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 Rejetting my q-jet, recommendations?
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cortcomp
Coyote

USA
5335 Posts

Posted - 20 Jul 2014 :  5:30:29 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I finally got my truck done and move my 71 back to the garage to swap the rear end (from olds 12 bolt/3.42/cone posi to 8.5 10 bolt/3.90/detroit locker).

While i was in there, i figured it was time to address the one or two clicky lifters and the rich carb. Hence my landing here. I know i could call cliff, but it's sunday and i'd like to get jets on the way to be here tomorrow. Here's my setup:

Cruising, i am in the 12.5-13.5 range, and WOT, 10-12 range.

I know i can probably tune some in the secondaries with rods on the car no problem, correct? Idle i am kind of lean and have the screws out a chunk, but it's a lumpy cam, doesn't get warm when idling, so i'm thinking that it doesn't ready accurately at idle (XE284/600rpm idle)..if you tap the gas AT ALL, like bring it up to 800-1000 rpm, it gets into that 12.5 to 13.5 range.

So, idle seems fine, but cruising and WOT throttle are rich. This is a cliff q-jet. I took it apart and it has 73 jets in it now. My plan was to go down a size or two, but i have no experience in shooting from the hip on what size to move to, and i HATE doing anything twice. I was thinking of re-jetting then working with secondary metering rods later if i had to.

I don't need to get to 14.7 or 15.0, but being in the 14.s cruising and WOT would be nice.

Ideas on which size to proceed with? Is one jet size a ton or should i go two?

Blued and Painted
Chief PONTIAC

USA
3406 Posts

Posted - 21 Jul 2014 :  01:25:09 AM  Show Profile  Reply with Quote
Sounds like a small adjustment

High 13 cruising should be sufficient for these antique clunkers IMO.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
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cortcomp
Coyote

USA
5335 Posts

Posted - 21 Jul 2014 :  09:43:27 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
If i get a chance i'll call cliff today. Looking at his book, i see in the second recipe (i think thats about where my setup would fall) he recommends 71-73 jets, if i recall (at work at the moment.) If that's the case, going down one step might be enough.
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cortcomp
Coyote

USA
5335 Posts

Posted - 22 Jul 2014 :  4:56:39 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Finally had a chance to call cliff, and he recommended sticking with the 73 jets and adjusting the APT. Admittedly, even after reading his book 50 times, i hadn't thought of that because i didn't realize you could gain that much adjusting that. Once i get springs in, will adjust that and secondary rods and see what i get.
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DanM
Buffalo

80 Posts

Posted - 24 Mar 2017 :  11:40:32 AM  Show Profile  Reply with Quote
12.5 - 13.5 what?
vacuum inches?
Air fuel ratio?
horsepower?

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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cortcomp
Coyote

USA
5335 Posts

Posted - 24 Mar 2017 :  3:34:55 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
AFR:

"but cruising and WOT throttle are rich."

I sold that car, but just adjusting the APT got me right at like 14.5 IIRC.
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DanM
Buffalo

80 Posts

Posted - 27 Mar 2017 :  10:21:15 AM  Show Profile  Reply with Quote
Last Wednesday at the dyno we learned I'm too lean in the secondaries at WOT. 14:1 most of the time then 16:1 just before it goes asthmatic and practically cuts out.

I had been using AX secondary rods. Those are about 0.040" at the power tip. I ordered a pair of 0.030" from Ames. The pair that arrived are CC rods. The power tip is shorter than with the AX and the profile is actually leaner than the AX rods.

So, I went to a friend's house who has a lathe. He makes opto-mechanical pieces for lasers for the defense industry. We spend much of an afternoon custom turning the CCs into what are best described as DXs. Still 0.030" tips but the transition comes in earlier and is more aggressive. Overall they should be richer.

Also I modified my hanger. I have a U. That hangs the rods as far in as they can go. There is misinformation in the literature and online about the hangers. The lower letter hangers are richer, contrary to what you might have heard.

Doug Roe has a nice book on Quadrajets. He suggests modifying the hanger by drilling a second set of holes. I did that and essentially made myself a D hanger. So now with my "DX" rods and my "D/U" hanger I should be able to solve my lean WOT problem.

When I get my AFR gauge mounted I can confirm this.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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DanM
Buffalo

80 Posts

Posted - 27 Mar 2017 :  10:31:37 AM  Show Profile  Reply with Quote
My '71 vintage carb doesn't have Adjustable Part Throttle so I can only set it via jets and/or primary rods.

Last winter I knew I was lean on part throttle due to a stumble while pulling away from lights. The problem did not exist in the summer so I knew I was just barely too lean. By swapping from 0.043" rods to 0.041" rods I solved my problem by richening that throttle range.

Meanwhile with my 0.073" jets I should be rich enough at heavy part throttle.

Well, now my fuel economy is horrible at 11.1. and I'm lean at WOT at 3000 rpm and above. The solution for my mpg is to lean it out at part throttle which will recreate my stumble issue.

If only I had APT.

Once I get my AFR gauge installed and running I can watch the numbers as I drive around for a couple hundred miles. Hopefully I'll figure out where in the curve I can lean it out for better mpg and where I can add fuel for power in a fashion which I can accomplish with jets and rods.

Maybe I should add a vacuum gauge to help me know when my power piston is engaged, or maybe even a little sensor that can tell me when it's up and down. I might not need that if I notice the AFR jump between two numbers in light throttle. That should tell me if it's up or down. And maybe another sensor to tell me the angle of the air valve...

Stay tuned.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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