Surging motor on NEW carb!?

l008com

l008com

Well-Known Member
Fixing up an old yardman mower. I filled it up with gas and started it and it ran... not perfect, but it ran ok. However it was leaking fuel everywhere. The fuel line had turned to dust, and the needle seat had as well. So gas was leaking everywhere.

So I replaced the fuel line and tried to replace the needle seat but couldn't get the needle valve to function properly. So at that point, for $10, I figured I'd just buy a new carb and call it a day.

New carb is in there and its running terribly. I'm pretty sure its running worse than before. It runs ok for a second or two, them almost stalls. Then repeats. With lots of misfiring in the process.

The fuel IS NEW. The oil is new. The carb and fuel line are new. The spark plug isn't new but it looks new. The air filter also looks new but I tried with and without it. I don't see any adjustments on the carb. But with the airbox off I can see plenty of fuel actually being sucked up and in. Makes me wonder if its not starving for fuel but getting too much fuel periodically? Everything else looks ok so I'm not sure what to do now. Any ideas?
 
$10 for a new carb??? Must have been an AutoScam or FleaBay special, definitely not OEM purchased from a dealer. I’m betting new carb is not the right one for your engine. Second, I don’t understand “couldn't get the needle valve to function properly”. Here is the procedure I gave students when rebuilding carbs. If you post back, please include all engine numbers, as most of the tech’s crystal balls have been out of condition since Trump took office. ICE has gone foggy.

Needle and seat replacement.

Remove the carb, and then remove the float bowl. Check the float bowl jet (which is the bowl screw) and make sure the jets both horizontal and vertical are clean and open. Tip the carb upside down and remove the float pin and float with needle attached. Look in the float needle passage and you should see the red float seat at the bottom of the passage. This is where a #5 crotchet hook would come in handy as you need to remove this seat. If you have no hook, but compressed air, you can blow through the fuel inlet and try to pop the seat out. Put your thumb over the passage to prevent the seat from flying who knows where. No air or hook try bending a stiff paper clip to dig the seat out.

I would either give the carb a good 24 hour soaking or have it ultrasonically cleaned at this time.

With the seat out clean the passage way with carb cleaner. Now you must find a drill bit slightly smaller than the passage way, to be used to press in the new seat. Apply a very, very small amount of a very light lube to the new seat. 3-1 oil or lighter, to help seat it better. Carefully insert the new seat in the passage way with the rings on the seat down toward the carb body. Slowly and carefully force the seat down with the back end of the drill bit. Once it is seated, check to see that it did not flip and the rings are up. Next check to make sure that the float does not have any liquid in it. If it does, replace. If everything looks correct, attach the new needle to the float and install with the float pin centered. It everything is correct, the float should seat level to the carb body, when looking at it upside down. If everything looks good reattach the float bowl, making sure that both the bowl gasket and the nut gasket seal properly. Reinstall on the engine and test unit. Remember to have patience and take your time. Good luck, but I don't think you'll need it.

PS: On the side of some Tecumseh carbs you will find a plastic cover. Under this cover will be an idle jet. Remove it and check to see that the jet is open both horizontally and vertically. You should be able to push the old float needle wire through the vertical opening.
 
Sounds like a carb mismatch, or defective one maybe the jets are plugged. Make sure the gaskets are correct, some carbs might have a small flange with a passage in it while another may not. Put the wrong gasket on it and it leaks. Also, I always try to find a good original carb if possible, even a good rebuildable one. Less chance of things like this happening. My time is too precious to save $100 then lose $500 worth of my time and life.
 
So I let it sit for a while, and the behavior is a little different now.
Now when I start it, it runs great for a few seconds, then just dies. It will start right back up, run great for a few more seconds, then just dies.

These carbs only have one spot for the linkage to connect to.
 
This is the new carb: https://www.ebay.com/itm/196279304487

Doesn't look like it has any adjustments. Or an idle jet, right? Just one jet for full throttle running?

It looks absolutely spotless, so I would be surprised if there was some kind of dirt clogging it up. But I guess I can't know for sure until I open it up. I wish it would run, even poorly, so I could at least pump the tank and then run the fuel line and carb bowl out of gas. I'm bummed, I thought I was going to be fully 'done' with this mower today (yesterday now) but these repairs are always stubborn it seems.

Rivets I think you already posted that same comment on one of my other posts.
I do have numbers for you. It is an old Yardman mower, I think around 25 years old. In fairly good physical condition, just old enough that all o f the rubber components rotted.
The engine number is: 120602 0131 E1 990820 FA
The carb's part number is briggs & startton 790120

I'm watching someone elses video now and this very similar mower does have an intake tube between the block and the carb. I thought my carb was connected directly to the motor but maybe not. If thats the case, that will give me more potential points of failure.

You know, my carb came with a new gasket to go between the carb and the airbox, AND it came with a big fat O ring that I didn't use. The old one used a gasket between the carb and the engine so I just reused it. I wonder if the O ring goes on there too? The parts diagram doesn't show one so maybe not. I guess I'm just going to have to get my eyes on it tomorrow and see.

As far as cleaning out the jet, is that just the screw that holds the bowl on? Or is there another part of the jet up in the body of the carb that can be removed and cleaned?

Also, that said. If the machine is able to run for a second or two, doesn't that mean that the carb is able to give the engine enough fuel, and does that suggest that the problem might actually be with the needle valve not opening properly? And not letting enough fuel into the bowl. Or, not letting enough fuel into the bowl, quickly enough? As soon as the machine dies, a few seconds later you can start it right back up again.

Note: Its 4 am right now which is why I'm doing endless speculating and no actual work on the mower :D I feel like I'm going to open up this carb and the float is going to be installed upside down or something!
 
So I'm looking through the footage I shot yesterday and at one point, before it started dying, I had the airbox off so you could look right into the carb. You can see the fuel being sucked up and going in, which is pretty cool. But you can also see that the throttle isn't opening. The machine is surging but its surging between fully closed, and practically fully closed. As if the linkage was in the wrong hole. But theres only one hole! Does this look like a problem, or does this look normal? I can't say I've ever looked down into a running carb before so I'm not sure what its supposed to look like while running.

I should be able to look at both carbs when they're removed and verify if the position of the butterfly matches the position of the linkage hole on both. Another thing to check.

 
Last edited:
l008com said:
This is the new carb: https://www.ebay.com/itm/196279304487

Doesn't look like it has any adjustments. Or an idle jet, right? Just one jet for full throttle running?

It looks absolutely spotless, so I would be surprised if there was some kind of dirt clogging it up. But I guess I can't know for sure until I open it up. I wish it would run, even poorly, so I could at least pump the tank and then run the fuel line and carb bowl out of gas. I'm bummed, I thought I was going to be fully 'done' with this mower today (yesterday now) but these repairs are always stubborn it seems.

Rivets I think you already posted that same comment on one of my other posts.
I do have numbers for you. It is an old Yardman mower, I think around 25 years old. In fairly good physical condition, just old enough that all o f the rubber components rotted.
The engine number is: 120602 0131 E1 990820 FA
The carb's part number is briggs & startton 790120

I'm watching someone elses video now and this very similar mower does have an intake tube between the block and the carb. I thought my carb was connected directly to the motor but maybe not. If thats the case, that will give me more potential points of failure.

You know, my carb came with a new gasket to go between the carb and the airbox, AND it came with a big fat O ring that I didn't use. The old one used a gasket between the carb and the engine so I just reused it. I wonder if the O ring goes on there too? The parts diagram doesn't show one so maybe not. I guess I'm just going to have to get my eyes on it tomorrow and see.

As far as cleaning out the jet, is that just the screw that holds the bowl on? Or is there another part of the jet up in the body of the carb that can be removed and cleaned?

Also, that said. If the machine is able to run for a second or two, doesn't that mean that the carb is able to give the engine enough fuel, and does that suggest that the problem might actually be with the needle valve not opening properly? And not letting enough fuel into the bowl. Or, not letting enough fuel into the bowl, quickly enough? As soon as the machine dies, a few seconds later you can start it right back up again.

Note: Its 4 am right now which is why I'm doing endless speculating and no actual work on the mower :D I feel like I'm going to open up this carb and the float is going to be installed upside down or something!
Click to expand...
Either the incorrect carburetor for your engine, or the carburetor is set up lean from the factory and you will have to drill out pilot jet to get it right. Third possibility is air leaks from gaskets. I have seen the hunting and surging issues hundreds of times over the years. It is both frustrating and annoying. Does the issue smooth out at wide open throttle and under load with the blades on? May have to try another couple of cheap carburetors until you get it right or close to right.
 
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