I'm having some trouble with slight engine misfiring with my 75 L-48. The car has no smog stuff or cats and aside from recently adding a 6A MSD box, no other alterations. The misfiring was present prior to MSD installation - msd has made an improvement but misfiring is hindering gains I'm sure. The car shows no vacuum leaks, plugs are new & good - gap set to .060 spec for 75, wires all high quality and new, new distributer cap, rotor & coil all new too. Engine has excellent compression at 9.0 Original Quadrajet carb was recently rebuilt and car is tuned well. What else could be causing this misfiring? I don't get any backfire, but it's very pronounced, almost bogging down at around 1500-1600 rpm. It's present at idle too. Any suggestions as to cause ? All replies are greatly appreciated !

You may have some carb problems and a lean condition. Could also be a PCV system problem. Or an intake leaking. The power brake booster could be leaking a bit of vacuum as well. Add a bit of carb cleaner or starting fluid and see if the engine smooths out or speeds up. If so, it's a lean or vacuum problem. Just a few things to check out.

Ken Styer

Thanks Ken, I've been thinking it's probably a carb issue too. I'll try your suggestion using the starting fuid or carb cleaner first. Thanks for your input on this one !

Mel Lenet

Groveboy- shoot a bit of carb cleaner at the aluminum base of the carb, where the throttle shaft goes into the base. Sometimes, judging on how much wear is on a carb, after you clean and rebuild a Q-jet, you develop a serious vacuum leak at this spot.
They do sell the base individually, and they also sell a bushing kit, which is a heck of a lot cheaper, but the carb needs to come apart again.

This happened to my 80 Caprice also, the carb. was fine, but I just had to mess with it, and rebuilt it. Now, a new set of bushings is in the base, and she runs fine. I was experiencing the same problem as you.

Good luck.

I've seen even new wires that were not worth a crap. Sometimes the factory crimping can break a spark plub wire core internally where you can't see it. I always check my wires with a volt meter, even brand new ones, before I install them.

Sounds like you may have a spark arc somewhere causing the misfire. I would Pop the hood at night and see if you can see any sparks arcing. I would check the ohm resistance on the wire with a volt meter.

I had similar problems with my 79 and After Shark hit it right on the head. spark plug wire arching. I have a hard time keeping them away from the exhaust manifold.

Thanks again to all for the feedback - it's appreciated
and when I finally figure it out I'll post the solution (hopfully soon)

Well, it turn out that the problem was carb related after all. Turns out I had a very lean condition (no smog stuff, no cats, larger true dual exhaust, aftermarket air intake) and when the carb was rebuilt it was set back to factory specs. Of course that meant as it was originally - WITH the air pump, cats, etc. By exchanging the metering rods inside the carb for thinner ones this compensated by allowing slightly more fuel and has solved the problem !! The car now runs great - as it should - no missing, just a happy nice low hum! Thanks again to all who posted replies and happy motoring

Good to hear you figured it out...it just goes along with my mantra about cars...

"It's always something stupid-if it ain't-it's $1.000.00"

Now go and smoke the tires off of that baby.