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I have a deere with a fc420v motor. It back fires when

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Hi. I have a john...
Hi. I have a john deere with a fc420v motor. It back fires when starting and pulsates when running. So far I have cleaned carb. seems to help some. any ideas.
Thanks,
Ken
Submitted: 7 years ago.Category: Small Engine
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Answered in 25 minutes by:
6/2/2011
Technician: Doug E., Kawasaki Master Technician replied 7 years ago
Doug E.
Doug E., Kawasaki Master Technician
Category: Small Engine
Satisfied Customers: 2,887
Experience: Professional mechanic since the late 70's on Kawasaki, Arctic Cat, Jonsered, John Deere and others.
Verified
Doug :

Hello

Doug :

If you give it a little choke, will that even out the running?

Customer:

does not seem to help.

Doug :

Does it seem to be loading up or flooding?

Customer:

Loading up a little, flooding no.

Doug :

Does it run better under load or when warm or cold?

Doug :

When did the problem first start?

Customer:

Runs better under load. and low idle.

Customer:

back fires alot when trying to start after warming up. problem started couple of years ago.

Doug :

If it had an adjustable main jet, you could probably adjust the problem away quickly. Only the air screw is adjustable, and that covers just the mixture at idle. It sounds to me like it's running too rich. Under load you need more fuel, so it smooths out. One thing to also investigate is the igniter. Some ignition problems mimic fuel issues, and the backfiring when hot is one that could go either way.

Doug :

If it were mine or if I had it in the shop, I would do a complete tune up and test the ignition.

Customer:

There are alot of comments about this motor having the same problem. is it just the a flaw in the motor. how do you test the ignition? since there is no adjustment on carb. do I need to replace it?

Doug :

The valves on those motors need to be adjusted on a regular basis. It's a very easy process and takes no special tools. Remove the valve cover and spark plug. Rotate the motor over with a wrench on the PTO or flywheel while watching the intake valve open and close. Go just a bit further until the piston is all the way up. Verify this with a straw down the spark plug hole. Adjust the clearances to .15mm using a feeler gauge to measure between the valve stem and adjuster screw.

Doug :

All motors these days have fuel problems. Carbs are not adjustable and are tuned for low emissions. Old gas is often left in the carbs over winter and that gums up the jets and passages. Any tiny obstruction will cause a problem being the carbs are so finely tuned.

Doug :

On your fuel system you should check your tank for water or dirt. Change the fuel filter if you haven't done so. The carb should perform fine unless there is a worn part or plugged passage or jet. The only part that wears would be the float valve needle. If you decide to clean the carb again, I would replace that. Check for any leaks on the mounting gasket and also check for leaks on the exhaust side. I don't know what model your motor is in, but there were some recalls on the muffler for internal breakage.

Doug :

On your fuel pump, pull off the impulse line to the motor. Is there any fuel in there? If yes, you have a pin hole in the pump diaphragm and it's flooding the motor a bit. Rebuild or replace the pump to cure that problem if it shows up. Remember that the fuel pump is vacuum driven so having your valve set will make the pump work better.

Customer:

It is a model GT 242

Doug :

Thanks. That's a good tractor.

Doug :

Your model is equipped with an electronic fuel shutoff in the carb. These can go bad and stick open or closed. This keeps the motor from flooding and backfiring by shutting off the fuel when turning off the motor. You can test them by removing them and putting 12 volts to the wire. The plunger should operate.

Customer:

I have checked that. seems to be fine

Doug :

There were some muffler issues on the GT tractors. Tap the muffler with a rubber mallet to see if you can hear any broken baffles on the inside. You can remove it and shake it, but sometimes the bolts rust, so I don't like to pull them unless I have to.

Customer:

How does that affect how the motor runs?

Doug :

That would cause backfiring and if the baffle is blocking the exhaust, it would run rich. If it was open, it would run lean. Same as if you put cheap glass pack on your car. The backpressure is messed up and the flow doesn't match your fuel system.

Customer:

Ok

Doug :

Mufflers aren't cheap but I have cut a few open and repaired them. That's only cheaper if you have all of the tools to do that of course.

Doug :

Give me a moment to post the ignition specs.

Customer:

Do you use any chemicals to clean a carb?

Doug :

Yes, if there is varnish then you have to. For just dirt you can wash and blow them out. The jets all have to be removed or you can't get in all of the passages.

Doug :

I use Yamaha carb cleaner. It mixes with water and is made to dissolve varnish.

Customer:

Which one do you use?

Doug :

It's also safer to use than some of the stuff at the auto store. That will kill you 10 different ways.

Customer:

ok

Doug :

ACC-CARBC-LE-NR

Doug :

It mixes with water. Heat it on a hotplate and soak the carb and jets for up to 10 minutes. It will be spotless. I've also had good luck with a handheld steamer with the fine point nozzle. I use that to restore vintage carbs. It does a surprisingly good job. Wash with water and blow with compressed air when done.

Doug :

It makes 40 year old carbs look like new without any chemicals.

Doug :

Chemicals are faster however.

Doug :

OK, that should be all of the info you need to do a full tune-up and test the ignition. If you go step by step there is no way you won't find the problem and get it fixed.

Doug :

My money is on a fuel issue, but don't overlook setting the valves and testing the igniter. I've replaced a bunch of the igniters.

Customer:

Thanks Doug! I will check the fuel pump, valves and igniter.

Doug :

I have to log out for the night. I'll be back on in the morning. Let me know if you need any additional information.

Customer:

Good evening. Thanks for your help.

Doug :

Goodnight

Doug E.
Doug E., Kawasaki Master Technician
Category: Small Engine
Satisfied Customers: 2,887
Experience: Professional mechanic since the late 70's on Kawasaki, Arctic Cat, Jonsered, John Deere and others.
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