Very basic carb questions.
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Tuned
Joined: Jul 2011
Posts: 350
Likes: 3
From: Sacramento, CA
Very basic carb questions.
66 240ci Holley 1940 if it matters.
1) is the butterfly at the top of the venturi suppose to open and close, even a little bit, when you step on the throttle? My just barely moves from 100% open to about 90% open. Without the manual choke attached.
2) sounds like a huge vacuum leak. Kind of. (It really sounds like someone sipping hot soup, loudly, from the edge of a bowl) The air / fuel mixture screw does nothing. Having it fully seated doesn't change the sound, speed or vacuum (measured with a vacuum gauge on manifold under 15psi) I used carb cleaner and WD40 to find the vacuum leak. Nothing. When I use a hose to my ear, that loud sipping sound is definitely coming from the venturi. With the engine at idle, I can change the position of the top butterfly but with the engine under power it really wants to stay at the 90% open.
SIDE NOTE: I think I might have vapor lock at well. I've tried to move the fuel lines as far from the engine as I can but I think it's still vaporizing the fuel.
1) is the butterfly at the top of the venturi suppose to open and close, even a little bit, when you step on the throttle? My just barely moves from 100% open to about 90% open. Without the manual choke attached.
2) sounds like a huge vacuum leak. Kind of. (It really sounds like someone sipping hot soup, loudly, from the edge of a bowl) The air / fuel mixture screw does nothing. Having it fully seated doesn't change the sound, speed or vacuum (measured with a vacuum gauge on manifold under 15psi) I used carb cleaner and WD40 to find the vacuum leak. Nothing. When I use a hose to my ear, that loud sipping sound is definitely coming from the venturi. With the engine at idle, I can change the position of the top butterfly but with the engine under power it really wants to stay at the 90% open.
SIDE NOTE: I think I might have vapor lock at well. I've tried to move the fuel lines as far from the engine as I can but I think it's still vaporizing the fuel.
Tuned
Joined: Feb 2010
Posts: 277
Likes: 21
From: Western Montana
I don't know much about Holley carbs, but I'm pretty good with small engines and the Edelbrock on my '62.
The top butterfly is the choke plate. It's only closed when cold starting the engine. During the winter I fully close it, crank till it starts, then back it off as needed. During summer I only need partial choke the first time I start it that day.
The air rushing into the carb and around the venturi will make a sucking noise. It's normal.
Since the adjustment doesn't work it sounds like your carb needs a cleaning and rebuild.
The top butterfly is the choke plate. It's only closed when cold starting the engine. During the winter I fully close it, crank till it starts, then back it off as needed. During summer I only need partial choke the first time I start it that day.
The air rushing into the carb and around the venturi will make a sucking noise. It's normal.
Since the adjustment doesn't work it sounds like your carb needs a cleaning and rebuild.
Thread Starter
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Tuned
Joined: Jul 2011
Posts: 350
Likes: 3
From: Sacramento, CA
Thanks for answering, I've rebuilt it twice already. It's not really the air sucking sound that has me scratching my head although it is a lot louder then I expected. It's the slurping sound. Like slurping the last few drops of soda with a straw. Just a really wet sound. I've rebuilt dozens of motorcycle carbs and a couple old truck carbs but I don't remember any of them sounding like that (or this loud).. I'm going to break out the carb cleaner again. I think there's a vacuum leak I'm missing.
Laughing Gas
Joined: Jul 2007
Posts: 753
Likes: 133
From: Phoenix
The slurping sound you are describing is fuel dripping from the venturis, you would visibly see that while idling. There are three possible causes, float level too high, the timing is retarded or both. You may also have an internal vacuum leak in the carb. The fact that you cannot get any adjustment from the bleed air screws leans towards that, pardon the pun
. One way to check for the internal leak is to put your hand over the air horn blocking air flow and see if the engine stalls immediately or engine stays running and or idle increases. if it stays running or increases there's a leak. In addition looking down in the carb you will see two pin holes next to the venturis, one for each. With the engine idling plug one of the pin holes with a pick or a ball point pen and watch for any changes in the idle, a drop in rpm's to near stall is what you want. As for the timing issue, if the timing is retarded the throttle plates would be open further than normal to compensate thus causing an increase in vacuum creating fuel to draw from the venturis. I hope these suggestions help.
. One way to check for the internal leak is to put your hand over the air horn blocking air flow and see if the engine stalls immediately or engine stays running and or idle increases. if it stays running or increases there's a leak. In addition looking down in the carb you will see two pin holes next to the venturis, one for each. With the engine idling plug one of the pin holes with a pick or a ball point pen and watch for any changes in the idle, a drop in rpm's to near stall is what you want. As for the timing issue, if the timing is retarded the throttle plates would be open further than normal to compensate thus causing an increase in vacuum creating fuel to draw from the venturis. I hope these suggestions help.
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