What did I do now?!?!

[Dave Z]

I'll try to lay this out the best I can, apologies for the lengthy description...
Last weekend I started my '75 and man did it sound weird. Out of the blue, it started running real rough, and didn't have the (stock only) throaty sound to the exhaust. In fact, the exhaust was doing more of a puffing with a pffft, pffftt, pffft noise. I took it out and it seemd to run just fine, plenty of power, just rough idle and sounding like crap. At first I thought exhaust leak, but I figured that carb had also gotten out of whack so I started playing with the idle and rich/lean mixture. Nothing helped. I did notice that the catalytic converter wasn't making it's usual clang clang, so I figured it might be pieces of the honeycomb may be clogging up the exhaust pipes. Since I am replacing the exhaust in the upcoming weeks, I figured I'd just bang the hell out of the cat and sure enough, two large chunks of honeycomb came flying out of the tail pipe. After that, it sounded normal (clanging was back too) and idled smooth again (Heimlich manuever for cars:L ). I adjusted the carb idle back 750 and thought I got the lean adjustment back to normal. However; now the car has literally no power, I mean a real slug here. Off the line there is slight hesitation but it will launch ok, but when I'm get to around 35 and over then nail it, it takes it's dear sweet time to go another 10MPH faster. The tach winds up to 3500+ and the motor's reving, but just no quickness. I can't believe my trans is the cause, simply because even when it was clogged, it still had power in those ranges. (Side note...my kickdown switch has never worked) It all started when I started monkeying with the carb. and the cat. Any ideas on what I did, or overlooked?

 

[wishuwerehere82]

If you played with the carb adjustments after it had power and now it doesn't, I think you better go back and do them again. Look for a vacuum line you may have knocked off inadvertently too.Step1) Adjust timing with vacuum advance disconnected.
Step 2) Adjust idle speed with the idle stop screw on the throttle linkage.
Step 3) Adjust idle mixture with the two mixture screws on the front of the carb. Use a vacuum gauge or tachometer to get the highest Vacuum or RPM, adjusting one screw at a time and rechecking again.
Step 4) Reset idle speed with the idle screw on the throttle linkage.
Step 5) Road test.
Step 6) BEER.

I have seen a few posts where people have adjusted their idle speed with the mixture screws. That's not the way to do it. It will result in a bad mixture all over the power range.

 

[jdp6000]

Sounds like a plugged CAT to me. Very common in newer cars.

Jim

 

[wishuwerehere82]

Time to gut the cat!:L

 

[1981corvette]

It's the CAT. Buddy had 2 motors meltdown due to plugged cats.
Same symptoms as yours but would not listen to others.
Nuclear Physisissisisttt. (can't spell)

 

[Dave Z]

Everyone's experience shines again. Adjsuted everything on the carb back again, then took the hammer back to the cat and started swinging...sure enough, more chunks-o-cat gut came flying out. Car runs like it should now, and no more clang clang from under the car. Ordering the exhaust this week. Hope to have it installed by months end. Thanks again everyone.

 

[goertz]

FYI for future reference, playing with the idle mixture screws at the base of the carb will have NO effect on anything the instant you open the throttle. that's why they're called IDLE mixture screws. they just adjust the mixture at idle, as soon as the throttle plate opens, the idle circuit is bypassed. glad you worked it out with the cat though. best idea is to eliminate it, or whack it open, take all the crap out, and weld her shut. i'm not convinced cats to a whole hell of a lot smog wise. I had a 1993 Chevy dually, 454. I cut the cat off it, and ran a single chamber flowmaster. when smog check time came, it ran just as clean without it. was totally within specs. the smog technician was too lazy to look under the truck to see if it was even there.