LS1-450

iTrader: (33)

From earlier this Summer, had installed Quickfuel w/ loose idle adj fueling screws on metering plate that would not stay where set. Quickfuel (actually their vendor, Summit Racing) exchanged it & the new one works great.

Carb is tuned almost perfectly. Idle AFR is 15-16:1. Normal driving is between 13.5-16:1 except the lean spots when very little throttle is applied in 5th & 6th gear @ highway speed 1500-2100 RPM. Can see it going to 18-19:1 lean when it gently stumbles. WOT is 12.5-13.1 after the initial stomp in the high 11's:1 (this was while using the current black accelerator pump cam.

Carb mod's:

Changed to black accelerator pump cam, which has a good early lift, but there's not enough throttle movement in the RPM range noted to get the extra fuel. Tried larger squirters & did not improve the condition, they just made everything else too rich. Shimmed the accelerator pump spring; no improvement. Maybe the answer is smaller air bleeds? Will smaller air bleeds lean out the rest of the fueling; especially @ WOT? Would rather not negatively effect the rest of the fueling just to improve the low RPM slight throttle movement condition.

Timing is conservative. Basically, from 30* @ 300 RPM down to 24* + 10* w/ vacuum advance 'till 2800 RPM where it climbs to 32* by 3000 RPM for WOT pulls. It's @ 26*-34* timing while in the 5th or 6th gear 1500-2100 RPM engine speed range.

I don't think it's a timing issue. Again, can see the lean spot while it's happening. Air bleeds or leave it alone & live w/ it? Thanks

Pop N Wood

iTrader: (1)

The other thing is measure how much of the transfer slots are visible. You need to pull the carb to check that, you should have 20 thou on both the primary and secondary. Mine were shut down too far causing a poor idle transition when cruising off no throttle. If you have to open up the idle speed screws to get more of the transfer slot showing then you may need to take out some timing at idle to get the idle speed back down. that is a lot of timing at idle.

If it is still lean up to 2200 RPM than adjust the idle air bleeds down a stop. You have a quick fuel, you could have done that in the time it takes to write a post. There are separate air bleeds for the idle and high speed circuits so you won't do anything to the WOT tune.

LS1-450

iTrader: (33)

It's a vacuum secondary. There is only adjustment for how soon it comes in. There is no slot visible on secondary vacuum butterfly valves when looking from underside of carb. The butterflies only move when commanded & their initial position is not mechanically adjustable.

There isn't an issue going from off throttle to on throttle. Issue is going from part throttle to just a little more throttle in order to to hold vehicle speed while @ low engine speed.

There is plenty of slot seen on the primary butterfly valves both above & below the butterflies.

Stock EFI PCM's use 40*+ @ idle straight from the factory. I don't understand why 30* is considered a lot on a carb set up. Am not claiming that it isn't; just don't know why it would be.


Problem isn't that it's lean up to 2200 RPM. Problem is that it goes lean while in the engine speed range between 1500-2200 RPM when transitioning from part throttle to just a little more throttle in order to hold vehicle speed. It's as if we are on the .020" part of the accelerator cam ramp & it's not enough throttle movement to reach the next higher ramp on the cam. So, there is no additional fuel added to the new amount of air & it leans out. Can see that it goes from 15:1 to 18-19:1 then stumbles.

Maybe the solution is to run a little richer early (stoich 14.6:1). Had set it up to run leaner than stoich in order to save fuel during normal driving. Maybe that was a mistake.

LS1-450

iTrader: (33)

Got it fixed.

Switched to the white accelerator pump cam which has more ramp in the transition from 20* to 30* & 40* rotation range & it holds the A/F ratio steady in the problem area noted. Originally thought the cam was on the earlier ramp when it happened, 0* to 10* & 20* throttle rotation.

Demonstrates the importance of a wide band 02 sensor if your really trying to dial it in. Thanks for the suggestions.