75 EGR delete
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
75 EGR delete
I am converting my stock L48 4-speed to dual electric fans. I have no choice because I got the car in pieces and do not have the shroud or fan blade. Also, I got a new aluminum radiator with the car.
After fabricating the brackets, I have the fans installed and am working on the wiring.
I will need to use the manifold port where the Thermo-Vacuum valve for the EGR vacuum is located for the temperature switch for the fans.
I plan to simply delete the EGR system since in Louisiana there is no emissions inspections on this car.
My question: The L48 still has the stock Rochester carb. When these carbs were set by the factory, they were adjusted to account for some volume of inert gas (exhaust) being pulled into the cylinder. If I simply block off the EGR and do nothing else, do I run the risk of having too much combustible air in my cylinder during times when the EGR would be open? Basically, I would be running too lean.
The EGR systems are blocked off at idle and when heavy acceleration. The mid-range cruising is the concern.
Am I being overly concerned or should I do something?
After fabricating the brackets, I have the fans installed and am working on the wiring.
I will need to use the manifold port where the Thermo-Vacuum valve for the EGR vacuum is located for the temperature switch for the fans.
I plan to simply delete the EGR system since in Louisiana there is no emissions inspections on this car.
My question: The L48 still has the stock Rochester carb. When these carbs were set by the factory, they were adjusted to account for some volume of inert gas (exhaust) being pulled into the cylinder. If I simply block off the EGR and do nothing else, do I run the risk of having too much combustible air in my cylinder during times when the EGR would be open? Basically, I would be running too lean.
The EGR systems are blocked off at idle and when heavy acceleration. The mid-range cruising is the concern.
Am I being overly concerned or should I do something?
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
You are 100% right. The quadrajet can be recalibrated to run well without EGR. The distributor ignition curve needs some help first. After the distributor recurve, I would be looking at recalibrating the carb, specifically the primary side.
If you feel up to a bit of experimentation you could play with the APT (adjustable part throttle) screw BUT be sure to write down exactly where it is now before you play around with it. Screw it down to lightly seat it and count the turns. You do need to know exactly where it was set by the factory. There is no reference for the original setting. Once it's gone, you have zero chance of going back if you do not know where it was.
If you feel up to a bit of experimentation you could play with the APT (adjustable part throttle) screw BUT be sure to write down exactly where it is now before you play around with it. Screw it down to lightly seat it and count the turns. You do need to know exactly where it was set by the factory. There is no reference for the original setting. Once it's gone, you have zero chance of going back if you do not know where it was.
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Mark -
The '75 Q-Jet is a 1-year-only carb, and it does not have the "normal" APT adjustment screw like the later Q-Jets. It has no piston stop for the power piston, so the rods go all the way to the bottom of the piston bore travel. It then uses the "3rd jet" with the adjustable aneroid substitution block single-needle "APT", which was a lousy system. You're best off re-jetting the '75 carb to get some fuel through it... #72 jets in a large main air bleed carb is just silly lean...
Lars
The '75 Q-Jet is a 1-year-only carb, and it does not have the "normal" APT adjustment screw like the later Q-Jets. It has no piston stop for the power piston, so the rods go all the way to the bottom of the piston bore travel. It then uses the "3rd jet" with the adjustable aneroid substitution block single-needle "APT", which was a lousy system. You're best off re-jetting the '75 carb to get some fuel through it... #72 jets in a large main air bleed carb is just silly lean...
Lars
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
Lars,
Of course you are right about the APT. I had no idea a '75 carb was a one year only deal. That said,
What primary rod/jet pair would you suggest as a good starting point? A "no EGR" calibration needs to be fattened up.
Of course you are right about the APT. I had no idea a '75 carb was a one year only deal. That said,
What primary rod/jet pair would you suggest as a good starting point? A "no EGR" calibration needs to be fattened up.
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Yup, it sure does. The problem with re-jetting and tuning the M4M Q-Jets is that you can't get passenger car primary rods (.026" power tips) anymore. With the large Main Air Bleeds (MABs), that carb needs #77 primary jets. But to prevent an over-rich cruise condition, it needs some 48 rods to go with them. This would provide a nice, modest enrichment at cruise from the 72/46 stock combo without getting too "fat." For a nice compromise, keeping the stock 46 rods, you can get it running pretty well with a 76/46 combo, which will provide a reasonable enrichment at both cruise and WOT. Run the single-rod "APT Fill Block" adjuster screw about 3 turns up from the bottom seated position against the stop ring. This would be my starting position for a fairly stock setup. If you want to use that carb with a bigger cam, you have to start restricting air bleeds to get some fuel flow through it at both idle and WOT. Then you can drop it back down to the 72 primaries.
Lars
Lars
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Thank all of you for confirming my suspicions about the EGR delete process. I don’t want to cause any detonation issues. I am certainly not in a position to rip that carb apart or send it off.
I have no problem keeping it intact since this will be a low speed, top-down, weekend cruiser.
For now, I will take SpeedReed’s suggestion and put the fan switch in the water neck. Simple solution for less than ten bucks!
I have no problem keeping it intact since this will be a low speed, top-down, weekend cruiser.
For now, I will take SpeedReed’s suggestion and put the fan switch in the water neck. Simple solution for less than ten bucks!
