Author Topic: Another wet airhorn gasket thread...  (Read 275 times)

Offline nUcLeArEnVoY

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Another wet airhorn gasket thread...
« on: March 27, 2021, 12:04:37 AM »
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:
« Last Edit: March 27, 2021, 12:26:31 AM by nUcLeArEnVoY »
1979 Pontiac Trans Am 400/4-Speed

Offline nUcLeArEnVoY

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Re: Another wet airhorn gasket thread...
« Reply #1 on: March 27, 2021, 12:06:01 AM »
(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.
« Last Edit: March 27, 2021, 12:19:29 AM by nUcLeArEnVoY »
1979 Pontiac Trans Am 400/4-Speed

Offline nUcLeArEnVoY

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Re: Another wet airhorn gasket thread...
« Reply #2 on: March 27, 2021, 12:07:24 AM »
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!
« Last Edit: March 27, 2021, 12:22:39 AM by nUcLeArEnVoY »
1979 Pontiac Trans Am 400/4-Speed

Offline von

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Re: Another wet airhorn gasket thread...
« Reply #3 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.

Offline nUcLeArEnVoY

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Re: Another wet airhorn gasket thread...
« Reply #4 on: March 27, 2021, 08:56:09 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.

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.
1979 Pontiac Trans Am 400/4-Speed

Offline Cliff Ruggles

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Re: Another wet airhorn gasket thread...
« Reply #5 on: March 28, 2021, 03:13:37 AM »
The problem is a poor fit between the airhorn and main casting plus adding a thick gasket.  Lacking clamping force the "wicking" is only worse with that configuration.

Warping of the parts isn't a big deal unless you are trying to sell something to flatten things or making Youtube video's trying to convince the rest of the World you weren't successful at your day job and certainly not much better at carb building.

All that is needed here 99.9 percent of the time is to very gently flatten the front of the airhorn slightly and let it re-conform to the main casting when you bolt things down.  Even though the two parts pretty much warp at the same rate the LONG BOLTS bolts tend to "hump" up the airhorn just a tad more than the main casting between them and the CENTER FRONT SCREW.

I would flatten the airhorn or I could do it for you, then bolt things back together with a thin OEM type gasket (denser material) so less "wicking". 

Not sure where you got all the parts to date, I'm showing a couple of sales last year and you mentioned getting the N/S from Quadrajet Power.

The correct seat should be .135" and no "windows" in it, I have them in stock. 

Replace the float with a new one if you are still using the original.

Also make sure the divider is in place between the fuel bowl and accl pump well as this reduces "splash" in that direction.

"K" metering rods aren't that great for fuel control with the APT system and you need to make some adjustments there with the larger main jets. 

I'd add a set of my 45C rods, new PP spring for the engine/cam combo, and 3/8"-16 set screw for the airhorn for quick access to the APT for fine tuning.  I would also get one of my accl pumps in there.  It has a lifetime warranty for a reason as the seal is NOT the same as the cheap POS blue ones showing up everywhere else, and those "soft" blue seals will NOT hold up in this new fuel no matter what some uneducated folks lurking around on the Forums will try to lead you to believe.......FWIW.......Cliff

Offline nUcLeArEnVoY

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Re: Another wet airhorn gasket thread...
« Reply #6 on: March 28, 2021, 06:58:14 AM »
Thanks for weighing in, Cliff.

That makes a little sense, but I still don't understand why  mis-fitting between the airhorn and main casting causes the gas to wick. The fuel should never even be reaching that high unless it splashes, especially at my float level. I know that fuel is constantly touching the gasket as part of the idle system, but that's all I can think of.

Interestingly, when I first got the carburetor, it was using a thinner airhorn gasket; but photos I found from at the time showed that it was also wet. Maybe not as much. I wasn't aware that the thinner OEM-style gaskets were made of a denser material. Do you stock those?

If I send it to you, will you need the main casting as well as a reference, or is there just a general amount you can plane off the front of the airhorn that should work?

The seat I'm using is .130 windowed from Quadrajet Power. I thought .130 was the stock seat size. The original part number for the needle and seat is 7035148, and NOS Delco units on eBay do verify that it's solid/non-windowed, so you're right there. I actually HAVE one of your .135 solid seats already from an order last year - can I re-use the metal sealing rings? If so, then I'll drop your .135 solid seat back in after vac-testing it with the needle. I don't drive my car hard, I prefer to cruise in it so honestly I didn't notice any difference in performance between the .135 and .130 seat. Is there a document somewhere that lists all of the factory orifice sizes for my carburetor? That would be like a holy grail for me.

By divider plate, you mean that thin metal strip separating the main fuel reservoir (located right next to the needle and seat) from the accelerator pump bore? Yeah, it's in there.

The float I put in is new. Maybe 3 weeks ago. Nitrophyl late-model type. When I got it, I weighed it and tested it in water. Came in at 6 grams and floated just fine.

1979 Pontiac Trans Am 400/4-Speed

Offline Kenth

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Re: Another wet airhorn gasket thread...
« Reply #7 on: March 28, 2021, 07:32:42 AM »
To flatten the airhorn i don´t like removing material from it, i prefer to use my shop press, add some heat on end needing straightening and some washers under screw holes and press the thing straight.
For inlet seat sizes i compiled this chart, it contains all Rochester inlet seats:

30-1 0.083
30-3 0.086
30-5 0.098
30-6 0.098
30-7 3.098
30-8 0.093
30-9 0.101
30-11 0.125
30-13 0.118
30-14 0.057
30-15 0.084
30-83 0.098
30-89 umbrella 
30-93 umbrella
30-96 0.130         Buick 1967, Cadillac 1967-68, Pontiac 1967-68 OHC6 + V8
30-97 0.135         Buick 1968, Oldsmobile 1967-68
30-98 umbrella
30-99 0.125        Chevrolet BB Late 1967 –68
30-130 0.108      Chevrolet SB Late 1967 -72
30-132 0.120
30-133 0.106
30-134 0.135      Buick BB HP 1970 + Buick BB 1971-Later
30-140 0.125      Buick SB, Chevrolet BB 1969-, Chevrolet SB 1973-Later
30-142 0.135      B BB –1970 (not 1970 HP), Cad 1969-Later, Olds 1969-Later, Pontiac 1969-72 (1972 RA, MT)
30-143 0.090
30-144 0.101
30-145 0.083
30-148 0.135      Pontiac 1972 (not RA, M/T)-Later   
30-150 0.125
30-152 0.120
30-156 0.090
30-157 0.110
30-158 0.116


Offline nUcLeArEnVoY

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Re: Another wet airhorn gasket thread...
« Reply #8 on: March 28, 2021, 07:46:25 AM »
To flatten the airhorn i don´t like removing material from it, i prefer to use my shop press, add some heat on end needing straightening and some washers under screw holes and press the thing straight.
For inlet seat sizes i compiled this chart, it contains all Rochester inlet seats:

30-1 0.083
30-3 0.086
30-5 0.098
30-6 0.098
30-7 3.098
30-8 0.093
30-9 0.101
30-11 0.125
30-13 0.118
30-14 0.057
30-15 0.084
30-83 0.098
30-89 umbrella 
30-93 umbrella
30-96 0.130         Buick 1967, Cadillac 1967-68, Pontiac 1967-68 OHC6 + V8
30-97 0.135         Buick 1968, Oldsmobile 1967-68
30-98 umbrella
30-99 0.125        Chevrolet BB Late 1967 –68
30-130 0.108      Chevrolet SB Late 1967 -72
30-132 0.120
30-133 0.106
30-134 0.135      Buick BB HP 1970 + Buick BB 1971-Later
30-140 0.125      Buick SB, Chevrolet BB 1969-, Chevrolet SB 1973-Later
30-142 0.135      B BB –1970 (not 1970 HP), Cad 1969-Later, Olds 1969-Later, Pontiac 1969-72 (1972 RA, MT)
30-143 0.090
30-144 0.101
30-145 0.083
30-148 0.135      Pontiac 1972 (not RA, M/T)-Later   
30-150 0.125
30-152 0.120
30-156 0.090
30-157 0.110
30-158 0.116

Incredible. How did you come up with that list?? Collecting parts, or is there an actual document somewhere?
1979 Pontiac Trans Am 400/4-Speed

Offline Cliff Ruggles

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Re: Another wet airhorn gasket thread...
« Reply #9 on: March 28, 2021, 10:52:02 AM »
If you need to do anything at all for poor fitting components 99 percent of the time all that is needed is to remove a small amount of material from the corners where the long bolts go thru.  Done correctly it will be just a tad flatter than the main casting under it and re-conform nicely when you tighten things back down.

I have the fixture to flatten airhorns and unwarp castings but rarely use it.  I've found it much less intrusive and no risk of cracking either part using the method described above.

I work on a LOT of very valuable and difficult if not near impossible to replace carburetors.  The best method is always the least intrusive with lowest risk of breaking something.  A 30 ton press and heating up 50 year old metal castings doesn't fall into that deal.

I stock a really nice .015" thicker gasket for the early carbs that is made of an excellent material, and it seals them up most of the time without doing anything at all to either part. 

The good thicker gaskets for the later 4 MAB models were unavailable for quite a while, but I just got some of those in about a week ago. 

Most of the later variety thicker gaskets will wick as the material isn't that great.  I have a lot of them here, but in almost all cases I'm going to make the parts fit better and use the stock high density gasket for those units.

As far as the N/S assembly you should be using a SOLID seat with a .135" hole in it.  They flow equally as well as a windowed seat and will NOT allow the carb to drain back nearly as far after shut down IF you are using the clip on the needle in the first place.

I have found ZERO negatives anyplace using the larger late style solid N/S assembly in any application.

Currently available rebuild kits are showing up mostly with .110" thru .125".  Some are solid but most are windowed.  Occasionally I'll see a .130" seat in a kit or in a carb sent here that was just "rebuilt", but in most cases it will be smaller.

Kind of interesting but at least 80 percent of the time one of the complaints about the "new" build and why it was sent here was LOOSING power at higher RPM's, and guess what?  Without exception every time that complaint was on the list the carb had a small N/S assembly in it.......FWIW.

Anyhow, as a reminder to anyone reading this thread I sell COMPLETE rebuild kits, the most complete in this industry for $45.95.  I will also put together "custom" rebuild kits with tuning parts for any application so it's basically just "plug and play" for the builder/owner.

It will include a high flow N/S assembly and complete accl pump assembly with a lifetime warranty.  The accl pump comes with new spring, stainless return and chrome vanadium spring wire upper for improved performance.  There is a garter spring under the seal to keep it in constant contact with the pump bore and a retainer to keep the pump from pulling apart.  It is currently the very best accl pump available for these carburetors and dating back almost 20 years now ZERO failures and never had to replace one.  IF you want a larger .140", .145" or .149" N/S assembly and/or an HP accl pump just ask and I'll upgrade those parts at no additional cost.

....continued

Offline Cliff Ruggles

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Re: Another wet airhorn gasket thread...
« Reply #10 on: March 28, 2021, 10:52:17 AM »
Also included in the rebuild kit will be a secondary cam/spring which need replaced during a complete/correct rebuild.  Your kit will also come with a new filter/spring, and a new float.  The small parts kit contains a new retainer for the power piston (split collar) so it slides right on w/o much effort.

The gaskets are the highest quality out there.  Those cheap POS black "non-stick" gaskets showing up in other kits are the worst parts ever sold for these carburetors.  I'll bet the Chinese are laughing their arses off selling us that part as they stick so hard to the main body and baseplate it takes about an hour with a razor blade to get them removed!

Rebuild kits can be found here:

https://cliffshighperformance.com/product-category/quadrajet-rebuild-kits

If you are not sure what you need or need to add items to a rebuild kit call the shop.  I stock every part currently available for the Quadrajet carburetor and over 30 parts that have been discontinued and NLA elsewhere.

Anyhow, thanks for listening, back to the shop.......working weekends as I'm pretty busy this time of year........Cliff

Offline lightning boy

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Re: Another wet airhorn gasket thread...
« Reply #11 on: March 29, 2021, 01:34:52 PM »
You test the needle and seat through the fuel inlet on the carb as per Cliff's book.

Offline nUcLeArEnVoY

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Re: Another wet airhorn gasket thread...
« Reply #12 on: April 02, 2021, 11:26:35 PM »
You test the needle and seat through the fuel inlet on the carb as per Cliff's book.

I tested the SEAT seal, yes. As in the metal ring gasket. The test where you fill the bowl with soapy water and inject compressed air into the fuel inlet while applying firm pressure on the needle and checking for bubbles at the seat gasket. Checked out fine.

As for testing the needle and seat, I vacuum tested them OUTSIDE the carb by slipping a vacuum line over the entire seat, inserting the needle and applying vac to see if it holds. It wouldn't hold at low suction, but if I pumped it up to 20" or more, it held fine. Not sure about the conclusivity of that test?

What's his procedure for testing the needle and seat through the fuel inlet?
I did remove the fuel inlet and threaded in a vacuum fitting and tried applying vacuum while the needle was in the seat... wouldn't hold vacuum, but neither did the other three needle and seats I have. Hard to believe all three were bad, so I assumed that it was normal, unless I'm doing the test incorrectly, so definitely please let me know.  ;D

FWIW, unless the soaked airhorn gasket qualifies as a flooding symptom, I'm not getting any signs of flooding. Car will idle all day, no nozzle drip, no overflowing, no issues with hot starts, no black rich exhaust, or anything indicating true flooding. Just the wet airhorn gasket, and it's seeming the consensus is a combination of airhorn warpage and the material my gaskets are made out of.

At the end of this month, I've decided to send Cliff my airhorn so he can flatten the front of it a bit. I don't trust myself to do it. Hopefully that'll help with the problem.
1979 Pontiac Trans Am 400/4-Speed

Offline lightning boy

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Re: Another wet airhorn gasket thread...
« Reply #13 on: April 05, 2021, 05:15:37 AM »
Install a barbed hose fitting into the fuel inlet. (where the gas line threads into). Put a hand vac on the hose fitting, install the N&S assm and see if it will hold a vacuum.
It's common for the casting to be corroded below the flat metal washer. I worked forever trying to fix one before!

Offline bry593

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Re: Another wet airhorn gasket thread...
« Reply #14 on: April 06, 2021, 08:49:42 AM »
I found that applying vacuum on the fuel inlet was not a positive way to evaluate seat leaks.  Instead, dial your air compressor down and verify the pressure using the same gauge you used to measure fuel pressure (9 psi).  Assemble your needle and float and add gasoline to the bowl.  With your low float setting, you should see the bubbles stop when the float rises to about 3/8 from the top of the casting (in my case, this was slightly above the top of the seat).  If bubbles don't cease, peen your seat and try again.

At any rate, if you are not having hot start issues or engine trying to die at a stop light, your float system is probably working just fine.

Another indicator of faulty seat is finding gasoline (wet) inside the hose to the choke pull-off diaphragm.

Most likely, you probably have just a spongy gasket as Cliff mentioned.