Odd engine behavior after first shutdown of the day

[Sledge]

I tried searching, could not find the right keywords to get meaningful results.

Engine—carburated Dart/Windsor 427, Quickfuel 750 cfm carb, Victor Jr. manifold, AFR/Smeding heads.

Previous engine was a 351W, 385 hp, originally with a Street Avenger carb, then a Pro Systems carb (no choke), Performer RPM heads and Air Gap manifold.

Both engines act(ed) very nearly the same, including a slight harmonic at about 2050 to 2150 rpm.

First start of the day or hot but after a very short shut down, it runs great. Nice idle, not too rich. Throttle tip-in from idle is crisp, as is the exhaust note.

But if it has been fully warmed up and then shut down for more than 5 to 10 minutes, it starts okay but then runs as if the mixture has become too rich, but there is no visible sign of being too rich. Off-idle throttle response is a bit hesitant and I need to rev the engine higher to get a smooth getaway from stopped. Nowhere near as crisp as before being shut down, and I also need to keep blipping the throttle to get it to run its smoothest at or near idle.

After a while--5 to 10 minutes of running--engine response returns to the way it was after the first start.

I have shut the engine down by shutting off the fuel pump and running the bowls dry. Makes no difference. I have disconnected the choke after the engine is up to temperature. Again, makes no difference

Recently, when getting off the throttle—like when stopping at a stop light—engine idle rpm is initially about 1100 rpm and then slowly reduces to about 950. As if the throttle cable needs lubrication, or if I need stronger return springs. I’ve even backed off the curb idle screw at least a quarter of a turn, with no change in response. Initially idle was set to 800 to 850 rpm. Perhaps the engine has loosened up some since new?

There is insufficient underhood room for a decent spacer between the carb and the manifold. I did install a .3 in thick Holley base gasket yesterday but no difference noted. The fuel system does not have a return to tank or regulator but does have an Aeromotive fuel pump set to about 7 psi according to product info. The car is at sea level.

I can live with this but it does not seem right. All help appreciated.

[sea2jet]

Vapor lock?

[Tom Wells]

Sledge,

I fought this on the Cobra and a subsequent car for years - literally. Their symptoms were essentially identical to yours except occasionally after a good high speed run the idle would go away and the engine would quit suddenly.

The solution was to switch both cars to fuel injection.

The reason for the problem as far as I have been able to determine is the low vapor pressure of the substance we now laughingly call gasoline.

When the lead was removed and alcohol introduced, the boiling point of "gasoline" went way down - at least for the alcohol portion.

I ran one of the carbed cars with the hood and air cleaner removed and observed "Old Faithful" after the engine was fully warm - the bowl vents would spew fuel several inches into the air, and that gasoline would fall back into the carb and literally make the engine run so rich that at idle it would choke down and quit...

I tried heat resistant gaskets, heat shield and everything else I and my group of carb-lovers could think of without any respite. The only one we didn't try was to direct the output of the car's air conditioning system at the carb.

Nothing got rid of the problem.

I'm convinced it is not the carb's fault! It is the gasoline which has changed that makes this happen.

That's why I went to EFI - and no more problem. At least not of that kind ;-)

My theory is that the float bowl temps become higher than the "gas" can withstand without vaporizing, the fuel boils in the float bowls, and "thar she blows!"

Good luck!

Tom

PS: One thing that I didn't get to try was the no-alcohol gas. Maybe you could try a tankful or two (if it wouldn't hurt your engine!) and let us know if it changed any of the symptoms?

[jhv48]

I also live in California and every carbureted engine I’ve had in my cobras over the last ten years have exhibited similar symptoms.
And I came to the conclusion that it was the low boiling point of our blended crappy California gas. Without a return line to the fuel tank, after shutdown, all that fuel in the pump and lines wants to boil and vaporize.
Tried spacers and different carbs. No change. Just learned to live with it.
After hot restart, it takes a minute or two to flush out the hot fuel with fresh cool fuel from the tank then all is well.
Winter California gas formulas have a higher RVP index which means it evaporates at a lower temperature than our summer gas formula. So, the problem is worse in the winter. Supposed to help with air pollution. Summer gas formulas have a lower RVP index which raises the boiling temp slightly.
And, yes, EFI should solve the problem.

[Sledge]

I wish I could afford fuel injection but at the moment I can't.

Is it the recirculation of the fuel that does the trick for EFI systems? If so I could install a recirc fuel system and keep the carb...wishful thinking on my part I suppose...

Thank you for confirming the symptoms are fuel quality related vs something I did wrong to the carb.

Cheers

[Tom Wells]

Bill,

Yep - been there - tried a return style regulator. This would help if the heat came in with the fuel from the tank. It didn't help. The problem seemed to be that the fuel already in the two float bowls got heated up after a while. Since that fuel didn't recirculate, the return didn't help.

The no-alcohol fuel is sold here in Florida all over the place but it was a rarity when I was having my carb misadventures. It is a couple of octane points lower (I think 89 vs 91 for our alcohol-laced premium I usually run); if that wouldn't cause your engine to knock it would seem to be worth a try. My engines are around 10:1 compression so I think it may have been OK.

The advantage to EFI is that it runs at 43.5psig all the way to the injector. The higher pressure prevents boiling. The pressure in a carb float bowl is 1 atmosphere, or 43.5 psi lower than the EFI.

Tom

[eschaider]

Originally Posted by Sledge

I wish I could afford fuel injection but at the moment I can't.

Is it the recirculation of the fuel that does the trick for EFI systems? If so I could install a recirc fuel system and keep the carb...wishful thinking on my part I suppose...

Thank you for confirming the symptoms are fuel quality related vs something I did wrong to the carb.

Cheers

Bill, check out the MS3Pro Plug-N-Play units at DIYAutotune. Use one of these systems with an OEM Ford wiring harnessing and you are off to the the proverbial races with a very easily tunable high quality EFI at a bargain basement price, click here > PNP System. This is the Mach I version system.

If you get a Mach 1 wiring harness from a salvage yard and set your engine displacement in the engine setup window (see below) of the included tuning s/w,



you can have a very capable system at a bargain basement price.

The reason EFI does not have the same fuel vaporization problems that carbs experience is because the base system fuel pressure is higher, typically 3 BAR (43.5 psi). This has the same effect as putting a radiator cap on with a higher pressure (temperature) rating. The higher pressure raises the boiling point of the coolant, just like the higher base system fuel pressure raises the vaporization point (vapor locking point) for gasoline.

Beyond the simple pleasures of being able to enjoy driving your car again you will get tuning capabilities that can only be dreamed about with carbs.


Ed


<Follow on Thought>

Did you ever notice how hot the under hood environment in your daily driver is? When was the last time you experienced the kind of problems the Cobra is having with your daily driver. My bet is never! There is a reason and it has nothing to do with good or bad karma. It is all about the fueling model you are using. Carburetors were made for cars up through the late 70's some early 80's. Fuels have changed significantly since then and most notably they have added at least 10% ethanol universally across the country. There is a website where you can look up "pure" gas, gas stations but they are almost always the better part of a tankful (sometimes more) from wherever you are.

If you want to purposefully end the headaches and aggravation you are going to need to do an EFI conversion. The longer you wait the higher the ante at the poker table. Your car, your engine, your dollars most importantly your decision.

[mrmustang]

Originally Posted by sea2jet

Vapor lock?

Given his location, that was my first thought, my 2nd is that he is on the ragged lower edge of the carb and even with custom tuning (needs a chassis dyno for it). it may be too much for the engine. Sure, runs great when cold, heats up, timing and subsequent vacuum to the engine changes when the choke heats up and kicks in, or out for that matter. I've had similar issues over the years, it all boils down to (no pun intended) to timing, jet sizes, vaccum......My full day, and unlimited carbs to swap out on a chassis dyno taught me an awful lot of what happens when the motor is in the chassis, and the parasitic pull of the rest of the drivetrain comes in to play. Apples and Grapefruit different between an engine dyno tune and a chassis dyno tune. Those that have done both know what I am getting out.

What is your initial timing, then what is your timing once the engine is warmed up? Jet sizes? Vacuum cold, hot? Manual or electric choke? Intake, a standard Vic jr, or a Super vic jr? Engine compression cold, engine compression hot? So many factors and items that can play it to what you are attempting to describe here.

Finally, have you checked that everything is torqued down, intake, exhaust manifolds, heads?


Bill S.

[LG57]

No one mentioned ignition issues. If you're running an MSD distributor with the GM style advance system, then the advance weights may be sticky and not returning fully as the RPM drops. This will give the high idle speed and not be responsive to the idle screw setting. Lube the pivot points for the advance weights and see what happens.
This is not to discount the fuel problems.

[saki302]

After a drive, I can HEAR the gas boiling in my carb.

At the track, I just pop my hood after a 5 min run. No hot start issues that way.

[dwathencars]

I’m in the “try a tank of non ethanol fuel” camp. That has helped me with similar problems, and helps narrow the fixes that can address it. In Michigan, its called “recreational gasoline” for boats and snowmobiles etc. But readily available at many stations. In the Cobra I add an octane booster, or mix in some high lead high octane like Sunoco. Dave.

[incoming]

My neighbor with a very hot tuned 408 was having the exact same problem. He had a flat piece of aluminum 1/4 or 3/16" cut as large as would clear everything and machined to mimic the top of the intake. He then installed it between the Carb and intake. The problem went away.

I always ran pure dino 93 in my "former race car" and never had any problems except for an empty wallet. She spun a bearing and my new build is lower compression, higher flowing experiment.

[patrickt]

I've dealt with this issue for many, many years. What has worked for me to fight percolation woes, and I fill her up with 93 octane 10% ethanol, is to 1) Use a Turkey Pan with a thick aluminum base for heat insulation; 2) Do NOT set your floats as per the 60 year old instructions of putting the fuel level at the base of the sight holes; 3) Set your floats instead to as high as they can go without dribbling fuel out of the boosters when you shut the engine off HOT and look down the top of the carburetor to see if there is a puddle on top of the butterflies; 4) Once you have your floats set to a point that doesn't have the gas percolating out when you shut it down hot, begin your classic tuning steps with an emphasis on the primary butterfly being set just above the bottom of the transition slot and the idle RPMs being controlled by the opening of the secondary (if you have a Holley 3310 replace the secondary set screw that is hidden under the base with an easy to adjust Allen head screw). Now, if you have really crappy gas, then you might have to live with it. But try this first before you do.

[DC COBRA]

Might try dropping the float level a little, sounds like too me that it is flooding a little after it warms up. Had a few holley carbs exhibiting your symptoms in the last couple of years. The older carbs never had these issues, newer carbs either don't like the fuel blend or just manufactured cheaply. Just not made like they used to be.

[xb-60]

A basic question, I know, but what octane rating fuel are you using?

Cheers,
Glen

[Sledge]

Thank you, lots of good advice and suggestions. EFI will have to wait a little bit as I blew right through my budget with the new engine and a respray. But I will start looking for the suggested harness.

Other than canned racing gas, I haven't found a source yet for ethanol-free frool. I use the highest octane pump gas available, which is usually 91 octane. I haven't flown an airplane in years so I don't have a friendly source at an airport.

Lubing the advance pivot points never occurred to me, so great suggestion I'll try it today.

Cheers +,

[Skip J.]

Hi Sledge - I have a QF 750 CFM carb on the 393 in my Lone Star Cobra Replica and also on the 383 in my wife's '67 Nova. Started having issues with the engine in the Nova doing what I suspected to be vapor locking on a regular basis. Pulled my hair trying to find solutions and after some research, I found that Quick Fuel made what they call their Quick Cool Carburetor Insulator Kit.

The main insulator plate consists of a poly material sandwiched between two thin sheets of aluminum. The cost was a little less than $100 so I ordered one, installed it on her 383 and we've not had a single issue since and that was four years ago.

Since Holley purchased Quick Fuel, I wondered if they still made the insulator kit. Looked up my four year old part number, found that although the part number had changed Summit Racing does carry it. Hopefully the following link works for you but if not, the part number is 300-4013QFT.

https://www.summitracing.com/int/parts/qft-300-4013qft

[MaSnaka]

In addition to what everyone else said I recently found my primary plates were catching at idle. Causing my idle to hang up while the rumble of the engine would shake it back down. I loosened the screws just enough (careful not to let one fall) so the plates could move and find a new happy place on the pivot shaft. Sure made a difference when slowly easing into the gas for 1st gear starts. It could be a little twitchy before.

Before I discovered this I thought my pedal or cable was binding but could not find any faults. I did spray some WD on the advance weights in the dist. too.

Good luck.

John

[cycleguy55]

Originally Posted by MaSnaka

In addition to what everyone else said I recently found my primary plates were catching at idle. Causing my idle to hang up while the rumble of the engine would shake it back down. I loosened the screws just enough (careful not to let one fall) so the plates could move and find a new happy place on the pivot shaft. Sure made a difference when slowly easing into the gas for 1st gear starts. It could be a little twitchy before.

Before I discovered this I thought my pedal or cable was binding but could not find any faults. I did spray some WD on the advance weights in the dist. too.

Good luck.

John

I can understand that - and how it may apply here. The carburetor body and the throttle plates expand as they warm, but the airflow of a running engine mitigates that. When a hot engine is then shut off the airflow stops and the heat soak allows the carburetor body and throttle plates to expand, creating an interference fit if the throttle plates aren't perfectly positioned. Re-positioning them eliminates the interference. Brilliant!

[Unique427]

Use non-oxygenated fuel.
There's a VP Racing Fuel location
in Long Beach. Buy some 260 GTX
98 octane non-oxy race fuel for testing.
If that solves your issue
go to this website to find
Ethanol free fuel near you:

Pure-gas.org