engine and drivetrain Fuel Pressure Problem - Electric Pump

[KFunk]

If you find that measurement,

please post it! It would be a worthy FAQ to

have known good fuel levels for Weber carbs.

t

I edited that post to show it. It's 25mm plus or minus 1mm. That came from a DCOE tuning guide written by dapbmw, and e-mailed to me from StaceyG.

[KFunk]

OK, here's where I got from, as written by dapbmw in an e-mail to Stacey:

Begin by setting the fuel level. If the fuel level is not set correctly, it will have an adverse effect on the ultimate state of tune you will be able to achieve. I’ll explain why it is so crucial later when discussing the main jet circuit. The standard procedure involves gapping the floats to the top cover. Unfortunately, this method is error prone. The floats are buoyant in a pool of fuel and should be measured as such in-situation. Construct a simple optical gauge for this wet level measurement. This tool works much like a dipstick. The split nylon sleeve clamps gently onto the acrylic rod, this allows it to slide freely along the rod but not slip. Rest the sleeve on the top surface of the well by removing one of the main, air and emlusion jet assembly and slowly push the acrylic rod down. When the bottom end of the rod just touches the fuel it will make the top end look dark (look carefully!). When it does, measure the tip to sleeve distance. Ideally, this should be done while the engine is idling and level. If the

engine is in a poor state of tune and shuddering severely it can be done within sixty seconds after shutting it off.

The desired level is predetermined by the design of

the DCOE carburetor. The passageway that connects the well to the auxiliary venturi is 23mm below the mating surface of the cover plate. The fuel level must be 2mm below that point or fuel will spill into throat when accelerating or braking. Therefore, the ideal fuel level is 25mm +/-1mm.

To measure the true air-fuel ratio (AFR) of the transition circuit, you must disable the main jet circuit by temporarily removing the emulsion tubes. If the floats are set correctly the fuel level will remain 2mm below the passageway leading to the auxiliary venturi (25mm +/- 1mm).

Consequently, no fuel should flood into the carburetor via the

auxiliary venturi, so this shouldn’t be a safety issue. With the emulsion tubes removed,fuel will continue to flow and the engine will run normally on the idle and transition circuits as long as the throttle plates are not opened past the last progressive hole. This occurs at approximately 10% of the pedal’s full travel.

Driving the car with the emulsion tubes removed provides a baseline for how the transition circuit was designed to perform.

Be careful when doing this test because you will be lacking about 90% of normal engine power to get out of harms’ way.

Drive the car gently at a steady speed on level ground and note the AFR measured by the air-fuel meter. Don’t move the throttle while taking a reading- the accelerator pumps will shoot fuel and make the AFR reading inaccurate. Moving the throttle plates past the last progressive hole will kill the engine so just don’t do it. Swap the idle jets until the AFR is approximately 12.5:1. I'd say start with 50 F8/F9 then 55F8/F9 then 60F2/F8/F9.

The next test measures the transition rpm at which the main jet system must be activated.

Shift the car into high gear and slowly increase the engine rpm until the engine dies. The maximum rpm at which the transition circuit keeps the engine running while in high gear is the transition rpm to which you MUST tune the main jet circuit to begin providing fuel. Typically, this target rpm is about 1400 rpm in high gear.

The main jet circuit must be contributing fuel at this transition point without the AFR deviating from the desired 12.5:1 value. Any gap in fuel delivery between these circuits produces the classic and much dreaded flat spot! At part throttle cruise in the higher gears the main jet circuit is actually providing all the fuel so concentrating on fine-tuning its low rpm performance is crucial to achieving the best tractability possible.

The “BEST LEAN IDLE ADJUSTMENT” method universally touted through the

years happens to be the worst tuning procedure possible, typically resulting in an AFR of about 22:1. This extremely lean mixture produces a number of undesirable results. It burns slowly and causes spitting and misfiring at idle. The slightest opening of the throttle plates causes a stumble. Furthermore, when the throttles are opened abruptly the

slow burning lean charge ignites the fuel vapor in the induction manifold, causing a backfire out of the carburetor. On overrun, the AFR will rocket to even leaner values and the chance of a total or partial misfire is very likely; unburned fuel will collect in the exhaust system resulting in backfiring or popping.

Avoid all these usual pitfalls by setting the idling AFR to 12.5:1. This should be done with a wide-band O2 air-fuel meter. To start, set the idle mixture screws as per the “BEST LEAN IDLE ADJUSTMENT” procedure to get all the same lean AFR contributed from every screw. Reduce the AFR to 12.5:1 by opening all the idle mixture screws by an additional equal amount of twist for each. A good indication that the AFR is equal on every cylinder is that the shuddering motion of the engine on its compliant mounts is at a minimum.

If the amounts of air entering the combustion chambers are unequal, the

combustion strengths will also be unequal and the engine will rock back and forth on its rubber mounts. Measurement of the airflow must be done with a high quality, calibrated gauge. I use and recommend the synchrometer made by STE. If the airflow is balanced correctly you should be able to place a brimming cup of water on the engine and slowly

increase the rpm via the idle speed screw to above 3000 without spilling a drop. Older carburetors might require small holes to be drilled through some of the throttle plates to balance the airflow to the throat with the highest rate of flow.

The Weber Tuning Manual

describes how to drill these holes. Newer carburetors have an adjustable air bypass bleed screw. The airflow synchronization between carburetors via the linkage is reserved for the cruise throttle condition at about 15% power so the engine pulls smoothly while in a cruise driving mode and this adjustment is done first. This cannot be done while in motion so raise the engine revs to where the engine shudders about worst and do the

synchronization via the linkage there to smooth out the engines’ torque reaction force. Next procedure is adjusting the airflow at idle of all the others to match that of the cylinder with the highest flow.

One last word of advice: Do not twist the throttle spindle to balance the airflow past the pair of throttle plates. The factory made an extremely fine adjustment of the throttle plate edge to first progressive hole when the carburetor was first assembled.

Close inspection may show that one of the first progressive holes has had additional metal scraped away downstream to make it even with its other paired hole. Twisting the spindle just makes the preset idling rpm less effective and increases the likelihood of an

off-idle stumble.

When you have done all of this, then we can discuss your jettings. just email me.

[brianstj]

I gutted the pressure regulator, turning it in to just a distribution block, and went back to the Carter 4070. Fuel pressure was sitting at 3.5 PSI running in the driveway. I'll have to take it out for a drive before I can tell if this will change things.

[mtriple]

I gutted the pressure regulator, turning it in to just a distribution block, and went back to the Carter 4070. Fuel pressure was sitting at 3.5 PSI running in the driveway. I'll have to take it out for a drive before I can tell if this will change things.

Brian,

How is the car running now? I am contemplating taking out my regulator as well since I noticed a few times "0" pressure on the gauge. Would you share how you gutted the regulator?

TIA

Hamada

[brianstj]

After the regulator was gutted I was seeing a nice 3.5 PSI at the gauge. I just pulled the regulator apart, took out the spring, and drilled out the shut-off ball. I drilled it out a little larger to make sure there was no flow restriction.

I am thinking now that the problem was my fuel line being too close to the exhaust. I never saw the fuel pressure act strange or the lean condition when the car was cold. I have been busy, so I just got back to the car yesterday and pulled the fuel line out. I am still trying to decide between a hard line along the pinch weld of the body or braided line through the inside of the car.