no gas in Rochester after sitting

[Warren L.]

Did we ever figure this out? Have a 70 or 72 400 CU in my Truck. Always had to turn it over 6-8 times if it sat for more than a day. As soon as there is gas that it fires instantly.

Had the carb rebuilt carb rebuilders in Toronto and asked a filter be put in that stops the gas from flowing back. Told him of the problem and when the carb was back on it was just the same though it was crisper than before the no gas thing was exactly the same.

If I squirt a syringe of gas in it fires as the key is turned, a second syring and it sucks enough to keep running. I just about cut a hole in the top of the air cleaner so I don't have to take it off. If this keeps up going to take the prime button of my lawnmower and stick it on the dash.

Have seen the discussions put in an electronic fuel pump etc. Also been told to put a plate between the carb and the intake and that maybe the diaphram is gone in the fuel pump and it pulls the gas back, also read something about plugging holes in the bottom which I am not sure if it has, so will have to take it off.

Just wondered if there was any test to derermine where this gas is going, if it is draining back, evaporating or or

Warren

[mike cobine]

Re: no gas in Rochester after sitting

Most of the time, it is the plugs on the bottom of the fuel body that are leaking. You have to take them apart and peen the plug. Then epoxy the plug sealed.

Corvette Fever has an article on this:

http://www.corvettefever.com/howto/53360

I thought I had a good picture of it, but I can't seem to find one.




Corvette Fever Q-Jet

[Warren L.]

no gas in Rochester after sitting

Thanks Mike,

Looks a bit involved for me. First thought is if I was clear about the problem and had it on a printed sheet when I took the carb in and also called back after it was done to tell him the problem was just the same why they did not take care of it.

Is this something that is unkown or common, since I spelled it out verbally and on my note with the carb do they not understand this carb problem.

Will give them a call and ask. My notes clearly said I wanted a filter that stops back flow of a specific make forgot the name now but it is in notes, when I picked it up they had not read the notes and put in a regular filter so changed that when I asked.

Warren

[Joe L.]

Re: no gas in Rochester after sitting

Warren------

I don't know of any filter originally used on a Corvette that will prevent back-flow. There may be one, but I don't know of it.

While I agree that leaking fuel bowl plugs are often the source of the problem you describe, I've repaired these with epoxy and still had the hard re-start condition thereafter. So, while it's a good idea to make sure that the plugs are sealed (also helps reduce gas fumes in the garage after engine shutdown), it might not solve the problem.

[Warren L.]

no gas in Rochester after sitting

Well I have smelled gas in the garage, thought that was the locking gas cap and checked today and can see the rubber has deteriorated on that so will replace it.

The filter he put in and I will have to check is some aftermarket thing and I was told right away to ask one of those be put in as it stops back flow. Maybe it does however behaviour was 100 percent the same in that respect after the rebuild, zero difference for starting. The kid that told me about the filter also said to get some special carb spacer that keeps the heat from the manifold off the carb and stops the gas from evaporation but that statement would be wrong, it starts perfectly as the key is turned for up to about six hours, after that it is a bit more like a K-car. If the gas was burning off as he said then after it cooled in an hour or so it should have the problem and not be fine for six or more hours.

Still seems to me this should have been approached by the carb shop on the rebuild, either they understand this problem or they dont and with the mass of these carbs and so many with similiar problems dont get it.

Will start with the cap and check for the gas smell for starters, if it is still there when the cap is replaced and if the lines are fine etc then will look at what has been said on the carb.

Thanks,

Warren

[Joe L.]

Re: no gas in Rochester after sitting

Warren-----

70+ applications should use an insulating gasket between the carb and the manifold. So, if you're using a thin gasket, that's not the right one. Some 1970, non-California applications may have used a thin gasket. In any event, change to a thick insulating gasket (about 1/4" thick). These are available in the aftermarket from a variety of sources. You may have to revise your carb choke rod (the rod between the choke thermostat in the manifold and the carburetor).

[Clem Z.]

the filters with a check valve will prevent

back flow if the carb has a "windowed" needle seat,that is 2 slots cut into it below the fuel level so the fuel does not have to go all the way up past the needle to get into the float bowl,and the fuel pump has a return line to the tank. most all GM higher performance engines have the "windowed" needle seat to allow max fuel to get to the float bowl because Q jets only have 1 needle/seat to feed the primary and secondary side of the carb where a holley has a needle.seat for each side. every new carb kit i see does not have this type of needle seat so i reuse the old one with a new needle or i drill 2 holes in the needle seat just above the area where the needle seals.since GM went to the "spun in" aluminum plugs in the bottom of the float bowl i have never seen a leaker.these spun in plugs must be drilled out to remove them as when i modified Q jets for racing i had to remove these plugs to enlarge the fuel passages to the secondary side of the carb. the ones that leaked are the early carbs that used a cup type expansion plug. you can check for these leaks by placing the carb main body on a clean sheet of white paper and putting lacquer thinner into the float bowl. if there is a leak you will see tha paper get wet.

[Clem Z.]

PS make sure the choke is shut tight

when the engine is cold. that way the lower pressure,vacuum, the booser nozzles see when cranking the engine will cause the air pressure into the float bowl thru the bowl vent to force fuel out the nozzles where if the choke is open the pressure drop at the nozzles at cranking speed is less because of the open area above the nozzles. JMHO

[John O.]

Re: no gas in Rochester after sitting

Hi Warren

Did you ever check the fuel pump? You might have a weak one or one with a hole in the diaphram that will allow the fuel to syphon back toward the tank, sucking gas out of the carb when you shut the engine off. I say this cause I had a simular problem with my truck a couple years ago. Just a thought.

Good Luck.....John

[Jim T.]

Re: no gas in Rochester after sitting

Warren this is just one reason I replaced the Q-Jets on my 68 and 70 Corvettes back in 1974. Bought two brand new Holley 650 double pumper spreadbores for about $60.00 apiece and installed them. Holley still sells the spreadbore to directly bolt on a intake used by Q-Jets. Another feature I like about the double pumper Holley is the direct control I have in applying the secondaries, my 68 327/350 performs very well for me with its old Holley today. It always fires right up after sitting overnight. My July built 70 Corvette came from the factory with a thin carb base gasket.

[Warren L.]

Rochester fun

Yes the choke is on when it is sitting cold but the high idle does not go on when I start it. I can do without that cause I dont use it in the winter and also with my wallywagon the choke works great but I always want to kick it off sooner than it wants me to. The truck you can babysit it for a minute or two then it idles fine. Idle is too low probably about 500 when it is warm. My cheap answer to wanting a lumpier cam idle sound.

Yes I was told to take the Rochester off and do have an old Holley but then people said get a Carter, aluminum intake and cam and it will wake up and be trouble free. Have kept it stock except did put on headers. The odd time that stop on a sideroad where no one is around and punch it it is pretty awake.

Was also told about the fuel pump diaphram so will invetigate that first as it is bolt on and easiest to start with.

Thanks for all the info it is appreciated, when there is time will do some poking around and see what can be done. In the meantime it is just an excuse to drive more often.

Warren

[Joe L.]

Re: the filters with a check valve will prevent

clem------

Do you know which of the fuel filters have the check valve? The most common filter which was used for 68-69 Corvettes, among other Chevrolets equipped with Q-Jets, was the GF-432. I was not aware that it had an internal check valve. But, it might. I've never "autopsied" one of these things.

Thinking about it, it would make sense that the GF-432 had the check valve since it was only used for Q-Jet applications (which may need the check valve) and not for Holley applications (which don't really need it).

On the other hand, my car was always equipped with the GF-432 and, if fuel drain back from the carb is the source of the hard-starting-after-lengthy-shutdown phenomenon, then any check valve which might have been part of the filter was ineffective.

The "spun in" float bowl BOTTOM plugs began about 1968, I believe. I've epoxied some of these, but I can't actually say that any of them needed it. I never actually saw one, either, that I knew, for sure, had been leaking. So, perhaps this is not a problem with 68+ carbs. I always wondered how these things could leak. The "spun in" type plug that I have removed and inspected (I've only done this for the one in the front of the fuel inlet) had a tapered aluminum plug with an integral 0-ring type seal which is retained in the fuel bowl by the "spun in" aluminum, peripheral "seal". These would seem to be pretty reliable. I did have one leak, though; that's how I had the "opportunity" to remove it.

[Clem Z.]

short one GF 470,long one GF471 *NM*

[Clem Z.]

PS, GM part #s

GF-470 GM part# 5651802, GF-471 GM part# 5651803