FOUND: elusive bowl vent [Marvel carb]

So make sure the venturi is not bent or that the gasket is not damaged or out of place so that the venturi is sealed top and bottom when the two halves are mated together.

I've worked on a number of different makes and designs of carbs and there is always a puzzlement about where some bleed-jets are in the real world..... ...Dell

Dell, All the makes and models of carbs you have worked on don't mean beans, the fact is the FO 4 fig. 50 drawing shows how the bowl is vented, just like Bob said it is, and just like my real world carb. is. If you disagree, show us how FO 4 is wrong, rather than generalize about "makes and models".

I have in my lifetime rebuilt many different makes and types of carbs and I have found it takes some imagination to match the real-world with the drawing. Has to do with trying to depict 360deg spatial locations in 180deg flatsheet..... ...respectfully, Dell, former aerospace draftsman

I guess you didn't realize there are a LOT of different "FO-4's" out here, depending upon when they were published.

For instance, mine (which has been around a while, since it caries a list price of $4.00), says very little about the carburetor, covering it in only ONE paragraph, Paragraph 42.

There are NO illustrations of the carburetor in the issue of the FO-4 I have.

Paragraph 50 covers the GENERATOR and photo 52 is of 9N and 2N gear shift rails!

Quite a difference between issues of the manual!

I will try to get the carb photos I have posted soon. The 'puter I normally use for photo work puked it's harddrive, and I have to install a new one and load the OS and photo software before I can post the photos.

My point was, I'll bet there's LOTS of other older manuals out that that folks are looking at, and I was bringing up the point they're NOT all alike!

Actually Ken, I suppose I opened this thread, or more precisly, this subject over on page two, when I called this vent "the elusive bowl vent". I don"t owe you an explanation but since you have gotten your britches all wadded up, here goes the long story. I got this tractor in June, it had sat up a long time. I re-wired it, did a tune-up, switched it to 12 volt,vatted the carb and put a TSC kit in it, after blowing it out with compressed air. I set the two adjusting needles as advised on this board and in my older FO4 manual. It started right up, idled fair but would not throttle up correctly without the high speed needle out 6-8 rounds. That in it self didn"t bother me too much except that it would sputter when under load if the fuel lever was at WOT. If I backed the lever down about 1/2 inch it smoothed out.I bought a new bucket of carb cleaner and boiled it out again. I blew it out good, ran a fine wire everywhere I could see to run one. Everywhere I blew air into I got return somewhere. After re assembling it again the problem was still there. I checked the distributor for correct point setting, made sure I had the right screws in the right place, checked for slack in the dist bushing. Everything looked good. It still did not run to suit me under load. I left the four screws a bit loose between the two halves and it did help. This led me and others on this board to think that the bowl was not venting which would cause poor fuel flow (if air can"t get out, fuel cannot enter the bowl). The float is set right and I did try changing the float level a tad each way. BTW-the compression is a litle over 100 psi on each cyl. Everyone here agreed the bowl vent had to be plugged. When I started asking the exact location of this vent, I recieved numerous replies, but they were as different as day and night. Not one reply back then posted an accurate picture or drawing of this vent. I tried Zanes trick of drilling a small hole in the top of the housing which helped a great deal but it is still not exactly right.I tinkered with the timing also which did not solve the problem. I am no novice to mechanical devices, am 58 yrs old, have been working as a mechanic since I was a young man (19) and am Ford and ASE certified
medium/heavy diesel tech. I like things to work right, and when they don"t I want to know why they don"t. Yes, this is a simple carb, espically when compared to say, a GM Quardajet or a Ford Varible Venturi, but that does not mean folks should get upset with others over it. I know fully well that I can order another carb but I want to fix this one, it is almost surely the original. I have kept all the old wiring and generator/regulator and plan on re-installing it at a later date. HOBO has posted a drawing on his site but it is in PDF format and as I have said this old computer has a small hard drive and only 28 MB ram, so I cannot install Acrobat Reader and open his site. I will do that tomorrow at work tho. Bob has been very helpful and has promised to post an accurate drawing also. We live in a small town with no local tractor shop, so no other FO4 manual is close by. I will order another one but had hoped to get this resolved this weekend. I apologize if this MINOR carb malfunction has ruined your weekend to the point you feel you have to get testy about it. From now on if you see a post with my name on it, please refrain from opening it as it may also cause you much anguish, and could result in high blood pressure and/or stroke or heart attack. If you wil post your whereabouts we will call ahead before we post again and have 911 send paramedics to your home just in case. :^)

You say "but it does go through with two small holes. One hole comes out above the throttle plate and the second larger hole comes out below the throttle plate."

HORSEPUCKY!!!

The holes above and below the throttle plate are the holes where the IDLE MIXTURE discharges into the engine's intake air stream.

The hole ABOVE the throttle plate is where the idle mixture discharges into the airstream at idle, the hole just below the throttle plate is sort of a "transfer passage" that begins to discharge fuel as the throttle opens, BEFORE the "load circuit" kicks in, to avoid "stumble" as the throttle opens.

The bowl vent is as follows:

There are 4 holes in the carb gasket, leading to 2 chambers ABOVE the gasket, left and right. From the 2 chambers ABOVE the gasket, there are 2 holes drilled into a common passage that leads to 2 holes that exit into the chamber SURROUNDING the venturi. The venturi is sealed at it's top and bottom, to the carburetor's top and bottom casting, isolating the air flow (and suction) inside the venturi from the VENT passage OUTSIDE the venturi.

This area SURROUNDING the venturi is upen to the rectangular port in the gasket that matches up with the rectangular hole in the lower carb casting that leads to the carb inlet area, at the top of the air inlet port in the LOWER carb casting, ABOVE the choke plate area.

Also, the air for the idle circuit is sourced from that same common area surrounding the venturi.

There is a drilled passage from the vent area up to the backwards-acting idle mixture screw.

As the idle mixture screw is OPENED, it allows MORE air from the vent area to flow INTO the idle mixture, LEANING it out.

I have several photos showing the vent circuit threaded with RED insulated wire though the passages, to make it easy to see. Within the next day or so, when I have a few minutes, I will resize them, and post them, HOPEFULLY clearing up "the elusive vent passage mystery" once and for all!

BOB, You need to hi-light the fact that this is the "Source" of AIR, Post Filter.

Regards,

Do you agree with my discription of the "idle" and "transfer" passages?

Regards,

Bob

Yes! also the air venting into the main Jet Nozzle cavity, acts like an accelerator pump, is an interesting feature.

Note: The Float Specifications are that the Wings are Bent such that the 1/4" distance is constant and results in the float been Parallel to the Upper Assembly. Therefore Max. Displacement. I think most PPl, & myself, just bent the contact Tab & that results in float Tilt (front to back)

JMHO

Are you referring to the rectangular hole in this picture which is marked "main Jet" (incorrectly)?
Or to the almost oval shaped indention to the right of that hole in this picture? Hobo's link below under "carb tips" does show a drawing of the vent and passages but I cannot post nor open that picture because I do not have Adobe Reader as I do not have room for it on this particular computer. You can scroll down to the "elusive Bowl Vent" thread below and click on Hobo's reply and see it on his site.

There ain't nothin' down there but the choke plate, and AIR!