Stumble off idle -

[decooney]

John,
are you referring to stumbling when you are just barely on the throttle, at steady cruise speed, say at 25-30mph mph in 2nd gear?

[Grubby]

Originally Posted by decooney

John,
are you referring to stumbling when you are just barely on the throttle, at steady cruise speed, say at 25-30mph mph in 2nd gear?

Yes and no. It did have a stumble when you moderately hit the gas from lower rpms. In addition it has some lean surge at steady cruise at the lower rpms.

Both conditions seem to have gotten better by going up 4 jet sizes. Seat of the pants power seems to be better as well at WOT.

John

[blykins]

We may need to look at the high speed air bleeds.

One thing that is different from when you had the 750 Holley and this new carb is the weather. We're around the same location as far as weather/temps go and the past few weeks have really seen a drop in temps.

[Grubby]

Brent,

It is certainly cooler out and I assume the gas suppliers have switched to the Winter fuel blend.

John

[patrickt]

Originally Posted by Grubby

Brent,

It is certainly cooler out and I assume the gas suppliers have switched to the Winter fuel blend.

John

It only takes about 20 minutes to switch the Holley back on there for a quick compare. Apples to Apples, temp to temp, gas to gas.... That would be definitive.

[blykins]

Temps are a lot cooler, air is a lot denser, which means more air than usual.

[Grubby]

No need to change carbs. I have put almost 8,000 miles on this car - most with the Holley 750. I drive it down to about 40 degrees. The cold did not have this much impact on the other carb for the last 3 years.

Reconfirmed no vac leaks. I used the propane torch method along the intake gaskets, carb base and carb fuel bowl gaskets. A leak on the inside of the intake is possible but i doubt it.

Quickfuel tech support suggested going down a few sizes on both IAB and HSAB. After reading up on the air bleeds; my guess is the IAB is the most likely. Decreasing the size of the IAB will richen the mixture at the high RPM range of the idle circuit (up to 2500rpm) and raise the RPM the carb transitions to main metering. The worst lean surge is between 1600 and 2000rpms in 5th gear.

I am inclined to drop IAB next. How many sizes? How many different sizes should I order and in what ranges? I assume the IAB change should be made to the primary side of the carb only. At least initially. Quickfuel tech says it is likely that with this change I will need to go down a few sizes in main metering jets.

I have been reading everything I can find to get up to speed on Holley 4150 tuning. I am feeling more comfortable.

I missed Brent today. So I will catch up with him or his carb builder later.

John