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Discussion Starter · #1 ·
ever since the temperature has dropped here in illinois, my 350 crate motor occasionally backfires through the carb. it's been particularly bad lately since it's getting cooler. once it warms up it's better and eventually goes away when the engine is hot. what kind of adjustments should be made for cold weather driving? i've heard your suppose to advance or retard the timing some. thanks.

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Rich
70 Chevelle
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You will need to retard the timing due to the gas quality of winter gas. I believe that it is propane or some other gas that is added to winter gas. I just had this conversation with my dyno man on Friday.

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1970 Chevelle SS Clone 502-540HP-560ft-lb
1992 Trans Am Conv(4Sale)
1992 Taurus SHO
Mercer, PA
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You will need to retard the timing due to the gas quality of winter gas. I believe that it is propane or some other gas that is added to winter gas. I just had this conversation with my dyno man on Friday.

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1970 Chevelle SS Clone 502-540HP-560ft-lb
1992 Trans Am Conv(4Sale)
1992 Taurus SHO
Mercer, PA
http://sswinger.home.att.net/
 

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Rich, Sam has a good point about gas quality. One other consideration may have more to do with the cold air charge that your engine is now seeing. Denser air allows a larger initial air charge into the cylinder and needs to be also offset with an increase in fuel. Otherwise, your engine could be running lean and will definitely spit back at you through the carb. What carb are you using and what size jets are installed? Try jetting up two numbers at a time until the popping and spitting stops. My initial guess is that you will just need to go up only two numbers. I know first hand what a lean engine sounds like when you try to nail it...been there and done that! Hope this helps.

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'70 Malibu - 383 'Fog Fed' Stroker! Houston, Texas Gold Member #39
Chevellz Alwayz!
 

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Discussion Starter · #5 ·
the carb is an 600 cfm edelbrock. could choke adjustment have anything to do with it since the backfiring eventually goes away?

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Rich
70 Chevelle
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Rich, I would have to guess that not choking the engine enough on a cold start would not necessarily cause it to backfire and spit back through the carb. A badly choked engine will simply die and have difficulty running before fully warming up.

It could be that once the engine compartment is warmed up enough to operating temps, that the colder air charge is also warming several degrees before entering the carb and your engine is no longer seeing the leaner air/fuel ratio. Maybe that is why the backfires dissappear after warm-up? This is just a guess though.

I only have experience with Holleys' and could not tell you exactly how to jet up the metering rods on your carb. I am sure that is really easy and I bet someone here knows how to do it - jetting up is still my best guess as the solution to your problem. It doesn't sound like an ignition problem at all. Let me know what you find.

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'70 Malibu - 383 'Fog Fed' Stroker! Houston, Texas Gold Member #39
Chevellz Alwayz!
 

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No No do not change the jetting, the choke is the problem, that is why it is okay when warm, adjust the choke so that the tension that makes it close is greater. This will make it more rich and stay closed longer. Every other part of carb can remain the same. As for retarding the timing, I wouldnt do that unless you can hear it pinging.

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Choke is probably the problem. I have had the same problem on several cars that were set up in the warm summer months, and when the cold mornings set in, the choke will need to finally be set up for what it's meant for. I've also noticed that the milder the motor, the more touchy it is about the choke, could be other factors, just my crude observations.


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Steve

72 Chevelle SS402/4sp
 

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Discussion Starter · #9 ·

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Most choke housings {the black housing} will have a pointer on it with a rich, lean, marking turn it a couple of marks richer for winter.Just don't forget to set it back come spring...FRED
 
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