Air lock carb on riding mower?

[Bo Loo]

Hi all,



I have an MTD RL638 riding mower. Recently it is very stubborn to start.



I replaced the lower bowl of the carb, installed new fuel filter and spark plug. Still wouldn't start.



Took it to a local small engine shop and the tech wrote on the invoice that the "carb was airlocked. cleared air lock and it started right up"



That was about 6 weeks ago. Now, it's acting up again.



The majority of the information I was able to get off Google searches is about vapor lock.



Is vapor lock and air lock the same? If not, how do I clear the air lock and how do I prevent it from recurring?



Many thanks in advance to all that assist.





Picture of the setup can be seen at this link:

 

[Richard L]

Not sure about the "AirLock" terminology and I am am sure that it is not related to Vapor Lock. Vapor lock refers to fuel in the carburetor getting so hot that it boils or vaporizes the fuel and cannot or will not start or run.....that cannot happen on a cold engine.



Most small engine carburetors are notorious for gasket leaks. That allows air into the carburetor and breaks the siphoning of fuel. Most small mower engines do not have a fuel pump and rely on engine vacuum to suck the fuel into the carb. I suspect that your carb need a new gasket kit and rebuild.



Also, if this is the first time using your mower this spring, you may have a clogged carb from fuel that has jellied from sitting in the carb over the winter. To winterize your mower you should run the carb out of fuel and drain the gas tank. In the spring, fill with fresh fuel and I always recommend a fuel stabilizer.



...Rich

 

[Todd Z]

I would bet as said it need a rebuild, Kits are 12-20 bucks and take no time at all to do....



Most common is the needle and seat get gummed up and stick over time shutting off the fuel....



I probably repair 20 machines a year from people using Stabil and not running machines dry over the winter, using old bad gas or just poor maintenance....



Ohh and the local repair shops charging more then the BMW dealer to repair these things !!



Todd Z

 

[Richard L]

I agree with Todd Z, who appears to agree with me...LOL



I only recommend a fuel stabilizer like Stabil during the mowing season when the mower may sit for a few weeks between use. After your last mowing for the season, drain or siphon out as much fuel from the tank as you can, then run the engine until all the fuel is used up and the engine stalls and will not restart. That ensures that your fuel system should not clog with fuel that has jellied from sitting in for the winter months. I recommend the same technique for all gasoline powered equipment like chainsaws, weed eaters, edgers, etc that are used infrequently during the spring/summer seasons, and then put up for the winter.



A good way to test if the carb is clogged or has leaking gaskets it to pour or spray some gasoline or starter fluid into the carb. If it starts, but dies quickly, it means that there is a fuel delivery problem. If the engine will not start with fuel or starter fluid sprayed into the carb, then you probably have an ignition/timing problem. You can get a good spark at the sparkplug, but it will not start because of timing problems



Ignition timing problems are often caused by the mower blade hitting a solid obstruction that damages the woodroof key that knocks the flywheel out of sync. Oddly enough the mower can continue to run after the flywheel is knocked out of time, but after the engine is shut down and allowed to cool, the engine will not restart...:angry:



...Richard

 

[Mark Stadnicki]

This has happened to me and few of my friends. The little vent hole on the of the gas cap gets plugged and gas won't let gravity feed, creates a vacuum and thus causes a vapor lock. As soon as you shut it off or open the gas cap it will start up and run for a while then stop again once the is created.

 

[Richard L]

Mark,

Good point ! and that can often be the problem on some really, really old mowers. However, due to EPA regulations, most newer mowers and small gasoline engines no longer vent the tank through the gas cap. None of my mower weed-wacker or chainsaw have a vent hole in the fuel tank cap. Not sure when it started, but I know my old 15-16 year old riding mower did not have a vented cap...and removing the cap never helped in starting.



...Rich

 

[Mark Stadnicki]

Rich-



Interesting, my 2013 Craftsman riding tractor has a vent hole in the cap. I learned of the problem on my 1999 riding tractor was doing it.

 

[Richard L]

Mark,

That's why I said "MOST mowers and small engines" do not use vented caps. I know some do, but many newer engines do not. Probably based on how the EPA worded the application specs in there regulations to determine what engines can or cannot have vented gas tank caps??



I think we may be in a transition period Similar to how EPA regulations used to be applied differently for passenger vehicles vs Light Trucks. Now the EPA emission regulations apply equally to both cars and light trucks....probably because light trucks and SUV's grew in popularity so fast



...Rich

 

[Mark Stadnicki]

:supercool:

 

[Bo Loo]

Update: It's very stubborn to start when cold. Cranks and cranks with no start. After I let it sit (30-60 minutes) it will usually start and keep running with no problems.



I can then turn it off and restart with no problems.



I use it about every 10 days...



"I replaced the lower bowl of the carb"



Is that what would be considered a rebuild?

 

[swshawaii]

Todd Z

Most common is the needle and seat get gummed up and stick over time shutting off the fuel....

Float bowl replaced, but no mention of replacing the needle and seat. Double check part for fitment.

The float needle allows fuel to enter the float bowl. If the float needle is stuck closed, the engine will not get fuel and thus will not start.

 

[Richard L]

Bo,

Replacing the lower bowl is not a rebuild. A rebuild involves removing the carb, complete disassembly and inspection. A thorough cleaning and reassembly with new gaskets.



Again, small engines typically do not use fuel pumps and use gravity feed. Some engines even have some parts of the carb integrated into the top or bottom of the fuel tank. Carburetors use engine vacuum passing through a low pressure ventura to draw a metered amount of fuel into the engine. Any air leaks or clogged orifices can cause starting and running problems.



It sounds like your mower is not getting enough fuel or too much air when trying to start it when cold. If your mower has a choke or a primer pump be sure it is working properly. I recommend trying a starting fluid spray directly into the carb, to insure you are getting fuel. If it fails to start or even fire once or twice, then you may have an electrical problem or the engines is out of time.



Hitting a rock, or tree stump can knock the engine out of time. Often an enigne that is out of time will keep running just fine until it's shut off and allowed to cool, then its very hard to start, but once it's running it runs OK and the out of time condition may not be very noticeable.



You need to determine if you have a fuel/air issue or an ignition or timing problem.



Spraying fuel or starting fluid directly into the carb should temporarily overcome fuel delivery problems and allow the engine to start or at least fire briefly. That calls for a complete carb cleaning and rebuild. If spraying fluel into the carb does not work, you may have a timing problem or there is a problem with the ignition...? Most small engines use Magnetos that can can fail, and those with electronic ignitions can have the ignition module fail.



...Rich