[pmd400]

I took the firebird out today to tune the carb. I recently installed a 17056262 qjet. AFR at cruise is about 15.5:1. WOT is about 13.5:1. After WOT the cruise AFR goes to 11:1 for a few minutes before it leans back out to over 15:1. What could cause this?

[77 TRASHCAN]

Got any specs on your carb, for the knowledgeable folks here to help you?

[lust4speed]

Float sticking? Full throttle puts a good drain on the Quadrajet bowl and the float would drop low as the fuel level drops, and possibly the arms of the float are hanging up? That would overfill the bowl and cause the rich condition. Could also be the needle valve taking time to seat properly after dropping down farther than normal cruising loads. Best I got.

[steve25]

Yup!
The float and or the needle is hanging up once the level of fuel in the bowl drops to a certain level for a while.

That cruse mixture ratio is going to lead to part throttle ping and maybe even hot running issues my friend !

[Kenth]

Inlet needle may/will bind in seat if clip is located in one of them holes in float arm.
Clip needs to be hanged in from behind of float arm.
With proper fuel delivery to carb, fuel level in bowl wont change much or at all at WOT.

[shaker455]

Originally Posted by pmd400

I took the firebird out today to tune the carb. I recently installed a 17056262 qjet. AFR at cruise is about 15.5:1. WOT is about 13.5:1. After WOT the cruise AFR goes to 11:1 for a few minutes before it leans back out to over 15:1. What could cause this?

Stock engine?

[pmd400]

Thanks, I'll check the needle and seat out. The engine is a 455, 9.5:comp, 6x heads, 232 at .050 cam.
Steve, what AFR do you like to see. I've never had any issues at 15.5:1 cruise. Runs to cold if anything.

[Formulajones]

15.5 at cruise with no load is fine, doesn't hurt anything as long as it's not to the point it's causing a surge or running hot, which you said it's not.

Some engines get away with it and work well, some don't. It's a trail and error thing.

The WOT AFR might be a bit on the lean side though. It's not dangerous but it's something to watch. Since you're not pushing the compression ratio and have a decent cam it probably won't complain.
With that said I found my Formula runs it's best times at the track when it's right about 12.9-13.0 AFR. It takes either a dyno or lots of track time to find a happy spot. Every engine is a little bit different in this department too. Some like richer, some don't. There is no real magic number, I just give the engine what it likes.

[Schurkey]

Make sure the power piston isn't sticking in the bore, and the little plastic retainer hasn't popped out of the bore.

WFO--power piston rises all the way up. If it sticks up, cruise mix will be rich until it drops back down again.

[steve25]

The mixture ratio changes with the octane level in the fuel that you run also and the 455 tend to scavenge pretty hard so if this is a new motor your tuning I would start off with 15.0 ratio at cruse for now just stay totally on the safe side.

As you prove out all phases of throttle usage / rpm temp and load then creep back up to being near lean at cruse.

[Navy Horn 16]

Originally Posted by steve25

That cruse mixture ratio is going to lead to part throttle ping and maybe even hot running issues my friend !

No it won't. 16.5 is the best fuel economy ratio for "lean cruise". His 15.5 is perfectly safe, damn near ideal.

https://www.afrplus.com/AFRplus-unit-functionality.asp

[Navy Horn 16]

Originally Posted by steve25

The mixture ratio changes with the octane level in the fuel that you run

No, it doesn't. The only thing that changes with your fuel is your fuel. The carburetor can't tell what the octane level is. He could be running race gas and the carb isn't going to know the difference. AFR is VOLUME, and the carb just moves fuel and air.

Everything you have written in this thread is wrong.

[Formulajones]

There is some truth to what Steve is saying but it's more in depth than octane.

The ethanol content will certainly affect the AFR, because stoich is much different (richer) with more ethanol content, and the carb should be adjusted and modified accordingly. Not all pump gas is the same.

As far as race fuel and pump gas, yes there will be different jetting requirements for it as quite a few race fuels are oxygenated.

There's a lot to it that Steve, and even I didn't elaborate on but I think that's what he meant.

[Kenth]

Originally Posted by Schurkey

Make sure the power piston isn't sticking in the bore, and the little plastic retainer hasn't popped out of the bore.

WFO--power piston rises all the way up. If it sticks up, cruise mix will be rich until it drops back down again.

I can´t see the power piston nylon retainer going anywhere as long as the airhorn is in place?

[77 TRASHCAN]

Originally Posted by Kenth

I can´t see the power piston nylon retainer going anywhere as long as the airhorn is in place?

Doesn't the boss, cast into the carb top press against the retainer???

[ponjohn]

would a higher volume pump help this in keeping the bowl full?

[pmd400]

I thought it may be the piston sticking up in the bore as well. But the piston would rise when the engine is turned off and it doesn’t run rich when initially started. The fuel pump is a carter competition mechanical pump. Im not sure the fuel bowl would run dry, it was a WOT run for a few seconds and didn’t get to 5000rpm.

[Navy Horn 16]

Originally Posted by Formulajones

The ethanol content will certainly affect the AFR, because stoich is much different (richer) with more ethanol content, and the carb should be adjusted and modified accordingly. Not all pump gas is the same.

Pump gas can be up to 10% Ethanol. That's all you know about your pump gas, other than the octane level. How much actual ethanol that is in your gas will vary from station to station and tank to tank. It depends on how much ethanol cost at the time they were blending, how much RINs cost, how much the blender was willing to pay for carbon offsets, and several other factors.

There are bean counters and chemists working at the refinery to make sure the right blend of gasoline and ethanol makes it to the premium and regular tanks.

It isn't something that you can tune your carb around. That's why I run a mix of 90 octane ethanol free and 100LL. Ethanol has less energy per unit of volume, so you can make more power by getting rid of it from your fuel system.

[Formulas]

Is the exhaust crossover blocked maybe fuel puddling after WOT and takes longer to work out?

[Kenth]

Originally Posted by 77 TRASHCAN

Doesn't the boss, cast into the carb top press against the retainer???

Not necessarily. It may help if the "retainer" is tight fit during installation of airhorn though.
While installed however, the "retainer" is held in place by the air horn gasket and due to the air horn design.
I see the nylon "retainer" mostly as a rich stop for the power piston.