Resolved!. Really need some help, Kubota dealer stuck, 2024 RTV520h engine issues

[omextreme]

Bought a 2024 RTV520h 1090 hours from auction, Not tested not running.
Got it home and running (ignition switch bad) But its not running well at low speeds but runs like a top at high speed. here is what I tried, and what the dealer tried so far nothing is getting better.

Starts instantly, idles smooth.

Idle up to 50% throttle, hesitates, sputters, backfires (in or out of gear)

I checked fuel pressure, injector pattern, air filters, changed plugs - No improvement.

Took to Kubota and they are still at the same spot,
Here is what they said/tried.
1. I cracked the plugs installing them (I have never cracked plugs before but who knows.. i was frustrated!)
They installed new plugs - no dice
2. Ignition coil
Replaced and nothing
3. Compression too high? I didn't check this. They are measuring 230 both cyls.
They suspected maybe valve issue, pulled heads adjusted valves - no improvement
4. ECU bad - no codes but bad?
Replaced and no better.
5. Tested crank position sensor (i asked them about this at the start)
No issues found.

Now they are waiting on Kubota engineering to call them back with next steps.

My best guess is I am $1000+ into them guessing at the issue at this point with no improvement. (they are not charging for the ECU since it didn't fix it)

So what causes one of these gas engines to run poorly at low speed but NOT at high speed? what would cause the high compression without any soot on pistons etc?

I am not a mechanic but get along well enough to understand the basics, but I cant sink endless $$ into this unit for Kubota to essentially do nothing to make it better.

Any thoughts would be welcome!

 

[Runs With Scissors]

#3 has me baffled.

Could someone please explain to me how "valves being out of adjustment" could make the compression too high?

 

[Workerbee]

Compression too high? Maybe a plugged up exhaust.

 

[Orangeglow]

How great a deal did you get at the auction ? It is always a huge gamble buying used stuff at auctions.

 

[NCL4701]

Restricted exhaust would be most likely suspect based on behavior and things y’all have already ruled out.

 

[GreensvilleJay]

know nothing about that engine but....
did you remove al the old gas, CLEAN the tank, and fillup with fresh gas and install new fuel filter(s) ??

 

[omextreme]

Compression too high? Maybe a plugged up exhaust.

During idle the tailpipe was blowing pollen around Like a leaf blower

 

[omextreme]

How great a deal did you get at the auction ? It is always a huge gamble buying used stuff at auctions.

I picked it up for three grand

 

[omextreme]

know nothing about that engine but....
did you remove al the old gas, CLEAN the tank, and fillup with fresh gas and install new fuel filter(s) ??

the 520 doesn’t have a fuel filter and has a fuel sock on the pump but yes, fresh gas no clogs good fuel pressure

 

[NCL4701]

During idle the tailpipe was blowing pollen around Like a leaf blower

So you know it isn’t completely obstructed, but it runs so we knew that already.

My thoughts aren’t specific to that engine so maybe irrelevant. Having dealt with a few gasoline engines with catalytic converters obstructed by collapse, the behavior described (including exhaust that appears free flowing at idle) is still consistent with partially obstructed exhaust.

 

[omextreme]

So you know it isn’t completely obstructed, but it runs so we knew that already.

My thoughts aren’t specific to that engine so maybe irrelevant. Having dealt with a few gasoline engines with catalytic converters obstructed by collapse, the behavior described (including exhaust that appears free flowing at idle) is still consistent with partially obstructed exhaust.

Interesting, Ill call Kubota an see if they have looked into that.

 

[Workerbee]

Long ago my uncle had a 10hp merc that he had taken to 2 dealers to try get it running right. They focused on the carb and each rebuilt it. Still wouldnt run right. He asked me what I thought, I told him bring the boat over. I checked the basics and all seemed good, but it just had a different sound to it. So I decided to pull the powerhead and lo and behold the mid section was about totally plugged with carbon. He just used that motor for trolling, it hardly ever got ran at higher rpm’s. Anyway, cleaned out the carbon and it ran like a top again.

 

[omextreme]

*** Small update - kubota engineering seems to think the misfire / timing issue is likely from the new ECU not being properly programmed. The local dealer is updating it and will retest.

Worth noting - Per the local dealer Kubota stated that the 230 psi compression test is high but not out of range?

 

[MapleLeafFarmer]

*** Small update - kubota engineering seems to think the misfire / timing issue is likely from the new ECU not being properly programmed. The local dealer is updating it and will retest.

Worth noting - Per the local dealer Kubota stated that the 230 psi compression test is high but not out of range?

Thanks for the update. Hope to hear about final resolution. plse keep us updated as I am following.

Good on kubota for staying on it though as being off warranty I guess you can't force a privately owned dealership to take on every odd ball job like problem auction cases, etc...

Hoping they can get to the bottom of it cheaply enough as those RTV's can be super useful workhorses.

 

[omextreme]

Thanks for the update. Hope to hear about final resolution. plse keep us updated as I am following.

Good on kubota for staying on it though as being off warranty I guess you can't force a privately owned dealership to take on every odd ball job like problem auction cases, etc...

Hoping they can get to the bottom of it cheaply enough as those RTV's can be super useful workhorses.

The dealership seems like a good old fashioned "local shop" when it finally gets resolved I am hoping they are willing to have a discussion about the invoice since the first few hours were in my mind based on guessing.

Heads, plugs, coil etc all things I feel were not logically the cause here

... Ill be happy to have it running and happier if we can be $1200(ish)

But I am reasonable as well, we cant always hone in on the exact issue and be in and out in under 2 hours. I have a team of over 100 techs in a different field and one of my fundamental principles is not to bill customers for my techs continuing education, learn solve then look back and price accordingly.

 

[#40Fan]

Every vehicle I've ever worked on with a plugged cat either did run good at all, or didn't run worth a crap at full throttle. None of them ran like crap under moderate load and then woke up at full throttle.

What is the compression ratio of this engine? 230 might seem high, but I've seen some 2 cylinder generator engines hit 210+.

If they had the proper equipment, scoping the crank signal versus the spark output would be a good place to start. Confirming base timing (proper crank to cam) is the first step in all of this.

 

[SDT]

#3 has me baffled.

Could someone please explain to me how "valves being out of adjustment" could make the compression too high?

Me too.

Sounds like dealer is trying to snow you. Dealers are want to do such things.

 

[#40Fan]

There are some engines that have compression releases where valve lash could be an issue. These are usually pull start engines where you want to make it easier to start. Yet, some SxS still use them, being electric start. Yamaha Rhino for example.

 

[Hugo Habicht]

Is the mix too lean or too rich at low revs?
Did you measure the exhaust gas?

 

[omextreme]

really starting to lose my mind!
Dealer reprogrammed ECU per Kubota and no dice.

Kubota engineering wants them to try the injector now. I verified pressure and spray pattern, The shop also said they checked pressure and pattern and didnt see any issue.

i am trying not to get frustrated but this seems like such a simple gas engine.