Webber-Fuel Question

[RMartin]

I'm trying to find out why my 68S misses at high rpm. After changing the condenser,checking timing and points I installed a new clear fuel filter. While reving the engine to 6k to check timing I noticed very little fuel in the filter. Laying on its side it was about 1/4 to 1/2 full. As I reved the motor the level would go down in the filter. I took the fuel line off with the filter on and put it into a container. The pump is working fine. Big stream of fuel.
Put the line back on and with the car running fuel barely trickels into the filter. This doesn't look right but I don't know what could cause it. Doesn't the air in the fuel bowl have to have some place to go? Are the carbs plugged somewhere?
I have the tool to set the float levels so I know they are correct.

[silverc4s]

Are you getting enough F L O W out of your present pump??
See Jamie's latest thread on this subject..entitled

carbs and fuel pumps... Pressure AND flow... jpnovak

[RMartin]

I haven't measured it but I am sure there is enough flow. The problem is there is a big difference between flow with the fuel line off and the fuel line connected.

[RMartin]

Read the post you were talking about. I definately have two of the symptoms. The pump on my car is a Carter rotary pump,rated for 72 GPH and 6psi max.

[silverc4s]

I think your pump could be okay, but the system may need a recirculating path to maintain the proper flow???

[RMartin]

Anybody else have any ideas?

[tswaney]

Great timing. I'm experiencing the same thing with my Zeniths.

What does it do when you just turn the key to engage the pump without starting? Does it seem to try and fill? Is there a difference with hot vs. cold engine?

I was thinking fuel vaporizing in the bowls from the heat was putting a gas (not gasoline!) pocket into the fuel line that the pump can't overcome. I've got approx. 31 gph flow when just pumping into a bottle. Maybe we're seeing the same thing!?

My reasoning is based on the fact that mine starts to show symptoms of running lean as it gets hotter (some pops through the carbs particularly at idle).

[RMartin]

My problem exactly. Early S Man told me he thought it sounded like fuel vaporizing in the bowls.
Here's something else mine does, If it starts missing,sometimes if I let off the throttle and just ease into it will go ahead and rev higher.

[tswaney]

Maybe it has something to do with the KS air! Where are you, unless it is a secret?

Do you have the phenolic spacers under the intakes (or maybe they go between the intake and carbs?) that are supposed to insulate from the conductive engine heat getting to the carb bodies? This seems like one solution to the vaporization problem, except that they are not woking on mine.

I find the recirculation pathway intriguing (sp?) and need to look at my set-up a bit closer.

Hopefully we'll both get the answer from someone with more experience than I.

[RMartin]

I'm thinking about building a cool can to see if it helps. I'm going to take a coil of copper tube in a can with ice and see if the cooler fuel helps.
Where in KS. are you located. I don't think theres an early 911 within a 100 miles of me.

[tswaney]

I'm in Kingman (about 40 miles west of Wichita). I've never seen another early 911 in these parts.

I intend to check my recirculation back to the tank and diagnose some more.

[RMartin]

I was told to put the spacers on my car to solve the problem. If it's not helping yours I'm not sure it will help mine.
Does your fuel recirc back to the tank? Kinda curious about this myself. I was told it could be another possible fix.

[jpnovak]

In my experience fuel boiling is not an issue when the car is running high rpms. It is usually when the car sits after being hot and heat transfer into the carbs prevent starting. Due to evaporative cooling, the carbs stay relatively cool during most operations. Put your hand on the manifold and you will fell a difference in temperature. Of course, heat insulating blocks are a must especially in the summer.

How strong is your battery? I have seen an intermittant miss at high rpms due to low voltage changing the RC constants of the spark/ignition circuit. Low voltage will take longer to charge the coil. Since the ignition system fires more rapidly at high rpm the system can't keep up.

btw, I did ignore the obvious thought of float bowls since you mentioned them already. I will also assume the carb is jetted right and is not going lean.

btw, it is common to see air pockets in a fuel filter for low pressure carb applications.

[rw7810]

Jamie - What sort of plumbing set-up do you use on your carbs. Do you run a return line?? Or is it one line from pump to a Y, then dead head into each carb?? If you have a return set-up, how does it flow?

[chibone_914]

Is it a high speed miss or a stumble. I would check fuel delivery first at idle and at the suspected RPMs. If fuel delivery is a a minimum of 3.5-4 lbs. Check for the high RPM lean condition (too much air, not enough fuel) You can verify this with an O2 gauge or reader and make the appropriate adjustments/jetting.

[RMartin]

My car sounds like a mis in the garage, but when driving it is definately a stumble. How do I check for high rpm lean condition?

[tswaney]

RM,

Have you made any new discoveries on this problem?

My car does not recirculate back to the tank. One line from tank to pump, then one line from pump into engine compartment that feeds a metal "T" , then to the carbs. My Purolator/Facet (#42S) pump sits in the little mounting shelf forward of the left rear wheel. I need to move the pump up front next to the tank, and see what that does.

[silverc4s]

Tom,
If that Facet pump is the little square one, it has been determined here to not have sufficient flow rating for 2 webers...;-(

[tswaney]

Thanks Bill,
I was thinking along those lines about my Facet (even though I measured the output at 31 gallons per hour). I was hoping that would be enough for the Zeniths on my T engine, but I guess that I should just bite the bullet and replace it.
What is your suggestion? How do you feel about the Carter like RMartin has? What do you have on your S and your 69T?

I hope I have not officially hijacked your thread, RM. I think we may be dealing with the same problem/solution. And I still haven't heard where you are in KS!?

[RoninLB]

carbs can foul plugs easily. If it's fouling plugs it's also building up carbon in the chamber etc etc. A miss may only start to come on at high engine loading in the beginning. If that is a possibility install NGK BP5ES for testing and carb adjusting.

Lean carbs for whatever reason may create detonation that is mild and un-noticable under low rpm and low engine loading and come on into a severe mis at high rpm and loading.

Beru plug wire connectors can make you crazy trying to track down a miss. Personally I think they are a prob waiting to happen.

A miss from worn alternator brushes is usually consistant.

MSD can draw big amps. Wiring Must be a serious consideration upon install.

party hearty.