Fuel and Weber gurus

[Ducman]

Help me diagnois this problem.

My heavily built-up 2.7 with 40 Webers has always performed spectacularly over the last 3 years I've had it. No hesitation, no bobbles - just perfect response in all weather and driving situations. Never touched a thing on the carbs.

About two weeks ago at an autox I experienced a hesitation during a high speed tight sweeper - hesitation almost like hitting the rev limiter.

Yesterday at the track I experienced a much more severe hesitation during almost every turn.

There is no problem on the straights, but after about one lap I get this severe hesitation about 1/4 way into the turn that then last through about 25 yards past exit.

I have not been able to get the car to do this on the street -I might have to go seek out a clover leaf : )

It must be fuel starvation right? - But what has changed to create the situation?

Bad fuel pump? Then why no hesitation on full accelerationin straight line? btw - I have 3.5 psi with engine not running

Float level? - how would this change all of a sudden?

Fuel slosh? - why did I not experience this before?

Pressure regulator? - why would this only effect turns? I've got 3.5 psi

Electrical? Possibly loose connector that is intermittent during lateral load...again doen't seem likely.


Despite the fact that I can see 3.5 psi static pressure - my thought is that either the fuel pump or the regulator or both are not able to maintain the required pressure and flow at full throttle and therefore bowls are draining on straights and then combined with the fuel slosh of turn-in I starve the motor.

Please - what say you?

Thanks,
Don

[jwetering]

When this was happening to me (quite a few years ago) it was fuel slosh. In fact the fuel was sloshing through holes drilled in the carb tops, where thes holes had been drilled (by a PO) to act as an overflow of some sort. I covered over the holes with JB weld and the problem went away. But why now for you? Good question.

[JP911]

Your carbs may just need a good cleaning. If the needle valves are gunked up, you could be getting insufficient fuel in the float bowls. Especially under WOT in hard cornering.

[Steve@Rennsport]

If this situation just started, the very first thing I would be checking is fuel VOLUME.

Your pump needs to provide a quart in under a minute to keep the float bowls full.

Have you changed the in-tank filter (located at the bottom)?

[Ducman]

Originally Posted by Steve@Rennsport

If this situation just started, the very first thing I would be checking is fuel VOLUME.

Your pump needs to provide a quart in under a minute to keep the float bowls full.

Have you changed the in-tank filter (located at the bottom)?

I was hoping you might chime in.

As I mentioned above I suspected volume as well - wasn't really sure how much I should measure. Have not looked into tank filter - suppose I have to drain to check that?

Thanks,
Don

[Grady Clay]

Don,

Good advice all.

Steve has the right approach. I would put an adjustable fuel pressure indicator light temporarily on your dash to help with diagnosis. For a track car, a low pressure indicator can alert you to inadequate fuel supply at maximum sustained performance.

What was your fuel level?

Cleaning the filter screen in the fuel tank is part of normal maintenance. This is accessible on the bottom of the tank. You might consider a circulating fuel system that keeps fresh (cool) fuel at the carb N&S inlets.

On some prior MFI posts I describe a good flow measurement procedure, part of CMA.

Does your ’74 still have the fuel pump at the rear? Even a slightly weak pump can show these symptoms simply because of the location. ‘69-‘70 and ’76 and later all have the pump at the front for great benefit. I’ll make the flat-out recommendation that every 911 ’71-’75 move the pump to the front and carbureted systems circulate fuel past the N&Ss.

I assume you have an original ’74 fuel tank and not an earlier (non-CIS) tank without the “swirl pot”. Has anyone put “sealant” in the tank?

Best,
Grady

[Steve@Rennsport]

Don:

Things like this usually happen after the car is refueled and thats the first place I begin the trouble shooting process in situations like these.

Gas station underground tanks are notoriously full of dirt-water-crap and if you refueled your car just after the gas station received a fresh load, that always stirs that junk up to contaminate a fillup. The Golden Rule here is never buy gas from a station for an hour after the tanker truck departs.

These older gas tanks are to prone to rust and dirt and all that stuff obstructs the nylon filter located at the bottom of the tank under the swirl pot. If you have never replaced that, this is the first place to begin as the carbs should not be touched until you can confirm they are getting sufficient fuel volume.

[Ducman]

Originally Posted by Steve@Rennsport

Don:

Things like this usually happen after the car is refueled and thats the first place I begin the trouble shooting process in situations like these.

Gas station underground tanks are notoriously full of dirt-water-crap and if you refueled your car just after the gas station received a fresh load, that always stirs that junk up to contaminate a fillup. The Golden Rule here is never buy gas from a station for an hour after the tanker truck departs.

These older gas tanks are to prone to rust and dirt and all that stuff obstructs the nylon filter located at the bottom of the tank under the swirl pot. If you have never replaced that, this is the first place to begin as the carbs should not be touched until you can confirm they are getting sufficient fuel volume.

I have a fuel filter with clear lens - I do in fact notice more contaminants than I have ever before (not a lot but still more) I suppose I could have clogged filter or pump.

To some of Grady's points - I had 3/4 tank, have rear mounted electric pump. I always purchase top octane gas from either Shell or BP (but I imagine this still doesn't rule out a bad batch). The tank has never had sealant in it.

But as I'm writing this I just remembered that I had an off road adventure at Road America just prior to these symptoms - not a particularly fast or violent one but none the less my car is very low and any trip through the weeds tends to rummble the underbody - could have loosened some crud that has since clogged or obstructed some fuel path....

Thanks,
Don