What Secondary Hanger and Secondary Metering rods are you using for your 455? I'm in the trial and error phase and have found best results from a G hanger and BP rods as well as a K hanger with AU rods. Still experiencing a slight stumble at higher (4200 +) RPM. Have a few more combinations to try out but was curious to see what others are having success with. Newly rebuilt full roller engine 9;1 compression .040 over with Performer Aluminum manifold / E heads with larger W-30 valves / Comp CTL1029 Cam / stock cast exhaust manifolds.

 

I know this is not a quick solution, but a wideband O2 sensor installed would be a huge help in looking at the A/F ratio when you experience the stumble.

Does it stumble only a little at that RPM, but then continue climbing?

 

Yes it will continue to climb after the stumble. Thanks for the advice about the wideband O2 sensor.

 

These guys make a great product. I have their stuff in both my Oldses
https://www.plxdevices.com/PLX-Wideb...uges-s/125.htm

 

The real upside of the qjet was its tune-ability. GM used it on everything from V6’s to the Caddy 502.

If you start with the factory primary/rod jet combination and the factory secondary hanger, you can tune from there. But that presumes you have a bunch of qjet parts fo play with. Which you may not have, or want to have. You may want to build a tuning kit with the parts organized to make this process easier. Also for each change, make notes about what it was before, what you changed, and how the experiment went. It’s very, very easy to chase your tail making changes you don’t remember and then repeat them. One change at a time is recommended so you can compare before/after and _know_ whether what you did had the effects you expect.

Check out Cliff Ruggles website, he has a few “recipes” which apply to various size engines. The information there may help you plot your next tuning moves.

I’m hoping your qjet is at least squeaky clean and new-ish. With carbs, cleanliness really matters since there are all kinds of tiny passages which can cause the carb to not work right if they’re plugged with dirt, even just restricted with grime.

If you haven’t tried, test loosening your secondary wrap spring about 1/8-1/4 turn clockwise as viewed from the passenger fender. That should cause a richer mixture with no need for new parts. You’ll know it’s too loose if the car bogs when you nail the throttle, then tighten it back up until it doesn’t.

The primary rod & jet combination also affect secondary metering. Before you go too far from stock on the secondaries, you might bump one jet size up to see if that cures the stumble. At that point in the RPM range the primary rods should be completely out of the jets, so a different primary rod shoudln’t matter, but a stumble usually indicates a lean condition which might be helped by getting more gas into the mixture - i.e. a bigger jet.

You’ll need to get a supply of qjet gaskets to have on hand so you can make changes as you need. Mainly these are the airhorn gaskets, and carb to intake manifold gaskets. Be sure to order the ones for your carb model, they all look a lot alike so a good step is to put the new right on top of the old and compare the holes to be sure they’re the same. I’ve bought stuff from Cliff’s Website and also qjets.com and maybe also quadrajets.com.

Get good quality rebuild kits too. Junky generic kits will just cause vacuum leaks and continued or additional performance problems.

I second the comment on the wideband 02 sensor, I use the Innovate DLG2 and like it a lot. One of the best, maybe the best, tuning tool I ever got. You can see the carb AFR or Lambda in realtime and adjust once you know which parts of the carb are affecting the mixture.

Hope that helps
Chris

 

Thanks for all the advice Chris. Much appreciated.