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It's a 914 ...
 
stownsen914's Avatar
 
Join Date: Jan 2008
Location: Ossining, NY
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PMO carb starvation under hard braking on track

I wasn't sure whether to post this in the Track forum or here, but I figured the traffic might be better here ...

I have a 3.5L vintage race 911 running 50mm PMO carbs. I believe I am having a fuel starvation issue under hard braking. When I blip the throttle to downshift, all I get is coughing and spitting. This only happens under hard braking. When not braking, the engine runs beautifully and throttle response is good. As a "control" test, I tried mimicking a downshift/blip near the end of a long straight but without braking. In that case throttle response is good on the blip. This all leads me to believe that the g forces of braking are causing some fuel sloshing in the carb float bowls. Note that this car has an external surge tank that is filled from the main fuel tank, and a second fuel pump feeds the carbs, so I don't think the starvation is happening at the fuel tank or surge tank.

I understand that Weber carbs used to have this issue in tracked cars, and that some people put baffles in the float bowls to solve that. I thought this wasn't supposed to be an issue with the PMOs, but I'm wondering if anyone else has experienced this issue.

On a related note, I have posted previously about a more fundamental fuel starvation issue with this car that was solved by switching to a higher output fuel pump (Holley black pump). Now that is solved, and I am hoping to address this issue.

Thanks in advance.

Scott
Old 07-09-2016, 08:46 AM
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What is your fuel pressure set at?

Float levels PERFECT?
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Old 07-09-2016, 09:18 AM
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Thanks Steve for chiming in. Fuel pressure is about 6 psi. I did a round of float level adjustment last season. I'd say they're close, probably not perfect, erring on the side of above the halfway point in the sight glass, measuring with the car idling.
Old 07-09-2016, 10:26 AM
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Well, the Weber problem happened during cornering. Because the main jet intake from the bowl was on the outside of the carburetors, the outside carb in a corner would have the fuel pile up away from the jet intake. Edit - I had this backward. The inside (relative to the corner) carb is the one which can have fuel starvation. The force acting on the fuel is pointing away from the main jet intake on that carb.

The solution was a horizontal plate covering the outside [edit - the sides toward the outside of the car, but inside the bowl - good picture in Bruce Anderson's book] of the bowl to the center of the carb. There are convenient ridges there to which the plate can be glued. This essentially moves the pickup point to the center of the carb, and as long as you don't corner so hard that this opening is uncovered the problem goes away.

I've not owned a PMO, but for sure Richard moved the main jet intake somewhere into the center of the bottom of the bowl. But heavy braking typically can result in G forces a bit higher than cornering.

A guess - the issue may be with one or both of the two bowls which serve two cylinders.

I have installed one size larger fuel inlet valves in these, on the premise that they need to flow more (or the one cylinder bowls don't need as much). Not sure that makes any difference in practice. And since you don't have WOT issues, probably not relevant here.

Rather than starvation, is it possible that, if a bowl has its level a bit too high, that the deceleration moves the effective fuel level up above the orifice in the venturi, causing extra fuel to run in by gravity, not suction?

Just because it should be easy to do, you might try reducing the fuel pressure (is 6 psi what PMO recommends?)

Another thing you might try is restricting the accelerator pump volume. On my Webers I cranked that way way down, and it seemed to cure acceleration stumbles coming out of corners (once the "slosh" plates were installed).

Otherwise, fortunately I never had this issue before switching to EFI.

Last edited by Walt Fricke; 07-12-2016 at 12:15 AM.. Reason: Fix mistake and ambiguous language on an off point topic
Old 07-09-2016, 12:18 PM
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Quote:
Originally Posted by stownsen914 View Post
Thanks Steve for chiming in. Fuel pressure is about 6 psi. I did a round of float level adjustment last season. I'd say they're close, probably not perfect, erring on the side of above the halfway point in the sight glass, measuring with the car idling.
IMHO, 6 psi is high and I'd knock that down to 4.5 psi. Then, take the time to get all the float levels perfect. The time spent to do that pays dividends.

Walt brings up an an excellent point,.....is this a starvation issue or a rich stumble?

Watching an AFR meter w/wide-band O2 sensor will tell you whether its starving or not.
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Old 07-09-2016, 03:53 PM
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Quote:
Originally Posted by Walt Fricke View Post

I have installed one size larger fuel inlet valves in these, on the premise that they need to flow more (or the one cylinder bowls don't need as much). Not sure that makes any difference in practice. And since you don't have WOT issues, probably not relevant here.

.
On PMOs, the bowl that serves 2 carbs already has a larger needle valve.

Todd
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Old 07-09-2016, 04:39 PM
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Thank you gentlemen for the input. Good point about the possibility of this being a rich stumble. I did recently get a wide-band O2 meter to help in this kind of troubleshooting, so hopefully that will tell the story and help solve this.

Scott
Old 07-12-2016, 04:01 AM
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It would be interesting to know how much the fuel surges forward due to deceleration.

I think Froude Numbers could be neglected and the fuel would try to climb up the wall of the reservoir by around 30 degrees at 2g deceleration.

If the reservoir has remained full to the float height it should be fairly easy to model when the outlet becomes uncovered.
Old 07-12-2016, 06:53 AM
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carb problem

Hi Scott,I am with Steve on the fuel pressure.When I built that motor 25 years ago for your father the pressure would be 3.5-4.0 lbs.Have you changed jet sizes since I originally set it up?If your meter shows it goes to rich try dropping floats 1mm.Set on very level surface with the motor off and they need to be equal.Float levels change over time.Then check it on the track and see what your meter says.Has this motor been rebuilt since I did it originally?If it has not it is probably a little looser and might need a smaller main jet but your meter will show you.I will ponder this tonight and get back to you.When I ran that car in IMSA it never any starvation issues and it run right up to 9200 rpm.Oh yeah.I ran slide valves not carbs.I am in the states right now so give me a call.267-772-4582.Fred Apgar
Old 07-12-2016, 02:12 PM
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I have two sets of PMO's and the float level is just below the dot in the glass site on both sets. I also have seen here on Pelican that this is the correct level for the floats. Just below the dot.I also agree that the fuel pressure is too high as others have stated.
Old 07-12-2016, 03:10 PM
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Have you checked to make sure you idle jets are clear? I have had this type of problem with 46 mm PMO's. No problem at high RPM, but coughing and backfiring under hard deceleration. Clogged idle jets were the problem.
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Old 07-12-2016, 03:34 PM
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carbs

Hey Tom,you might be on to something.Great thought.Idle circuit is a part of the whole flow.Could be a job for ULTRASONIC MAN.Fred
Old 07-12-2016, 05:21 PM
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Idle jets are simple to clear. Remove them and blow them out with compressed air.
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Old 07-12-2016, 06:34 PM
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