[jarretts70]

I'm struggling a bit with the last couple tuning issues with my quadra-jet. It's a 1978 T/A W72 carb (original to my car) on a reasonably stout 428.

My issue is that I lose all the fuel out of the fuel bowl if the car sits for more than a few days. If I start it every day or two there is no problem, it starts as well as my late model Chevy truck. If it sits for 5-6 days its hard to start. If I let it sit more than a week, forget about it. I could crank it all day....I use a squirt bottle of gas to prime the carb and it starts right up.

The last time I had it started was about 3 weeks ago. Today I pulled the carb & carefully took it apart, so as not to spill any remaining fuel. It was bone dry; as well, there was no fuel sitting in the intake plenum.

When the carb was rebuilt I re-sealed the plugs with marine epoxy - I don't think they are leaking. But I can't figure out where the gas is going. Any thoughts?

[shaker455]

The plugs could be leaking and the gas will evaporate faster with the ethanol content
Also, how old is the fuel pump and what's the float level set to?

[Formulas]

It shouldnt take any more than about 5 seconds of cranking and foot stomping to start our stuff with a good mechanical pump and no cracked rubber lines with a dry bowl

[TedRamAirII]

If the vent works to push air "out" when filling the tank, it should also vent air "in" when the engine is running.

[jarretts70]

FWIW I've never had any indication there is a problem with the tank vent. No whooshing sound when pulling off the gas cap or other weird noises.

I've tried lots of different cranking/ pumping gas routines like the ones suggested and but all I wind up with is a non running car with a dead battery....

Fuel pumps are pretty cheap on Rock Auto, think I'll give that a try. It snowed about 10 inches this weekend, not going to be driving it any time soon anyway...

[78w72]

pretty sure thats a common issue, even without plugs leaking or ethanol in the fuel. gas does evaporate & the q-jet fuel bowl is very small.

i have 2 of the same 78 t/a q-jets in use on my cars that were both rebuilt by cliff r, both do the same thing after about 4-5 days of sitting. i dont use fuel with any ethanol.

my routine for when the cars sit that long is to just crank the engine for about 5 seconds to prime the pump/carb, then pump the pedal a few times to squirt some fuel & set the choke, it starts right up.

[Race City Pony]

Originally Posted by 78w72

pretty sure thats a common issue, even without plugs leaking or ethanol in the fuel. gas does evaporate & the q-jet fuel bowl is very small.

i have 2 of the same 78 t/a q-jets in use on my cars that were both rebuilt by cliff r, both do the same thing after about 4-5 days of sitting. i dont use fuel with any ethanol.

my routine for when the cars sit that long is to just crank the engine for about 5 seconds to prime the pump/carb, then pump the pedal a few times to squirt some fuel & set the choke, it starts right up.

My '79 jeep also does the same when I use ethanol additive gas. I usually try to purchase non - e and when I do no problem.

[David Jones]

What you're experiencing is normal with todays fuels, ethanol laced or not. I have an electric pusher pump out back by the tank that helps after a long non-running spell.

[Gary H]

I agree with everyone else. It's evaporation. I have the same issues with all my cars equipped with Q-jet carbs if I let them sit over three or four days.

[77 TRASHCAN]

Yeah, modern gasoline's evaporate sooo easily. I was working on a 2 Bbl carb for a friand. I set it in a small steel bucket that had enough capacity to completely submerge that carb. I let it set for a couple of days, that helped with all the slime and grime, inside and outside...

I left the dirty gas in the bucket, put a 5 gal bucket over top, to keep the dirty gas from evaporating.........I needed to move that bucket of gas 3 month's later, it was completely gone...

I don't drive my 77 T/A much. Something I learned from an old job (we had lot of old gas powered buses that sat around for a long time between starts..
I pump the throttle to set the choke, then crank the engine for 5-10 seconds, some times it will maybe hit on one cylinder, I then turn ign swi off, THEN pump accelerator 3-4 times, it usually firs right off. The point being, will that carb with gas before you utilize the acc pump to get the extra needed in engine to start.....

[Mister Pontiac]

Same here.

When having sat several days, crank it a bit then pump the throttle several times.

When having sat only a day, one or two pumps and she fires right up.

When warm, don’t even have to touch the throttle most times.

[TedRamAirII]

If the carb is already off, it would not hurt to test the plugs for a leak.

[jarretts70]

The car had a full-bore resto, so everything is new from the gas tank to the carb. That said, the fuel pump was nothing special, just a stock style replacement. To be honest I don't even remember what brand it is. I was trying to keep everything looking pretty stock....

I'll check the plugs just to be sure....I didn't realize that modern fuel was that prone to evaporation. I run 91 octane pump premium which is supposed to have minimal ethanol content, but it's hard to find anything without at least some ethanol around here....

I've tried the pump gas once/crank/shut off & pump gas multiple times/crank thing, but I have to repeat it many times, and crank the heck out of the thing. Sometimes it works, sometimes not...If I prime it then it starts easily so that's what I normally do.

One other thing that might(?) be related - when I shut the car off after it runs for more than a few minutes I can hear gas running back into the gas tank. Residual fuel left in one of the return lines? Not sure if this is normal or not.

["QUICK-SILVER"]

Originally Posted by jarretts70

The car had a full-bore resto, so everything is new from the gas tank to the carb.

Questions about this later.

Originally Posted by jarretts70

That said, the fuel pump was nothing special, just a stock style replacement.

Changed the mechanical pump on my 79 a few years ago. Sitting level, with a full tank of gas, I had to plug the line to the tank to keep fuel from running out. It siphons out because the line up front is lower than fuel level in the tank.

.

Originally Posted by jarretts70

.I didn't realize that modern fuel was that prone to evaporation.

Think about how big the bowl is and how much of that is displaced by the float. Carbs stay cool when the engine is running. Heat soak after you cut it off, especially with a functional exhaust cross-over, and not all but most of the gas will evaporate in a hurry.

Originally Posted by jarretts70

One other thing that might(?) be related - when I shut the car off after it runs for more than a few minutes I can hear gas running back into the gas tank. Residual fuel left in one of the return lines? Not sure if this is normal or not.

The 79 gave me more trouble with fuel starving (after engine upgrade) Than any Pontiac I've ever owned. Electric pusher helped a lot but it still happened at the worst possible times. More gas the tank had in it...the fuel starve happened more often.
Smog era tank vent does not let enough air in the tank for what gas is coming out at WOT. Fuel pump was pulling a vacuum in the tank. Worse when full (because of less air space) and fuel would cover the vent line making it harder to let air in.

Adding old school vent and vented cap stopped the vacuum in the tank that the fuel pump couldn't over power.

On your car...Vacuum in the tank would/could be pulling fuel out of the pump through the return line when you cut it off. The factory vent line can also get fuel in it when the tank is full.

A lot of cars, sitting parked, would actually flood the carbon canisters when conditions were right. Like cutting the car off right after fill-up on a hot day. A nasty wet carbon canister will barely let any air go back to the tank.

Just some past expierence with this
Clay

[1968GTO421]

Originally Posted by "QUICK-SILVER"

The 79 gave me more trouble with fuel starving (after engine upgrade) Than any Pontiac I've ever owned. Electric pusher helped a lot but it still happened at the worst possible times. More gas the tank had in it...the fuel starve happened more often.
Smog era tank vent does not let enough air in the tank for what gas is coming out at WOT. Fuel pump was pulling a vacuum in the tank. Worse when full (because of less air space) and fuel would cover the vent line making it harder to let air in.

Adding old school vent and vented cap stopped the vacuum in the tank that the fuel pump couldn't over power.

On your car...Vacuum in the tank would/could be pulling fuel out of the pump through the return line when you cut it off. The factory vent line can also get fuel in it when the tank is full.

A lot of cars, sitting parked, would actually flood the carbon canisters when conditions were right. Like cutting the car off right after fill-up on a hot day. A nasty wet carbon canister will barely let any air go back to the tank.

Just some past expierence with this
Clay

Interesting info to have! Thanks for posting this, I think it may be applicable to my problem.

[dmac]

Don't pump the pedal at all before cranking it. Crank it for several seconds at a time. You need to pump gas all the way from the tank to the carb. as even the fuel lines can completely drain after a few days. If your car usually starts easily, once you get fuel to the carb, just a single pump of the pedal should start it if your choke is set right.

An electric pump near the tank would help. Run the pump on a switch for a few seconds to fill the lines and fuel bowl before trying to start. Then again, one (possibly two) pump(s) of the pedal before turning it over should be enough.

If you don't want to add an electric pump, you will eventually learn exactly how long you need to crank it before starting. I've had this issue with both Holleys and qjets.

[jarretts70]

Thanks for all the advice!

I have the gas tank vented thru the charcoal canister and the original gas cap so I never really considered that there might be a vacuum in the tank. But that would explain why gas runs back to the tank when I shut it off..

Once I get the carb back on it I'll try get it running without a as cap so the tank can breathe, and when I shut it off I'll listen to see if I hear gas running back into the tank.

[mgthejudge]

I worked as a mechanic at a Pontiac dealership in 1979. Your Q-Jet carburetor started coming with a paper fuel filter with a spring loaded trap door at the receiving end to prevent your problem. This insert is easily removable and most people do if they can't buy the earlier filter which has no door at all.

When you run the car obviously all of the lines and the card are full of gas. When you turn the car off and it sits the fuel tank systems actually reverse and siphon all of the fuel back to the tank draining even the carb bowl. The trap door prevents the reverse flow from the carb. Get a new paper fuel filter in the carb with a trap door and hopefully the Chinese will have made it so it seals correctly and this will solve your problem.

[jarretts70]

FWIW, I've been thru the carb a few times. When I first got the car running the tune was way off. Idle mixture screws did nothing/ bad hesitation issues/ dirty spark plugs/ etc.

I bought Cliffs book - between that and advice on this forum I've got it running well. Idle circuit mods & air valve adjustments were all it took. Idles great, cruises great, good throttle response. Even gets reasonable gas mileage. Two remaining issues:

1 - After 2-3 seconds of WOT it runs out of fuel & I have to lift. Raising the float level helped a little but did not cure it. Float is set at 1/4" below the top of the airhorn. I'm installing a windowed needle & seat to see if it makes a difference.

2 -The starting issues.

The fuel starvation at WOT and the seeming inability to refill the fuel bowl when trying to start have got me thinking maybe my new fuel pump is not up to scratch?

[mgarblik]

Having problems at WOT now has me leaning toward a fuel pump volume issue. Yes the Q-jet bowl is small, but with the stock 3/8" line, there should be sufficient fuel available to run the car to the factory red line. The fuel you hear going back to the tank I suppose could be leaking past the valves in the fuel pump. The valves could also be leaking during running, unable to maintain fuel in the bowl. The fuel pump won't cure the bowl fuel loss of course, but as said an empty bowl Q-jet should fill and start the engine in 2, 5 second crank intervals. My next move would be to verify fuel pump pressure and flow, or just replace it.