Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Another wet airhorn gasket thread...
nUcLeArEnVoY:
Yep. Nice for a first post, right?
Cliff has been saving my butt through e-mail now for nearly a year since I got my car, which is a 1979 Trans Am 400/4-Speed. Motor is non-numbers matching. The factory cast-iron 10003395 intake; carb; emissions equipment; valve covers; etc are from the original PWH W72 motor so it looks original at a glance, but the block and heads are from a plain jane XA L78 motor from late 1976/1977.
ANYWAY... that's neither here nor there. I'm tired of bothering Cliff through e-mail, so let me go to his many Quadrajet expert brethren and if he wants to respond to this, he can.
So yes, another wet airhorn gasket. I thought this was an issue that only started after I did a rebuild on the carb this past August, but I came across some photos I took of the carb when I first got the car and it turns out it's always been doing this since I got the car.
Now let me preface, I have no problems here with how my carb or my car RUNS. I have the usual 4-6 second cold startups after 3 or more days due to modern fuel evaporation and my low float setting, but it starts up at the turn of the key cold if I start it the next day. Starts up the moment you flick the key when hot or heat soaked, and will idle at 700 RPM till' the next coming of Christ. A little boggy when getting the car to move from a dead stop in 1st, but that's probably because the car has a mild cam and I like my idle speed very low. My only issue left with this carb is the soaked airhorn gasket, which is more of an annoyance, than anything.
Carb in question is the original carb to the car: 17059263. Only "mods" to the carb is an electric choke conversion from Cliff, and a small kick in jet and rod size: 73 jets/42k rods. (Stock is 70 and 40K). Otherwise, it's been rebuilt to totally stock specs. No orifices drilled, new idle tubes are stock .038", float level is stock 17/32", and ATP is untampered (still has aluminum plug in airhorn). Secondary rods are stock DB. I've done all the usual rule-outs for a soaked airhorn gasket:
nUcLeArEnVoY:
(Continued... sorry, I ramble a lot...)
1). Three weeks ago I put in a new needle/seat, and float. Parts were from QuadrajetPower; and I know Mark doesn't peddle trash since all of the stuff I've bought from him as well as Cliff so far have worked great. Seat is a .130 like stock. I weighed the float and it came in at 6 grams, and needless to say, it floats in water. :P LOL. I vacuum tested the needle/seat and it held vacuum, though I'm not sure how to test it? What I did was just squeezed on a 3/8" fuel hose over the seat while it was out of the carb, inserted the needle and applied the vacuum with a MityVac pump. It wouldn't hold vacuum at like 5", but the more vacuum you applied, the more it held. At 20" of vac, it held just fine... not sure if this is still considered a failed test or a passed test? What is the PROPER way to vacuum test a needle and seat? Also pressure tested the metal seal ring for the inlet seat by filling the bowl with soapy water, pressed down on the needle and I injected 8PSI of compressed air into the fuel inlet - no bubbles. So it passed.
2). So what's next? Ah yes, of course: fuel pressure. For awhile I had a Carter M6122 installed, which is one of the better-reviewed stock replacement pumps - I put in an inline gauge and was getting 7.0-8.5PSI of fuel pressure, which HAPPENS to fall right in the exact range that the Pontiac 400 is specified under in the GM 1978 service manual, page 6C-120. That, and I know that the late 70's carbs handle far more fuel pressure than the early ones. STILL, because I'm anal about this... JUST IN CASE, and to absolutely RULE OUT fuel pressure as a problem since these new pumps are known for excess pressure, I went ahead and splurged on a NOS AC Spark Plug Division, completely original #6471561 GM fuel pump correct to the car. It's the high capacity three-fitting one with the vapor return line. Despite being nearly 50 years old, it works great and fuel pressure gauge with the NOS pump showed 6.5-7.5 PSI. I know that is not too high for a late 70's carb, so it's not a matter of fuel pressure.
nUcLeArEnVoY:
And well, that's really all I've done. The only other things I can think of that are often cited as a reason for wet airhorn gaskets are...
A. Warped airhorn, which yes, mine is (of course). About .020" warpage. A thick .062" airhorn gasket seals it just fine so that the carb and therefor the car runs as intended. One thing I'm curious about is WHY a warped airhorn would cause the airhorn gasket to soak? I see all the time that a warped airhorn causes gasket soaking, but WHY? Nobody ever explains WHY. It doesn't make much sense... especially since my airhorn and fuel bowl are both original and warped to generally the same degree.
B. Maybe I'm getting fuel boiling during heat soak? Doesn't make much sense, either... the fuel pump has the vapor return line. Just in case, I applied vacuum to the return fuel rail back to the tank and was able to suck fuel back right from the tank, so it's not clogged. Besides, I have no problems with hot starts or any true flooding symptoms, so I'm guessing the vapor return is working the way it should. (I live in Florida, FYI.)
One thing I should note, however; is that even though the car fires and starts up just fine even after a heat soak, it will idle rough until you give it some gas for 15-20 seconds or a few revs of the motor, then it's all good and I can hit the road. Maybe I am getting some minor degree of fuel boiling? For what it's worth, I'm using a 1/8" thick flange gasket, a Fel-Pro one, along with the factory heat plate under it. Did the factory originally use 1/4" insulator mounting gaskets on these cars? Just in case, I have a 1/4" one on the way and will try it out and see if that fixes the problem. Since the fuel pump is NOS, I only use REC-90 ethanol-free fuel. (I'm well aware that even ethanol-free fuel of modern formulation boils way easier than the fuels of yesteryear.)
C. And then finally, as has been stated on this forum before, maybe these modern airhorn gaskets just aggressively wick fuel no matter what you do? As in, to a point where the simple transfer of idle fuel up the tubes, across the transfer opening in the gasket and to the DCRs, is enough to soak the gasket over time? But then that wouldn't explain why not everybody has wet airhorn gaskets.
So what's going on here? I've tried just about everything, but this is still happening. Like I said, more of an annoyance than anything else. Car runs just fine, just wish my airhorn gasket would remain dry. ::)
Thanks, everyone!
von:
It's been my experience that modern airhorn gaskets will wick fuel no matter what you do. Every Q jet I've rebuilt for my own use did it. The part of the gasket that protrudes from the front of the carb will be wet after the engine runs a bit. I'm sure others will chime in.
nUcLeArEnVoY:
--- Quote from: von on March 27, 2021, 02:46:28 AM ---It's been my experience that modern airhorn gaskets will wick fuel no matter what you do. Every Q jet I've rebuilt for my own use did it. The part of the gasket that protrudes from the front of the carb will be wet after the engine runs a bit. I'm sure others will chime in.
--- End quote ---
I'm starting to wonder if that is the case. Mine gets fuel-logged at the areas you mentioned, and on the sides, too. Not so much in the rear part of the gasket. It doesn't get so bad that it actually drips but it's pretty damp and shiny, as in you can tell it's saturated.
Like I said, I'm almost wondering if the idle system operation is what's causing it over time.
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