Well, I thought I had finally gotten my Q-Jet setup on my 383 well enough to start tuning, but now that it has gotten cold, I'm getting (what I believe) to be lean popping out of the exhaust, especially at low RPMs (< 1500).

So, now I'll leave it to the forum to get me a good estimate for a starting point for my setup.

Currently, I've got .73 jets and .50 rods on the primary and DR secondary rods with a P hanger. I'm really only concerned with the primary side right now, but I'm open to suggestions on the secondary side as well.

My combo is:
383 with 10.5 compression
Edelbrock #1904 Q-Jet
AFR 195 heads with 2.02 and 1.60 valves
Comp Cams 274XR solid roller cam with 236/242 duration @ .050, .016/.018 lash, and mid .500 lift
Weiand Team G street ram single plan intake (#7525)
15 in of vacuum at 800 RPM

Any help on which rods/jets to order would be greatly appreciated.

 

Chris,

You need to get in touch with Lars (aka the Q-Jet guru). I am running a 74/47 primary combo with a much milder engine setup (see sig). I would also get a copy of Lars tuning paper from http://www.corvettefaq.com .

My $0.02. My gut says lean. Remember, it is the relationship between jet and rod that governs part throttle mixture but since all rods have the same tip (.026), only jet size governs WOT mixture.

 

Steve, I have read quite a bit on the Q-Jet, which includes Lars's papers. I agree that I most definately need to run a smaller rod. The problem is that I don't believe that I can get them locally, so I'm trying to get input on what range of rods/jets I should order for my tuning so I can just pay shipping one time (which is the only reason I even mentioned the secondary rods and hanger).

I have also read in one tuning guide that the difference between the jets and the rod should be about .30, so my .73 jet should be matched to a .43 rod. To me this would seem to be not too acurate since by modifying the diameter and keeping the difference the same, the area of the difference is changing.

 

FYI, I'm running 71 jet & 39 rod with 6 psi power piston spring. With my new setup, vacuum increased to 17-19 psi so I'm thinking of using the 8 psi spring & maybe a 72 or 73 jet. Hope you get things sorted out soon. Good luck!

 

Absolutely, I was working under the same bad info until Lars set me straight. Trouble is that there are very few new jet/rod combos available. Edelbrock has .068-.077 jets available but only 4 rods (.048,.050,.052,.054). Also only 5 secondary rods and they are all long tip rods which aren't good for economy (good from drag racing :D ).

I also recommend the Doug Roe book on Quadrajets. Good info there as well especially about all the secondary rod combos available.

BTW: How far from Wylie to Denver? May be worth packing a couple of sixes of Lone Star and heading up to Lars' place for a little tuning session. :cheers:


[Modified by SteveG75, 3:42 PM 1/2/2002]

 

Chris -
I e-mailed you some info on this - for some reason I was unable to post replies to the Forum earlier today. Perhaps you could post my e-mail to you for others to see - I think I gave you some part numbers and info that may be of general interest, and now I can't remember what I wrote...

 

Chris,
I think you have 2 problems, both are a result of the carb you got is way too lean for a 383 with your cam.
1) The really tough thing with the Edelbrock carbs and primary rods is that - despite everything ya read - they use have fatter tips than the car rods! I bought their tuning kit a few years ago and found that the rod tips are 0.035", not the always-stated 0.026" !!! I have both of Doug Roe's books and neither mention this tip width. By calling around, I was told that Edelbrock sell primary rods for TRUCK carbs, and they have different tapers and tip widths. So, you can only tune what you have by changing jets. Anyway, your jets are way too small. Go to a 75 - 77 jet with a 50 rod. Your engine will handle the jet okay, but you will have to play with the APT (adjustable part throttle) screw to get the right cruise mixture. If you feel slight surge when cruising at 60 mph, you are lean so then richen by 1/2 turn of the APT screw (up) at a time to get it right.
2) Your LOW rpm leaness is from the idle channel restrictions being too small (again, wrong calibration of the carb fror your engine). Changing the primary rods / jets will not affect this since the throttle is not open enough for the main system to be feeding fuel - you are on the off-idle system. The off-idle feeds up to about 40 mph at light throttle. Opening the idle screws will help the off-idle, but you will idle really rich. Opening the channel restrictions is very delicate work, but if you are careful you can do it if you have the right tools.
Unless you want to do all this, you are probably best to get a carb specifically calibrated for your situation.

 

Here is the info Lars had emailed me:
And then I said:
And then Lars responded:

 

Richard, you are right about my primary rods. They are the 50M rods. The "M" declares them to have a small diameter of .032. I have just rescaned the Doug Roe book and I may have the primary throttle plates open too far up the transfer slots. I did have to play with the idle adjustment screw a lot to get it to idle. I'll check this out before I go drilling anything, but if I have to, I will. I just want to exhaust all other options first.

Thanks

 

the M means mod quad or 75 or later, the shorter rods.