How much does the cold weather lean out carb engine.

[Murd]

Being an age where I’ve never driven anything but efi, how much does the mixture lean out when temps drop? Next couple weeks look like low 50s at best,, mostly 40-50deg daytime highs, expecting salt could be on the roads within the next 2-4 weeks so I’d like to get some driving in first.
Never had a tune done on my Holley 650 DP so I don’t know my AFR, last I checked plugs they were nice and tan in 75-80deg weather. Do I need to worry about it getting lean in say 40deg weather?

My only experience with this is with snowmobiles. When it gets really cold, like -20 or colder, they like to lean out and go pop.

Thanks

Murd

[NAZ]

Yes, temperature can affect AFR but typically not enough to worry about unless you are trying to extract every ounce of performance out of your engine. Temperature is one factor that changes density altitude and we usually refer to density altitude when tuning AFR. A rule of thumb for Holley (and clones) carbs is to change one main jet size for every 2000' of density altitude change (smaller jets for higher elevation). If you're tuning on your race car you'll want to know the density altitude not just the air temp and base your changes off DA rather than just temp. Again, not something a street car should need.

[BadAsp427]

Ditto Naz.... For a street car, you most likely not see or feel any real difference. If anything, a bit more power due to the denser air. You should be fine... Enjoy the driving before the salt is out...

[Murd]

Thanks guys. Just making sure it’s not going to run too lean and blow something up.

[rich grsc]

So you think all the millions of cars on the road before EFI, changed the carbs every winter then back in the summer?

[SSNK4US]

X3 on BadAsp427.... nothing to worry about.... enjoy!!!! And of course stay away from salt lol

[Murd]

Yes Rich.
That’s what I think, my grandfather always told me about the fall carb change when he put on the snow tires up here in the land of igloos and polar bears.
I have no fricken idea how stock carbs were tuned then, I do know that stock carbs had some more complex stuff going on than an aftermarket performance carb does.
I also know if you have a snowmobile jetted to run sweet at freezing, and you hammer on it at -20 it will live about 10 seconds before lean burn down.

I feel like it’s a fair question.

[CraigS]

...
I feel like it’s a fair question.

It is a fair question. Maybe Rich didn't get his nap in today.

[rich grsc]

If you're out driving your cobra at -20* then I guess you better rejet

[Jim1855]

I'd be more concerned about the tires than the A/F ratio.

See the post: Summer Tires in the Cold

Jim

[scottiec]

From the research I have done, you typically start to see AFR changes in 20 degree increments. Now that is all reading I have done, not real world. Also, the delta may not be noticeable or enough to be concerned about.

However, I just tuned my car and know what the AFR was on a 67 degree day. Now that we are dabbling in the high 40's low 50's, I noticed a little hesitation on tip in, which I didn't have before. I will be putting my wideband back in to monitor my AFR this week and will let you know how much of a change I see. It will be a good test to get an idea. I am glad you are worried about running lean and melting a piston, it is good to pay attention to that stuff.

[NAZ]

Scottiec, you may find that hesitation on tip-in is likely caused by poor atomization and fuel dropping out of suspension as the intake air, manifold, and heads are likely much colder than when you tuned it. If on tip-in you see an instantaneous extremely high spike in AFR and a second later everything recovers; that can usually be tuned out with the accelerator pump system. It's a common thing with performance carbs and manifolds in cold weather and why OEMs used low rise manifolds and exhaust heat below the plenum.

[scottiec]

Scottiec, you may find that hesitation on tip-in is likely caused by poor atomization and fuel dropping out of suspension as the intake air, manifold, and heads are likely much colder than when you tuned it. If on tip-in you see an instantaneous extremely high spike in AFR and a second later everything recovers; that can usually be tuned out with the accelerator pump system. It's a common thing with performance carbs and manifolds in cold weather and why OEMs used low rise manifolds and exhaust heat below the plenum.

Naz, I completely agree with you. It is especially noticeable when the car is cold. I find that these motors are truly warmed up when the oil is at its operating temp, not just coolant. I will look into more accelerator pump nozzle, or a more aggressive cam for the accelerator pump. Also, although there was no indication of pinging, detonation, or knock on the dyno, I do think I can back my initial timing down from 16* to 14*, then just adjust the limiters for my mechanical to keep my total advance at 32*. I figure this just rules out too much initial timing when cold and keeps things safe.

[Yama-Bro]

It's a totally fair question, especially considering your background with snowmobiles. While I don't have any experience with snowmobiles, I rode ATVs for about 20 years. When I rode 2 stroke quads it seemed like we changed the jets every time we rode, either due to temp changes or elevation changes. I never worried about the jetting for temp changes after I switched over to 4 strokes. They are much less sensitive to what the thermometer says.

[Murd]

I'd be more concerned about the tires than the A/F ratio.

See the post: Summer Tires in the Cold

Jim

On it.
I have 15” cooper cobras. All season tire, good down to 7c or low 40s f before they are supposed to start getting hard. Not the main reason I have them but one of the considerations.

Thanks for the replies everyone, hopefully a few more weeks left!