Troubleshooting my FE

[CompFi]

An update to this thread is overdue. But after weddings and graduations, finally able to get back to tuning the carb per Holley’s instructions. Along with the assistance of a knowledgeable buddy, since I was making little progress on my own,

My carb is the Holley Ultra XP 80805, equipped with an Idle Bypass Valve. Lots of trial & error, we tried Fully Open, Fully Closed, and many settings in between on the Idle Bypass Valve. But we could not detect any noticeable difference. So, we left the Idle Bypass Valve on a neutral setting and moved on to the Idle Speed Screws. The car proved to be extremely sensitive to any adjustments to these screws, particularly the Primary. But Holley instructs that “this adjustment should be made to both the primary and secondary screws in equal amounts, so that the throttle plates are opened the same amount.” We didn’t notice much sensitivity to the adjustments on the Secondary Idle Screw, but I stuck to the instructions to make equal adjustments to primary and secondary screws together. Once warmed up, we were able to get the car to idle at around 950 RPMs. And then, we returned to tweaking the Idle Bypass Valve. This did NOT seem to do much to the idle speed itself, but as we adjusted this valve, we could hear changes to the air flow and therefore the fuel / air mixture. And then something encouraging happened – the engine began to cool itself down. From about 85 degrees C to 75 degrees. Yes, the fans were already on by the time the temps hit 85. But last summer, the fans alone were not enough to cool the engine back down to 75 degrees.

Was the engine running too lean and therefore that was the root cause for running hot? Unclear, but promising.

So, I finally had a break in the weather yesterday to take the beast for a short test drive before the rains rolled in for the weekend.

The engine got hot, but maybe not too hot. Short drive, maybe 5 miles. Seemed like lots of back pressure/ popping and cracking when I got off the gas above 3000 RPM. More popping than I recall. Responsiveness seemed good, though I didn’t really push it very hard. But, I did NOT notice any hesitation or sluggishness. When I got home, I noticed some coolant had spit from the overflow tank but no coolant in the driveway which is progress.

I declared this initial test a success. Certainly more tuning and testing to do before declaring victory, but progress.

Does anyone here have experience with Holley’s Idle Bypass Valve? Holley claims it is a new feature, but I don’t have a sense for what “new” means at Holley. Wondering if I might now be close using the Idle Speed Screws, and so I should be attempting to fine tune the carb via the Idle Bypass Valve.

The Holley instructions describe additional steps to set the mixture using the four mixture screws. I have NOT done this. Not yet anyway, since Holley recommends installing a vacuum gauge to a vacuum port in the manifold. I have zero experience with vacuum gauges etc, but if that is what must happen next then ok.

As always, insights and guidance are welcomed and appreciated.

[twobjshelbys]

Lean engines run hot.

Rich engines backfire.

[patrickt]

Originally Posted by CompFi

Does anyone here have experience with Holley’s Idle Bypass Valve? Holley claims it is a new feature, but I don’t have a sense for what “new” means at Holley. Wondering if I might now be close...

Once upon a time guys used to drill holes in the primary butterflies to help them idle and transition better with big performance engines. Why would they do that? Because they had a mill that put off a good bit of heat, the air under the hood would thin out, the idle would drop, maybe even stall, and when they "upped" the idle by turning in the idle adjustment screw it opened the primaries up past the transition slot which causes 98% of the carb problems in performance autos.

There are two solutions to the hot idle blues. One is the idle bypass valve and the second is to install a cap screw that lets you open the secondaries a bit for more air. Both do the same thing. Read this thread and you will understand:

Best Way to set the idle mix on a 4160

There's a pic in there of my installation of the cap screw, which you don't need because you have the idle bypass. There's also a lengthy explanation of why setting the primary butterfly properly to the transition slot is so important. Super important. As important as tightening your spinners before you tackle the twisty mountain road.

[Grubby]

There is likely a plug in your intake that can be replaced with a vacuum fitting. It won’t be in the ideal location, but will work to give you the vacuum signal needed to adjust the idle mixture screws.

On Edlebrock Performer intake it is on a runner in front of the carb and to the Left.

John

[Grubby]

There is likely a plug in your intake that can be replaced with a vacuum fitting. It won’t be in the ideal location, but will work to give you the vacuum signal needed to adjust the idle mixture screws.

On Edlebrock Performer intake it is on a runner in front of the carb and to the Left.

John

[Gaz64]

The purpose of Holley's bypass valve is the same as a Barry Grant Idle-Eze, or any other adjustable "vacuum leak".
You should set both Primary and Secondary idle speed screws so the transfer slots resemble a small square as viewed from the bottom of the carb.
At that point, the bypass valve has enough range to have almost any engine idling at respectable speed.
Back off your idle speed screws, open the primary side 1/2 turn, secondary side 3/8 turn, and the mixture screws at 3/4 turn out. Bypass valve in the middle of it's range.
Warm the engine up, adjust the mixture screws 1/16 turn either way, monitor engine vacuum and rpm, monitor ignition timing, adjust bypass to bring idle speed to near target, adjust mixture either way, depending on what the engine wants. Highest vacuum, then check AFR at idle.