Mule 550 unstable hot idle

[Werhat]

Hi folks,

I have a Mule 550 (FE290D engine) that sat for several years and would not start.  A few years ago I replaced the fuel filter and carburetor, at which point it started right up and developed full power.  More recently, I've changed the air filter, fuel pump, ignition coil (gapped to spec), and spark plug, and adjusted the valves.  The engine still has a lope/surge at part throttle and at idle.  Rather than idling at a stable speed, it speeds up and slows down until it quits.  It snaps to life right away if I open the throttle, so I don't think it's starved of fuel.  I can always get it to restart immediately if I choke it while cranking, then it will die after several seconds again.  It takes some cranking to start hot when not choked.  I've tried adjusting the idle mixture, but no setting seems to make it stable.  It won't idle at all with the throttle fully closed; I have to turn the stop screw to hold it slightly open.

I suspect it's getting idle air, since the passages are open, but that the idle jet isn't providing adequate fuel, such that I am having to hold the throttle open and idle it off the main jet which isn't stable.  This is just speculation.  I've cleaned the idle jet and tried a larger size, but it made zero difference.  The carburetor seems to be clean.  It's been doing this since I installed the new carb.  The gas tank seems to be clean, although it's hard to see inside.

I posted a video of what it's doing:

What's next?  Try another carburetor?

Thanks.

 

Edited May 5, 2022 by Alex
video embed

[Travis]

Could you post a link to the carburetor used.

I had a similar issue with mine last year and it turned out i was not adjusting the idle jet screw properly... the way to set it like it came from the factory is to unscrew, or screw it in until the engine reaches the highest idle, then adjust the throttle stopper screw to 1300 RPM

Those chinese aftermarket carbs are crapshoot if you get a good one or not. I have one on mine that is a KIPA brand from amazon.

 

[Werhat]

9 hours ago, Travis said:

Could you post a link to the carburetor used.

I had a similar issue with mine last year and it turned out i was not adjusting the idle jet screw properly... the way to set it like it came from the factory is to unscrew, or screw it in until the engine reaches the highest idle, then adjust the throttle stopper screw to 1300 RPM

Those chinese aftermarket carbs are crapshoot if you get a good one or not. I have one on mine that is a KIPA brand from amazon.

 

I got it a few years ago, so it will take some digging to find it.  It was sold as an OEM carb in a Kawasaki box and says Mikuni on the side.  I think it was around $170.

It's possible it was a copycat, but it did look identical to the original.  I specifically tried to avoid risking a knockoff!

[Travis]

Oh okay, then that is the OEM Japanese carburetor.

Do you have a tachometer? I would highly suggest you get one and set the idle at 1300 rpm and see how it does.

[Werhat]

9 hours ago, Travis said:

Oh okay, then that is the OEM Japanese carburetor.

Do you have a tachometer? I would highly suggest you get one and set the idle at 1300 rpm and see how it does.

Today I readjusted the valves (0.005" or 0.127mm) on a cold engine, swapped in another spark plug gapped to 0.027", and added some Techron to the tank.  The old (about two years old) spark plug had a lot of dry carbon buildup except right at the electrode, which was tan.  I reset the idle to what i think is around 1200-1300 rpm.  It seems a little high because it clanks/slams into gear, but it's definitely more stable at the new speed.  (I seem to remember 1100 rpm from the service manual.)

I don't have a tach.  I tried wrapping some wire around the spark plug lead to see if my multimeter would pick up anything, and it reads about 4x what I expect.  Does this engine fire one spark per rotation?

What did work was to record audio by the exhaust and open the waveform in an audio editor on the computer.  Pulses * 2 * 60 / seconds = rpm.  I did this a few days ago and need to redo it after fooling with the carburetor.  It's not real time but does allow verification after the fact.

I also noticed that the throttle doesn't return fully against the stop when I release the pedal.  There's a little bit of free play which allows the idle to hang high until I nudge it with my finger.  It doesn't seem like the governor is doing anything to control the idle.  I'll see about adjusting the governor next.  Maybe part of the problem is that the governor isn't adjusting the throttle as needed to maintain a constant idle speed because it's set to low.

[Travis]

also, there is another screw you could adjust that may be causing the governor to hang,  where the throttle cable hooks to the control panel (on the engine) there should be a phillips head screw poking out from that cover, you can use that to raise or lower the RPM as it presets the tension on the governor spring. The manual calls it the Accel Lever screw.

Engine fires one spark per revolution as it is a fixed  coil and magnet  on the flywheel. just like a lawnmower.

[Werhat]

17 hours ago, Travis said:

also, there is another screw you could adjust that may be causing the governor to hang,  where the throttle cable hooks to the control panel (on the engine) there should be a phillips head screw poking out from that cover, you can use that to raise or lower the RPM as it presets the tension on the governor spring. The manual calls it the Accel Lever screw.

Engine fires one spark per revolution as it is a fixed  coil and magnet  on the flywheel. just like a lawnmower.

I backed off on the carburetor stop screw and advanced that screw on the governor a bit.  It seems to work the same as the carb screw, namely it sets where the linkage rests when the accelerator is released.  It put an inch or so of slack into the accelerator pedal.

Is this supposed to work as an all-speed governor, where the governor is always moving the throttle to control the speed?  Or is it more of a speed-limiting governor only?  The arm from the engine only seems to move when it hits maximum RPM.  There's another screw under the cover that I assume is setting the max speed.  I didn't mess with that one.

[Travis]

It's both... If you're driving along and say you go up a hill or a heavier load gets put on the engine, the governor opens the throttle to maintain your speed, while also limiting Max engine RPM within a safe range.

[Werhat]

3 hours ago, Travis said:

It's both... If you're driving along and say you go up a hill or a heavier load gets put on the engine, the governor opens the throttle to maintain your speed, while also limiting Max engine RPM within a safe range.

Adjusting the idle up definitely helped, but it's idling way too fast and grinds/slams into gear.  Any slower and it eventually stalls.  I suspect it won't stay running at slower speeds because the mixture is too rich.  When I snap the throttle open, it responds instantly, suggesting it has plenty of fuel.  This happens even when the air adjustment screw is all the way out. 

I can't remember if my replacement carburetor came with a pilot jet or if I had to move the one over that originally came with the Mule.  I've seen a few carb diagrams for the FE290D with different air jets in them.  This has me wondering if maybe the carb came with air jets that are too small.  I'm running the stock #40 idle fuel jet.  Here is the diagram I'm using for reference.

[Werhat]

18 hours ago, Werhat said:

Adjusting the idle up definitely helped, but it's idling way too fast and grinds/slams into gear.  Any slower and it eventually stalls.  I suspect it won't stay running at slower speeds because the mixture is too rich.  When I snap the throttle open, it responds instantly, suggesting it has plenty of fuel.  This happens even when the air adjustment screw is all the way out. 

I can't remember if my replacement carburetor came with a pilot jet or if I had to move the one over that originally came with the Mule.  I've seen a few carb diagrams for the FE290D with different air jets in them.  This has me wondering if maybe the carb came with air jets that are too small.  I'm running the stock #40 idle fuel jet.  Here is the diagram I'm using for reference.

I pulled the jets out.  The diagram shows a #1.3 air jet on the left and a #1.8 on the right.  What I pulled from the carb was #1.3 on the left and #2.0 on the right.  The pilot fuel is #40 per the diagram.

I can tell that the left jet goes through the idle air adjustment screw.  Not sure where the right-side one goes.

[Travis]

very odd problem.... I'm curious too,  did you purchase a carburetor with a Kawasaki Mule part number, or one for a FE290D engine?  That may be the fly in the ointment as Kawasaki made them run somewhat leaner than the general purpose version of the FE290D to conform to all 50 state versions of air quality regulations that existed then.

 It also Sounds like there is a restriction, of some kind in the pilot circuit....  it's probably idled up high enough it's pulling some from the main jet as well.

Just to be sure,  The jet I  circled in red is the pilot jet, which you adjust until the engine reaches the highest idle,  now this idle may be too high too high for it to go into gear properly  so you adjust it to the 1300 rpm spec with the screw in blue

 

[billycalexander]

Werhat.

 

Did you ever figure out your problem?  It sounds like a gummed up carb circuit to me?

[Werhat]

On 9/13/2022 at 9:24 PM, billycalexander said:

Did you ever figure out your problem?  It sounds like a gummed up carb circuit to me?

Sorry about taking so long to respond.  I did resolve the issue by replacing the carb again, but I'm not sure exactly what was wrong with the one that was on the engine.  It definitely wasn't gummed up, because it behaved that way right after installation.  I believe it was a factory assembly, but I seem to recall having to migrate the jets from my factory original carb.  I suspect I forgot a part or otherwise messed up when moving them.

With the carb that was acting up, I tried to back out the slotted plug from the underside (the bowl area), and it was seized into the aluminum threads.  After mangling it with my screwdriver, I ordered another factory carb that came with the jets preinstalled.

Before mounting it to the engine, I compared that new replacement to the one that was having issues.  They were 100% identical except, when viewed from below, the slotted brass plug on my previous one was recessed about 1/4" higher up in the carburetor than the new one. 

So either I significantly over-drove it when installing the jets  or I missed a part, such as an orifice disc, when assembling.  I'll never know for sure, though, because there was no way I'd get that assembly apart without cutting it.

The new-new factory carb has worked great for a couple seasons now with only a minor adjustment to the idle mixture.