Kubota 400 ci backfire issue

[Rollo]

Just purchased this 2012 Kubota 400ci and changed the oil (has gas smell) and plug. It will backfire and boggs down and I let off and get back on it and it will repeat. It starts very easily. I want to find out where the fuel filters are, I think one in tank and one ??? The previous owner only drove it a very short distance and then would shut it off numerous times a day. Maybe 100 yards every trip. My understanding these models are not designed to be ran short distances. The engine does not get hot enough to properly deal with the fuel. Therefore the gas makes its way down into the oil. I paid a whopping dollar for it and hope I did not get ripped off. lol

I could use a parts catalog for it also. I have the users manual.

 

[Russell King]

I assume that the model number is for an engine but if it is in a Kubota product then you can get to illustrated parts lists at Kubota USA dot com and Messicks.

I don’t know how to find anything on just the engine model but if you can find some vehicle models that use the engine then you can find out parts that way

 

[Rollo]

Now the new plug is black as heck (fouled). Still backfiring and bogging down. Need air filters then try that.

It has a subaru
Kubota RTV400ci Specs
Here’s a list of specs that the Kubota RTV400 offers.

Engine Specs

• Power: 14.6 HP
• Max Speed: 25 mph or 40 kmph
• Fuel Tank: 5.2 gal
• Fuel Type: Gas
• Displacement: 24.6 in3 (cubic inches)
• Number of Cylinders: 1
• Drive System: 4WD Standard

The engine in the Kubota RTV400ci has 14.6 hp and can achieve a maximum speed of 25 mph.

 

[Russell King]

Now the new plug is black as heck (fouled). Still backfiring and bogging down. Need air filters then try that.

It has a subaru
Kubota RTV400ci Specs
Here’s a list of specs that the Kubota RTV400 offers.

Engine Specs

• Power: 14.6 HP
• Max Speed: 25 mph or 40 kmph
• Fuel Tank: 5.2 gal
• Fuel Type: Gas
• Displacement: 24.6 in3 (cubic inches)
• Number of Cylinders: 1
• Drive System: 4WD Standard

The engine in the Kubota RTV400ci has 14.6 hp and can achieve a maximum speed of 25 mph.

Here is a link for the illustrated parts list.

Kubota Tractor Corporation - Illustrated Parts List

- K-PAD - Parts Lists - Illustrated Parts Lists - diagrams

apps.kubotausa.com

search for the rtv400ci after agreeing with their terms

i was assuming that it was a carburetor issue but I’m not seeing anything about that so I guess it is injection based.

I would assume it has something wrong with the fuel or the air intake systems.

I would probably start with replacing the air filter and drain all fuel and replace it with non-ethanol fue. Might replace the injector if not expensive (and it has one).

 

[whitetiger]

The Kubota RTV400ci uses the SUBARU GH4120 single-cylinder engine.
Attached are the specs and the fuel system pdf. There is no fuel filter other than the filter sock on the fuel pump located in the fuel tank.

 

[Rollo]

I already drained and replaced the fuel with recreational gas. The air filter was horrible, dirt in the box and everywhere else. Its like they had it in the dunes for its life. But it was not. I am going to order new filters (crazy expensive) and try that. The hoses coming out of the airbox has dust inside also. Thinking since the dust issue, may be I should take the throttle body off and clean it also?

 

[whitetiger]

If there is dust there, it couldn't hurt to clean it.

 

[cthomas]

Not a RTV tech, but, automotive, the ECT helps determine the injector pulse, therefore, maybe Subaru uses the engine case temp. sensor. Also be aware the MAP sensor determines altitude which the ECM determines injector pulse. This is based on theory, but should hold true. Make sure both are plugged in and a good clean connection. If you find a WSM please give me a link as I would like to know how this system work, also verify the correct spark plug is installed and not just matched to the old one(YES, I have been fooled by this). Subaru has some weird ideas(spare tire mounted on top of engine and 4 speed Hi-Low Dual range transmission ) that don't always work out, but nice that they try(too many Subaru's I have owned to count).

 

[lugbolt]

cthomas, you'r epartially correct

rtv400 is an efi engine

it uses engine oil temp since it is air cooled, to determine how "warm" the engine is. I forget but I think there is also an air inlet temp sensor in the intake. Maybe part of the map sensor (like polaris calls it, t-map)

it's basically a hybrid alpha-n/map system. It's poor, in my opinion but it is what it is. It is cheap to manufacturer. Kubota built the 400's to compete with the Mule 610. And they did a poor job of competing with the Mule 610. I worked at a dealer who had the highest Mule sales in the region at the time. Kubota sent engineers and a prototype "RTV" out to our little dealer, to compare them so that the engineers could make changes to the 400. The only thing the 400 does better is accelerate. It's ever so slightly quicker to 25mph than the Mule is. The mule destroys it in every other aspect. Even the fuel delivery system (at the time it was carbureted, and the carb was a much better design than the RTV's efi).

yes if the engine is used for short distances, the engine oil never warms up. Combustion gases are normally introduced to the engine oil via normal blowby. Over time it builds up, UNLESS the engine oil gets hot enough to evaporate it, i.e., run the machine at full throttle against it's governor in low gear every so often, for a length of time to sufficiently warm the engine oil. In doing this, I'd be surprised if it doesn't also blow a bunch of soot out of the exhaust system-as the exhaust gets carbon built up in it and it's worse with short trips that don't warm the exhaust system fully.

 

[Rollo]

It is not backfiring as often but it still is. So far I am waiting for the valve to get here for the carbon filter cannister. Not 100 % I might need the actual cannister also. I blew into the valve and it seemed to backfire less after. Possible gummed up?
I have been looking for air inlet temp sensor but no luck so far.
I have replaced the air cleaner, transmission air cleaner, plug, oil...
Hopefully the air inlet temp sensor is not the issue since it is quite costly. Is there anyway to clean it? I will mess with it more to check the connections on the weekend. I am hoping to have this working soon.

I am only getting 15 miles per hour at this time.

Thanks everyone for your help.

 

[lugbolt]

check the fuel pressure. I'd be surprised if it was within spec, based on your description of how it's running

 

[PoTreeBoy]

Fooled with one once. If you need a fuel injector, you have to buy the controller too, $$$$$ Some guy had bought 4 which were the same as a Subaru car for his 400 and was selling one on eBay, but that was over a year ago. I didn't buy one, the people decided to get a 900x.

 

[Rollo]

Fooled with one once. If you need a fuel injector, you have to buy the controller too, $$$$$ Some guy had bought 4 which were the same as a Subaru car for his 400 and was selling one on eBay, but that was over a year ago. I didn't buy one, the people decided to get a 900x.

This is the video you are referencing.
Kubota RTV 400Ci will not start - YouTube starts at the 2:30 mark.

 

[Rollo]

I am going to run mystery oil through the system. Hoping to clean mainly the injector and valve. The temp sensor is about $200 which I am hesitant to replace not knowing for sure the fix vs cost. Backfiring has gotten a little better since i have put mystery oil through it. It also does seem to get a little worse when it does warm up. This weekend I am going to take the electrical connections apart and check them.

 

[DaveFromMi]

Backfiring is a sign that the engine is running too lean. Check for intake air leaks between the carburetor and engine.
I had a dirt bike that was backfiring when decelerating. The exhaust was glowing red as well. I added copper based sealant to the boot that connects the carb to the engine intake. It fixed the problem.

 

[Riverbanks]

Seeing how it's fuel injection, I would run some techron cleaner in the tank, you possibly have build up on the back side of the intake valve, you did say it's dusty. The gunk will absorb the incoming fuel and alter the flow, creating a bad combustion process, another thought, if the temperature sensor is a two wire, put a jumper across the connector, that will bring it to the upper temp. limit and lean it out, if backfires disappear, bingo

 

[Rollo]

Seeing how it's fuel injection, I would run some techron cleaner in the tank, you possibly have build up on the back side of the intake valve, you did say it's dusty. The gunk will absorb the incoming fuel and alter the flow, creating a bad combustion process, another thought, if the temperature sensor is a two wire, put a jumper across the connector, that will bring it to the upper temp. limit and lean it out, if backfires disappear, bingo

I changed oil and ran it with more Marvel Mystery Oil in the gas. Going to add some more and run it some more tomorrow. Hoping it just needs a good clean out. When I ran it tonight it seemed to go much further without backfiring. It is still not going over 16 mph when it says top is 25.

 

[CiscoRanger]

I changed oil and ran it with more Marvel Mystery Oil in the gas. Going to add some more and run it some more tomorrow. Hoping it just needs a good clean out. When I ran it tonight it seemed to go much further without backfiring. It is still not going over 16 mph when it says top is 25.

Did you compression test or cyl leak down test it? If the Intake valve is sticking open that will show you. one of the ways I dealt with a crudded up sticking intake valve is

-pull the spark plug
-put the motor at top dead center Then advance it 90 degrees by hand. The intake valve should be open and piston halfway down.
-fill the cyl thru the spark plug hole with sea foam or marvel
-reinstall spark plug and by hand rotate the engine 45 degrees opposite it’s normal direction ( halfway back towards top dead center).
-let it soak overnight or however long you want.
-pull the spark plug and crank it to blow out that oil.

this essentially lets the cleaner soak the open valve and the valve seat at the same time.

 

[Rollo]

Did you compression test or cyl leak down test it? If the Intake valve is sticking open that will show you. one of the ways I dealt with a crudded up sticking intake valve is

-pull the spark plug
-put the motor at top dead center Then advance it 90 degrees by hand. The intake valve should be open and piston halfway down.
-fill the cyl thru the spark plug hole with sea foam or marvel
-reinstall spark plug and by hand rotate the engine 45 degrees opposite it’s normal direction ( halfway back towards top dead center).
-let it soak overnight or however long you want.
-pull the spark plug and crank it to blow out that oil.

this essentially lets the cleaner soak the open valve and the valve seat at the same time.

Cisco, without taking a bunch of stuff apart, not 100% sure how to rotate the engine 90 degrees back. It appears that I would have to remove the flywheel plastic cover.

 

[CiscoRanger]

Cisco, without taking a bunch of stuff apart, not 100% sure how to rotate the engine 90 degrees back. It appears that I would have to remove the flywheel plastic cover.

you might not be able to rotate it or find the TDC timing marks without some disassembly. I’ve never worked on that specific engine. i had a single Cyl Yamaha that had compacted carbon in the valve seat and running it with treated gas never cleared it up. From the leak down test I knew it was leaking down from the intake valve tho so I went ahead and tore it down enough to soak it. Same with a 4.3L Chevy buried in a boat bilge. I’d probably do some compression test before I started tearing into it tho. And just suggesting this based on the other comments about it possibly having a dirty build up on the intake Valve.