67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

[Bill C.]

hey ...

Car:
67 327 350HP 67K miles. engine never taken apart. stock point ignition, new repro wires, all new NOS point/cond/cap,coil, new plugs.
fuel: Ethanol free 93 octane.


I have a interesting situation --

A:
if I have the VAC ADV disconnected, regardless of engine temp, I don't get a gas starvation when going WOT in 1st gear.

b:
if I have VAC ADV connected and engine and fuel cool, I don't get gas starvation WOT 1st gear.
if the engine temp is above normal (180-210) and been driving for a bit, and fuel is hot in carb - I get terrible gas starvation.
the car seems near dead, will not accelerate. if I go up to higher gear like 3rd, it seems to not drain the gas out of the carb.



the initial timing (VAC disconnected) is 6 degrees advanced. it will go up to about 16 degrees soon as I connect the vacuum line.
idle is set to 900 approximately.


if it is running the fuel down low in the bowls I have no idea if is the front or back.
the carb is original and has been professionally restored. 99 % sure the jetting is stock front and rear.
the fuel pump is like brand new and AC Delco.
no place for air to get sucked in - all rubber lines are new.

So why would I not have issues with no VAC ADV but they show up when I connect it?

this one has me confused somewhat.

Any help is appreciated..

Thanks
Bill

[Mark E.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

Can you be more descriptive of the symptoms- "car seems near dead, will not accelerate"?

Gas starvation and not draining gas out of the carb (not sure what that means) are diagnoses.

[Bill C.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

it seems like the carb is emptying out of gas.
if I back off for a few seconds and let more fuel build up, the car will accelerate.

this only happens with the VAC ADV connected and when the car is running 180+ from driving.
and when the fuel is real hot in the bowls.
when the car is cool this issue does not happen.

I had the issue on a hemi challenger, I added a electric fuel pump behind the mechanical one and all issue went away.

but as for the Vette, Im thinking more a jetting issue or a fuel bowl float level issue.

[Mark E.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

More ignition advance likes a richer mixture. Also, higher temperatures tend to make a carb run leaner.

So so one possibility is these two conditions combined are enough to tip the engine towards being too lean. Although this is just a guess at this point. I'm leaning away from fuel delivery to the carb since it runs ok when cool.

Have you you checked the float levels? Next step might be to make sure the primary metering circuit is clear.

[Gene M.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

Bill,
The vacuum advance has nothing to do with fuel supply. The lack of advance may be just covering up the problem by not letting the engine rev and attain power and acceleration. I suggest your Holley is just fine. The issue may be with your fuel supply ......... fuel pump or even blockage on fuel pick up in the tank. Easy to check pump output into a jar or jug when return home after a ride.

[Mark E.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

Another thought- If the symptoms only occur at WOT, then after checking float levels, check power circuit and secondary operation including their rate of opening.

[Bill C.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

thanks for the great ideas....

tomorrow I'm going to check the float levels and pump output/pressure.
will also give the carb a good cleaning too while I have to off.
maybe junk got somewhere it was not meant to be.

if I can't find anything there I will pull the sending unit and make sure the sock is clean.

thanks again!


Bill

[Duke W.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

hey ...


the initial timing (VAC disconnected) is 6 degrees advanced. it will go up to about 16 degrees soon as I connect the vacuum line.
idle is set to 900 approximately.

Your data indicates that the VAC is not functioning properly. If original it is stamped MS 236 16. It should start to pull at 4" and add a maximum of 16 deg. at 8", so with 6 initial, total idle advance should be 22.

The only possible correlation I see between having the VAC connected or not and "fuel starvation" is a failing breaker plate ground wire, which is interrupting the ignition circuit. Search for a thread started by me in December 2012 - L-79 distributor blueprint/overhaul for more discussion on this issue, which is usually overlooked. Even with your car's relatively low mileage, the distributor probably needs some TLC. The '65 L-79, subject of that thread, only had about 50K miles.

A common cause of WOT fuel starvation is a clogged fuel filter. Has the tank ever been removed, inspected, and cleaned? There is a primary filter on the fuel inlet inside the tank (80 microns). The sintered bronze filter inside the carb inlet is 10 microns, and because it is rather small it doesn't take much debris to clog it.

A properly running L-79 should idle at 14-15" at 750, in neutral, and the best idle mixture adjustment is usually one turn out from the seat on the mixture screws.

Bottom line is your "fuel starvation" problem may actually be an ignition system problem, but both the fuel system and the distributor should get some attention.

Duke

[Bill C.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

Duke -
thanks for the info.

I am pretty sure the filter at the carb side has been replaced, but I will give it a good cleaning when I take the carb off later today.
The original tank is in the car and never been taken out. If I can't get the car to run well after going through all the items (short of taking the pickup out) then I will take the pickup out of the tank.

FWIW -
drove the car earlier today. outside air temp was around 65-70 degrees.
I let the car warm up so the thermostat was open etc..

I got on the car at a light after 10 minutes driving time.
It pulled hard all the way up to near 5.5K RPM and never missed a beat.

[Duke W.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

I recommend you investigate the distributor, first.

From my experience, breaker plate ground wires don't fail hard, but start out as intermittent ignition interruption. The weak point may be the attachment to the breaker plate, but problems can also develop under the insulation that you can't see. The wire begins to break. one strand at a time, and eventually causes high resistance that reduces primary current leading to low ignition energy, which can cause high speed misfire.

Copper work hardens rapidly, which causes it to become brittle. In a 100K miles this wire is flexed millions of times. Time can also take a toll due to oxidation of the exposed copper.

It's a wonder that they last as long as they do, but with age and miles the failure becomes more common. The thread I referenced previously gives a part number for the two-dollar repair wire from NAPA that can be installed without removing the distributor.

Duke

[Timothy B.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

How about a dirty carburetor (internal).

[Bill C.]

Re: 67 350HP - gas starved at WOT 1st gear, when between 180-210 deg operating temp

I plan to give the carb a good cleaning tomorrow after work.

and I will replace the breaker plate wire. It is a good maintenance idea regardless of what I visually see.

Thanks everyone for all the great ideas!

I love this board