[tooski]

Carb is a Holley 4779-1, no choke tower.
Getting ready for a cruise in. Warming up engine till it idles without feathering the throttle. AFR in gear is high 12's to mid 13's, temp about 120 +/-. Idles nice, not too stinky, not hunting. 40 min @ 62mph to cruise in. Cruise afr basically mid 15's. Wot at mid 12's. Get to city. Stopped at lights, idle afr now at low to mid 14's, temp 180. Idle steady. Left lights, cruise and wot same. Couple of hour later, leaving cruise in, temp at 110, idle afr at high 16's low 17's. Engine hunting by 100 to 200 rpm. Dies in gear. Drive to store, engine fully warm, 180. Idle afr high 15's to low 16's. Engine hunts and dies in gear before I shut it off. 40 min drive home. Cruise and wot same. Pull into garage, idle afr high 14's mid 15's, engine hunts slightly, stays running till I shut it down.
Went to same car cruise two weeks ago, and previous trips into town, no problems. Earlier in the summer I dialed in the heavy part throttle and wot using the secondary jets and pvcr. I have two primary metering blocks with different size pvcr's. Ended up with the one I started with.
Comments/suggestions?

[Tom Vaught]

Describe the fuel system feeding the carb, please.

Tom V.

[steve25]

Is this a Hydro Camed motor?

[JSchmitz]

Fuel pressure, and subsequent float bowl level, dropping is my guess. Do you have a gauge near the carb? Clean or replace all filters. I had a pleated fine mesh stainless steel inlet filter on my Mallory 110. Fuel pressure would drop off while driving. Screen looked spotless until I held it up to a light. Flushed it with brake clean and crap poured out of it. No more problem.

[steve25]

If think you have a different root cause to this issue.
I think need to skip looking at your AFR readings and instead track your engine vacuum in relation to rpm and engine temp.

[tooski]

Fuel system is 3/8 copper line to short rubber hose to carter m6907 mechanical fuel pump. 6 an braided line to summit adjustable dual feed line for Holley's with sintered bronze filter.
Cowl mounted fuel pressure gauge tapped into the summit feed. Idle and steady cruise it reads 4 1/2 psi (which I think is low, but....). When I'm hammering it it's at 3 1/2 well into 3 rd gear (th350) when I get out of it because of speed. Car does not nose over.
None of that is different before and after my car cruise sojourn.
HFT cam.
The afr readings are only a reference point indicating something has happened to create that much of a change at idle. Indicates no change in the cruise and wot circuits.
If I eliminate the readings, then the idle before was steady 850neutral/750 in gear. When I arrived at the site I idled up the aisle and into the parking spot. After a couple of hours I fired up to leave and the neutral idle was dropping to 600, recovering a bit, then dropping and stalling. If I kept it at 1000 it would be steady. I needed to feather the throttle at lights and stop signs. After the 40 min drive home the idle in gear would hunt from 750 to 650 and not stall.

[77 TRASHCAN]

Take out the idle mixture screws, squirt some spray carb cleaner in the holes with the straw, reinstall...........
I'm guessing you have not been able to get back to your AZ project yet???

[Tom Vaught]

How did you arrive at the 3/8" copper line size?
Inside dimension, outside dimension, identification tag?

Tom V.

[tooski]

Originally Posted by 77 TRASHCAN

Take out the idle mixture screws, squirt some spray carb cleaner in the holes with the straw, reinstall...........
I'm guessing you have not been able to get back to your AZ project yet???

I'll try that. Maybe some junk got into the metering block during changeover.
No to AZ. Really been missing it. We have flights booked for the fall. I'm still kind of apprehensive over the covid thing. The county our place is in has had a resurgence and lately was the worst in the Phoenix area. So far still going for it.

[tooski]

Originally Posted by Tom Vaught

How did you arrive at the 3/8" copper line size?
Inside dimension, outside dimension, identification tag?

Tom V.

I did not. It was on the car when I bought it over 20 years ago. It looks like the non rigid line that comes in a coil. I did not inspect it for an ID tag, nor did I measure it for size. 3/8 id fuel injection hose fits well over it. This is the third engine and third holley carb with this fuel line.

[Tom Vaught]

Well you do not have a 3/8" fuel system.
A True 3/8" fuel system should have a inside diameter of 3/8" so the outside diameter needs to be larger due to the wall thickness.

Assuming a 3/8" outside wall diameter (you said 3/8" FI hose goes over it) would mean your inside diameter would typically on copper tubing .375 minus .040 (one wall), minus .040 (the second wall) or .080 total to be subtracted from your .375" outside diameter.

So you are trying to feed your engine with a fuel line that is .290" in inside diameter approximately.

I run TWO 3/8" steel fuel lines to feed my street engine.
I did this because I could purchase a properly designed (fit the frame) GTO fuel line
for the drivers side and a Chevelle fuel line for the passenger side of the frame.
AND THE STEEL FUEL LINES ARE MUCH SAFER IN A CRASH VS COPPER FUEL LINE AS
I DO NOT WISH TO BE A BURN VICTIM IN THE LATER PART OF MY LIFE.

Many plumb the vehicle for 1/2" fuel line for any performance work with one 3/8" fuel return line for low speed driving.

Food for thought

Tom V.

[77 TRASHCAN]

THe walls of copper tubing are pretty thick........

[Tom Vaught]

Yes they are, and a SCCA Scrutineer would never allow a copper fuel line on their race tracks. Totally unsafe in a crash.

Tom V.

[77 TRASHCAN]

Frank,
So how long has this fuel system been on the engine?
I am assuming it has been performing properly the whole time it has been installed, AND
this is the first "hiccup" you've had "performance wise" since installing???

[Tom Vaught]

My Opinion: You can live with "Hick-ups", they eventually go away.

Burned Human Flesh and Pain never goes away. Remove the copper line and install a safe
fuel line on the vehicle.

Tom V.

[i82much]

maybe an intermittent vacuum leak? i had a bad power brake booster check valve that gave me fits with intermittent lean conditions. no problems since i replaced it.

[Tom Vaught]

Good point there. Used to working with manual brakes or Hydra-boost brakes.

Tom V.

[tooski]

Originally Posted by 77 TRASHCAN

Frank,
So how long has this fuel system been on the engine?
I am assuming it has been performing properly the whole time it has been installed, AND
this is the first "hiccup" you've had "performance wise" since installing???

20+ years. 3 different engines. 3 different Holley's. Had some wot troubles 3 years ago new engine...trouble resolved with carb refresh. Never any previous idle troubles. However I doubt that I would have driven 40k miles.

[tooski]

Update. Yesterday morning I fired It up.Still would not idle once warmed up. Other duties required my attention so I left it for now. After supper I thought I would try it once more thinking I'll have start with cleaning it again. Fired it up and it idled well. Went for a 20 spin/blast, stopping and idling and shutting it off a couple of times. So, whatever gremlin had descended on me had it's fun and left. The intermittent vacuum leak has me thinking. A few years ago after cleaning one of the other carbs I forgot to install the mixture screws. The car started but would die without constantly stabbing the throttle. Massive vacuum leak.
Thanks for all the replies.

[JSchmitz]

Idk why I didn't think of this before. What about a gas tank vent problem? Might want to look at that.