Hi all,

I'm having a frustrating issue in the cold weather. Granted, cold weather usually brings around some drivability issues for carbs, but this is different.

The engine starts like a champ. It was -15° degrees a few weeks ago, and it fired up the first half turn of the key. Then, only about two more starting attempts before it idled on its own. Then, once it warms up, it drives just fine. Overall, it does really well in the cold.

However, I'm having an issue that's driving me crazy, and I can't seem to find any source for it. Also, keep in mind that this happened with the 600cfm Holley I had on there and with the 450cfm QuickFuel I have on there now. So it's not isolated to a single carb.

What happens is, the first 20 - 30 seconds after first firing up, it idles beautifully. Around 12 AFR, exhaust is smooth, etc. Then, suddenly, it'll start sputtering and stumbling and my AFR drops to around 10 (as low as it'll go.) I can listen to the exhaust audibly change as I'm outside scraping ice off the windows.

The AFR will then stay at 10:1 for the next 10 - 15 minutes of driving, even well after the engine's warmed up. It'll struggle to idle around 200 RPMs (often stalling out), the engine's shaking, sputtering, etc. and I have to finagle the gas, brake, and clutch to keep it running at stop lights (which is not my favorite thing). The AFR remains flatlined at 10:1 from 0 to 4000 RPMs whether I'm accelerating or cruising. The whole cab just reeks of gas filled exhaust.

Then, suddenly.... it'll just go away. At the next random stoplight it'll return to a smooth 12:1 AFR at idle, and cruise is around 14 - 15 AFR. It's like it never happened. There's no middle ground. It's just plain done acting up and will not do it again until it sits for a while, and it'll drive beautifully for the rest of the day.


Any clues as to what would cause this? As said, it's persisted through two carbs, so I unless it's an issue with the carb style (ie. any Holley carb will do this), it must be something else. Something in the valve train? Ignition freezing and firing poorly? I have no idea.

Thoughts?

 

stuck choke?

 

Justin, are you running an open element air filter?

 

I'd investigate the ignition and its wiring connections, crappy connections can explain that behavior as can failing electronic devices.

 

I know the choke works, but I hadn't thought about if maybe the cold weather was keeping it stuck closed (like moisture in the air freezing, etc.)
However, it was 15° this morning and I popped the hood after I fired it up and it opened up like normal. However... it didn't act up this morning. So I'll keep checking until it acts up and see if it's opening then.

I am. Just a normal, open element filter.

I'll look it over. Almost every single wire is brand new as of last year, but that doesn't mean I missed something, or something isn't loose. A weak spark would make sense. I just find it strange that there's no in between. It just *poof*, goes back to normal all the sudden. But maybe something to do with the HEI? I have no idea.

 

Holley carbs are not known for having good behavior chokes.

I would try leaning out the choke 1 notch and raise the fast idle speed a little.
also that open filter and I bet no heating of the intake manifold does not help any.
Dave - - - -

 

If the choke isn't opening properly, that would make sense as to what the deal is, since it's super rich all throughout the power band. Then, suddenly, comes unstuck and flings open. Problem over.

Otherwise, the intake is heated. And if I'm not having this issue acting up, it drives just fine even when it was -15° out (after its warmed up).

The open filter and coolant heated intake really only cause an issue when the engine is cold as I get massive lean spikes when I push on the gas.

The issue I'm having here is a very different thing.

 

Keep in mind that with both carburetors, you experienced this condition and both shared the open element air cleaner. With no heated air going to the carburetor, the evaporating gasoline drops the temperature in the venturi and moisture in the air freezes causing an ice buildup, which later melts as engine temp comes into play.

There was a reason the engineers designed the air filter and snorkel as they did.

Here is a good read on the subject: Air Cleaners - ???Gary's Garagemahal

 

Oh I definitely know there's a reason for them. It's something I've had to just settle dealing with after doing performance upgrades. Ice cold air has a lot more oxygen in it than warm, and until my engine warms up, I have some serious lean issues and often struggle to not kill the engine if I drive it when it's ice cold. A snorkeled / heated air intake alleviates this quite a bit by taking warm air from around the manifolds.

However, the lean spike issue from the open air element (and using coolant to warm the intake) alleviates gradually over time as my engine warms up. Once the engine is at full operating temperature, my drivability issues from cold air are almost non-existent, as I would expect. Every little smidgen my temp gauge moves equates to less and less lean spikes on acceleration.



This issue is different as it's a sometimes it happens, sometimes it doesn't phenomenon and when it does, it's PIG RICH no matter if I'm idling, cruising, or accelerating, often persisting long after the engine has fully warmed up. Then, like a light switch, it just immediately goes away and full drivability is restored with a good, smooth idle, and a healthy stoich cruise. It's done acting up for the day.

There are also other times, like this morning when it was 15° out, where it didn't happen at all.

 

I'm not a carb guy but have you checked fuel pressure when it goes rich?

 

Carb icing can happen at different temperatures because of moisture content in the air.
And it does not have to be way below freezing to happen.

Heating the in take does nothing for carb icing only helps keep the fuel atomized after the carb.
What you are seeing when it runs better is the engine bay heating up so warm air is pulled into that open air filter assy.

I also think you have a choke issue that needs to be looked at.
As I said Holley's don't have good working chokes.
It is not uncommon to have 1 setting for winter and 1 for summer. Or you get it to work ok for both but it is never great.
Just my .02
Dave - - - -

 

Kind of had a similar problem in my '61 on a very cold winter day - ran fine until I tried going up a really long hill (which prompted me to give it a lot of gas) - and it puked and ran like crap. An older and wiser mechanic (when I finally made it to work) suggested that my problem was that as air was being sucked through the venturi, it was chilling to the point where the vaporized gas was condensing out (and we all know liquid gas doesn't burn.) Is it possible you have a similar problem and as the engine warms up and you hit the magic temp where gas stays vaporized, it clears up? Would also explain why your fuel ratio dropped like a stone and then(when the manifold warmed up enough to let all the fuel/air get through) rose back to proper levels. Try arranging in addition to the manifold heat, to suck some warm air through the carb? (I wound up making an impromptu cardboard housing around the air filter that day and ran some VW preheater hose down to just behind the exhaust manifold - never had another problem, and at that time it was the coldest winter in Kansas history). Just a thought.

 

I do think there's merit to the icing idea. And yeah, it seems to be more prevalent at "higher" freezing temperatures, like 20 - 30°, rather than 15° and below. At least around here, things tend to get a crisp layer of frost when it's extremely cold, and a more larger crystal frost at higher freezing temps. The larger ice crystals would probably be more problematic. As said, I'll keep checking it every morning to see if something happens, like the choke sticking or freezing closed. Otherwise, this isn't displaying any of the "carb icing" symptoms I'm reading online.

I know the drivability improves due to warmer air in the engine bay.
My reason for explaining how it behaves is because that's a very gradual thing, that correlates directly with increased engine temps. The warmer it gets, the better it drives. And I don't have a pre-heated air filter assembly anymore, so the difference when it runs cold vs warm is extreme (especially when cold).

The issue I'm having here is not related as it is an ON/OFF scenario. There is no gradualness to it.

I adjust my choke several times throughout the year, the last time being just a few weeks ago. It also snaps alive on the first half turn of the key, even when frigid out, so the closed setting is correct for the time of year. And once fully warmed up, it's 100% open. I double checked it again this morning.

I appreciate all the thoughts and brainstorming.

 

^ X2


........

 

Icing up is usually easy to see if you remove the air filter & look down into the carb, when it is acting up (obviously, be careful doing this - fan & belts etc).

In my experience, the ice will look distinctly white in the limited light of the carb throat, & be visible down at the venturi level.

The other benefit of having the complete & working stock intake system, is that in hot weather you are ducting cold air to the carb, instead of hot under-hood air.
This means more oxygen, so more power when tuned accordingly.

The rule of thumb is 1% power increase for every 10F drop in air temperature.......so a free 5+ hp gain, & mileage, is pretty do-able in summer.