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12-20-2015, 08:52 AM | #1 |
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Location: Danbury Ct
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Need Marvel Carb Expert
My Marvel carb mystery continues. As I started ti disassemble my Marvel Carbs to build a couple of very good ones I am still puzzled by a mystery third adjusting screw (see picture). Some of my carbs have it and others don't. On many of the sources I have found online this location is labeled "fuel bowl vent".
On the carbs without the adjusting screw there are no threads in the holes, and there is no sign that they ever had this adjustment. So maybe - yes - just a vent. But what about the ones that have this (mixture??) adjustment screw? What are they, what are they there for, and why do some marvel carbs not even have them? I know lots of questions! Thanks in advance for your response. Last edited by pgerhardt; 12-20-2015 at 10:55 AM. |
12-20-2015, 09:22 AM | #2 |
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Re: Need Marvel Carb Expert Needed
pgerhardt, do a search on this forum for Marvel Schebler carbs. There was a very in depth post made by one of our members covering everything about these carbs with an ample amount of great detail photos. You will not be disappointed.
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12-20-2015, 10:23 AM | #3 |
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Re: Need Marvel Carb Expert Needed
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12-20-2015, 10:53 AM | #4 | |
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Re: Need Marvel Carb Expert Needed
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12-20-2015, 11:57 AM | #5 |
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Re: Need Marvel Carb Expert
The main nozzle (only nozzle in a Marvel) sits in a well with several air bleed holes drilled into the tube at various levels. At low throttle demands that well is full and both levels of bleed holes are covered with fuel, at the same level as the bowl setting.
As the throttle reaches half or more open the fuel feed through the main nozzle exceeds the feed rate of the main jet and the vent holes become uncovered. That leans the mixture. The well that the nozzle sits in either has a fixed, calibrated air vent rate to control the overall mixture ratio as the throttle increases or. as in some Marvel variants, an adjustment screw as you have. During normal part throttle low speed running the nozzle well vent adjustment does almost nothing. During heavy throttle the screw will either cause the nozzle well to empty faster (leaner top end, economy) or slower (richer top end, power). |
12-20-2015, 01:02 PM | #6 | |
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Re: Need Marvel Carb Expert
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12-20-2015, 01:19 PM | #7 |
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Re: Need Marvel Carb Expert
I thought it was a "load" adjustment. I have a marvel on m'lady and after working on dozens of Marvel tractor carbs, I just thought that was the load adjustment setting (for when the car's heavily loaded, climbing a hill, towing, ect.), just an adjustment that helps supply the engine with extra fuel during those situations. And I set it up on my car exactly as I would have on a tractor, lol. But I haven't had her out on the road yet to test my theory. I'm interested to see what other members say about this, thanks for posting this question.
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12-20-2015, 01:29 PM | #8 |
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Re: Need Marvel Carb Expert
How would this accomplish anything?
It reduces the volume of air in the chamber a bit. It's hard to see how that would affect the A/F mixture. |
12-20-2015, 01:58 PM | #9 |
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Re: Need Marvel Carb Expert
Just nipped out to the garage to check my Marvel carb.
No extra needle screw, just the vent hole. Someone will probably explain that there was a series or variant from the factory that had this needle screw, but for the moment I would assume that many years ago, someone found a needle valve lying on their work bench - and then went looking for a hole that it would fit! In 80 years +, a lot of dumbass things were done to these cars and engines for expediency for just ignorance. The needle screw thread would probably tap into the vent wall quite easily. Incidentally - I missed the photo tear down by Jeff in BC (2014) - many thanks to him as it is terrific. Last edited by Richard Redmond; 12-21-2015 at 07:28 AM. Reason: word duplication |
12-20-2015, 02:04 PM | #10 |
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Re: Need Marvel Carb Expert
Sorry to make a second comment -
I looked again at the initial photo - assuming that the screw is not at an angle in the carb float chamber top (as it looks a little ? due to tilt) would it not be very awkward to adjust the needle valve once the throttle spindle and throttle arm are in place? |
12-20-2015, 03:27 PM | #11 |
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Re: Need Marvel Carb Expert
>>but for the moment I would assume that many years ago, someone found a needle valve lying on their work bench - and then went looking for a hole that it would fit!
I thought of it possibly being a one off mod by someone, but two out of the five Marvel I have obtained have the extra adjusting screw. I know that one of our members (Steve Becker) is from Berts. It would be good if Steve could take a look at their stash of Marvel carbs to see if any more than my two have the extra adjusting screw. >>I looked again at the initial photo - assuming that the screw is not at an angle in the carb float chamber top (as it looks a little ? due to tilt) would it not be very awkward to adjust the needle valve once the throttle spindle and throttle arm are in place? Not only that, but it is difficult to remove and install the valve, and impossible to adjust the screw when the throttle arm is not in the idle position. BTW Although I enjoy solving this puzzle, unlike the fellow who just MUST know who built the 31 tudor body, I have a practical reason for this query. I have enough good parts and bodies to build two complete Marvels either with or without the third adjusting screw. I am wondering which way to go. Is their an advantage to having the extra adjustment or not? |
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