The Ford Barn

pgerhardt · 12-20-2015, 08:52 AM

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.

leo · 12-20-2015, 09:22 AM

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.

jthomas6 · 12-20-2015, 10:23 AM

Pgerhardt, this might help:

https://www.fordbarn.com/forum/showth...ight=disection

pgerhardt · 12-20-2015, 10:53 AM

Quote:

Originally Posted by jthomas6

Pgerhardt, this might help:

https://www.fordbarn.com/forum/showth...ight=disection

Thanks for the link. I have read through that thread. That is one of the sources that labels the hole as a bowl vent, but does't explain the presence and use of a third adjusting screw. ???

MikeK · 12-20-2015, 11:57 AM

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).

pgerhardt · 12-20-2015, 01:02 PM

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Quote:

Originally Posted by MikeK

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).

Interesting, sounds like the one idle adjustment controls fuel while this other one controls air. But upon further disassembly of the carb I am still puzzled as to how this would work. I removed the adjustment screw and found what look like a typical needle valve (picture 1). However there is no seat of any kind for this valve. The needle simply enters a large chamber (picture 2). When installed and fully closed the needle simply crosses the chamber an hits the far side. There is no hole being closed by the valve on that side of the chamber (picture 3). How would this accomplish anything?

Pinesdune · 12-20-2015, 01:19 PM

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.

ModAfan · 12-20-2015, 01:29 PM

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.

Richard Redmond · 12-20-2015, 01:58 PM

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.

Richard Redmond · 12-20-2015, 02:04 PM

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?

pgerhardt · 12-20-2015, 03:27 PM

>>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?

12-20-2015, 09:22 AM	#2
leo Senior Member Join Date: May 2010 Location: New Jersey Posts: 734	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.

12-20-2015, 10:23 AM	#3
jthomas6 Senior Member Join Date: May 2013 Location: E Tn. Posts: 169	Re: Need Marvel Carb Expert Needed Pgerhardt, this might help: https://www.fordbarn.com/forum/showth...ight=disection __________________ If at first you don't succeed, skydiving isn't for you! You don't need a parachute to skydive, you only need a parachute to skydive twice.

12-20-2015, 11:57 AM	#5
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	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, 01:19 PM	#7
Pinesdune Senior Member Join Date: Oct 2015 Location: Canada Posts: 414	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.

12-20-2015, 01:29 PM	#8
ModAfan Senior Member Join Date: Jul 2014 Location: Near Boston Posts: 135	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
Richard Redmond Senior Member Join Date: Oct 2010 Location: Scarborough, Yorkshire Posts: 119	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
Richard Redmond Senior Member Join Date: Oct 2010 Location: Scarborough, Yorkshire Posts: 119	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
pgerhardt Senior Member Join Date: Jul 2014 Location: Danbury Ct Posts: 1,254	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?