. I️ replaced fuel pump, motor clip connector, and ran it on an open can of fuel.
SAME PROBLEM!!!!
It dies down after 10 seconds WOT.
Pumping primer bulb gets it back running.

What am i looking for in terms of pressure from the crank case that runs the fuel pump diaphragm?

I️ don?t feel like it is carbs due to fact that it runs while pumping primer.
The motor also runs fine at idle and low rpm.

 

[racerone]

Did you act on post # 6 , yes or no ?

 

[toddschubert]

No. Not yet. Would that lower PSI in cyl #1 cause this type of problem?
Are we talking about piston rings?

 

[toddschubert]

Also worth noting:
had these same numbers on compression test when it was running great at 5000 RPM

 

[racerone]

Removing those covers is elegantly simple to do.----Not sure why you are hesitant to do this !-----If you put a smaller pitch prop ( less load ) on a motor it will again rev up to 5000 rpm !!-------If your gauge is accurate and your test procedure is good then your motor has a problem.------And yes you can have a look at the piston rings by taking the bypass covers off.

 

[toddschubert]

I just need to order gaskets before I️ remove covers. Remember though, it runs great at wot for around 15 seconds and will continue to run great if someone pumps the primer. This tells me that mechanics are ok. It?s something to do with fuel supply.

 

[Joe Reeves]

1998 90hp Johnson

The 10 or 15 second run would be burning the fuel that was forced in via the fuel primer solenoid.

The open tank scenario eliminates a clogged vent or anti siphon valve problem from existing.

You state the pump is cheap?? I assume this engine does not incorporate a VRO?

You state that by having someone constantly pump the fuel primer bulb, the problem vanishes and the engine runs fine. The new fuel pump should have corrected the problem... it didn't!

* If you haven't already, check the incoming fuel lines for obstructions, also from the pump to the carburetors...

* Check the pressure line leading to the pump if it exists for any obstruction.

* If the pressure hose doesn't exist... remove the pump to expose the pressure passageway and crank the engine over to check the pressure that would be applied to the fuel pump via the pistons down mode.

It would seem that either that new fuel pump is faulty OR the pressure passageway is blocked OR the pressure that would operate the fuel pump is escaping via another route... cracked crankcase, blown leaf valve, whatever. Let us know what you find.

 

[toddschubert]

Thank you Joe. i needed a comprehensive reply that understood what i have posted thus far.
It seems when there is fuel in carb bowls it runs fine. When that empties it fails.
Pressure from crank case is my main focus right now maybe a vacuum hose leak. I?m in process of tearing everything down to inspect fully.
Thanks again

 

[toddschubert]

It is the spl version without vro and fuel pump draws directly from crank case

 

[Joe Reeves]

It is the spl version without vro and fuel pump draws directly from crank case

Okay, I appreciate you jumping back in to answer that question. I sort of thought it was the straight small fuel pump version but I did need a answer.

Keep us updated as either the new pump is bad or the pressure needed to operate the pump has gone south somehow.

If you run the three tests/checks that I suggested in post reply #13... let us know the results.

 

[Fed]

Did you check the fuel filter under the cowl or only the external one?

 

[toddschubert]

I have yet to remove the intake manifold but at close inspection I see that one reed valve doesn't lie completely flat (closed) It is very close, not quite a hair width space but all the others are completely closed/flat.
Would this be a problem?
All vacuum line are in good shape and no detectable leaks. No signs of cracked crank case unless it's leaking into exhaust which I guess is possible.
Joe,
I'm still trying to get a gauge capable of measuring fuel pressure. I'm going to have to try measuring it while cranking starter as the carbs are all removed.
With the fuel pump removed, how much pressure should I see from crank case? Provided I had a way of measuring it. I gather it is a pulse fluctuating positive and negative pressure. Is that right?

FED,
I am always open to input but please note that I have even changed the fuel pump which would have included the screen filter I think you are talking about.

 

[Joe Reeves]

I have no idea what the fuel pressure should be.

The positive pressure "from the pump" is the only pressure you should be seeing. The two spring loaded valves would prevent any negative pressure... I'm sure you knew that but I wanted to slip that in....... Pressure and vacuum would register at the hose that operates the fuel pump BUT the vacuum would be less as that would be opening the reed plate leaf valves also.

The very slight space between a leaf valve and the plate usually does not have any effect on the fuel pump as the sudden downward plunge of the associated piston would slam that shut. I've had leaf valves jammed open with butterfly retaining screws... and the fuel pump continued to function perfectly. A lot of fuel got blown out the carburetor throat though .

A pressure gauge? I never used one... seems like my finger tip on the end of a hose registered whether it was good or bad quite well. Imaging that cupped piston rushing down at a minimum of 650/800 rpm into the crankcase... hellava lot of pressure!

 

[toddschubert]

The book says around 4PSI so its not a lot. I really do not like this SELOC manual! It states that when the engine cuts back from WOT it could be a crank case problem but never goes into detail about that. Just the standard fuel pump rebuild stuff