Hi All: 1971 Cutlass Supreme convertible. It has an 350 from a 1974 Olds, original had a bad knock and I replaced it many years ago.
The car has always had a very carbon dioxide( I guess) smelly exhaust and I'm in the process of changing out the charcoal cannister but I thought that maybe hotter plugs might make the engine burn more efficiently and reduce the exhaust smell. The original 4 barrel carb is on the engine and has been rebuilt so it runs great - except for the smelly exhaust.
Any suggestions as to if hotter plugs might be a solution ?
Thanks in advance

Joe JVCCutlass

 

If your exhaust is smelly; like unburnt fuel, you need to properly tune your carburetor and timing. Changing the heat range of your spark plugs will have little effect if your car isn't in proper tune.

You can start by removing a spark plug. Is it sooty and carbon fouled? That is an indicator that your car is running too rich.

BTW, what you're smelling is unburnt HydroCarbons.

 

X2.to the above from Olds64, going to add do a compression test while changing/checking the plugs.

 

Joe,

Your carburetor rebuilder cleaned and reset everything to factory specs. That's what makes your car run well.

But the rebuilder can't set the idle mixture. He merely returned it with a generic setting.

The mixture adjustment at idle depends on your individual engine and it's up to you to set that. If it's too rich, your exhaust will stink.

But it's easy to adjust the idle mixture.

1. Turn the idle mixture screws in so they lightly touch their closed position.
2. Open both screws two full turns.
3. Be sure the engine is warmed up, the choke is off, and the idle speed is correct.
4. Close both screws 1/8th of a turn. You will be repeating this 1/8th turn process in the steps below.
5. Pause after each of the 1/8th turns to listen for a tiny drop in idle speed. (You can use a portable tachometer if you have one.)
5. Continue with the 1/8th turns until you hear the tiny idle speed drop.
6. Stop there and smell the exhaust to see if you have eliminated most of the smell. It won't all go away.
7. If you want less smell, you can try another 1/8th turn in. But if your idle gets unstable or if there's any slight bog when barely cracking the throttle open, you will need to return to your previous setting.

There's variation on this where you use a vacuum gauge to detect the idle drop. But if you are careful, the numbered steps will get you there.

Good luck.

 

You really need a tach to set the idle speed.

 

Charcoal canister is for fuel vapor not exhaust. You need more timing. It seems like folks are always needing more timing. The earlier spark allows for more time for the crappy fuel to get thoroughly burned off. I like idle timing at 23 plus with todays fuel. Hoskinson runs 36 degrees locked out most of time, sometimes more. Really cleans it up.

Get an understanding of how much vacuum advance you have and adjust the base accordingly. You're actually better off with more idle timing and restricting the advance.

 

She can run great but stink when the air fuel ratio is off. I know first hand because I have dealt with this issue in my car.

VC455s post covers it like a wet blanket.

 

do a air/fuel mixture test and go from there..

 

First of all, carbon dioxide is an odorless, colorless gas, so you're not smelling that. Carbon monoxide is the same way and is much more deadly.

Second, I always thought exhaust was SUPPOSED to smell. What, exactly, does it smell like? And who stands at the back of the car while the engine is running and inhales the exhaust?

 

I definitely agree, these Olds V8, low compression like this one, like a lot of part throttle timing. Honestly set it for 38 to 40 revved up to 3000 to 3500 or so for maximum advance without the vacuum advance hooked up. You can still run 20+ degrees vacuum advance off the manifold. The mixture screws may be improperly adjusted. Adjust for highest vacuum and or rpm. Old cars stink without cats. I added cats to my 70 to get O2 bungs for a wideband and not stink me out.

 

A quick check, pull a vacuum line that's sucking at idle (but doesn't affect distributor timing). If the idle goes up you're probably rich, if idle goes down you may be lean. I remember Cliff Ruggles saying the rich smell can be caused by a LEAN mixture! This was contrary to most theories but he explained it in a way that made sense.

 

Thanks to all for the really good advice and especially the step by step instructions.
I have all testing equipment that is needed for a better tune-up.

This is a great resource !
JVCCutlass Joe iin NJ

 

Let us know how she runs once you get her in tune.