Help me understand carbs

Barneyrb

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A carb is a device that actually draws air through a venturi, past a fuel nozzle, which draws fuel out of the carb body, mixes fuel with the high speed air, then sends the air/fuel mixture down the intake boot, and into the crankcase for going through the transfers before being ignited by the spark plug, thus creating an explosion of said air/fuel mixture which creates power and makes the chain thingy go round and round.




Hope this helps
 

H 2 H

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I might have a few of those carb's laying around here if you get in trouble
 

heimannm

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Always pressure test a carburetor to insure the metering needle is seating and sealing properly. On a few occasions some won't hold pressure, others may have body parts that won't seal. A jar with solvent work for a dunk under pressure to find leaks is convenient.

Mark
 

DSS

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8433jeff

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Always pull the screws, one at a time, and I use brake cleaner. If you don't have three separate places on the low side spraying the jets needs to be opened, wire from a equipment tag gets 95% of these. That's the only time I would pull the plugs the first time.

The needle and lever is replaced if I have the rebuild kit and haven't heard it run. Ethanol raises hell with the tips of these, as does carb cleaner, I think. A visual check, a squirt to confirm the passage is clear to fuel line, replace, done. Gotta have to screen in place.

Diaphragm and gaskets in order, should go. Anything that was clean or flooded when brought to me throws red flags.
 

Genius

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Can actual carb body's go bad, or would they be bad from the factory?

Last night I had a carb not holding pressure, I sprayed soapy water at the seat and welsh plug and there were no leaks. I dunked the carb and it was leaking from the top. I swapped gaskets, different top and I was still getting a bunch of bubbles. The surface of the carb body was clean and free gasket or diaphragm pieces.
 

heimannm

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It is possible that the carburetor body or one or more of the plates may be warped either from manufacturing or from use, abuse, or physical damage.

Many Walbro SDC carburetors have cast plates with a "crush line" cast in that concentrates the forces against the gaskets to insure a good seal. I have had a few with the stamped plated that would not hold regardless of the condition of the gasket or how tight the screws were. Replacing the stamped plate for a cast plate solved the issue.

The only photo I have available that shows the "crush line" is from the flat back or bullfrog carburetor used on some older McCulloch -10 models but I think you get the idea.

DSCN1737.jpg

Mark
 

srcarr52

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It is possible that the carburetor body or one or more of the plates may be warped either from manufacturing or from use, abuse, or physical damage.

Many Walbro SDC carburetors have cast plates with a "crush line" cast in that concentrates the forces against the gaskets to insure a good seal. I have had a few with the stamped plated that would not hold regardless of the condition of the gasket or how tight the screws were. Replacing the stamped plate for a cast plate solved the issue.

The only photo I have available that shows the "crush line" is from the flat back or bullfrog carburetor used on some older McCulloch -10 models but I think you get the idea.

View attachment 29355

Mark

I've run into similar issues where a small bit of the crush line is corroded and they will no longer seal. Usually 066 carbs.
 

Eccentric

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It generally thake many pulls to pull fuel into a freshly kitted carb. I use and eye dropper off mix squirted into the carb throat with high idle set and choke off. It usually takes 2 or more primes to get the carb drawing fuel on it's own

Watch using high pressure air on carbs with mechanical check valves. That can ruin the check valve.

Sea Foam Engine cleaner is great stuff to soak dirty carbs and poorly acting carbs in. I recycle it.

Here's good info on Walbro carbs.

Good stuff here.

http://www.walbro.com/media/21936/SERVICEMANUAL.pdf Well worth the read for how they work.

Carb specific

http://www.walbro.com/servicemanuals.aspx



Tillotson Carbs.

Google Tillotson and the model number. ie Tillotson HS


Zama

http://www.zamacarb.com/pdfs/TechGuide_2007.pdf

http://www.zamacarb.com/images/techsupport/DisassemblyC1U.pdf

Look at you going all circus bear on us


Yep. Tim's post was almost exactly what I wuz furmulatin' in my mind while I wuz readin' through this here thread. Scary...


Can actual carb body's go bad, or would they be bad from the factory?

Last night I had a carb not holding pressure, I sprayed soapy water at the seat and welsh plug and there were no leaks. I dunked the carb and it was leaking from the top. I swapped gaskets, different top and I was still getting a bunch of bubbles. The surface of the carb body was clean and free gasket or diaphragm pieces.

Carb bodys with non-serviceable high speed check valves can go bad (or be ruined by certain chemicals and/or high pressure air). Walbro WA/WT carbs come to mind. A Poulan rep warned us about that in 1994 or so when I was at Sears Service. At that time, basically all the Poulan made Craftsman equipment had Walbro WA or WT carbs......
 

fossil

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Is there a way to tell if the check valve is bad?

If you look in the 1st link I posted (Good stuff here) which is the Walbro Service manual, pages 15 - 17 will tell you how tho test it. All that is required is a piece of tubing and someone to suck and blow on the tube. (Drain the gas out first unless you have someone else do the suck and blow part).
 

dboyd351

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If you look in the 1st link I posted (Good stuff here) which is the Walbro Service manual, pages 15 - 17 will tell you how tho test it. All that is required is a piece of tubing and someone to suck and blow on the tube. (Drain the gas out first unless you have someone else do the suck and blow part).

There was also a post on AS by Maico490, which explained not only how to test the check valve, but how to push one out and replace it.

Don't think I saw mentioned the need to seal around welch plugs with fingernail polish. Sometimes fixes an erratic idle.

Good thread and comments.
 

countryhog

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There was also a post on AS by Maico490, which explained not only how to test the check valve, but how to push one out and replace it.

Don't think I saw mentioned the need to seal around welch plugs with fingernail polish. Sometimes fixes an erratic idle.

Good thread and comments.

you got a link to that thread?
edit - never mind. found it. naturally pix are gone
 
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dboyd351

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you got a link to that thread?
edit - never mind. found it. naturally pix are gone

Jerry,
Why don't you PM Maico490 directly on AS. He is the one who posted the pics in the first place, so would be the best bet to resurrect them.
 

countryhog

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Jerry,
Why don't you PM Maico490 directly on AS. He is the one who posted the pics in the first place, so would be the best bet to resurrect them.

did that right after I posted. no response yet.
 
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