You are correct. You were lean at the top of the idle circuit. IAB's control lean/rich at that area. You richened that circuit up (smaller hole, less air) and it got worse.

We have introduced an additional problem, but still didn't fix the initial problem, right?

May have been rich at the top of the idle circuit.

Go up to 74 IABs as first test, if it gets better then maybe drop your IFRs and try the original IABs.

How far out are your mixture screws now?

Brent, let's just suppose that Grubby's problem is not fixable by changing/adjusting the carb. And, remembering that the highest vacuum reading he could muster was 7", what scenario would result in the conditions he's seeing and be consistent with the low vac readings?

Well, it could be a few things....

I can't remember his cam specs exactly right now, but it didn't stick out as a big cam to me. However, the overlap and ICL it's installed on can play a role of course.

I think the Quick Fuel carb will make a noticeable difference though once he gets the stumble ironed out. I think he's on the right track...

Another thing to watch out for John, is that if you have raised the idle speed at any point in time, you possibly could have put it into the transition circuit if you adjusted it all on the primary side.

I'll betcha a bottle of cam break-in goo that if he removes the breathers from both valve covers, fires it up, and puts his hands over the holes he'll feel the leak. Or use aluminum HVAC tape over them to see the suction. That's what we used to do when we were young, dumb, and broke....:cool: If it tries to suck the tape in to the valve covers then he can tweak the carb 'til the cows come home and he's not going to fix the problem.

I always assume everything is mechanically sound in something like this....otherwise the brainstorming/parts chasing would be endless.

Always a possibility for a leak though.

John, maybe you could humor us and just put some tape over the valve cover holes, start her up, and see if it looks like it's trying to suck the tape in. You might even go one step further and stick the vacuum gauge on the dipstick tube after you've sealed up the holes, too. If you eliminate the specter of an internal vacuum leak, and propane around the outside does nothing, then that will put your mind at rest.

When I put the carb on I tested for vac leaks with my propane torch. Blow gas along intake gaskets and around carb and look for idle changes. None.

With the PVC unhooked, the engine blows some oil mist from the valve covers.

Vac is close to 11 at normal idle speed of about 900.

John

Alright.... I can only imagine the frustration that you're going through with what usually is a one-weekend job, if that.:(

watching this tread have simular engine set up...went from a holly to a quick fuel...most of the stumble went away...(notice it more in warmer weather...not noticable in cooler temps)...

The Holley 750 ran really well at steady cruise and part throttle. That indicates to me this is a set up issue with the Quickfuel.

The car is no fun to drive with the Quickfuel. It was much more pleasurable with the Holley and my wallet was fatter.

I will try a few more things and then throw in the towel. It is foolishness to keep spending $25 a whack to try air bleeds or jets. I can only think of one other thing to try anyway - go up in the IAB size.

Idle was mostly set without touching curb idle screw. I went up about 1/4 turn. I am sure the transition slots are still good. The four idle mixture screws are out about 1.5 turns.

John

1 carb - 8 pages, many experts, sound logical advice and poor Grubby, still no positive outcome... :CRY:

What chance would a poor bloke have if it were multi-carbed like webers... :eek:

Grubby - Persistence always wins!
Best wishes with knocking this on the head!

At any time, I will buy the carb back from you, how's that? Like Patrick said, I know it's frustrating not having any time to work on it.

Did you get a hold of Scott?

John,
Sounds like you are done with it, but I only had one last thought - back to my original question when you started with the new carb. In any of your conversations with Phil at Quickfuel did he ever recommend increasing the size of the idle feed restrictor jets for the transition circuit? I realize you bought an HR series carb vs. a Q series version but don't know if the HR has the adjustable metering blocks or not since I don't mess with the HR versions carbs. The only reason I ask at this point is purely for reference and notes.

Note: this was the fix on my buddies QFT carb after working through it with QFT techs. Changing primary/secondary jets, acc pump cams, air bleeds, power valves, all had nothing to do with getting his setup to run clean below 2500 rpms at cruising speeds. It was those darn idle feed restrictors that allowed more fuel at slow cruising speeds and resolved the "stumble off idle" issue which is your original post for this thread. Its a bummer you did not get a chance to try the Q series carb.

John, you and I are not Morris, we're not even Rick Lake.:rolleyes: We don't race our Cobras competitively and, for us, tuning for WOT is over-rated. Personally, I can't hit WOT at low speeds -- she'll just break the tires loose -- and I can't hit WOT for more than two or three seconds at any other speed. In going on seven years with this car, I don't think I've ever held the pedal to the floor for more than three seconds when the car was actually moving. And if I tried to speed shift from third to fourth at 6500 RPM, while holding the throttle to the floor, the next day's discussion would not be whether there was hesitation or stumble, but rather if I get an open-casket funeral or not. That said, what I think you should do is give the QF carb one more weekend of tweaking and monkeying, and then put it in the classifieds here. Put the Holley 750 back on, put the #70 jets in the primary, put the .031 squirters in, orange cam, a 65 power valve, and a black secondary spring -- yes, black. Then carefully set the floats and idle mixture, and see how she runs at part to half throttle. Forget WOT. Only when you have the car running nicely at light to part throttle do you start to change out the secondary spring, going lighter, until you feel hesitation or stumble at the heaviest throttle you can safely hit on the street. Then go back to the spring that was in before that happened. If you do that, I think you can get it so that you are happy 99% of the time.

He doesn't have to put it in the classifieds and take a hit. If he's that unhappy with it, I'll give him his money back.

I would personally never use anything less than an 850 on an engine of this displacement, whether it gets raced or not.

These QFT carbs are certainly capable of excellent part throttle and cruising performance, and I think John has already stated that the WOT difference is very noticeable. He can have the best of the both worlds.

But he's got a nice 750 conveniently positioned on his living room coffee table.;)

My carb did much the same as yours until I removed the idle feed restrictors completely. Try it and see. You'll need a tiny allen wrench to remove all four from the metering blocks.

Brent, you are a stand up guy, we're lucky to have you on this forum.
Lou

It is not a time to work on the carb issue. It is a time to get parts issue for each next step.

I started this carb change not wanting to get a tool box full of jets and air bleeds, and not relearning carb tuning. So much for that. I have read up on the Holley tuning and feel comfortable which things to tweak, and my tool box is filling up with each step.

This should be straight forward. My engine is a pretty standard big block.

Quickfuel tech support and Holley tech support are hit or miss. Some of their answers make sense and some sound like the guy is late for lunch and throws out the first thing he can think of.

I will filter all "expert" advice. If it makes no sense or they can't explain how it works, then I am not going to waste my time and money.

More IABs are inbound. I may have them Thursday.

The plan is;
1. Install IABs that are 4 sizes larger
2. One more try with Brent's carb expert
3. One more try with Phil at Quickfuel (not going to waste a call to their tech support)

If the above don't get it, then I will call Brent and discuss.

John