Very Rich light Throttle Cruise AFR readings

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Walker434

Walker434

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Hey guys,

I have recently installed a Holley HP 750 carb that has been professionally converted to blow through. Recently installed a P-1SC Procharger on my 5.9 Magnum build. Also, installed a water/injection kit to keep the air charge cool.

I have a AEM wideband AFR guage. My readings are perfect at idle and wide open throttle. Also good at half or 3/4 throttle. The only problem is a very rich condition at light throttle cruise. Reading in the 9.5:1 range. The first few drives out with the new set up it didn't do this. I was too rich at WOT so the only change I've made is jetting down the secondary's. After that I all of the sudden had this rich condition at cruise, which obviously doesn't make sense. Fuel pressure is good, float bowl levels are good. Just can't figure this out.

Any help would be greatly appreciated. Thanks FABO.
 
If it's at lower RPM cruise, it's likely still on the idle circuit. Have you tried smaller idle feed restrictors? You'll need to open up the mixture screws to compensate. How far out are your idle mixture screws now?
 
mopowers said:
If it's at lower RPM cruise, it's likely still on the idle circuit. Have you tried smaller idle feed restrictors? You'll need to open up the mixture screws to compensate. How far out are your idle mixture screws now?
Click to expand...

I set the 4 corner idle screws to highest vacuum reading which got me around 14.0 at idle. When you first put it in gear and start to move the AFR readings are still good. Get on the street and give enough gas to get up to say 20 MPH and it goes way rich. Stays rich at light throttle, even at 50 or 55 MPH. But, give it half throttle and it's good

I haven't yet messed with the idle feed restrictors, that's probably my next move. This is the first carb I've owned that they are adjustable.
 
Just by reading the title....

You always get a rich spike when you dip the throttle from the pump shot.... so consider that when you lightly dip the pedal rolling at parking lot speeds...and in the meantime it's probably a lil lean off idle cruise..so it might even jerk on ya....that's just fuel dribbling onto the throttle blades.

If you ever do a chassis Dyno or watch a dyno test in general you'll see a blip when the secondaries start in with your typical dp carb that everyone uses. Almost no one uses a vac sec carb on the dyno.

If the carb is setup for blow through... the pv won't be a problem.
 
Make sure the air bleeds are clean. You may also try going larger on the IABs and see if that helps the off idle cruise.
 
Update: called the guy that built this blow through carb. He also thought power valve staying open. He asked if the vacuum port on the main body was hooked to intake manifold vacuum/boost. It was not. That was my problem. No it's fixed!

Thanks for the help and thoughts FABO!
 
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