440-6 rich idle challenge

[70runner]

.030 over, stealth heads, SRP pistons approx .006 in the hole, stock iron manifolds, Lunati 60303, stock rockers, 10:2CR, TTI 2.5 exhaust, pertronix II with flamethrower II coil. Carbs are new 71 repros, 65jets, 6.5PV, #35 squirter. Engine is on test stand with 727 attached. Engine idles at 850rpm, 20deg base timing, 14-15" vacuum, however, to get sufficient idle center carb throttle blade is well up the idle transition slot. Center screws at approx 1.5 turns, outboards approx 1/8 turns. Idle at approx 13 on wideband AF meter. Here's a short video of a recent startup/run: http://youtu.be/JN5rMmg_bhg

First procedure: with center carb removed, adjusted the throttle/transition slot to "square", then adjusted timing for max rpm/vac without touching idle screw. Highest rpm I could achieve was barely 600 with timing at 32deg. Engine won't start with that much base.

Second procedure: returned center to transition circuit (so it would start and idle), seated outboard idle screws. No significant effect on AF, approx 13.2, rough idle, hard start.

Third procedure: enlarged outboard throttle air bleed holes from .082" stock to .086". Some improvement in AF, to about 13.4, otherwise no change in rpm or vac (so still in transition circuit).

Should I continue to enlarge the bleed holes or is there something else I can try ?

 

[Meep-Meep]

It sounds like you are on the right track with looking at throttle blade/transfer slot position in relation to timing and idle speed. On the outboard carbs are you seating the mixture screws or the throttle plate screws? The secondary throttle blades shouldn't stick in the bores so having that little stop screw to just hold the blades from seating should be all you need. Then you would adjust the mixture to compensate for the extra air. No vacuum leaks I assume? Hot idle compensator is not leaking? You could try sealing off two outboard carbs by sandwiching a piece of thin cardboard under the base gasket and see if it will idle off the center carb, or if the outboard carbs are contributing to the problem. If you are sure you can't supply sufficient A/F mixture with the center carb only, and at idle with the throttle in proper relation to the transfer slot, then you may need to drill a small hole in the primary throttle bores. Usually big overlap cams require this mod.

 

[lewtot184]

in my opinion, the idle air bleeds are already too big in the center carb. there is an idle feed tube in the main well that i believe is the real fuel restriction in the center carb. the oversize bleeds in the center carb require using the end carbs to make up for it. i adjust the mixture screws on the end carbs at 3/4 turn out from seated. with the engine warmed up and running cover the idle air bleeds on the end carbs and check whether the rpm increases or decreases. if there is an increase the mix is lean, decrease mix is rich. usually the 3/4 turn out will put you in the middle and is a good starting point. now go back to the center carb and make adjustments. usually 3/4 to 1 1/4 turn out is all i need on my cars.

 

[70runner]

It sounds like you are on the right track with looking at throttle blade/transfer slot position in relation to timing and idle speed. On the outboard carbs are you seating the mixture screws or the throttle plate screws? The secondary throttle blades shouldn't stick in the bores so having that little stop screw to just hold the blades from seating should be all you need. Then you would adjust the mixture to compensate for the extra air. No vacuum leaks I assume? Hot idle compensator is not leaking? You could try sealing off two outboard carbs by sandwiching a piece of thin cardboard under the base gasket and see if it will idle off the center carb, or if the outboard carbs are contributing to the problem. If you are sure you can't supply sufficient A/F mixture with the center carb only, and at idle with the throttle in proper relation to the transfer slot, then you may need to drill a small hole in the primary throttle bores. Usually big overlap cams require this mod.

Seating the mixture screws on the outboards, the ones that are REALLY hard to get to. No vac leaks as best as I can tell. I've seen folks talk about drilling the primary throttle blades, but was gonna leave that as a last resort. I might swap to a 5.5PV just to make sure the 6.5 isn't contributing. Interesting idea on the cardboard seal - been trying to figure out how to isolate the carbs during idle to help pinpoint the problem.

in my opinion, the idle air bleeds are already too big in the center carb. there is an idle feed tube in the main well that i believe is the real fuel restriction in the center carb. the oversize bleeds in the center carb require using the end carbs to make up for it.

not sure I follow - are you talking about the carb design?

i adjust the mixture screws on the end carbs at 3/4 turn out from seated. with the engine warmed up and running cover the idle air bleeds on the end carbs and check whether the rpm increases or decreases. if there is an increase the mix is lean, decrease mix is rich. usually the 3/4 turn out will put you in the middle and is a good starting point. now go back to the center carb and make adjustments. usually 3/4 to 1 1/4 turn out is all i need on my cars.

that's exactly how I did mine, tho I'm 1 1/2 out on the centers; anything between 1 and 2 out on the center doesn't seem to make much difference (clue?)

 

[Meep-Meep]

The mixture screws in just about every carb only supply a corrected A/F ratio to the idle discharge hole and transfer slot. On the top of the Holley metering block you will see some small plugs that look like miniature freeze plugs. The ones aligned with the idle passage is where the fuel restriction is but it's almost never messed with. There is an accessible passage on the metering block right where the mixture screws go in and that's referred to the idle feed restriction. Some carbs have a small orifice in there but mostly it's just a big hole. One trick to restrict the fuel is to bridge a piece of wire across the holes. Here's a pic of what I'm talking about http://c564296.r96.cf2.rackcdn.com/Articles/2011/09/13/h12-1011ae.jpg

Looks like the rest of the article is here. Hope this helps. http://beta.amosauto.com/Articles/Corvette/Tech/tuning-holley-four-barrel-carbs

 

[70runner]

Yea, the IFRs - THX! Good article. Looks like they drilled out the primaries before doing the IFR wire trick.

 

[Meep-Meep]

Yeah, I forgot all about those on my first post. Hopefully this will get it sorted out.

 

[70runner]

Seems like there are a couple options at this point to get the engine some more air (and idle): continue enlarging the outboard throttle bleed holes or drill some new holes in the center throttle plate.

I could try the wire in the IFR surgery, but my sense is that won't get it off the transition circuit...could be wrong about this tho (?).

Other option is install it in the car as is and see how it runs under load...tho that shouldn't have much effect on the idle issue.

 

[70runner]

Update: still wrestling with the rich idle. Took out a couple spark plugs with about 20min run time - interesting pattern, black on one side, white on the other. Checked a couple others, same appearance. Can anyone tell me what this means?

 

[Meep-Meep]

Those plugs do look odd. And also strange the color is on the same side of the plug since I doubt they index exactly the same in two different cylinders. Just to throw out a theory here, maybe it's possible the dark side could be where the spark is initiated and starting the burn cycle because the plug is sharper at that point. And maybe after the plug fires more cycles it finds a new area to spark and keeps finding new sharper areas throughout it's life. I don't recall seeing any new plug with very little run time have complete coloring.

Going back and refreshing myself by reading your first post nothing seems out of whack except the base timing being 20 deg. Normally I find 15 deg I and 36 - 38 total all in by 2000-2500 for a mild cam engine to be about right. But that's no small cam and is also interesting it produces 14-15" Hg. Perhaps 20 deg is what it wants?

Having the transfer slot half way exposed seems normal to me. When you say "square" does that mean the throttle blade is adjusted to a position that when it covers the slot it forms a square hole? If so that might not be enough throttle opening. Does the engine run on after shutting it down? If not then you can probably open it more and be OK. Believe it or not a properly running factory six pack will tend to run on, which is why they came with the anti-diesel solenoids. I believe that's because of the multiple carbs all cracked open just enough to provide too much air during shut down.

The carb should try to produce a consistent mixture from idle to tip in and WOT. It's very possible the IFR's are only in play for the idle discharge passage and the other restriction hidden up top feeds the transition circuit and the IFR, so restricting the accessible hole may not affect the transition much at all. Easy enough to try and no harm to the carb. Have you tried a single 4 BBL on that engine? Or the six pack on another? You are sure the power valve is not leaking or open somehow?

 

[a440plus6]

I assume you got the right metering block with the carbs. If I remember correctly it is a 6022.
If so it sounds like your emulation tubes might have crap in them.
All idle circuits are controlled by center carb....in my opinion you should never open the plugs up on outboards....they should be controlled like the factory intended....by adjusting rate of opening by diaphragm springs.
I agree with lewtot184 that the idle air bleeds are already too big.
Holley is notorious for putting out screwed up six pak carbs.
I have customers send them to me all the time after they get them back from the Holley Custom Shop to make them run right.
But I don't take things for granted and always tune them on an engine before I send them out.

 

[70runner]

Those plugs do look odd. And also strange the color is on the same side of the plug since I doubt they index exactly the same in two different cylinders. Just to throw out a theory here, maybe it's possible the dark side could be where the spark is initiated and starting the burn cycle because the plug is sharper at that point. And maybe after the plug fires more cycles it finds a new area to spark and keeps finding new sharper areas throughout it's life. I don't recall seeing any new plug with very little run time have complete coloring.

Fellow on another forum said this pattern could indicate oil being drawn from the valve seals or intake gasket. Not good. He suggested I put some new plugs in, mark the porcelain such that when removed the orientation of the darkened area can be determined, that is, does it map to the intake valve.

Going back and refreshing myself by reading your first post nothing seems out of whack except the base timing being 20 deg. Normally I find 15 deg I and 36 - 38 total all in by 2000-2500 for a mild cam engine to be about right. But that's no small cam and is also interesting it produces 14-15" Hg. Perhaps 20 deg is what it wants?

18-20deg seems what it wants, although the rpm will increase if I move the it up to 30deg, but the engine won't start with this much base timing.

Having the transfer slot half way exposed seems normal to me. When you say "square" does that mean the throttle blade is adjusted to a position that when it covers the slot it forms a square hole? If so that might not be enough throttle opening. Does the engine run on after shutting it down? If not then you can probably open it more and be OK. Believe it or not a properly running factory six pack will tend to run on, which is why they came with the anti-diesel solenoids. I believe that's because of the multiple carbs all cracked open just enough to provide too much air during shut down.

Yes, the throttle blade intersects the transition slot in a square hole. If I set the carb this way, as noted in orig post, I can't get the engine to idle by adjusting the timing. Does not run on.

The carb should try to produce a consistent mixture from idle to tip in and WOT. It's very possible the IFR's are only in play for the idle discharge passage and the other restriction hidden up top feeds the transition circuit and the IFR, so restricting the accessible hole may not affect the transition much at all. Easy enough to try and no harm to the carb. Have you tried a single 4 BBL on that engine? Or the six pack on another? You are sure the power valve is not leaking or open somehow?

Have not tried a single 4BBL or the 6pack on another. Fairly certain the PV is working properly and sealed. The mixture screws will stall the engine when seated, though they don't seem to have much effect on rpm/vac between 1-3 turns out.

One other thing - about a week ago I used the wrong metering block gasket when I reassembled the carb and dumped some raw fuel into the engine. Once it started, it was immediately evident, so I shut down. Changed the oil twice and used some crankcase cleaner on the second oil change. Engine has been harder to start since that senior moment. This is getting pretty frustrating but I'm determined to make it work.

 

[70runner]

Removed the intake manifold - wanted to get a look in there. Here's some pix. There is definitely oil residue in the runners, but not sure how much is normal. From what I can see, the valves look ok. I used paper gaskets on both sides of the valleypan and copper gaskacinch on all surfaces to seal.