1985 GT complete factory stock exhaust

[mooncrickett]

Greetings all,
The exhaust on my GT is factory stock front to rear. However I believe the cat is shot, when at an idle the exhaust smells like sulfur and rotten eggs, plus at idle it makes a gurgling sound kinda like its bubbling. I feel like the exhaust is not flowing properly and possibly clogged or restricted.
Anyhow I am trying to decide on a few different routes.
Complete exhaust overhaul from uneven shorties, H pipe, flow master etc
Keep stock and just replace cat.
Keep stock and just cut the cat out, replace with straight pipe in its stead.

Car has 50k miles all original. thoughts?

[grabbergreen84]

It really depends what you want to do. If the car is really original, I'd maybe try to bolt another converter in there. Hard to argue with new duals though.

But try to not cut off the old system, as someone might buy it if it's otherwise good.

[art5059]

What ever you do, don't ignore this. I didn't realize my converter was plugged and parked the car outside running while i cleaned the garage. I looked out later and saw smoke pouring out of the interior. Almost set the car on fire; as it was there were more than a few bits melted or scorched.

It's not hard to gut the converter. I pulled the pipe off exiting the converter and rammed a stick in the end to break up the core. I don't recall it being difficult to clean the chunks out, but it has been twelve years.

Just a bit of advice from one who's been there.

Best Regards,
Art

[JACook]

If you're smelling sulfur, the catalyst is still working, but it's probably not going to stay working for long,
because the engine is running very rich. If you still have the stock carb, most likely the accelerator pump
transfer tube o-rings are gone.

If your converter is still functional PLEASE do not gut it. There are people here that would love to have
an original one. Get yerself a piece of straight pipe instead. Or better yet, fix the problem that's causing
it to overheat and leave it be.

[mooncrickett]

JA, if I do go straight pipe or new x pipe with high flow cats, how do I work out the air pump tube?

[grabbergreen84]

Most aftermarket H-pipes have an inlet for the air tube. You can connect it with hi-temp hose.

[mooncrickett]

I have an exhaust shop here that will install a magnaflow cat and weld the airtube into the pipe right behind the cat. Does this sound like a good idea?

[Haystack]

Get a $10 infrared laser from harbor freight and measure exhaust temps before and after the cat. If it is much higher before the cat is probably shot. You can also try drilling a 1/4" hole before the cat in the exhaust and see if temps drop way down or not, which would also verify it being restricted or clogged.

[JACook]

As I mentioned earlier, if your exhaust smells like rotten eggs, it is actually proof that the catalyst is working,
but is probably having to deal with more unburned fuel in the exhaust than it should. The excess fuel is what
causes the catalyst to heat up enough to produce the hydrogen sulfide.

Some of this is normal, such as when you come to a stop after a hard run up through the gears. But if you're
smelling the sulfur when you've been driving easy, that's not normal, and you need to address the problem
before it overheats the catalyst enough to damage it.

And a new catalyst will -not- fix the stink. In fact, a new catalyst will probably produce more of the rotten egg
smell than the one you have now. A new catalyst will run much hotter than an old catalyst, if the fuel mixtures
are too rich, because the new catalyst is more efficient at converting the unburned fuel to heat.

Removing the catalyst will certainly make the rotten egg smell go away, but your exhaust is still going to smell
bad, it's just going to be a different bad smell. You need to address the problem at it's source.

[mooncrickett]

JA,
I hear ya, The o rings on the transfer tubes are good, matter in fact the carb is freshly re-built. I actually dialed in the air/ fuel screws yesterday( by ear). The only thing left is to adjust the choke, it revs 2500- 3000 at cold start and runs rough until warm. The idle is also high, sitting around 1200 under normal conditions. With that said , the exhaust sound is real funny at idle, kinda like pulsating or gurgling. Add in the funky odors is which leaves me to believe there is blockage in the exhaust .

[2nd chance cobra]

Please listen to ja. A second thing on your "plugged" cat, if it's plugged your engine will not run up the rpm range. The engine under w.o.t will likely stop review at 3000rpm, or so, and lack power severly. Also it will also backfire back through the carb. Have someone knowledgeable look at the carb, it doesn't sound right to me, but I only worked for the factory for 12 years

[homer302]

Wow, so many replies, LOL. So the easiest (and FREE) way to see if your cat is clogged will take about 10 minutes. Get the car in the air and loosen the two bolts at the end of the Y pipe and in front of the converter. You don't have to take them off, just back them off enough to slide the connector back and let the exhaust out in FRONT of the cat before it has to enter. If all problems are solved, the cat is clogged, it not, it's not the cat. Obviously the pre-cat on one side is not causing this issue so I assume you mean the only REAL catalytic converter on the car.
I have seen one of these clogged and it was as red as a fire truck and even heating up the carpet inside the car. We had to crawl under and do exactly what I said above and we were able to drive it home just fine by letting the exhaust exit in front of the cat. It sounded like a tractor but it got us home for free.

[Walking-Tall]

Whereabouts are those mixture screws you dialed in? How many turns out? Same number of turns out side-to-side? They actually meter in fuel mixed with idle bleed air, to mix with the air coming in past the throttle plates at idle. Yeah, a 2500-3000rpm choke's fast idle speed is far too high. More like 1500rpm is what it should be. If the curb idle is too high (1200rpm most definitely is far too high, assuming a stock or mild camshaft) I'd guess those idle mixture screws probably had little to no affect on the idle quality. A too-high idle speed setting takes the idle system and those screws out of the loop, and causes fuel flow that's normally reserved for off idle, from the transfer slots, which would be all kinds of reason the engine's probably running rich regarding your rotten egg smell references above. A blubbery sounding exhaust is classic over-rich conditions territory. Check your spark plugs, they're likely good and black too.

Are the float levels set? Pending the answer to that, the engine-up-to-temperature curb idle needs to be adjusted to a much lower idle speed, to ~750rpm in neutral, assuming a manual transmission. This will allow the carburetor to only actually then meter air-fuel from the idle system by the mixture screw adjustments, instead of the transfer slots (or maybe even the mains), and a more pronounced difference should be had with adjusting the idle mixture screws for smoothest idle. The smelling of rotten eggs should then be minimized or over.

[homer302]

Alright, so I am guilty of reading the original question and then responding. I often find most replies are useless and I don't read them normally, LOL. However, in this case, I don't believe that is true. So you "rebuilt" the carb and set the adjustments "By ear" according to your account and now you magically feel the cat has somehow gone bad? I am going to call BS on that and say please take your carb to someone that actually knows what they are doing. I doubt very seriously that the cat became clogged at EXACTLY the instant you rebuilt the carb and started noticing fumes and poor idle.

[TruperTim]

This entire post just answered my question I was here searching for...! My stock 85' cat gets so hot it turns red orange... Smoke billows from all over it. I didnt know what the problem was... My car will be a show only car that cant be driven on the roads, i want stock appearance. If I gut the cat, it wont heat up like it currently is? How do i properly gut it? I dont need it to drive from trailer to show spots...

[Walking-Tall]

This entire post just answered my question I was here searching for...! My stock 85' cat gets so hot it turns red orange... Smoke billows from all over it. I didnt know what the problem was... My car will be a show only car that cant be driven on the roads, i want stock appearance. If I gut the cat, it wont heat up like it currently is? How do i properly gut it? I dont need it to drive from trailer to show spots...

Why not fix the fuel swilling, like suggested above? Maybe then you won't have to provide gas masks for spectators... lol

[TruperTim]

Walking Tall, would fixing the fuel swilling rectify the molten glowing cat?

[Walking-Tall]

Walking Tall, would fixing the fuel swilling rectify the molten glowing cat?

People that know better than I do all about this condition and subject said exactly that above, so I would say so, yes.

[TruperTim]

I didnt get that same read out of the thread - but thanks Walking Tall... It is obvious you are well versed about autmotive mechanics and I am not. Ill leave your threads alone.

[grabbergreen84]

Too much or or maybe even any unburnt fuel will heat up and clog up the cat.

Get your carburetor working properly, and then try running it normally if the cat isn't totally clogged, and see if it will clear out with the more optimal mixture of
exhaust gases. Then try to run only ethanol-free gas. Ethanol EATS carbs.

I suppose only running the engine when cold, and never allowing it to warm up, will also expose the cat to more unburnt fuel, because of the choke enrichment. Over a long period of time, that might harm the cat, even if the carb is OK.

[erratic50]

What was stated is every single classic rich mixture condition IMO.

As someone that was once a punk kid and LOVED to throw too big of a carb on just about anything and always made them richer when I rebuilt them, etc, looking for the little value up top in the R's I totally agree. Now go read the plugs!! You'll see carbon.

As opposed to the other source of high idle - overly advanced base timing which often has associated started stopped hard starts and white plug tips.

[erratic50]

Oh and before I forget - if it only has 50K original miles do yourself a big favor and drive it in nice weather and enjoy it for pity's sake. Otherwise, why own it?!

[gr79]

Wrong engine timing can cause the bubbling sound. And the rpm issues.
Check for vac leaks. A hand held vac gauge is very useful for tuning.
So useful, i always include one in my in car gauge packages.

Carbs and engine timing, both ign and cam, go hand in hand. Basic tuning.

Happened to me when i changed out the timing belt on my 86 2.3 Ranger.
Vaguely remember went around the block to test, then came back, noticed exh manifold was orange, and adjusted prob the dist.
Did not take long to correct.
Another time when replacing the plug wires, somehow were not in the correct place on the dist cap.
Correct firing order, but #1 was not indexed with #1 terminal. 180 out.
Timing and dist position was way off due to that.

Watch out parking in long dry grass or the like at car shows if that cat is real hot.

[vintageracer]

How do you "Gut" the cat?

Long screw driver or other long rod stuck down the throat of the car to break up the honeycomb inside the cat. A little banging on the side helps also. Then pour the broken honeycomb out the ends.

You still look totally stock with nothing inside the cats that will pass the visual part of the air inspection and in my case has always passed the emissions test portion of the "Sniffer" test with a pint of rubbing alcohol added to the gas before the test.

[JACook]

Given that good original cats are hard to come by, I would certainly hope no one would ever do ^^^ that to an original
factory catalyst that had not already gutted itself.

Beyond that, it solves nothing to gut a catalyst that is overheating because of a pig rich fuel mixture, and/or an air
injection system that is not switching between upstream and downstream air. It only creates another problem where
one didn't exist before.

An engine that makes catalysts glow will most assuredly not pass the sniffer.