Rebuilt 273 won't idle

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RichB

RichB

CherryCuda65
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I can't get my newly restored 1965 Barracuda with a 273 and original 4bbl carb to idle down.
Dropping below ~ 1500 rpm it cuts off. It was built with a mild performance cam, hydraulic lifters, and 318 heads. The heads are ported and polished and the heads are shaved. Advance is set at 10 deg. BTDC. I blew out the carb adjust holes(3) to no avail. The idle screw is turned all of the way in and it won't stay running.
What should I do?
 
Turn the curb idle screw out until it calms down for starters. Put a vacuum gauge on it so you can tell what it wants. Once you get it to idle at 1000 rpm, start tuning the idle circuit screws watching the vacuum gauge. Cant speak to a stick car but an automatic car with your build should idle about 800 rpm in neutral and 650 +- in gear. Could be it wants a little more initial timing as well but you should be able to get it from where you are.
Good Luck!
 
check for vacuum leaks...

is this a new carb or rebuilt?
 
Same issue for most part with my 273. Major part of my problem was shitty distributor.
Check floats in carb too
 
What cam specs? New fuel pump? Fuel pressure?
 
Yep, it sounds like you don't have enough vacuum to hold the power pistons down and it's just to rich when it idles down, you probably need lighter springs in the power pistons. Does it have a vacuum advance on the dist, unplug it and see if it will idle. Shut the throttle plates all the way and turn the idle screw in about 1 turn after it hits the throttle shaft, turn the idle mixture screws out 2 turns, disconnect vacuum to dist and see if it runs.
 
Pull the plugs to see if it's running rich. Could be flooding over due to too much fuel pressure. AFB's only tolerate 5# max.
 
If the base timing advance was set at 1500, then 10 degrees is not enough. If the vacuum advance is in, with throttle open too much, as you try to set idle speed down, timing will be way too retarded. If you know how to set static time try 15 degrees or a few more, otherwise try 20 to 25 degrees at 1500 RPM, then reset as idle and mixture are tuned.

Also check the throttle plates, primary and secondary, see if they are able to close properly. The primary needs to be per post #8, to insure idle circuit functions.
 
KitCarlson said:
If the base timing advance was set at 1500, then 10 degrees is not enough. If the vacuum advance is in, with throttle open too much, as you try to set idle speed down, timing will be way too retarded. If you know how to set static time try 15 degrees or a few more, otherwise try 20 to 25 degrees at 1500 RPM, then reset as idle and mixture are tuned.

Also check the throttle plates, primary and secondary, see if they are able to close properly. The primary needs to be per post #8, to insure idle circuit functions.
Click to expand...
Way too many un-answered questions.
 
Here is timing spec for 66 273 4B.
The degrees and RPM are at distributor, not crank, so 2x both of those. It is easy to see how timing may be too retarded. At 1450 RPM the mechanical advance is10 to 14 above base. Vacuum could be measured with T, to estimate what is happening on vacuum adv.
IMG_20170720_190534.jpg
 
brian6pac said:
Yep, it sounds like you don't have enough vacuum to hold the power pistons down and it's just to rich when it idles down, you probably need lighter springs in the power pistons. Does it have a vacuum advance on the dist, unplug it and see if it will idle. Shut the throttle plates all the way and turn the idle screw in about 1 turn after it hits the throttle shaft, turn the idle mixture screws out 2 turns, disconnect vacuum to dist and see if it runs.
Click to expand...
KitCarlson said:
If the base timing advance was set at 1500, then 10 degrees is not enough. If the vacuum advance is in, with throttle open too much, as you try to set idle speed down, timing will be way too retarded. If you know how to set static time try 15 degrees or a few more, otherwise try 20 to 25 degrees at 1500 RPM, then reset as idle and mixture are tuned.

Also check the throttle plates, primary and secondary, see if they are able to close properly. The primary needs to be per post #8, to insure idle circuit functions.
Click to expand...

Very Good suggestion - I need to check the secondary. It may be open. I reset the timing at 15 deg. (no vacuum) no change.
 
The original AFB carb in 65 has a large screw in front between the two smaller idle mixture screws found on most AFB's. It's my understanding that this screw controls air flowing through an idle circuit that bypasses the throttle plates. I believe this screw is used to set the idle speed, not a screw on the linkage, which I believe wasn't even present in 65. The linkage will have a fast idle screw for when the choke is on, but not the standard idle screw found on most carbs.

You stated that you can't get the engine to slow down. Try turning this large center screw in until the engine will idle with the throttle plates completely closed.

Then adjust the idle mixture with a vacuum gauge the way you would any other carb. Adjust the mixture screws for max vacuum and then reduce the idle speed using the large center screw and repeat until you get it idling nice at the proper RPM.
 
Thank you all. I see many of you diagnosed a low vacuum condition and too rich near low speed. That seems exactly right since I see many flame ups and backfires from the carb. I'm going to check tomorrow to see if my secondary is closing. That would explain it.
It's hard to see all of this when you're working alone and can't keep it running. I'll let you know when I get it running.
 
Just re-read your initial post. Now I understand that it stalls below 1500. Based on that, I would say open up the large center idle air screw to get some air flowing through the idle circuit. Then you should be able to back out the idle screw on the linkage.

Also, I think backfires and flames out of the carb indicates timing is too far advanced.
 
I have found a few carbs with messed up throttle plates, things like flipped plates, or miss adjusted, so they bind in bores. One way to inspect is with a light, look for gaps when closed.

How are you setting timing? If with a light running at 1500 RPM, it needs to be at least 20 to 25 degrees. Changing to 15 with no change indicates, other issues, 5 + degrees more should be noticeable.
 
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RichB said:
Thank you all. I see many of you diagnosed a low vacuum condition and too rich near low speed. That seems exactly right since I see many flame ups and backfires from the carb. I'm going to check tomorrow to see if my secondary is closing. That would explain it.
It's hard to see all of this when you're working alone and can't keep it running. I'll let you know when I get it running.
Click to expand...
Backfires are usually timing and not carbruation unless extremely lean.
 
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I brought my Carter AFB into an old, experienced mechanic to look over. He didn't see any obvious problem with the carburetor. I re-installed it , covered over the secondaries and the port to the brake booster and the engine ran much better. I uncovered the secondaries and it made no difference. It looks like the main problem is a vacuum leak through the power brake booster.
 
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