Fuel Cutout only on Track

[rcwaiz]

I am working on diagnosing a fuel cutout issues during DE days.
First off the engine is a 2.4 bored to 2.7, Weber 40IDA, MSD ignition, and Pierburg 5.5psi fuel pump, regulated to 4.0psi.

The issue I am having is when the car gets really warm(210), and driving aggressively the fuel cuts out to the point where there is no acceleration under throttle and the car bogs as throttle is applied. The problem is progressive and once it appears gets worse unless the car is let cool down.

I have only experienced the problem once on the street, where the car typically maintains 180 deg. unless pushed.

The problem occurred at my first track day this past spring, where I noticed the fuel level in my clear fuel filter was only at half full. Tested the fuel pump and max psi was 3.5psi and only dripping fuel out at the filter. Replace the fuel pump with a new unit and saw 5.5psi and good flow. Ruled out fuel level in the fuel tank.

I thought the problem was fixed until last weekend and only appears when the engine is hot and taking very long hard turns. Has anyone experienced this before? TIA

[T77911S]

I dont think it is heat related. i think you are running out of fuel either from the pickup inthe tank or a problem with the carbs. note, bruce anderson talked about modifying the float bowls in his book. you may want to see if that applies.

i would pull the bottom part off the fuel tank and look up in there to see if it is clean and clear.

then i would get the spec on the fuel pump and check fuel flow.

perhaps a carb rebuild is due.


the reason i say it is not heat related is when you start running the car hard, the heat will go up BUT, so do the fuel demands. so the carbs run out of fuel in the bowls.
maybe next time just let it idle for a few minutes to fill the bowls back up and see what it does.
i am leaning towards a carb problem. spec the fuel pump first just to make sure it can deliver enough fuel. i would do it with and without the pressure reg.

[HawgRyder]

In my experience there are 2 types of fuel regulators.
The first is an inlne type...that just adjusts the pressure that flows thru.
The second is a return style that bleeds off the excess fuel and returns it to the tank.
We always found that the return style was better at regulating pressures when large demands were being made on the system.
The MFI engines work this way in our cars.
I'm not familiar with carbed Porsches...so I can't say if yours should be this way.
Bob

[rcwaiz]

Originally Posted by T77911S

I dont think it is heat related. i think you are running out of fuel either from the pickup inthe tank or a problem with the carbs. note, bruce anderson talked about modifying the float bowls in his book. you may want to see if that applies.

i would pull the bottom part off the fuel tank and look up in there to see if it is clean and clear.

then i would get the spec on the fuel pump and check fuel flow.

perhaps a carb rebuild is due.


the reason i say it is not heat related is when you start running the car hard, the heat will go up BUT, so do the fuel demands. so the carbs run out of fuel in the bowls.
maybe next time just let it idle for a few minutes to fill the bowls back up and see what it does.
i am leaning towards a carb problem. spec the fuel pump first just to make sure it can deliver enough fuel. i would do it with and without the pressure reg.

I have not heard about the bowl modifications but this seems logical with the way the problems occur. I could consistently get 2 good laps in and then would have cutout issues.

[Walt Fricke]

If this problem occurs when you have a full or almost full fuel tank at the start of a track session, it is not related to fuel pickup in your tank. Those issues usually start down at about a quarter tank. They are very real, but only happen on long hard corners. Once the engine comes back alive on a straight it works fine until coming out of the next corner.

So have you modified your carb bowls? The stock Weber pickup from the bowls is at the outside of the bowl - outside as in toward the outside of the car. So when you make a hard right sweeper at 1.2G or so, the fuel moves to the left in the bowls, which can uncover the pickup holes on the right bank, leading to stuttering as you accelerate out of the turn, and a little way after straightening out until the fuel runs back.

The modification effectively moves the pickup point to the center of the bowl (which is why I have always carefully made sure I had epoxy all around the home made aluminum plate for a good seal). It does make quite a difference.

The other thing which can cause carbs to stutter coming out of corners is too much gas per accelerator pump squirt. A lot of us have ours dialed down to well less per stroke than stock.

None of this seems to have much to do with heat, though.