Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Test and tune
Cliff Ruggles:
Did you rebuild your carb following one of the recipes in the book? Need some help with tweaking it for maximum performance? Trying to dial it in and get your mileage up? Need a few more tenths? Want advice on a combo change? New intake? Spacer? Jetting?
Post it here.
JimC:
I am intrigued with the idea of Q-jets working like FI. I have 3 Q-jets, I am wondering which would be the best platform to start with?
17081226
17082226
Edlebrock 1904
I have your book and 3 other Q-jet books. Right now, I have a Holley on my truck, but I believe I can do better. My vehicle is an 81 Jimmy. I have a new 249hp 350 crate motor with an EPS intake and a 570 Street Avenger. (I have 2 more Holleys). I still use the cast iron exhaust manifolds, but have gone to 2" dual exhaust. Since the warranty is still good on my engine, I have not made any real changes to it. I have, however, changed the drive train. It came stock with 2.54 rearend. :o(GAG) Now I have 3.73 front and rear with LSD's. I am only interested in fuel economy and torque.
So where from here?
Jim
Pontiac Collector:
Hi Cliff, I am working on dialing in my 68 Firebird carburetor you built for me a few years back. Time flies.... Anyway, I am trying to work through what I perceive as a lack of WOT performance. I wont' go into detail on the motor unless you need it, but it is basically stroked 400 (462ish), Eagle crank and rods, Comp Camps roller, mildly ported 16 heads, stock intake, doug's headers, 3.55s, and a Cliff's 7028265 built Q-Jet, altitude 5K in the foothills of Colorado. I figure it should be just a little scary, but it really isnt that impressive. Anyway, I have an AFR wideband O2 setup from innovate motorsports to get a good AFR read. It runs like a top, idle, part throttle, no stumble, etc... I took some AFR readings this weekend and at cruise it is in the 14s, but when I mash it, the AFR meter dips into the 9's and works its way up to the low 11s over about a 5 second span of WOT. The choke pull-off rate is about 1-1.5 seconds. The car seems to respond immediately and then just goes flat on the way from 3K to 5K RPM. If I can trust my equipment, I am definitely too rich. I know you tweak these in to a customers specs (including altitude). Where should I start? Secondary metering seems to be a likely choice, but I want to get your take. I can share the innovate logs if you want ( no RPM data in them yet, just AFR...). I think the software is free from innovate motorsports.
Cliff Ruggles:
I agree, start with slighlty leaner secondary metering rods.
Even with that said if you are around 11 to 1 A/F ratio, you aren't leaving much power on the table.
I've changes metering rods at the track on back to back runs, going from "pig" rich to slightly lean, and the entire spread of ET is never more than about a tenth of a second. I run my own engine slightly rich, using .041" secondary metering rods, which are pretty close in the cool/cold months, but pretty rich in hot/humid weather.
When drag racing, it simply makes ET predictions easier as evening approaches and the temps cool down some. On the few occassions I've chased ET and MPH with metering changes, it's just too difficult to accurately predict how much the car will pick-up during the day and into the evening, not to mention knowing exactly which metering rod to install for the next round?
What are your engine specs, compression ratio, camshaft, etc?.....Cliff
Pontiac Collector:
Cliff, here is what I have done so far.
I noticed quite a few lean misfires on my AFR readings (the LM-1 really takes the guess work out of this effort) so I installed slightly hotter plugs NGK v-power XR-5s and gapped them to .40 where they were at .35 with NGK v-power 7s. I have a pertronix flamethrower coil so I felt that was probably a good gap. I have absolutely no lean misfires at all, so I guess that was a good start. Incidentally, this motor will run @ 150-160 F all day long...
I went ahead and changed from a B hanger to a L hanger when I changed my plugs (I know changing two things at once isn't good science). If I'm not mistaken, this will only modify the rate of change of the AFR while the secondaries progress to fully open. Between those two changes I noticed a much better 3K-5K acceleration and it pulled really strong. My AFRs seems to hover around 12-12.5 once I get past the rich dip which is around .5-.7 seconds. I have continued to use the metering rods you sent with the carb. I am going to put the B hanger back in and do some more testing because the final AFR should be about the same I would think. I also want to re-validate my original results.
Since I am greedy I would like to address the rich dip to an AFR of 9.0 as I am pretty darn confident I can feel this in the transition to WOT. Essentially, it is a very short duration power drop before it cleans out (this is not a stumble). I moved the primary side accelerator pump lever to the outer hole in an attempt to reduce the pump shot, but that didn't really do much to the measurements or feel. I am not sure how to experiment from here to see if I can tune this out. If this is more a function of the secondary accelerator ports, could I close off or reduce the slot size in the air valve flaps as a test? If so with what?
I am interested in your thoughts.
Per your previous inquiry, my cam specs are the following. I don't claim to understand them all....
BTW, Kauffman put this combo together for my local builder.
Comp Cams roller grind# 3315/3316 HR112+4
Intake Exhaust
Gross Valve lift .510 .520
Duration @ .006 tappet lift 281 287
Valve Timing @ .050 7 BTDC 43 ABDC 54 BBDC 2 ATDC
Specs are for cam installed @ 108.0 Intake Center Line
Duration @ .050 230 236
Lobe Lift .3400 .3470
Lobe Seperation 112
~462 C.I. Eagle Crank and Rods
Ross pistons
Balanced and blueprinted (I am pretty sure it was zero decked and given all the Pontiac performance treatments)
16 heads with mild porting
Stock Intake
Cliff's Q-Jet 7028265
Doug's headers
Muncie M22
3.55 gears
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