Jetting for performance...

[69chevyguy]

Good day - I'm new here and I hope I'm not asking a question that's already been asked.

Rebuilding my '69 Quadrajet (7029202) for my bone stock '69 Impala, 350, 255 horse auto. The intent is to build to factory specs BUT since '69, we've taken lead out of gas, added 10% ethanol, and I run 93/94 octane instead of 87.

That being said factory specs aren't going to yield factory performance when the fuel is very different.

I need some advise on if I should size up the jets or change the metering rod sizes. I've read about 10 different articles on this with about 9 different opinions. All I know is you need more ethanol fuel than regular to yeild the same amount of power, and the higher octane slows the burn rate also. I figured out how to adjust timing to accomodate the octane, but now's the time to mod that quadrajet if I'm going to, before it's all back together.

Thoughts?

[omaha]

I am not sure if the 69 chevy carbs were set up on the lean side or not. {im gonna say probably not lean} that being said, I think it would be safe to go up one jet size to compensate for alchohol. Of course the ideal would be to use some sort of O2 reader while driving. I am using a mid eighties carb from a Dodge van originally used on a 360 mopar and the jetting SEEMS spot on for my 318 with mild cam and 360 heads. [i know it seems strange to think of Mopars with a Q-jet] but the Mopar Qjets were not lean at all. The engine does not surge at all and the exhaust pipe is not "sooty" {is that a word} and I am getting really good fuel mileage BUT...I cannot be 100% sure if the A/F ratio is spot on. That would be the function of a wideband O2 reader. I am guessing by the "old school" methods to determine the correct jet, no surging, no popping and no soot but that does mean it has the right jet. I can understand not wanting to take apart the carb to change jets but with the early style of Q jet, it may be necessary. Newer carbs [77 up] have the height adjuster on the piston for the primary jets. the old carbs have one also but it is hard to access. if possible this could give you some leeway as far as adjustability goes. One of the drawbacks of the Q-jet is that you need to take it apart to change the primary jets.

[von]

I have the 255 hp 350 in my '69 Chevelle and that 29202 Q jet. I increased the jet size to 69 from stock 67 and left the stock 42B primary rods in. Also left the stock AN secondary rods in. The only other mod I made was to increase the secondary POE tube restrictions to .031. It runs excellent and with the recurved distributor and openings I cut in the bottom rear (hidden) of the air cleaner, it's surprisingly quick with a 3.36 rear gear. I also blocked off internally the exhaust heat riser to the air cleaner. I installed a 200 4R trans for the OD. It's a great combo IMO, runs on 87 octane without ping and gets great gas mileage.

[Cliff Ruggles]

I would employ the APT system in the baseplate if you have the skills to do so.  Then install one of our external APT screws, (pic attached).  This will provide full control of part throttle A/F from rich to lean with the right jets/metering rods.

While the carb is apart, put one of our kits in it, so the correct parts are in there and it's up to par for this new fuel.  I would also replace the jets, metering rods and PP spring, those items wear out, and we consider new parts essential for a good end result.

I can spec out the correct jets, metering rods and PP spring for what you are doing.....Cliff