[98 SNAKE EATER]

So my 70 T/A is still having fuel drain back/evap issues

Earlier this year, it was pretty bad as the bowl would run dry after just a few hours of sitting.

I replaced the bronze filter with a regular paper filter from VatoZone (w/check valve) and it seemed to help slow the drain

Instead of just hours, it would take days before the bowl was empty enough not to give me a pump shot.

I left it as is and figured I'd mess with it when I was back in town

Came in last week and sure enough the bowl was bone dry.

Took about 20 seconds worth of cranking, but she finally fired up

Looking at possible causes and fixes:

One thing I noticed yesterday while on my lift is that the return line to the tank is plugged (I'm guessing it has a replacement tank that doesn't have a return provision)




Hoses show some mild cracking, but no signs of leaks





Now, I'm not too familiar with Q-Jets as I've always ran Holleys, but from what I've read, there can be a few causes:

1) Leaky well plugs

2) Faulty check valve in fuel pump siphoning fuel

3) Windowed seat allowing bowl to empty




A list of possible fixes:

1) Epoxy well plugs (sumthin I'd rather not do unless I have to)

2) Replace fuel pump with functioning check valve

3) Assuming the carb was rebuilt using a standard kit with windowed seat, replace it with a non windowed seat

4) Install an inline fuel pump for priming the bowl only



Open to all suggestions

[Formulabruce]

Number 4 is your fix. WE ALL have this problem, even in New England.
The problem? NOT LEAK BACK.
It's ALCOHOL... your fuel evaporates.
I never crank my engine that long, rather squirt some fuel in carb, and save the wear on the starter AND CARB.
Countless men slam the gas pedal up and down 20 times and think they are pumping gas, but they are just wearing the carb fuel pump bore.

[98 SNAKE EATER]

Originally Posted by Formulabruce

Number 4 is your fix. WE ALL have this problem, even in New England.
The problem? NOT LEAK BACK.
It's ALCOHOL... your fuel evaporates.
I never crank my engine that long, rather squirt some fuel in carb, and save the wear on the starter AND CARB.
Countless men slam the gas pedal up and down 20 times and think they are pumping gas, but they are just wearing the carb fuel pump bore.

Not doubting you as I'm sure evap is a problem over a long time, but going from a few hours to a few days by just replacing the fuel filter with one that has a check valve leads me to believe it's leaking down rather than evaporating.

Also, I've never had this issue with any of my Holley carbs





I've left them for weeks, sometimes months on end and always had enough fuel in the bowls to give me a decent pump shot.


I'm actually leaning towards a non-windowed seat since it's the cheapest fix (assuming she has a windowed seat in her now).

As for adding an inline priming pump, do you have any photos of your setup?

I'd want it to be as stealth as possible without cutting into the hardlines, which is why I was eyeballing the hoses near the tank yesterday.

[Gary H]

X2 on fuel evaporation being the cause of your hard starting problem. It is common on all my carbureted cars. The reason that Quadrajets are more prone to have the issue though is that have only one smallish fuel bowl versus the Holley's two. It simply will take the Holley twice as long for the fuel to evaporate.

[padgett]

Was a TSB on Quadrajet Well leaks. Early ones were notorious.

[Formulabruce]

Every Q jet with a fuel vent to the Air cleaner has evap issues with this fuel.
Personally I see about 3 days at 70 degrees and the bowl is near empty. So, it doesnt take that long.
trying to keep it up where its hot will only keep more up to wherever you have a check valve, BUT
fuel systems have a vapor line to reduce pressure..
Tough battle to win with alcohol.
I have used a cheap Jeggs 4-7 psi in line pump mounted in thick rubber near the tank, but up off the frame rail.

[unruhjonny]

are the fuel bowls in the Holley carburetors a closed system?

Not many people realize that these (Quadrajet) carbs have a passive atmospheric vent;
For 1970-California, and 1971-1979 Pontiac 350-400-455 Quadrajets they all vent into the aircleaner, but it's still a more or less atmospheric vent;
The 1970-non-CA, and earlier Pontiac Quadrajets vent outside of the air cleaner.

[98 SNAKE EATER]

OK, so as a test, I pinched the fuel line at the pump and let her sit for a couple of days



This morning the bowl was still full and gave me a solid pump shot for an instant start

I guess I can rule out leaky well plugs as it's most likley just a bad check valve in the fuel pump causing it to leak down into the tank

That being said, rather than replacing the pump, I think I'll join the club and add a small electric pump for priming only

Reason being I only see this car about every 6 months, so even with everything in top shape, the fuel will still dry up by the time I get back.

Been eyeballing this Airtex E8251 as it only puts out 4.5 psi max (no regulator required) and appears to have zero restriction when not running




Found this install video on YouTube and it seems to work well

https://www.youtube.com/watch?v=sUUJd8ECvXU


Now I just gotta plan out where to mount it and where to put the switch and relay (all gotta be hidden)

[padgett]

My 78 Sunbird was hard to start so I added a pushbutton on the dash to run the fuel pump for a few seconds before cranking and oil pressure was high enough to turn pump on. Later GM added a few seconds prime with the computer cars.

[Formulabruce]

Originally Posted by 98 SNAKE EATER

OK, so as a test, I pinched the fuel line at the pump and let her sit for a couple of days



This morning the bowl was still full and gave me a solid pump shot for an instant start

I guess I can rule out leaky well plugs as it's most likley just a bad check valve in the fuel pump causing it to leak down into the tank

That being said, rather than replacing the pump, I think I'll join the club and add a small electric pump for priming only

Reason being I only see this car about every 6 months, so even with everything in top shape, the fuel will still dry up by the time I get back.

Been eyeballing this Airtex E8251 as it only puts out 4.5 psi max (no regulator required) and appears to have zero restriction when not running




Found this install video on YouTube and it seems to work well

https://www.youtube.com/watch?v=sUUJd8ECvXU


Now I just gotta plan out where to mount it and where to put the switch and relay (all gotta be hidden)

Put rubber around the can, And thick rubber on the screw mount to wherever its going. Looks like great solution, hopefully rated for e85.

[Formulajones]

Sounds like your fuel pump was the culprit. From the pictures it looks relatively new too. I'd be more inclined to try another pump myself but with todays parts you never know what you'll get.

I've never had a problem with starts on my 70 RAIII Formula even if it sits for a couple weeks. I'm still running a 25 year old AC pump on the car. Actually I don't have a start issue with any of the carbed cars here.

I've been wanting to swap the pump on the bird just for preventive maintenance reasons but it's problems I see like this that has me hesitant with the garbage available today.

[jhein]

I thought the fuel filters in the Qjets had a check valve to prevent leak down. Is that correct or not?

[98 SNAKE EATER]

Originally Posted by Formulajones

Sounds like your fuel pump was the culprit. From the pictures it looks relatively new too. I'd be more inclined to try another pump myself but with todays parts you never know what you'll get.

I've never had a problem with starts on my 70 RAIII Formula even if it sits for a couple weeks. I'm still running a 25 year old AC pump on the car. Actually I don't have a start issue with any of the carbed cars here.

I've been wanting to swap the pump on the bird just for preventive maintenance reasons but it's problems I see like this that has me hesitant with the garbage available today.

I guess there's a reason why NOS pumps go for $300+ while replacements are ~$20

Originally Posted by jhein

I thought the fuel filters in the Qjets had a check valve to prevent leak down. Is that correct or not?

Mine had a sintered bronze filter in it and would leak down within the hour, but after I replaced it with a cheap paper filter w/check valve from VatoZone, the leakdown time was extended quite a bit

A non-wondowed seat should keep some fuel in the bowl, but it will still dry up by the time I see her again next year

https://quadrajetparts.com/needle-se...ed-p-2048.html

[78w72]

the Q-jet bowl is literally about a tablespoon of fuel compared to holleys with closer to 1/4 cup of fuel in each bowl, big difference between the 2 bowl sizes & why Q-jets have issues after sitting for awhile.

also i dont think its an alcohol issue since all 3 of my cars do it after about a week of sitting & i dont use ethanol fuel, we have options in my state for both 87 & 91 non ethanol fuel. im sure todays gas evaporates faster than the older stuff regardless of ethanol content but alcohol itself isnt to blame based on my experience.

i notice it much worse on my robbmc dead head pump car with a 1978 cliff built carb, about 5 days in the summer & maybe a week in spring/fall i have to turn the starter for 3-5 seconds 2 times to get fuel to the bowl, then 1 or 2 pumps of the throttle & it fires right up. any longer & i squirt some fuel into the carb vent to fill the bowl.

its almost as bad on my 78 t/a with cliff built carb & repro OEM large canister pump with vapor return, pretty sure its an airtex brand, i just know to crank it a couple times before giving it throttle to fire up. so i dont think its well plug related since cliff fills them with epoxy.

on my 81 with CCC q-jet carb rebuilt by another reputable place its not as bad but it has the original pump, that car can sit for 2+ weeks sometimes a month & after 1 5 second crank & a single throttle push it will fire up, sometime on its own with no throttle push but i usually do it to set the choke.

so after seeing the line clamped above, i think its fuel pump or filter drain back related causing most the problems combined with some evaporation depending on the fuel used.

[unruhjonny]

Originally Posted by 98 SNAKE EATER

I guess there's a reason why NOS pumps go for $300+ while replacements are ~$20

actually the correct replacement pumps for 1970 cars just became available again, last time I checked they were not cheap.

LINK

I just did a quick check, it's not currently available on Rockauto, Summit, or Jegs.
I even checked and none are on ePay at this time.

[Kenth]

Originally Posted by jhein

I thought the fuel filters in the Qjets had a check valve to prevent leak down. Is that correct or not?

Incorrect.
Check valve in fuel filter was implemented 1975 as a part of the "crash roll over safety policy" so the fuel tank would not be emptied via the carb if car is going off the road and ends up on roof.

[jhein]

Originally Posted by Kenth

Incorrect.
Check valve in fuel filter was implemented 1975 as a part of the "crash roll over safety policy" so the fuel tank would not be emptied via the carb if car is going off the road and ends up on roof.

Since my new carb was installed I've had some cold start problems. Not too bad but I have to crank it for a few seconds a couple times before it fires. With the old carb it never did that, even if it sat for a week. When I had the carb installed I told the shop to check and make sure the carb had a filter in it. I don't know if it did or not, not in their notes but he did tell me he put in a in-line filter so I'm thinking carb filter is not installed.

So maybe I had a check valve filter before in the old carb but not now, which is why I'm having the issue?

[unruhjonny]

/\ also if the "high flow" seat is installed, and the fuel is draining back through the pump, that would seem to enable the carb to more fully drain;
The standard needle and set for our carbs (as far as I know) was not the "high flow" one with openings at the base...
As mentioned earlier, with the regular seat there, then even if fuel is draining back, the fuel would only be able to drain back to a point - and not empty.

[Kenth]

Originally Posted by jhein

Since my new carb was installed I've had some cold start problems. Not too bad but I have to crank it for a few seconds a couple times before it fires. With the old carb it never did that, even if it sat for a week. When I had the carb installed I told the shop to check and make sure the carb had a filter in it. I don't know if it did or not, not in their notes but he did tell me he put in a in-line filter so I'm thinking carb filter is not installed.

So maybe I had a check valve filter before in the old carb but not now, which is why I'm having the issue?

One has to realize our old carburetored cars uses atmospheric pressure and vacuum controlled devices to control engine management and to run.
And as the carburetor is vented to atmosphere at all times fuel WILL evoparate at shut-off.
NOT like a modern vehicle with closed fuel system computer controlled with electric fuel pumps preload the fuel system before "start".
Our old Pontiacs will NOT start as fast as the modern cars, especially with fuels that are composed for closed fuel systems.
Not always a bad thing as the engine will have oil pressure at the lifters when engine starts (if correct viscosity engine oil is used).

Also, how many enthusiasts have read the Starting The Engine procedure in their Owner Manual? From 1970 Pontiac Owners Manual:
COLD ENGINE
-Fully depress accelerator pedal- and slowly release. (to set choke closed). With foot off the accelarator pedal crank the engine by turning the key to the Start position (to fill carburetor with new fuel) and release when the engine starts.
If the engine starts, but fails to run, repeat above procedure.
When engine is running smoothly, the idle speed may be reduced by slightly depress the accelerator pedal and then slowly releasing.

A lost art!

[Formulabruce]

If you crank for 15 seconds, then pump once to fire up, you STILL have the same problem we all seem to.
Proper tuned, with working choke in 60 degree weather, the engine. should at least fire in 5 seconds.