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Discussion Starter #1
I have a edelbrock 1406 carburetor that's backfiring through the carburetor on the initial drive of the day when the engine is cold. Once the engine warms up after you have been driving for a few minutes it runs fine, no backfiring. I've had the accelerator pump adjusted and other springs in the carburetor swapped out with no luck. It's a brand new carburetor. The engine is less than 1 year old and it's a 350 long block. Anybody experience this before?
 

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Most situations where I have seen backfiring through the carb or throttle body have been a combination of problems, generally at least two or three of these four:

1. Bad timing.
2. Lean mixture.
3. Bad gas.
4. Sticking or improperly adjusted valves.
 

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Discussion Starter #4
The timing has been checked and re-checked and then checked again. I have been convinced since the start that its my carburetor. I had a Edelbrock 1407 carburetor on my 350 engine and was told that it was too much carburetor for the engine and it was suggested to step down to a small block carburetor. So, I went with that advice. I put on a Edelbrock 1406 carburetor with an electric and ever since, I had hesitation, stumbling, stalling... Then backfiring started. Since the backfiring started.. Springs have been changed, jets, and everything else has been adjusted with no luck of getting the backfiring to go away. Before I drove the car to the shop 2 days ago I changed out my spark plugs and I have uploaded pictures of my plugs to see what you guys think of them. Finally the shop has replaced the carburetor yesterday and I drove the car home last night. The car runs a LOT better, the hesitation is gone, the power is back, however I did get the carburetor to pop a few times while I was in a parking lot and i was in park and thumping on the throttle quickly but its nothing even close to what it was before. I drove the car this morning when the engine was cold and the carburetor did not backfire, but the RPM's were very high and I had to thump on the throttle to get the Rpms to settle down. So, with all that.. What do you guys think?
 

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Discussion Starter #6
Yes, the choke is functioning fine. I have visually seen the choke working like it's supposed to be. The first 1406 carb that was on the engine was terrible and was hesitating, and backfiring horribly. After swapping it out. 90% of it was gone but as I said, I sat in a parking lot and thumped on the throttle quickly and was able to have the carb backfire a few times but it was nothing like the previous carb. I also drove it in the morning when it was dead cold and there was no backfiring with just simple driving around. The previous carb wouldnt have even let me get up the street with out popping all over the place. So, this new carb is a marketable improvement for some reason. Whether it's the carb that was bad, the plugs or something else... I don't know.. Could the distributor have something to do with all of this??? I'm worried that the distributor is the cause of all of my problems and I'm chasing my tail with the carb. If the plugs are so black like they are.. Then what could be causing them to be like that? What else engine wise could cause the plugs to be black and the threads oily like that?
 

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If the plugs are black the carb is running way rich. If you started the car and let it run for a minute or two. Then pulled the number one plug does it smell like gas. I had the same problem once and found out the real problem was that junk from the tank was getting into my carb and causing all sorts of issues like popping from the carb. I went with a new tank and cleaned out the carb and never had a issue again. You could try just cleaning up the carb and test it again by running a rubber hose from your fuel pump to a clean can of gas blocking the line from your tank to the fuel pump while you test.
 

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^ It's an Edelbrock carb, no pump cams.


Is it popping when you lightly open the throttle (to accelerate from a stop light) or when you slam it open like you are trying to launch it?


Can you look and see what position the accelerator pump arm is in on the front right of the carb?
 

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If it backfires with a quick opening of the throttle, you need a bigger accelerator pump shot.
But first the float levels should be checked, as they are never rift from the factory.

Then put the accel pump arm in the top hole, closest to the pivot. This will give the biggest shot.
Then bend the linkage arm as needed, so that the s link almost touches the carb body at full throttle.
Lastly, buy the edelbrock accel pump
Kit, and if the hesitation is still there after doing the above changes, go to the next larger squirted size.

Also if your choke is adjusted properly, the choke will stay on as well as on fast idle until the engine is warmed up.
 

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Discussion Starter #11
Accelerator pump arm is on the middle hole. I have to slam on the throttle to get the carburetor to pop. I have driven the car around the last few days lightly and it ran fine with no backfiring but I never got on the throttle since I had my 8 year old in the car. What I do notice is once I first start the car, the Idle or RPM is way high. I always pump it once to start it now, if I dont it will start but die almost immediately. I have sat there and waited for the RPM's to come down but they just dont. Ive thumped on the throttle and that makes the RPM's come down a little each time I do if I space it out a bit. I dont know if the choke needs to be adjusted or just the idle.. Im not very knowledgeable with either of them frankly. But if walked through it.. I can figure it out. I watched somebody adjusted the electric choke the other day so I may be able to figure it out again if thats all it is. I dont know, its strange.. I have had 3 fuel pumps on this car in the past 3 months... all 3 of them leak at the engine block including the new one. Im concerned obviously as to what the heck could cause all 3 fuel pumps to leak at the engine block. The oil has been changed several times in a few hundred miles and each time it was mixed with fuel.
 

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I'd move that arm to the top hole and see if that helps. That's usually where I ran all of my Edelbrock carbs.


What fuel pumps are you using? I'm running a 15+ yr old Edelbrock and it's still going strong...
 

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improper intake valve adjustment or a valve sticking or burned!
A very unlikely coincidence that valve adjustment or a burnt valve would suddenly appear as soon as the OP swapped carburetors. It's not impossible, but unlikely.

What I do notice is once I first start the car, the Idle or RPM is way high. I always pump it once to start it now, if I dont it will start but die almost immediately. I have sat there and waited for the RPM's to come down but they just dont. Ive thumped on the throttle and that makes the RPM's come down a little each time I do if I space it out a bit. I dont know if the choke needs to be adjusted or just the idle.. Im not very knowledgeable with either of them frankly.
I'm somewhat confused because before you said the choke was definitely set properly, but now you're descriptions reveal that you don't really understand how the choke system works. It's fine that you don't know and that you admit you're not knowledgeable but want to learn, but I question from where you're getting all the information about the timing and carb adjustment. Is this particular shop doing all the work and telling you everything is fine? I ask because first and foremost, if you paid a shop to install and tune the carburetor, you shouldn't have to be worrying about figuring out this issue; you paid a shop to do that, and they should honor that agreement. If the car didn't have the issue before the shop worked on it, then it's their problem.

Now let's talk about the choke: When the engine is cold, in order to actuate the choke you must open the throttle (so you floor the pedal once). This action swings a cam onto the fast-idle step and also closes the choke butterfly most of the way to restrict air and enrich the mixture. When you turn the ignition switch to "run," electricity flows through the electric choke coil (a bimetal spring) in a function that will gradually open the choke butterfly as the choke coil heats up and expands. Note that an electrical coil has nothing to do with the engine (aside from some residual heat) but is a type of electric timer. Technically, if you turn the ignition to run but don't fire the engine, the choke butterfly will still open, which is why you should only turn the key to run as you go to fire the engine. The engine fires and idles on the fastest idle step of the cam in order to allow the engine to run in cold conditions. Once you hear the engine's rpm rising higher and higher, at some point you can lightly blip the throttle (don't mash it), which should swing the fast-idle cam onto a lower fast-idle step, thus lowering idle rpm. If you mash the throttle hard at any point after carb is on the highest fast-idle step, the cam will disengage and place the throttle shaft up against the hot-idle (normal idle) adjustment screw. Essentially, the engine is supposed to idle fast (at a set speed per your setup/carb specs), you should be able to decrease that speed gradually if you like by lightly blipping the throttle to get on smaller fast-idle cam steps, and then the entire fast-idle should go away as soon as you open the throttle wide enough.

Now, the choke butter is controlled by the electric choke coil, so even if you open the throttle enough to lock out the fast-idle, the choke might still be partially closed, which is normal depending on the engine temp. The purpose is so you can begin driving the car sooner when it's still not at running temp because the choke is functioning to lean out the mixture, yet you won't be experiencing fast-idle when idling down the road or at a stop. Are you following me?

to check the choke, with the engine cold, remove the air cleaner and open the throttle wide by hand. The choke butterfly should close for the most part. Go and fire the engine without touching the throttle. Check the choke butterfly. It should have about a 1/4" gap between it and the wall: too little=too rich, too much=too lean. The engine will stumble/stall in either case on a cold day. Once the engine rpm increases with warmth, give slight throttle by hand and let the throttle return to closed. Does the rpm decrease but not all the way down to normal running idle speed? Once you know the engine will idle at normal speed, give some more throttle by hand and see that the throttle shaft stop comes to rest on the normal idle adjuster screw (fast-idle is locked out). While you're doing all of this, allowing the engine to warm, watch the choke butterfly. How quickly is is opening compared to engine temperature? You can adjust the opening rate by turning the choke case, which will expand or contract the spring. You'll want to set the fast-idle speed per Edelbrock's instructions. Engines aren't designed to received full throttle blips when cold, so a poorly tuned choke could cause backfire issues when cold.

As for other things, where is initial timing set? Is this with or without vacuum advance hooked up? Is vacuum advance hooked to manifold or ported vacuum? It should be hooked to ported, no matter what other people might claim about the myth of manifold vacuum.

One more thing: how was the idle air-fuel mixture adjusted? Do you know how many turns out on each adjuster (this isn't the way to adjust, but it'll give us a ballpark idea? Let us know.
 

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A very unlikely coincidence that valve adjustment or a burnt valve would suddenly appear as soon as the OP swapped carburetors. It's not impossible, but unlikely.



I'm somewhat confused because before you said the choke was definitely set properly, but now you're descriptions reveal that you don't really understand how the choke system works. It's fine that you don't know and that you admit you're not knowledgeable but want to learn, but I question from where you're getting all the information about the timing and carb adjustment. Is this particular shop doing all the work and telling you everything is fine? I ask because first and foremost, if you paid a shop to install and tune the carburetor, you shouldn't have to be worrying about figuring out this issue; you paid a shop to do that, and they should honor that agreement. If the car didn't have the issue before the shop worked on it, then it's their problem.

Now let's talk about the choke: When the engine is cold, in order to actuate the choke you must open the throttle (so you floor the pedal once). This action swings a cam onto the fast-idle step and also closes the choke butterfly most of the way to restrict air and enrich the mixture. When you turn the ignition switch to "run," electricity flows through the electric choke coil (a bimetal spring) in a function that will gradually open the choke butterfly as the choke coil heats up and expands. Note that an electrical coil has nothing to do with the engine (aside from some residual heat) but is a type of electric timer. Technically, if you turn the ignition to run but don't fire the engine, the choke butterfly will still open, which is why you should only turn the key to run as you go to fire the engine. The engine fires and idles on the fastest idle step of the cam in order to allow the engine to run in cold conditions. Once you hear the engine's rpm rising higher and higher, at some point you can lightly blip the throttle (don't mash it), which should swing the fast-idle cam onto a lower fast-idle step, thus lowering idle rpm. If you mash the throttle hard at any point after carb is on the highest fast-idle step, the cam will disengage and place the throttle shaft up against the hot-idle (normal idle) adjustment screw. Essentially, the engine is supposed to idle fast (at a set speed per your setup/carb specs), you should be able to decrease that speed gradually if you like by lightly blipping the throttle to get on smaller fast-idle cam steps, and then the entire fast-idle should go away as soon as you open the throttle wide enough.

Now, the choke butter is controlled by the electric choke coil, so even if you open the throttle enough to lock out the fast-idle, the choke might still be partially closed, which is normal depending on the engine temp. The purpose is so you can begin driving the car sooner when it's still not at running temp because the choke is functioning to lean out the mixture, yet you won't be experiencing fast-idle when idling down the road or at a stop. Are you following me?

to check the choke, with the engine cold, remove the air cleaner and open the throttle wide by hand. The choke butterfly should close for the most part. Go and fire the engine without touching the throttle. Check the choke butterfly. It should have about a 1/4" gap between it and the wall: too little=too rich, too much=too lean. The engine will stumble/stall in either case on a cold day. Once the engine rpm increases with warmth, give slight throttle by hand and let the throttle return to closed. Does the rpm decrease but not all the way down to normal running idle speed? Once you know the engine will idle at normal speed, give some more throttle by hand and see that the throttle shaft stop comes to rest on the normal idle adjuster screw (fast-idle is locked out). While you're doing all of this, allowing the engine to warm, watch the choke butterfly. How quickly is is opening compared to engine temperature? You can adjust the opening rate by turning the choke case, which will expand or contract the spring. You'll want to set the fast-idle speed per Edelbrock's instructions. Engines aren't designed to received full throttle blips when cold, so a poorly tuned choke could cause backfire issues when cold.

As for other things, where is initial timing set? Is this with or without vacuum advance hooked up? Is vacuum advance hooked to manifold or ported vacuum? It should be hooked to ported, no matter what other people might claim about the myth of manifold vacuum.

One more thing: how was the idle air-fuel mixture adjusted? Do you know how many turns out on each adjuster (this isn't the way to adjust, but it'll give us a ballpark idea? Let us know.
Best explanation I've seen in a long time of how to properly set the choke and idle. Nice job.

Gerry
 

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Given then OP said it's just when it's cold and he has to slam on the throttle for it to happen (with the new carb at least), isn't this just how it is with a carb and considered a lean pop due to low attomization of fuel?

I guess really detailed tuning can help with this situation, but is it even possible to be able to get into a carb'd car and floor when it's cold?
 

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Discussion Starter #17
The choke has been set and re-set so many times now I can't keep up. The car seems to run fine then fall apart the next day with not wanting to start or loss of power and hesitation to backfiring. Now I have a new carburetor on the engine and it seems to run much better. Today I drove over to Ft Carson and back and this is how it went... I started the car but it was cold out side so I decided to let the car warm up, so I gave her 1 pump and she fired up no problems. The rpms were way high but I hoped they would come down and I went inside to get my son and the rest of my stuff and let the car warm up. After about 3 or 4 minutes I realized that there was a loud noise coming from the garage and I ran out side and the car engine rpm was so high it was crazy. It was just going up and up and up! I jumped in the car and thumped on the throttle with my foot and the rpms came down immediately. I don't know why the rpms were going up like that but it's worrisome for sure. Anyway, I grabbed the rest of my stuff and left. The car seemed to run fine all the way to base with good power. I finished up on base and I left and headed home after about an hour there. On the way back at a traffic light I came to a stop and the rpms dropped so low that the car stalled. I had to re-start the car and keep my foot on the gas and brake at the same time to keep it running at each light for the rest of the way home. Once driving the car ran fine. I have no rpm gauge so I don't know the exact rpms, all I could watch was the voltage and oil pressure which kept dropping close to zero unless I gave the car some gas with my foot on the brake and then they would go back to a normal range. everything has been changed from the carburetor to the gas tank and this is carburetor number 2! It did smell like it was running rich.
 

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Best explanation I've seen in a long time of how to properly set the choke and idle. Nice job.

Gerry
Thanks, Gerry. I did make a typo in saying that "the choke is functioning to lean out the mixture, yet you won't be experiencing fast-idle when idling down the road or at a stop." Obviously from my other details, I meant the choke enriches the mixture.:eek: I was in a rush, and hungry when I said choke butter.:D Mmmm.
 

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The choke has been set and re-set so many times now I can't keep up.
. . . .
The rpms were way high but I hoped they would come down and I went inside to get my son and the rest of my stuff and let the car warm up. After about 3 or 4 minutes I realized that there was a loud noise coming from the garage and I ran out side and the car engine rpm was so high it was crazy. It was just going up and up and up! I jumped in the car and thumped on the throttle with my foot and the rpms came down immediately. I don't know why the rpms were going up like that but it's worrisome for sure.
I have a serious question: Have you even read what anyone has posted trying to help you? I just explained the workings of the choke system and why the engine rpm is supposed to be higher on cold start within spec, yet you say you seem confused as to why the "rpms were way high" and that you "don't know why the rpms were going up like that" and seem amazed that the rpm drops once you "thump on the throttle."

I know why the engine rpm was too high: the choke is not properly set up, If you don't want to take the time to read other people's posts trying to help you, what's the point of this thread?
 

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Discussion Starter #20
I am reading everything guys, don't get angry. This is a learning process for me and I read everything that you guys write. A lot of time as I'm leaning over my engine I read your posts on phone. It's a lot of information to take in for someone who doesn't know carburetors or what things are called. Your also right... The choke was not set up right like I thought it was and like I was told. I drove it back to the shop and had them adjust it. Seemed like a fairly quick fix actually. They agreed that the rpms were set way too high and it was running at over 2000 according to their instruments. So that got set right. A few other things got changed and now it's running much better and the backfiring has stopped, no more hesitation, has good power and all seems well. All the engine needed was a replacement carburetor. This new one seems to function better after some tweeking. I'm sorry you guys got irritated with me, you can't believe how frustrated I have been trying to get this car running properly so that I can just drive it around town. All I have been able to do is drive it around the block then back in the garage because it was backfiring so bad, leaving me searching for answers as to what could be wrong. But, hopefully now the problem is gone and with this replacement edelbrock I'm good to go. I'm sorry again for all the trouble fellas and I hope I haven't caused anybody too much grief, but I really appreciate all the help... Seriously!!
 
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