78 L-48, stock except true duals.

Sick & tired of smelling rich exhaust fumes & stinking up my clothes. Have been through everything, quadrajet rebuilt, exhaust gaskets replaced, confirmed no leaks at welded joints. Also had fuel smell (now gone) after dropping tank, replacing hoses, new fuel pump hoses & new vapor canister.

Standing at the back of the car while idling gasses me out. Will putting a Cat back in solve this problem? Really don't want to go back to stock exhaust but will if it will put an end to this stink.

 

Even though you've gone through everything, something is still out of adjustment. If it's that rich, I'd look at the carb again. A stock exhaust won't fix it.

 

Your engine will run rich during the first few seconds after a cold start if your choke is operating like it's supposed to. Once your choke butterfly is fully open the air/fuel mixture should be correct for your engine's needs. If the butterfly stays closed that will make it rich too.

Check the carb with the air filter off and running to see if there is a lot of fuel dumping, could be the float is set to high. Also, is the air filter too restrictive? Try running without the air filter and see if it runs rich.

The mixture screws may be turned out too much causing the air/fuel mixture to be too rich.

When are you detecting a "rich" smell? When it's cold or when it's hot? If it is running rich standing behind your car will make your eyes water profusely and you will often see a visible black smoke haze around the rear of your car.

Late ignition timing will also cause a rich condition as a lot of the fuel isn't getting burned so make sure your initial timing is set correctly with the vacuum advance disconnected.

Also check your gas cap to see if it is sealing good.

 

Still sounds like a carb problem. At this point in Q-jet history, there have been so many rebuilds and parts swaps to make one good carb out of 3 or 4 who knows what's supposed to be there for jets and metering rods. Maybe send an email to Lars and see if he could give you some guidance. If it's running rich, I don't think adding a cat would help

 

If the smell is that bad the carb is way out of adjustment or the engine is badly worn with lots of incomplete combustion.

My rebuilt/upgraded (2014) L-82 355 with AFR heads, roller cam, and 10.2:1 compression with a Holley 4175 650 CFM vacuum secondary Q jet replacement carb on the car since 1985 (rebuilt by me 2X...very easy) has virtually no gas fume's smell once warmed up...very slight!

I definitely would not add a cat to the OEM type 2-1-2 exhaust if you go that route. Add 2 high flow small cats to the true duals you have now up to as close to the front of the engine as possible (you want the exhaust gases to be hottest).

 

Smell is cold or hot. Had adjusted the choke totally open cold to & has been that way for a while.

 

Are you the original owner, so you know that a longer duration cam was not added in the last 40 yrs? (That allows unburned gases to leave the cylinder)
Wrong heat range on the plugs?
Weak spark due to ignitions issues?

 

if you are trying to be factory correct ,work on the carb and check timing.harmonic damper could have moved giving you a false timing reading.if you don't care about factory correct I would put on a sniper fuel injection with a o2 sensor.Take a good look at the plugs.

 

Thanks for all the replies, I'm the third owner and don't believe the engine has ever been modified (52,000 miles). Will look into the heat range on the plugs, hadn't thought of checking them. Have played with the timing but made no change in the smell.
Thanks again for the ideas,

 

It's possible someone long ago tried rejetting the carb, going richer. Factory jetting was very lean. You should pull the primary jets and see what you have.

 

I think AirBus hit on something here. It was somewhat common practice to tamper with carbs back in the 70s. Remember the tamper resistant plastic caps on the idle mixture screws? Pair of pliers and snap, off they came. Bubbas would drill out shooters on Holleys and jets on others instead of buying the proper sizes. Only a fine drill bit set and a jet chart size will determine what someone else did years ago.

 

So the choke is is wired open? How does it run cold? If it seems to run ok cold with a wide open choke, the carb is way rich.

 

I can walk through a Corvette outdoor show and smell the difference in exhaust fumes between my 1978 L48 and older vettes without exhaust system emission control devices. I can tell a 1978 without a cat converter and those with the original cat converter. What you are smelling are carbon monoxide (like cigarette smoke), hydrocarbons and nitrides of oxide. I had a neighbor who was successful in committing suicide by sitting in his car, in the garage with a hose stuck in the rear pipe and pump exhaust fumes into his car with the windows stuffed around the hose. This **** kills!

The purpose of the converter is to change hydrocarbons in the exhaust to water vapor or carbon dioxide. When contact is made with a catalyst, a chemical reaction takes place that weakens the molecular bond of the polluting chemicals and allows them to easily convert into the more desirable byproducts of combustion: H20 (water), CO2 (carbon dioxide) and N2 (nitrogen).

The 1978 used a dual-bed converter to meet 1975 emission standards for HC, and CO by oxidizing and reducing emissions. Uses catalyst material to oxidize and reduce HC and CO to H2O and CO2.

Emission control devices targeted exhaust emissions (briefly described above), fuel vapor emissions (others described here) and crankcase vapors. The latter are managed through PCV (positive crankcase valve), A.I.R. (in 1978 only L82 equipped), EGR and various other devices. The point is emission control works through various systems to include a carb and its setting as described above. So all work together...so, what is the carb part number installed on your vette?

Hope this helps!

 

Carbon monoxide is odorless which is what makes it so dangerous. What he's smelling is probably unburnt fuel. Either a carb that is running rich or a weak ignition system with misfires.

Even without emissions controls a well-tuned engine with a short duration cam will have very little smell. But without a catalytic converter there's still carbon monoxide to be wary of.

Notice that you can start up a modern car with a cold engine and there's almost no smell even though the cat doesn't work until there's some heat in it. But even a modern car can hurt or kill you with carbon monoxide poisoning if it's started cold in a closed garage!

 

We have got so used to modern Fuel Injected Cars that any old Carbed car seems to run so rich it gases one out. Hard to believe that all cars ran this way. If the car is really running rich a catalytic converter will burn up in no time. What I would consider is having it dyno tuned with a air fuel meter in the exhaust and find out whats really going on. The last shop I had my car on a dyno the operator was younger than my car by quit a bit. He was amazed that the distributor could be twisted and a crude timing light used to adjust it. At least I had a FI system ( Accel Gen 7 ) ( windows XP ) which he could re late to. So You may want to call around and make sure they understand what a "jet " does.

 

This is true.

Make no mistake about it though, the late 70's/early 80's car engines even with cats, computer controlled carbs, lean idle screw limiters, EGR, retarded ignition timing, AIR pumps etc STILL were very dirty...due to a multitude of issues that included carbs that simply cannot meter fuel precisely like port fuel injection. GM spent 20 years trying to refine the Qjet to precisely meter fuel and finally gave up the ghost in the 80's...bad idea.

Shockingly, my 78 L-82 4 speed stock from the factory saddled with every emissions piece available at that time only had to pass a hydrocarbon limit of 300 PPM...that is VERY high by today's standards and extremely dirty! My 1994 5.0 V8 mustang GT with fuel injection and full emissions (EGR disabled-BTW-operates at all throttle off idle on the mustang) was tested this year with hydrocarbons of 13 PPM at idle and 2,500 RPM!!! That's right 13 PPM!!!! That is unbelievably clean!!! and a 1994 high performance V8.

Many vehicles in the late 70's/early 80's actually could pass the federal emissions standards with no cat in the exhaust since the standards were so high and if you leaned the mixture enough and retarded the timing enough the car would pass the sniffer...I did it on multiple cars at that time...NO CAT!

My 78 L-82 355 rebuilt in 2014 with 10.2:1 compression, AFR heads, and 219/225 roller cam (.525/.525 lift) has very little exhaust smell once warmed up to 180 temp on the gauge...you can smell it but not bad at all compared to the OEM L-82 exhaust that would kill you before the rebuild with zero emissions hardware.

With all that said, modern car's today actually do NOT put out any measureable amounts of carbon monoxide, even when cold 30 seconds to 1 minute of running and the cats heat up VERY quickly to eliminate the CO.... .02% (not 2% but .02%) on the 94 mustang when warmed up after 30 seconds for the cats to heat up. My 10 C6Z06 puts out 0% carbon monoxide. All cars today emit CO2 carbon dioxide and water as the primary pollutant..if you can call C02 a pollutant since we all exhale C02.

The danger from autos is that C02 can suffocate a person quickly if there is not enough oxygen in the air..think C02 fire extinguishers......

 

Be aware that if your motor is running so rich due to a problem, cats will have issues too. It needs to be running properly or the cats will be working too hard. This will wear them out in short order and in some cases with too much unburned fuel entering the cats they will get red hot-leading to problems like fire.

Get the issue sorted, then if it is still too much for you add cats, not before.

I think the posters talking about carb or cam mods may be on to something, if the car is properly tuned now, otherwise you have some sort of issue.