strange fuel starvation

Turbo Den · 10-11-2009, 03:27 PM

car is 911sc on PMO carb.
i understand once carbs are involved, it can be endless possibility...
but, please give your thoughts on the most possible cause for the following issue...

i have a wideband o2 sensor, so i know rich or lean.

car runs rich at low speed, low rpm situation.

The car can run to 90mph with full throttle. Lift throttle, and try to hold a steady part throttle to keep the speed. But cannot stay there.
seems like the engine is coughing (literally)... and runs lean like 15, 16.
After about 2 minutes,the engine is willing to run again. And this will repeat everytime with the same attempt in any weather. And, this problem killed my trackday.

another situation is running at autox. it was like this: rev it high on a straight, brake, turn, exit with full throttle, but no immediate power. instead, it coughs a while, and ruins the run. So, everytime it was full power in first 20-30 secs, and then super slow in the later half of the run. It used to be power in any rev range anytime. This situation happens usually in the 2nd or 3rd runs when the engine is fully warm up to 180-200. 1st run usually starts with 150-170, and no engine problem.

any comments? suggestions?
thank you

mpetry · mpetry's Garage

I don't know PMOs but a lot of experience with Webers. This really sounds like a float level problem or a stuck needle valve. The other possibility is that your fuel pump is not delivering enough gas for sustained high speed running, and the fuel level in the float bowls is dropping at high RPM.

Don't PMOs have a window on the side to check the float levels ? Can you see the fuel level in the sight glass ? Wonder what you could do to observe it at high RPM?

kepa75 · 06-06-2010, 04:19 AM

I bring up this thread as I've the same problem. When I come out of sharp corners (there are a lot here in the mountains of northern Italy) I always have a few seconds where I've no power at all and the engine only suppters. RPMs are at 2000-3000. It doesn't bother if I go on full throttle or only partial. After 2-3 seconds, ones I'm straight again, power kicks in and everything seams fine...
I'm runing 46 PMOs on a quite fresh SS 3.2 L in a quite light ST-replica. Floating levels should be ok (around the dot in the glasses), and fuel pressure is around 3.5 - 4 psi.
The plumbing is classical as suggested by PMO.

Any suggestions??

Thanks,

Peter

Steve@Rennsport · 06-06-2010, 08:02 AM

Peter,

A few thoughts,....

1) I would check fuel pump volume. You should get 1 litre in 30 secs if the pump is healthy. Check the in-tank filter screen for any obstructions as they are most oftentimes ignored.

2) Float levels must be perfect, not simply "close" enough.

Once you know the delivery system is in perfection condition, make sure the float levels are spot-on.

kepa75 · 06-06-2010, 09:27 AM

Steve,

Thanks for your help.
I'll check the pump volume first (I'm running a MFI pump) and then the float levels again (although I've a hard time defining if the level is exactly on the dot with running engine...)

Best regards,

Peter

rennworks · 06-06-2010, 10:09 AM

Are you running solid motor mounts?

Christien · Christien's Garage

I had a very similar problem that I chased through ignition components, carburetors (tore them down and rebuilt them, then replaced with webers), all sorts of stuff. Basically, after 1 or 2 laps at the track, it felt like I was down about 100 hp. Could barely maintain speed going uphill, even in the heart of the powerband, around 4500rpm.

My problem was the fuel filter in the tank. Too much crap at the bottom of the tank, and after it got swished around from a few turns at the track, it was significantly hampering fuel flow out of the tank into the lines. This also explains why the car drove perfectly normally on the street.

I pulled the gas tank, cleaned it out really well, removed the filter and there you go, no more problems. Took a couple hours one afternoon. The worst part is emptying the tank, and that's not a really big deal. It's a rather simple job, easy to DIY, and the total cost is a jerry can full of gas, because you don't really want to clean it with anything but gasoline. Pour a few litres in, swish it around really well, pour it out. Repeat 5 or 6 times until the gas runs perfectly clear.

Try this before spending any money on carbs or ignition.

JP911 · JP911's Garage

I had the same experience as Christien. Gunk in the tank would slowly get sucked into the filter (after a few laps) and start to limit the amount of fuel making it to the carbs. The situation was worse when the tank got to about 1/4 full.

kepa75 · 06-06-2010, 12:31 PM

Quote:

Originally Posted by rennworks

Are you running solid motor mounts?

No, I'm running Porsche motor mounts, maybe a bit harder ones (CUP), but not solid.

I'll check my fuel tank, as it is something I haven't done yet. The tank was rebuild with the car about 4 years ago, so there might be some junk in there still. I'm just wondering why I have the problems almost only in very sharp turns (hairpin bends)??
It seams to me more like a slosh in the the carbs, cause it almost disappears after 3-4 seconds, ones the car is straighted out.

JP911 · JP911's Garage

As I understand it, the PMO design all but eliminates the potential for fuel starvation in high-G turns. My issues were also worse in turns, which is likely the combination of a partially clogged filter AND fuel sloshing to one side in the tank. As an experiment (before you go draining the tank) try cleaning and/or changing out your fuel filters to see if that temporarily remedies the situation.

Christien · Christien's Garage

The turns are what causes all the crap in the tank to cover the filter. When you're driving straight with no lateral forces, the crap all settles to the bottom below the filter. In case you're not familiar with the layout of the tank, the output sits about 1/2" above the bottom of the tank, and the output line extends inside the tank, covered in a filter. So you have to stir the gunk up enough to cover that. It only happens when you really get stuff moving, like in turns.

kepa75 · 06-06-2010, 10:17 PM

Quote:

Originally Posted by Christien

The turns are what causes all the crap in the tank to cover the filter. When you're driving straight with no lateral forces, the crap all settles to the bottom below the filter. In case you're not familiar with the layout of the tank, the output sits about 1/2" above the bottom of the tank, and the output line extends inside the tank, covered in a filter. So you have to stir the gunk up enough to cover that. It only happens when you really get stuff moving, like in turns.

That sounds quite logic. I'll first check the pump volume and then drain the tank (to bad I just filled it right to the top day before yesterday...) and let you know.

Peter

kepa75 · 06-08-2010, 07:30 AM

Hi,

I started working on the car yesterday, but have not done much yet.

A question arised: What do you think about the plumbing of my fuel pump (it is an old Bosch MFI fuel pump)?

wwest · wwest's Garage

Ice formation in the carb(s) throat..?

Christien · Christien's Garage

Plumbing looks fine, though I'm no expert. That's not just a guage, it's a pressure regulator. You need it if you have an MFI pump, because MFI pressure is about 25psi, whereas carburetors need it down around 4-5 psi. Return line should be plugged - not needed with carburetors. Good news that you've got the extra filters in the line. But I couldn't see them being the cause of your problem, because you're getting starvation in turns - if your inline filters were clogged, you get issues with driving in a straight line, maybe even just idling.

kepa75 · 06-08-2010, 08:27 AM

Quote:

Originally Posted by Christien

Plumbing looks fine, though I'm no expert. That's not just a guage, it's a pressure regulator. You need it if you have an MFI pump, because MFI pressure is about 25psi, whereas carburetors need it down around 4-5 psi. Return line should be plugged - not needed with carburetors. Good news that you've got the extra filters in the line. But I couldn't see them being the cause of your problem, because you're getting starvation in turns - if your inline filters were clogged, you get issues with driving in a straight line, maybe even just idling.

I was basically just wondering if the plugged exit of the pump could harm the system someway. Tonight I might measure the pump volume...

Christien · Christien's Garage

I don't see how. Like I said, carbs don't need a return line to the fuel tank. Conversions from MFI to carbs will usually leave the line in place and just plug it.

kepa75 · 06-08-2010, 09:49 AM

Quote:

Originally Posted by Christien

I don't see how. Like I said, carbs don't need a return line to the fuel tank. Conversions from MFI to carbs will usually leave the line in place and just plug it.

My understanding is that the return line is needed for the excess fuel after the pressure regulator (like on the MFI system, where there is a pressure regulator in the filter unit).

avendlerdp · avendlerdp's Garage

There needs to be a return line otherwise the pump will overpower the regulator. An MFI pump will generate a lot of pressure if the return line is capped. This pressure will be too much for your regulator and you will end up with a lot of problems.

kepa75 · 06-08-2010, 12:23 PM

Quote:

Originally Posted by Steve@Rennsport

Peter,

A few thoughts,....

1) I would check fuel pump volume. You should get 1 litre in 30 secs if the pump is healthy. Check the in-tank filter screen for any obstructions as they are most oftentimes ignored.

2) Float levels must be perfect, not simply "close" enough.

Once you know the delivery system is in perfection condition, make sure the float levels are spot-on.

1) I've just checked the pump. It delivers approx. 0.9 liters in 30 secs. This was measured after the filters, just on the inlet of the carbs, just running on the battery. Both lines are also quite balanced.

The next days I'll check the float levels and ones I've emptied a bit the tank I'll check the in-tank filter screen...