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Ideas on AFR and carb tuning?

hunt2elk

hunt2elk

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Trying to get my 451 in my Road Runner running a bit better. Just hooked up an AEM wideband gauge and need some direction as to what to try first. I am happy with everything, except from idle to about 2500 rpm when I floor it. It doesn't stumble, but seems lazy until it gets to the 2500 mark. Carb is a Quick Fuel 750Q mech secondaries. 71 primary jets and 80 secondaries. 33 primary squirter and a pink cam in the first hole. 35 secondary squirter and a pink cam in the second hole. Idles at 850 rpm with an afr of 14.4. Cruising afr 13-14. WOT afr 12-12.2. So what I notice is as I hit the throttle hard from an idle or slowly moving, the gauge quickly blips to 15 and then quickly blips to 8 and the settles out smoothly at wot.
 
hunt2elk said:
Trying to get my 451 in my Road Runner running a bit better. Just hooked up an AEM wideband gauge and need some direction as to what to try first. I am happy with everything, except from idle to about 2500 rpm when I floor it. It doesn't stumble, but seems lazy until it gets to the 2500 mark. Carb is a Quick Fuel 750Q mech secondaries. 71 primary jets and 80 secondaries. 33 primary squirter and a pink cam in the first hole. 35 secondary squirter and a pink cam in the second hole. Idles at 850 rpm with an afr of 14.4. Cruising afr 13-14. WOT afr 12-12.2. So what I notice is as I hit the throttle hard from an idle or slowly moving, the gauge quickly blips to 15 and then quickly blips to 8 and the settles out smoothly at wot.
Click to expand...

Could use more info:
Cam specs?
Transmission?
Rear end gears?
Tire size?
Converter stall?
 
Cam is a Racer Brown SSH-44 hydraulic flat tappet. 242/242 @ .050, 108* lobe separation, Harland Sharpe 1.6 intake 1.5 exhaust rockers. Edelbrock RPM intake. 4-speed. 3.55 rear. 28" tall rear tires. Fire Core distributor 18 initial and 33 all in, black and silver springs all in by 3400, no vac advance. Been driving it a bit more, and now am not noticing it going lean when smashing the pedal. It is however still dropping to a rich condition before leveling out at wot.
 
Nail the throttle and let up around 2500. If it kinda burbles its to fat. If it seems to increase rpm its too lean
 
IMO

More initial timing if starter can handle it.

Start with the idle and work up from there. Think house and a foundation. Everything starts with a good foundation.

Don't tune to a number, let the engine tell you what it wants. It may be a bit lean at idle with cam overlap. Try turning the idle mix screws out a bit and see if it helps. Use a vacuum gauge to assist here. Highest vacuum you can get is what you want. See what the AFR is at that point. Being a 4 speed makes idle tuning MUCH easier.

It may need a IFR and IAB adjustments, but, that's bit deeper. If the idle mix screws are out less than about 1.25 turns, you may need a smaller IFR.

Squirters and pump cams would likely be the next stop.

It's fat on cruise just a bit.
 
Last edited:
Crackedback said:
IMO

More initial timing if starter can handle it.

Start with the idle and work up from there. Think house and a foundation. Everything starts with a good foundation.

Don't tune to a number, let the engine tell you what it wants. It may be a bit lean at idle with cam overlap. Try turning the idle mix screws out a bit and see if it helps. Use a vacuum gauge to assist here. Highest vacuum you can get is what you want. See what the AFR is at that point. Being a 4 speed makes idle tuning MUCH easier.

It may need a IFR and IAB adjustments, but, that's bit deeper. If the idle mix screws are out less than about 1.25 turns, you may need a smaller IFR.

Squirters and pump cams would likely be the next stop.

It's fat on cruise just a bit.
Click to expand...
I have run it with as much as 24* initial timing. Runs great there, but the starter bucks back when hot and it will also "run on" when shutting off at anything more than 18*, so that is where I ended up with that.
All 4 idle mixture screws are out 7/8 of a turn. That is where I get the best vacuum.
 
I
Frank Mopar said:
Nail the throttle and let up around 2500. If it kinda burbles its to fat. If it seems to increase rpm its too lean
Click to expand...
I'll try that tomorrow morning as it is already 94 degrees outside and I have settled in the ac house.
 
I just did the rod and spring dance with my edlebrock 1405. I borrowed a O2 clip on fabricated by a mechanical engineer I work with.
I leaned mine out from 12.8 to 13.3 to 13.9 to 14.1 depending on cruise mph.
WOT is 12.4.
Much smoother at cruise now, mpg came up from 13.3 to 14.0, and the throttle response is amazing.
 
Clip on O2 set up.

20180501_083722.jpg
 
You clip that in your tailpipe and it actually gets hot enough to react properly ?? I could see on an open header.
 
65wgn440 said:
You clip that in your tailpipe and it actually gets hot enough to react properly ?? I could see on an open header.
Click to expand...
You do have to get it hot. Takes about 10 minutes of driving to settle down.
 
Sounds like a bigger shooter is needed. Try next several sizes. Changing shooters s easy & cheap. Go up a couple steps & see if it likes it. My 451 motor with a 4781 Holley and TM7 wanted 38F & 42R . You just need to try stuff.
 
66Satellite47 said:
Sounds like a bigger shooter is needed. Try next several sizes. Changing shooters s easy & cheap. Go up a couple steps & see if it likes it. My 451 motor with a 4781 Holley and TM7 wanted 38F & 42R . You just need to try stuff.
Click to expand...
What size acc pump are you using? Thought I read somewhere that a standard 30cc was only good for up to 35 squirters?
 
hunt2elk said:
What size acc pump are you using? Thought I read somewhere that a standard 30cc was only good for up to 35 squirters?
Click to expand...
Not in my experience, I did use up to 42 shooters with the 30 cc pump on a 4781 in the front. I did also use 37 to 42 with the 50 cc secondary pump. Try what it takes. I also used the high flow screws.
 
66Satellite47 said:
Not in my experience, I did use up to 42 shooters with the 30 cc pump on a 4781 in the front. I did also use 37 to 42 with the 50 cc secondary pump. Try what it takes. I also used the high flow screws.
Click to expand...
Yes, it was the high flow screws that are needed over 35's that I was thinking about. Just checked, and I have those already. Put a 35 in the primary and now it seems better. When I floor it at 1k, the gauge blips to 10 and then settles at 12.5 at wot rather quickly. What should the afr be doing ideally in this situation? Not sure what I am aiming for.
 
Try some different pump cams. The blue 427 cam will wake it up. Don’t worry so much about the A/F Meter when you initially go WOT. It’s more about what the engine likes. Usually smaller squirters with an aggressive pump cam is the route I try first. Then from there I will walk up on the actual size of the squirter if needed.
 
Last edited:
68 HEMI GTS said:
Try some different pump cams. The blue 427 cam will wake it up. Don’t worry so much about the A/F Meter when you initially go WOT. It’s more about what the engine likes. Usually smaller squirters with an aggressive pump cam is the route I try first. Then from there I will walk up on the actual size of the squirter if needed.
Click to expand...
Tried a green cam this morning and it was terrible. Went to a black in #1 hole, way better than the green, but not as good as the original pink. Put #2 in the black and it got better, but still not as smooth as the pink #1. I'll try pink #2 and then the blue.
I am doing all this on the primary side of the carb. Is this where I should be concentrating, or do I need to be putting the same in the secondaries at the same time?
 
If the primary side functions well you want to focus on the secondaries. Figure you’re always using that primary squirter so you don’t want to go crazy on that one to cover up a secondary issue.
 
68 HEMI GTS said:
If the primary side functions well you want to focus on the secondaries. Figure you’re always using that primary squirter so you don’t want to go crazy on that one to cover up a secondary issue.
Click to expand...
I feel as though I have it running really strong above 2500 rpm. My issue is from idle to 2500. Shouldn't I then be focusing on the primary side?
 
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