8n carb or valve or...

Have i believe to be a 49 or so 8n, front mount 12 v electrical, 12v coil. Have been doing a complete refurbish of this tractor. Have new fuel tank, fuel cap, sediment bowl assembly, fuel line. Old marvel (aftermarket) carb was wonky. Constantly flooded and ran like crap. Rebuilt twice to same outcome. Float either stuck open and flooded or stuck closed and starved. Bought new aftermarket marvel carb...same problem. Unscrewed drain plug to check fuel flow and it was cross threaded....drain plug fell to ground taking all known threads with it.

Purchased new made in USA zenith 13876 carb. Out of box and floats stuck closed. No fuel to the bowl. Return carb. Ordered another 13876 and returned faulty carb. Received the next zenith put new carb on and floats stuck open gas pours from screen in bottom of carb. Tractor will sputter but not run.


Just for giggles i check spark and spark will jump 3/16" gap every time with bright blue spark.

Also should be noted it has a fresh reman governor as the old one was destroyed.

Sediment bowl shows no debree. That coupled with everything in the fuel system is new leaves me to believe floats are not sticking because of debree.

Any ideas? Is my carburetor luck THAT bad or am i missing something?

What are symptoms of stuck valves? Would this cause the flooding of carb?

"gas pours from screen in bottom of carb."

Does it do this just setting when you turn the gas on, or after you try to start it?
One reason they leak gas like this after trying to start is excessive choke.
That could be caused by the operator or by a plugged air cleaner if it's hooked up.

(quoted from post at 18:23:23 11/16/18) Have i believe to be a 49 or so 8n, front mount 12 v electrical, 12v coil. Have been doing a complete refurbish of this tractor. Have new fuel tank, fuel cap, sediment bowl assembly, fuel line. Old marvel (aftermarket) carb was wonky. Constantly flooded and ran like crap. Rebuilt twice to same outcome. Float either stuck open and flooded or stuck closed and starved. Bought new aftermarket marvel carb...same problem. Unscrewed drain plug to check fuel flow and it was cross threaded....drain plug fell to ground taking all known threads with it.

Purchased new made in USA zenith 13876 carb. Out of box and floats stuck closed. No fuel to the bowl. Return carb. Ordered another 13876 and returned faulty carb. Received the next zenith put new carb on and floats stuck open gas pours from screen in bottom of carb. Tractor will sputter but not run.


Just for giggles i check spark and spark will jump 3/16" gap every time with bright blue spark.

Also should be noted it has a fresh reman governor as the old one was destroyed.

Sediment bowl shows no debree. That coupled with everything in the fuel system is new leaves me to believe floats are not sticking because of debree.

Any ideas? Is my carburetor luck THAT bad or am i missing something?

What are symptoms of stuck valves? Would this cause the flooding of carb?

Click to expand...

oes gas pour out bottom carb screen even if you just turn on gas and NOT try to start it?/

Air cleaner is totally refurbished from carb to cyclone air cleaner and is all hooked up.

Carb leaks from screen on bottom a few seconds after fuel ia turned on without cranking engine. Will continue to leak from screen and after a while will begin to leak from gasket leading me to believe the floats are stuck open.

Carb did not do this right from the box. The last two carbs do this after attempting to start the engine. The engine will spit and sputter then carb starts dripping. After it starts to drip i shut gas off. I can wait an hour or a day...whatever, as soon as gas is turned back on carb starts dripping again.

If it does it as soon as you turn the gas on, without trying to crank
the engine, then there is a problem with the carb. Needle valve,
seat, gasket under the seat, loose seat, etc. It's not shutting off the fuel.

Thats the conclusion that i keep coming to. Just doesnt seem right that 4 carbs have the same problem. 1 was rebuilt the other 3 were brand new out of the box. I keep wondering what could be causing this. I guess it seems strange that 4 seeingly perfectly good carburetors fail immediately.

Is quality control THAT bad on carbs now days?

If i have a stuck intake / exhaust valve would some kind of back pressure or wierd vacuum cause all these floats to stick open?

I might be able to dream up a scenario where pressure from the
compression stroke would collapse the brass floats. Then they
wouldn't float to close the needle valve. Any evidence of that?
A compression test should give you an idea of valve health.

There is no quality control on new carbs that I know of, that's why
we rebuild the original M/S carbs. If I read your first post correctly,
your tractor came with an aftermarket so you didn't have that option.

Correct on my tractor coming with an aftermarket. An original Marvel was my first choice but havnt been able to find one on ebay that looks good enough for a rebuild.

Before tear down on the refurbishing i took a compression test. It showed a solid 105 on each cylinder. Once head was removed i noticed all pistons and valves looked fairly new with minimal run time. Pistons were laser engraved with "std" on the top of each one so i assume the engine was freshened prior to my purchase which is consistant with the previous owners story.

I always rebuild the OEM Marvel-Schebler carbs and have never had an issue after rebuild. I have done a few Zeniths as well. Also, I've mucked with a few junky aftermarket models, but won't touch them anymore. Clarification: "...Bought new aftermarket marvel carb...same problem. Unscrewed drain plug to check fuel flow and it was cross threaded....drain plug fell to ground taking all known threads with it..." -so on a 'new' unit the drain plug was already cross-threaded or did you do that? The drain plug uses an NPT thread. Often guys try to chase the threads with a standard nominal tap size and end up boogering it up beyond repair. Likewise, the fuel line and brass elbow use a special thread -7/16-24 UNS, used on brake and fuel lines. Obviously you have been taking the new carbs apart and examining them before installing. Do you check the float level per instructions? After install, do you perform a fuel flow test to carb? You say you have spark but are plugs wet or dry? For safety sake, it is best to do a spark test before a fuel flow test. You don't want to be sparking anything if there's a chance fresh gas is around loose. One other thing, before you start cranking and testing for fuel flow, and after the spark test, look at the fuel sediment bulb for signs that leaking is coming from it. Often it is worn out seal in the valve stem causing the leak. A new valve stem is $6 and is the only wear part in the assembly. That and cross-threading the fuel line into it are the main reasons for leaks. When doing the fuel flow test, As you open the sediment bulb valve 2 full turns, look closely for fuel leaks. After cranking and no start, remove the air cleaner hose at the carb and observe if fuel spews out from there. Don't worry about the governor -not the root cause. I am not buying that you've tried 3 or 4 carbs all with the same issue. Zenith makes a good carb. The problem with the aftermarket units sold today is that they are just cheap-a$$ clones of the OEM M/S carb and/or the Zenith carb. They were reverse engineered no doubt from some 3rd, 4th, 5th generation already cloned model and they find ways to lessen costs with shortcuts because they don't have a drawing to go by. Do you think the 8 year old kid assembling carbs in Cheena actually sets the float via a gage? Hardly not. Years ago one could get a new TISCO USA-made Marvel-Schebler-like unit which adhered to the OEM part manufacturing. But M/S has been out of business for many years so no such thing as a 'Marvel aftermarket' unit. Most are junky clones as I said. Debris in the sediment bowl would only possibly inhibit fuel flow, not cause excessive. When you do the fuel flow test, do you first remove the fuel line from the brass fuel inlet elbow on the carb and then open valve to observe that fuel flow passes to that point? Have you removed the elbow to ensure no debris blockage is present in the screen filter there? If fuel flow up this point passes satisfactorily, then move on to the drain plug. I normally suggest one remove the gas tank cap as a plugged vent can cause flow issues, but that is usually when there is no fuel flow. Since you also say you have a new gas tank, chances that the vent is plugged is low. But, have you tried that anyway? Don't crank, just observe. I'd say your root cause is most probably in the carb and/or fuel line system, not stuck valves, governor, or risen creek. If you still have your OEM Marvel carb, send it to me and I will rebuild it -you pay for parts and shipping only.


Tim *PloughNman* Daley(MI)

Drain plug on new carb was cross threaded from factory. Sorry you dont believe that all the carbs ive tried are junk...im having a hard time believing it myself...hence the post.

Tried the fuel flow test. Sediment bowl assembly is new. The old one leaked. Rebuilt the old one and it still leaked. Bought a bew one and no leaks.

I dont remember saying that ive opened or worked on these new carbs in any way.

Apparently i mis-spoke. I have tried a new carb that looks like an old Marvel style carb.

I do not have an original marvel carb and have not had one in my posession at anytime ive owned this tractor.

Rh zeniths ive tried have a zenith made in usa cast into the crb body. I assume theyvare legitimate. If i thought my credibility was going to be questioned i would have video taped this process so there would be no question as to my actions.

I have nothing to gain by lying about any of this so i would appreciate if we keep that to a minimum. No reason to make this personal just trying to solve a unique problem im having with a tractor.

If you are getting fuel out of the bottom with it not running and you have not tried to start it, then for whatever reason the float needle is not closing. If you have tried to start it and get fuel out the bottom it COULD BE from over choking.
I got two 8n machines is original MS carbs on both and have never had any fuel dripping out the bottom. One I shut the fuel valve off and one I don't.
There is TSX 33 on fleabay right now that looks rebuildable to me. It would have to be pretty bad to not be rebuildable IMHO.

Wishing you luck with your problem.

I believe the only problem you have is the floats need to be adjusted. Do you have a service manual that shows how to adjust the floats? New carbs have no quality control thats why we always rebuild our originals. Same with govrenors and distributors.

"Rebuilt Twice" had me thinking you did open new ones, but, not unusual for a new Cheena-made aftermarket part to be junk out of the box. Perhaps you should open one up and check the float setting. Like I said below, no one is setting floats or even the jets at the factory in Beijing. I see OEM M/S carbs on fleabay everyday. I'd look for a TSX-241 unit over a TSX-33 unit but either will work just fine on your 8N. Like anything else one buys used, caveat emptor, "buyer beware". As long as the unit has not been boogered up like a stripped out thread or oversize reamed hole, a used unit should work fine. I've rebuilt probably over 300 carbs in my lifetime. My offer stands to do your OEM M/S carb as well...

Tim Daley(MI)