Ignition & carb finetuning

[Eljaro]

I took my Contemporary out today after a mayor upgrade process which included a new TKO 600 gearbox, new waterpump ,a new differential cover, shimming the dif, and new yoko tires.
All very nice, especially the gearbox. Unfortunately all this did not help do do away with one simptoms which I can not resolve.
The engine stalls when breaking hard.
I have the jet extenders and special floats with the cutouts, the fuel level is set right just dripping out of the check hole, the vent pipes are in both bowls, the idle is set to 700 and the idle screws 2 turns out. I have raised the gas pedal so I can brake and when almost full stopped push with the foot sideways on the gas to keep the engine running.
There are no pops or backfires, so it seems to be a simple lack of gasoline. What can be done?

[lineslinger]

Eljaro,
Are you runing an electric or mechanical fuel pump?

[Rick Parker]

Very common. Fuel in the rear bowl is forced out of the vent tube under braking and sloshes onto the secondary throttle plates creating an overly rich condition, causing the engine to stall. Get the white plastic vents (sometimes called "Whistles") that fit inside the float bowl and metering block. It's actually a relatively easy fix if you have the right parts. To test for this condition, fit a piece of 1/4"-3/8" diameter rubber hose and slip it over each vent tube forming an arch inside the air filter housing. Cut a small "V" in the hose to vent it and then drive the car. If the condition disappears, follow the above remidy and then remove the hose.
This "hose trick" used to be very common in Drag Racing circles because the cars "Left Hard".

Rick

[trularin]

Rick, good call on the carb and yes that is the way we used to do it, but how did you know he has a carb and not EFI?

Did I miss something?

[SCOBRAC]

Before you install the whistle... simply extend both (or all) the bowl vents with a length of rubber hose. Some even use a single hose connecting the front and rear bowl but leaving a segmet of hose cut away at the highest point to allow both bowl (or all 4 boals if dual quad) to breath.

This allows you to verify this is or is not the problem without dissasembly and minimal effort. The hose to use is 5/16 fuel line.

Something else which may be happening, especially on long duration, dual carb engines is the amount of vacuum being taken away at the point of braking to feed the vacuum brake booster actually goes to zero in the intake manifold, paricularly if you are not down shifting when braking "hard" (most really hard stops are "panic" stops in which there is no time or need to keep the rpms up.

The resulting loss of vacuum creates a rich condition and with no vacuum stalls the engine.

Hope this helps,

[Eljaro]

I have the whistles installed in both bowls and have a mechanical fuel pump.
I will try the extension tube on the vent pipes, see if it works.

[ack fia]

In my Holley days I had the same problem even with the whistles and vent tubing. I eventually had to lower the floats a little. The lower floats didn't seem to hurt and the stalling problem went away.