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10-27-2023, 03:24 PM | #1 |
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Join Date: Jan 2013
Location: Portland, Or.
Posts: 35
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Air Fuel Mixture Adjustment
Still messing with carb issues, but making some headway. The issue now is that no matter where I set the air/fuel mixture from inside the cab, there's no difference. Engine still runs even when it's fully closed.
The needle valve looks to be in good shape. Perhaps the seat is worn or something is out of spec? Not sure where to go from here. thanks |
10-27-2023, 04:25 PM | #2 |
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Location: Germany, near Aachen
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Re: Air Fuel Mixture Adjustment
Rex,
pull out the needle and blow through the canal with compressed air. Then try again.
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10-27-2023, 04:44 PM | #3 |
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Re: Air Fuel Mixture Adjustment
Rex, can you clarify which needle you are referring to? The statement "air/fuel mixture from inside the cab" has me confused. Are you asking about the air/fuel mixture screw or the GAV?
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10-27-2023, 06:54 PM | #4 |
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Location: Portland, Or.
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Re: Air Fuel Mixture Adjustment
Sorry, I'm referring to the GAV. It doesn't seem to want to seat, nor does it have any effect on the operation of the engine. I start with it out one turn. When I turn it in clockwise, nothing, even when it's bottomed out.
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10-27-2023, 07:02 PM | #5 |
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Re: Air Fuel Mixture Adjustment
If the cap jet is drilled out too large, the GAV valve will have no effect. That is why it is important to have all the jets flow tested. The GAV does not have to seat because you normally run with it open from half a turn to three quarters of a turn.
Tom Endy |
10-27-2023, 07:03 PM | #6 |
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Re: Air Fuel Mixture Adjustment
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If the choke rod acts properly, a loose knob is not the problem. Time to move on to Plan B. Marshall |
10-27-2023, 08:33 PM | #7 |
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Location: Huntington, NY
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Re: Air Fuel Mixture Adjustment
Unless it's in a former post, you have not given a lot of info on the proceedings so far. While I can agree with Tom's suggestion, it could also be that your lower carb body is the 28/29 style that has a removable seat for the GAV needle. If this is missing, the needle will never seat and fuel flow to the cap jet will be massive. Then, there have been several different manufacturers of both the GAV needle and GAV housing over the years. The needles vary in length and threaded area length. SO, in some cases, even if the lower seat in the body is good, the needle will not seat. Are the parts you're working with new or used? Do you have an extra carb so you could interchange these parts from one to the other? Are you missing that removable GAV needle seat in the housing?
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10-27-2023, 08:43 PM | #8 |
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Join Date: Jan 2013
Location: Portland, Or.
Posts: 35
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Re: Air Fuel Mixture Adjustment
All good things to look for. I have two carbs, both of unknown origin. They came with the two A's I purchased long ago. Both have worked well in the past, but they sat too long with old gas in them and now I'm trying to get them going again. I'm learning as I'm going.
thanks |
11-01-2023, 08:53 AM | #9 | |
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Join Date: Feb 2013
Location: MICHIGAN
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Re: Air Fuel Mixture Adjustment
Quote:
I also always start off recommending that you try and drive your car with a GAV closed as it was designed that way and only open the GAV enough to provide extra fuel as needed for the current conditions. Running around with the GAV half to 1 turn open is a red flag to me that you have a vacuum leak as no engine needs that much extra fuel typically. The main and compensator jets would have to be severely under sized to require that much extra fuel. Max flow is achieved through the GAV orifice at one turn open it is pointless to open it more than one turn. The GAV is a high speed adjustment active only above 600 rpm. It should have absolutely no effect at idle rpm's, 600 and less. David ( I would gladly discuss this carburetor theory with anyone who is interested, feel free to call me. ) |
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11-01-2023, 08:56 AM | #10 | |
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Join Date: Feb 2013
Location: MICHIGAN
Posts: 38
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Re: Air Fuel Mixture Adjustment
Quote:
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11-01-2023, 09:24 AM | #11 |
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Join Date: Apr 2018
Location: Mebane NC
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Re: Air Fuel Mixture Adjustment
Would you recommend this for the Marvel carbs as well? I assume so but wanted to check. I run my Marvel with the GAV open 1/4 turn on startup to help the engine power through the high-torque upshifts needed to get on the main roads, but I close it once it's warm.
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11-01-2023, 10:52 AM | #12 |
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Join Date: Oct 2023
Location: Mansfield, Missouri
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Re: Air Fuel Mixture Adjustment
Following, this is helpful information for a newbie
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11-02-2023, 08:30 PM | #13 | |
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Join Date: Feb 2013
Location: MICHIGAN
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Re: Air Fuel Mixture Adjustment
Quote:
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11-05-2023, 08:45 AM | #14 |
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Join Date: Jan 2016
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Re: Air Fuel Mixture Adjustment
Dave, you are a wealth of knowledge on carburetors. Thank you very much for sharing.
Over the past few years I have spoken with you and I learn something every time. |
11-06-2023, 02:31 PM | #15 | |
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Join Date: May 2010
Location: Southern California
Posts: 3,131
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Re: Air Fuel Mixture Adjustment
Quote:
You are absolutely correct. My apologies. I identified the wrong jet. Memory is hazy, I should have referred to my own sketch. What I meant to say, if the comp jet is drilled out the same or larger than the cap jet, the GAV has no control. The sketch is attached. https://www.santaanitaas.org/wp-cont...ay-Diagram.pdf Tom Endy |
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