All Small Six 1966 ford 200 Backfires on Downshift and Running Rich (?)

[gavincorneliusedmonds]

I've been putting some good miles on my 1966 200cid Mustang this summer and it's been a lot of fun. Carb was rebuilt (Carter YFA) along with some other tune-ups in the spring and it's been running smoothly and reliably. Recently though I've been getting backfires while downshifting (3 speed manual) from 3rd to 2nd and the idle feels a little rough/rich. If I sit at a light and then move, I get dirty exhaust come out for a couple seconds as I accelerate. New York metro area so temps have been 75-90 degrees at the hottest lately. I'm young and not incredibly experienced with carbureted engines so I'm curious to hear from folks more experienced what some initial things to check or adjust should be. I already replaced my plugs after this started. Should I check my distro cap/spark/cables? Is it a carb thing even though it was just rebuilt? Time to replace fuel pump/filter? Time to put on an electric choke?

 

[awasson]

I've been putting some good miles on my 1966 200cid Mustang this summer and it's been a lot of fun. Carb was rebuilt (Carter YFA) along with some other tune-ups in the spring and it's been running smoothly and reliably. Recently though I've been getting backfires while downshifting (3 speed manual) from 3rd to 2nd and the idle feels a little rough/rich. If I sit at a light and then move, I get dirty exhaust come out for a couple seconds as I accelerate. New York metro area so temps have been 75-90 degrees at the hottest lately. I'm young and not incredibly experienced with carbureted engines so I'm curious to hear from folks more experienced what some initial things to check or adjust should be. I already replaced my plugs after this started. Should I check my distro cap/spark/cables? Is it a carb thing even though it was just rebuilt? Time to replace fuel pump/filter? Time to put on an electric choke?

Hmmm… That’s a tricky one to figure out remotely online. Backfiring on deceleration usually indicates a lean condition (not enough fuel when decelerating and the carb is closed). It can also be caused by air entering the exhaust system (bad exhaust manifold gasket or cracked pipe and igniting unburned fuel. It can also be caused by ignition timing being too retarded but it sounds like the engine is running well from your description.

Check your distributor and cables just to be sure and also make sure you don’t have any vacuum leaks on the intake or exhaust leaks.

 

[B RON CO]

Hi, my 66 Bronco would pop through the exhaust sometimes when I let off the gas, while I still had the original LOM-SCV ignition system. I admit it was fun. That all stopped when I installed the Holley 2 barrel and newer distributor. I think the cause was the ignition timing "catching" up to the engine. Good luck

 

[awasson]

Hi, my 66 Bronco would pop through the exhaust sometimes when I let off the gas, while I still had the original LOM-SCV ignition system. I admit it was fun. That all stopped when I installed the Holley 2 barrel and newer distributor. I think the cause was the ignition timing "catching" up to the engine. Good luck

That’s a thought. Maybe it is the ignition catching up and igniting unburned fuel on the way out. My old hot rod BMW 2002 used to pop on deceleration and it had a points ignition. To be honest, I was surprised that my mildly hyped up inline-6 doesn’t do that too but I’ve got an HEI electronic ignition on mine.

 

[John Ha]

Check your choke to make sure it's open all the way after the engine is warm.

Offering as a thought - I was always taught not to downshift to slow down because it puts a strain on the driveline, especially if you don't do it correctly. Brakes are much easier and less expensive to replace than are clutches, transmissions and driveshafts.

 

[gavincorneliusedmonds]

Hmmm… That’s a tricky one to figure out remotely online. Backfiring on deceleration usually indicates a lean condition (not enough fuel when decelerating and the carb is closed). It can also be caused by air entering the exhaust system (bad exhaust manifold gasket or cracked pipe and igniting unburned fuel. It can also be caused by ignition timing being too retarded but it sounds like the engine is running well from your description.

Check your distributor and cables just to be sure and also make sure you don’t have any vacuum leaks on the intake or exhaust leaks.

New header and gasket were put on in April but I will check for any vacuum leaks. After being hot for a while I notice I spit out dirty exhaust on acceleration (especially when going into first from idle.) Not sure if it's burning oil or perhaps another part of fuel delivery that is causing her to run rich. I was thinking of checking my pressure from the fuel pump... PCV is clean, Fuel Filter is clean, and plugs aren't oily but they do have carbon deposits that I just cleaned off of em.

Anyways, my starter just busted so I'm going to deal with that first haha.

 

[gavincorneliusedmonds]

That’s a thought. Maybe it is the ignition catching up and igniting unburned fuel on the way out. My old hot rod BMW 2002 used to pop on deceleration and it had a points ignition. To be honest, I was surprised that my mildly hyped up inline-6 doesn’t do that too but I’ve got an HEI electronic ignition on mine.

I have not yet messed with the ignition timing, I suppose I will look into that. Any recommendations or things to look out for on stock components?

 

[awasson]

Check fuel pressure for sure. The replacement mechanical pumps that have been available recently are known to put out too much pressure and overwhelm the carburetors.

As far as ignition parts to look out for. The little capacitors in the distributor cap have been known to fail from time to time. That and just making sure the points are clean and the gap is correct.

 

[Frank]

After being hot for a while I notice I spit out dirty exhaust on acceleration (especially when going into first from idle.) Not sure if it's burning oil or perhaps another part of fuel delivery that is causing her to run rich.

Are you also having long cranking time restarting a hot engine? Reason for asking: it's possible the fuel is hot-soaking enough in slow stop-go traffic in summer heat that fuel pressure is increasing, causing a rich condition on take off.

You have 2 opposite symptoms here. Popping in the exhaust, if fuel related is a lean scenario. Black smoke taking off is rich.

 

[gavincorneliusedmonds]

Are you also having long cranking time restarting a hot engine? Reason for asking: it's possible the fuel is hot-soaking enough in slow stop-go traffic in summer heat that fuel pressure is increasing, causing a rich condition on take off.

You have 2 opposite symptoms here. Popping in the exhaust, if fuel related is a lean scenario. Black smoke taking off is rich.

Good call. I do have long cranking times in after hot and being in Brooklyn I have a lot of stop and go traffic. Anyway to fix this? Heat wraps on the fuel pump or lines? There have been times I use some cold water on the fuel pump to cool it and crank it up again, especially after gas stops out on Long Island. The smoke coming out the back is white-ish (and to my eyes kind of blue) which leads me to think I am burning some oil too.

 

[Frank]

Good call. I do have long cranking times in after hot and being in Brooklyn I have a lot of stop and go traffic. Anyway to fix this? Heat wraps on the fuel pump or lines? There have been times I use some cold water on the fuel pump to cool it and crank it up again, especially after gas stops out on Long Island. The smoke coming out the back is white-ish (and to my eyes kind of blue) which leads me to think I am burning some oil too.

Yes insulating the fuel line will help a good bit.