So who doesn't want a free 12-14 RWHP on your '70 Chevelle/Elco SS LS-5?!?!

[JMB]

Do you want a to add a FREE ~12 to 14 RWHP to your Factory stock LS-5 (and likely L-34) and still have it remain appearing 100% OEM factory stock??? Read on and many thanks to Eric Jackson at Vintage Musclecar Parts for his help!

Short Story (for those with little patience): Spend 5 minutes to re-jet your overly rich “as delivered” Q-jet by changing out the secondary hanger and secondary metering rods to safely lean it out roughly 1 point from ~11:1 A/F to 12:1 A/F, In my case I had to also swap out the needle and seat to a larger diameter orifice due to an overly lean secondary condition where the car was actually starving for fuel due to the float bowl draining too quickly which killed performance.

Long Story (in detail for those that want an education): Something that I’ve NEVER seen talked about when restoring Muscle Cars is how to take a Factory stock car and make it actually run properly….seems almost EVERYONE is content with whatever the Factory sent out the door because they want the car 100% OEM “as delivered”….unfortunately, I’m not that guy, I like to go for the “win-win”, keep it 100% stock appearing but optimize what’s there. NOTE! Although my carb isn’t stamped 7040204 it is the equivalent with the “78” primary jets and “BG” secondary rods.

There is a ton of data on this dyno chart so let me walk you through it. It took me just over 100 dyno runs to iron out all of the issues I was having with the Q-jet on my stock 1970 LS-5 SS 454 El Camino. I was having three main issues: 1) extreme lean condition in the upper RPM range causing the motor to not rev; 2) a severe secondary bog when you nailed the throttle; and 3) excessively rich secondary fueling even with 91 octane E10 (10% ethanol fuel) premium gas which should have slightly leaned out the secondaries.

Extreme Lean Condition in upper RPM’s (dyno baseline run 12): after replacing fuel pumps and trying various other things I contacted Eric at Vintage Musclecar Parts to review my problem. He suggested either drilling out the 0.107” seat to 0.135” or replace it with 0.135” and re-set the float level. He noted that it is common for the cheaper Q-jet rebuild kits to include the smaller needle/seat. Once I installed the larger needle and seat and installed a new float (just in case mine was bad) the high RPM lean condition was gone (see Dyno Run 53).

Overly Rich Secondary Fueling (dyno run 70): I actually did several things here in an effort to lean out the secondaries, I replaced the hanger to a shorter hanger (which holds the secondary metering rods to the carb via 1 small screw) and tried several leaner (physically thicker) secondary metering rods. I settled on swapping the base “H” hanger (0.550” height) to an “M” hanger (0.575” height). The hanger height controls the fattest portion of the rod in the jet (orifice) at the initial onset of the secondaries. Since the secondary cam raises all hangers the same amount, it also controls how much of the tip is in the jet at full lift. After trying several fatter rods I settled on swapping the stock “BG” rods (0.397” medium tip) to “DP” rods (0.686” medium tip) which leaned out the WOT A/F about 1 full point from roughly 11:1 A/F to 12:1 A/F and this was good for around 12-14 RWHP!!!

Secondary Bog Elimination (dyno chart 97): the secondary bog (when you nail a Q-jet at any speed) is VERY frustrating and can be easily fixed! I copied an pasted the process I used below from the Internet that I found on v8Buick.com by d7cook: It is a very common speed trick to loosen the secondary windup spring so that the secondaries will open very quickly. This is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries.

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening.

Prior to these Q-jet changes, with the T400 still in the car, it would just bog off the line and at 25MPH when I nailed it. After the fixes, it now has lightening quick throttle response off the line spinning the tires at will with only part throttle and would spin the tires nailing it at 25MPH from a steady cruise when it downshifted! It's also interesting to note the difference in dyno reported power between the T400 and M22, roughly 20 RWHP and 40 RWTQ and this was done with a 12 bolt 3.31 axle ratio.

The good news for you 100% “as delivered” guys/gals is that the Q-jet mods that I performed use OEM parts and you just have to decide, is it worth making my car ~1.5 seconds/~1.5MPH faster in the ¼ mile, which is a seat-of-the-pants noticeable change? Personally, I say you can pop out the replacement secondary rods and hanger to swap back in your OEM units for a MCACN Show and still be 100% truthful advertising your car as 100% “as delivered” stock and you enjoy the best of both worlds, dramatically improved performance without compromise! Thanks for reading and PM me if you need help!

[Xplantdad]

Nice write up! Eric is the best with carb stuff. Hope you are feeling better, too!

[JMB]

Yes he is! Thanks Bruce, I see a Orthopedic Surgeon on Monday, fingers crossed for a fix!

[Xplantdad]

Fingers are crossed!!