gassy odors

[Chas Henderson #28127]

My 71 sb seems to have a gassy odor coming from the air openings in the dual air snorkel. After I drive the the car and park it in the garage, I have to leave the door up for a while so the garage doesn't smell too bad. I have checked all the fuel lines and the fuel pump doesn't leak nor the gas tank. I have it down to the carb. And it isn't leaking where the fuel line goes in to the carb. As I said, after about thirty to forty minutes the strong smell of gas is gone, but you can still smell it from the snorkel. The carb was rebuilt about three years ago, but I don't really know by who because I bought it from a major corvette supplier and they had it rebuilt for me, but I am a little suspect on his abilities. The nightmare story behind all the problems I had getting it would make it seem that I bought it from the "Three Stooges". And that would be demeaning them. I know that the car will get the hottest after you turn the motor off and it would vaporize the remaining fuel in the carb, but I can't think that much smell is normal. Is it? It seems to run o.k. I have a 3:36 rearend and it is a auto, the original motor was rebuilt three years ago and gets 13.5 to 14 mpg a 65 - 70 mph. Is that normal? The carb is properly torked down on the intake and it has the proper mounting gasket. All the C.E.C. and pollution equipment is hooked up and working. That leaves me to think I need a new rebuild on the carb. I know this is long but before I send it off, I thought I would try to give you all the most info I can.
Thanks for anyhelp you can give me,

Chas

[Joe L.]

Re: gassy odors

Charles-----

What you are describing is very common for 50s, 60s, and early 70s cars. Folks today aren't used to it since evaporative emissions controls have come a long way and just about eliminate such vapors on modern cars. Your 1971 will have an EEC (evaporative emissions control)system. However, keep in mind that this was among the very earliest of such systems and it's nowhere near as efficient or effective as more modern systems. In fact, 1971 model year cars were the first year to have the system nationwide. 1970 model year California-delivered cars were the very first.

Nothwithstanding the above, I would suggest the following:

1) Check the carb soft plugs in the bottom of the float bowl assembly. To do this, you will have to remove the carb from the engine and remove the throttle body assembly. There will be 4 soft plugs in the bottom center of the float bowl; 2 large which go straight down and 2 smaller which are angled. These are notorious leak sources and almost invariably leak to some degree. After shutdown, leakage can be responsible for a LOT of vapors in your garage. To seal these permanently, clean them assiduously and seal them with an epoxy like JB Weld;

2) Check the charcoal cannister in your EEC system. The system sometimes fails and gasoline accumulates in the bottom of the cannister. This renders the system completely inoperative. The charcoal cannisters also just wear out over time. A replacement for your car is available under GM #17063011 which GM lists for about 70 bucks

[Chas Henderson #28127]

Re: gassy odors

joe,
Thanks for the advise. I will try and take it apart this week. I have also already purchased a charcole canister and the replacement filter pad. Just haven't put it on yet. Isn't J.B. weld a two part epoxy for metal?

[Joe L.]

Re: gassy odors

Charles-----

Yes, JB Weld is a two part epoxy which is primarily used for metal, but may be used for just about anything. It's the best stuff I've ever found and available just about everywhere.

[Joe L.]

Addendum

By the way, epoxies, due to their long-chain molecular structure and other factors, are completely impervious to gasoline and other organic solvents. Sometimes, folks think that epoxies will be "attacked" by gasoline. They won't be, though.

[Jack H.]

Add-on

Great input(s) from Joe! Only a few additional words....

The EEC portion of the '71 emission system has it's own 'quirks'. The charcoal cannister is intended to be a temporary storage vehicle for fuel tank vapors. It joins the tank (lines along the LH chassis rail) to the carb (rubber hoses to carb and PCV). Stray vapors created by thermal soak on engine shut down are intended to flow to and be trapped in the charcoal cannister, then burned on the next engine start when the carb starts to draw vac and can suck the vapors out of the cannister.

There is a liquid separator mechanism in the fuel tank to thwart raw gas from flowing to the cannister. It's a 'first generation' design since this system emerged only 1-year before your car was built (1970 NA9 California Emission component). If you're in the habit of 'topping up' when you fill the gas tank, you're 'tickling' the integrity of the tank's separator....

If you REALLY top up the tank (MAX it), you can expect to see droplets of raw gas beneath the charcoal cannister indicating you 'abused' the system by overfilling the gas tank and denying it sufficient vapor expansion space. If this is the cause of your odor problem, the fix is easy. See if odors reduce significantly with lower fuel levels in the tank and, if so, learn where to stop when you fuel up.

Last, the cloth filter at the bottom of the charcoal cannister Joe mentioned was intended to be replaced frequently. Most guys drive 'em into OBLIVION and when she's plugged, the system has a hard time purging itself when the engine starts and begins to pull vac. It's a cheapie item from the Fram shelf at your local Wal-Mart and once you do the R&R (drop LH splash shield), you'll find it's no big deal to change. Should look clean & white on the bottom exposed surface. If she's black as coal, that's a part of your problem....

[Patrick H.]

Re: Add-on

In addition, the hoses for the entire system have to be present for any of Jack's description to be working. They run from the carb to the canister on the left hand side of the motor. I know that seems obvious, but I've seen plenty of cars with those hoses cut off or otherwise absent and removed.

Patrick

[Joe L.]

Re: Add-on

Jack-----

It's real hard to overfill a 70 with EEC or any 71-74. The fuel filler neck was redesigned for EEC applications with an extended filling tube. Once the fuel reaches the bottom of the filling tube, it's hard to add any more and this ensures that air space will be present in the tank. This filling tube extension is the reason that the fuel capicity was reduced from 20 gallons to 18 gallons for 71-74 Corvettes.

[Chas Henderson #28127]

Re: Add-on

Guys, I do have a tendency to fill her up all the way and then burp the air out and I fill up to the bottom of the neck. All the hoses are correct. She has been TF twice and I keep her tuned up to specs. I have changed the little filter once or twice before. I'll try to leave some room in the tank for some more air. Any idea what type of mileage she should be getting? Auto, 3:36 rear, driven easily?

[Jack H.]

Re: Add-on

Joe and I had some off-line chat on the 'top up' issue. This MAY NOT be the problem, but humor me a tad (costs nothing). Intentionally leave 'er down about 1/2-inch from the bottom of the fill neck and see if your odor problem doesn't go WAY down and/or disappear altoghether....

On milage, your engine configuration is pretty close to that on my '71 (also have 3.36 rear but with manual trans). Fuel efficiency will vary with driver and habits, but compare to my averages of 13-15 MPG around town and 16-18 on the highway with 'reasonable' speed due diligence. I expect it's easy to deteriorate by 30% based on how heavy one's foot is....

[Joe L.]

Re: Add-on

Charles-----

If you fill the tank ONLY to the bottom the filler neck (or, better yet, 1/2" below as Jack suggests), then you should have no problem, at all, with overfilling. The 71+ filler tube was extended deeper into the tank for this reason. When you fill to the bottom of the tube ONLY, there is still about 2 gallons worth of of air space in the top of the tank (or, perhaps, somewhat less depending on the angle of the car when the tank is filled). Overfilling occurs if you attempt to keep adding gas AFTER the level reaches the bottom of the filler tube. It can be done since the fuel in the tank will reach whatever level that the filler tube is filled to. So, if you keep slowly adding fuel after it gets to the bottom of the filler tube, you can continue to increase the fuel level in the tank. It's a very slow process and I doubt that you actually go this far due to the time involved. Also, trying to add fuel at this point usually results in "splash-out" that gets on the rear deck of the car.

[Craig S.]

Re: Addendum

JB Weld is kind of like the liquid version of duct tape...a great fix all! However, I tried the 5 minute version once but didn't care for it, the regular version is my norm....Craig

[Joe L.]

Re: Addendum

Craig-----

Duct tape is a very temporary fix. JB Weld is quite permanent. Also, the fast-curing version of any epoxy is not as good as the standard cure product. If it were, there would likely be no standard version.

[Craig S.]

Re: Addendum

Thx Joe....you are right, JB weld is a permanent fix. The duct tape comment was my attempt at sarcasm. I have used JB weld for thread repair in aluminum intake manifolds in the 1/2" NPT fittings with great success, even after tapping the threads. And as you say, gasoline impervious...great for the well plugs on Rochesters, have even seen it used on Holleys around the accelerator pump diaphram plate but that would not be my choice on usage...I usually buy the industrial size tubes that are about half the size of the large toothpaste tubes for each part. Great stuff!...Craig