Re-jetting the q-jet
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Safety Car
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From: San Diego CA
Re-jetting the q-jet
Alright so I need up richen up my Edelbrock Q-jet (#1904 M4M).
So I need some suggestions on what size jets and rods to get to do so. Any input would be greatly appreciated on this little process.
Reason of course is I went with headers and sidepipes and am now running lean since it was originally tuned for the stock exhaust.
So I need some suggestions on what size jets and rods to get to do so. Any input would be greatly appreciated on this little process.
Reason of course is I went with headers and sidepipes and am now running lean since it was originally tuned for the stock exhaust.
Thread Starter
Safety Car
Joined: Dec 2004
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From: San Diego CA
Alright some more info for you guys. It's currently using the stock jets and I believe the rods are, also. The stock primary jets are .073 and the stock metering rods are .050." According to lars's tuning paper I should leave the secondaries alone. So I'm thinking I should bump up to .075 jets and probably the .048" rods. This sound about correct?
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Are those "M" series primary rods? If they are, they have the fat .036" power tips on them, and will run you lean at WOT.
Your current 73/50 combo is WAY too lean. The 75/48 is a good setup for cruise and off-idle throttle response, but WOT mixture will still be lean with those fat .036 tips.
If you're going to keep "M" series rods in it, run a set of 77 jets with a pair of 48 or 49M rods. This should get you good WOT while making part-throttle nice and crisp.
Your current 73/50 combo is WAY too lean. The 75/48 is a good setup for cruise and off-idle throttle response, but WOT mixture will still be lean with those fat .036 tips.
If you're going to keep "M" series rods in it, run a set of 77 jets with a pair of 48 or 49M rods. This should get you good WOT while making part-throttle nice and crisp.
Thread Starter
Safety Car
Joined: Dec 2004
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From: San Diego CA
Yeah, I was thinking the 75s might still be too small. I'll grab a 77 jet and 48 and 49 rods and see which pair of rods works better. There's a shop called San Diego Carburetor that will tune it in for the correct air:fuel ratio for like $170, but I don't want to have to spend that much now then have to pay that much again after I put on new heads.
So with that jet and rod configuration it should be pretty good till I'm ready to crank up the compression, yes? (Current compression is estimated at 8.5 - 8.7:1 amazingly)
Edit: Just ordered the jets and rods. Apparently they don't make any 49 rods so I just went with the 48s.
So with that jet and rod configuration it should be pretty good till I'm ready to crank up the compression, yes? (Current compression is estimated at 8.5 - 8.7:1 amazingly)
Edit: Just ordered the jets and rods. Apparently they don't make any 49 rods so I just went with the 48s.
Last edited by Kalway; Feb 14, 2006 at 06:51 PM.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Keep in mind that on your carb you have the APT (adjustable part throttle). When you do the jet/rod change, pop the little plug out of the airhorn for APT access. You can then dial in your cruise mixture from outside the carb by raising/lowering the power piston stop. 
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Changing the metering rods will only change the A/F ratio at part throttle (light cruise). Changing the jets changes the mixture across the entire range, including WOT. Make changes based on what the engine needs - you have to try it out and see what changes are needed, if any.
Thread Starter
Safety Car
Joined: Dec 2004
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From: San Diego CA
So basically it's wait and see, eh?
When those new jets and rods get here I'll change my plugs, too. I don't want to make my new ones all nasty from the lean condition. New headers going on this weekend, again, this weekend. Ya know changing the spark plugs becomes very easy with the headers off.
When those new jets and rods get here I'll change my plugs, too. I don't want to make my new ones all nasty from the lean condition. New headers going on this weekend, again, this weekend. Ya know changing the spark plugs becomes very easy with the headers off.
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Thread Starter
Safety Car
Joined: Dec 2004
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From: San Diego CA
Alright got the new jets and rods in. Much better! I used the .077 jets and the .048 rods and it has great throttle response and is nice at WOT.
Now I've forgotten which screw to turn to get it to idle a bit lower... idling around 1k RPM or so right now, needs to be around 800, I believe. Which screw is the one to twist again? The one hidden in the cove or the one outside with a spring? I always get them mixed up.
Now I've forgotten which screw to turn to get it to idle a bit lower... idling around 1k RPM or so right now, needs to be around 800, I believe. Which screw is the one to twist again? The one hidden in the cove or the one outside with a spring? I always get them mixed up.
Thread Starter
Safety Car
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From: San Diego CA
Alright base idle seems to be fine. I think it's the fast idle I need to adjust because it's between 1200 and 1700 on fast idle. Normal idle is fine at 650 or 750 now (can't recall which one it was at.) Drove it to work today to see how she's doing with the rejetting. However, I'm still noticing a bit of a popping sound when I let off the accelerator at high speeds. It's not as much as before but it does it a little bit. Could I still be a touch too lean? Would hate to have to order new jets again. I guess I'll have to look when my new spark plugs get here. Also, the clip that holds the primary metering rods (forget the official name for it) bent a little when I was putting it together so I bent it back to where it felt proper but I'm not sure if I got it to the correct spot. Suggestions on how that should be bent to let the rods sit correctly? I just made sure they were even and hoped for the best.
