I had a friend come over a couple weeks back, truck not passing emissions.


The carb was a carter afb performance series 750 cfm. The prior owner had the largest jets available on that carb on it.


We went and bought an edelbrock aftermarket kit that had different needle and jet sizes, got the jets and needles swapped out. The jets were 6 sizes smaller than the ones in there, I want to say they were the 403's to begin with.


Still failed, my friend still had needle and seat for this carb in his tool box from when he was 16 and he seems baffled at the failure. It's failing at idle.


We're swapping out the harmonic balancer tomorrow, somehow it's bad. Could that be the reason for timing being off? I had a shop verify a while back it needed replaced anyway so I'm going to swap it.


I replaced the plugs and wires, we're going to throw a cap and rotor on tomorrow even though it looked fine with only a little oxidation.


It sounds like it's missing, he's messed with the advanced timing. The timing light will show timing marks at times and at other times the marks seem to disappear like they're rotating around. (possibly the harmonic balancer issue?)


Anyone here think of other things we can do, he said he doesn't think the coil could be the culprit. As stated, it does sound like it's missing but it's not a consistent miss it's random.


Thanks for any feedback.

 

The balancer is isolated from its hub by a rubber ring and the outer portion with the timing marks could have slipped. A way to determine if it has is to set the engine and mark the balancer at TDC. Then start it up and check with a timing light.


..could be a sticky advance mechanism in the dizzy too.

 

how much is it failing by ? in Ca. were allowed 3 degrees of timing either way from spec. my truck is supposed to be at 10 degrees before. if you wanna clean up the emissions, back it off 3 degrees to build more heat in the engine. also, if your idle test has a max allowable idle, to raising your idle as close to that as possible. higher idle = less emissions. try dialing in your carb with a vacuum gauge to obtain the highest vacuum possible. a stock engine should get around 20-22 inches of vacuum.
hope that hopes you out

 

Thanks Hio and Chris. I'm engine dumb, I learned more about carbs watching him swap the needles and jets 6 times than I've even known about them.


The timing thing still confuses me, I mainly made this post so he could read the feedback and advice so I'll take any more feedback anyone here's willing to give.

 

Chris, forgot to mention the truck has an rpm guage it's idling about 900 rpm anyone know if that's about right? When I paid the professional tuners to do it two years ago for an outrageous price they had it idling about 700.