Kawasaki 25 hp V-twin

Fuel filter pics, it's almost all air space, just a little fuel at the bottom. Not sure how much this matters, assuming carb is typical bowl/float setup, just surprising.

 

Not that unusual...mine is the same.
I agree that it seems like it should be full, but they never are.
So when you had it apart, how did the governor mechanism look? The flyweights able to move free and smooth...no worn spots? Just wondering if the governor could have been sticking, allowing RPM to drop?
But didn't you say it was cutting out/missing too? Maybe it had/has an intermittent ignition issue too? Hopefully whatever gremlin it was has been exorcized...got a fresh engine now anyways! You do the rings while it was apart too?

 

The flyweight rig is on the camshaft in this engine, and I had to remove the camshaft to get to the breather reed valve. I checked that the return spring was there and good, and the dome that pushes on the governor arm seemed to move freely. I even checked the little ball bearings in the cam lobes, which are lifted by the flyweight assembly pushing on a rod that runs thru the cam to adjust valve lift. All seemed good and moving freely, but I did not disassemble it.

I did not touch the bottom end, mostly because when working on car engines, removing crank and pistons usually meant a lot of boring and honing work, you never put those back together without a few weeks lost to teh machine shop. The shop manual said piston bores should maintain their cross hatching, which I thought was insane... I thought they'd be gone after just a few hundred hours break-in, and mine were. The ridges at the tops of the bores were between minimal and non-existent though, so at least compared to most car engines I've pulled apart, it looked pretty good. There was one faint wear spot on one of the piston walls, just a slight discoloration, but no rock or slop in the pistons within their bores or their connecting rod bearings, so I decided to leave well-enough alone there. Compression was above spec and leak-down was good, all along. so I didn't feel any great desire to tear into that.

When this all started, I thought ignition might be a culprit, since I at least imagined it only started after warmed up. Intermittent or temperature-driven stuff like that is often ignition coils. But the behavior I saw yesterday seems to go against that, in that it started right away after firing up, and went away when I brought the governed high idle down from 3800 rpm to 3600 rpm. Weird.

 

Any possibility of ignition related over speed protection? Not seeing how 200 extra rpm gets into valve floating.

 

Yeah, that's typical. Back to the valve float issue, that's typically a high rpm issue.

 

Yes it is...and I wouldn't expect it to occur at ~5% over rated RPM...heck, I'd think even a worn engine would take 4k RPM without too much issue.
But even if it was valve float, that should basically just act as a "rev limiter", but not affect power below "float" RPM.
Drag racers that deal with valve float are usually looking for several thousand RPM (+) more than OEM redline

 

There's no ignition control module on this one. Just a single wire to the coil primary, which I assume is just DC 12V, and a single magnet on the flywheel that passes below each coil, basic dual magneto type setup. I did try to measure coil primary resistance, and got nothing, but manual says you need a special Deere meter to measure that, there's apparently a built-in rectifier circuit.

Yeah, I was thinking the same, actually. I've dealt with valve float on my old drag racing car, but that was over 6500 RPM. But then again, these are wimpy little springs, so who knows. It does seem to be bad design to have an engine that could float at 3800 rpm though, if the intended operating speed is 3600 rpm.

I did measure the free length of each spring while I had it apart, and they're all well-within spec for that. Deere doesn't publish a spring tension number, but maybe we could find something from Kawasaki.

I've been running it all morning, catching up on the walnut harvest, and behavior has been "pretty good". I've caught just the hint of the rough running a few times in the first hour, but now it seems to have smoothed out. There's still something not quite right.

I had tested fuel pump output previously, but given the intermittent nature of this, maybe the conditions were not right. Likewise on spark gap, it was good when I measured it cold, but if it's varying...

If I could find some way to monitor and record fuel delivery and spark while running over the course of 30 or 60 minutes, I suspect we'd have this debugged pretty quickly.

 

Its not just bad fuel/water in fuel, is it?
Water can cause intermittent issues as it sloshes around and gets sucked up now and again...E10 blended gas can separate and cause issues as the ethanol absorbs moisture too...

 

Good point. Bad fuel will do all sorts of things

 

I think I mentioned that back in the first post, that yes... this could be bad fuel. You know, you wouldn't believe how long it takes to run one of these machines dry, but after about 10 hours, I think I'm just about there.

The fuel that was used in this machine hasn't been tested in any other, as circumstances and weather have dictated this mower is the only straight-gas machine that's been in use the last few weeks. I did pick up a fresh can of gas, which will be dumped into this tank as soon as I run it dry and inspect the sump.

I have another theory, with some merit. This machine has a tendency to boil gas in the carburetor, usually when I park it for ten minutes to blow it clean and refuel after mowing in summer, and then re-start it to back it into the shed. The behavior I was seeing last week seems similar to that, and initially appeared after several hours of low-load running, towing a walnut harvester. Today, it misbehaved a bit when towing the walnut harvester again, but not so much when actually mowing.

I'm wondering if low fuel usage under low load, combined with continuous high RPM temperatures, are conspiring to cause a fuel boiling issue.

 

What type of fuel pump does it have...mechanical or electric?

 

Impulse. Basically mechanical, which operates off air pressure or vacuum ported off one of the valve covers. This is one of the reasons I was concerned about crankcase pressure.

 

Is that pump mounted in an area that gets particularly hot? Should be able to extend the hoses and move it to a cooler area if heat is a problem, no?

 

Or you may be able to recirc some fuel to cool it too...Ford did that on some of the ems service 460s back in the carb days... IIRC they simply used a fuel filter that had a small return line back to the tank...I don't remember how they controlled fuel pressure on that...I suspect that the return line was small enough to not affect the minimum pressure the carb needs to have to keep a full bowl...which is all it really needs

 

I suspect that if the fuel is boiling, it's happening in the carb bowl itself. But unlike my good old Holly 650's, this one only has a single inlet to the carb bowl, so no chance to recirc through that.

The pump hangs on a tab off one valve cover. It's definitely warm there, and might soak enough to get hot during a shutdown, but shouldn't be that hot when running and air is moving everywhere. The carb is heated by direct conduction, although they did at least put a 0.2" phenolic spacer between intake and carb, which might have been partly to improve that.

 

Could the root of the problem been the clogged oily fins raising the overall temperature thus boiling the fuel?

 

This was explained to me by a chemist saying that it wasn't air but rather fuel vapor separation because of the increase in volume separating the inlet and outlet. He went further into it but at that point I was lost.

 

It definitely could explain why it was so bad before rebuild. But everything is sparkly-clean now, and it's still doing it just a little, albeit much less. Moreover, this is a new problem that just popped up a week or two ago, so it's not really a design problem.

My tank is very nearly dry. Here's to hoping a fresh fill will resolve my woes!

That makes total sense to me. I wasn't aware the pressure drop was enough to cause that behavior, but the phenomenon is one I understand well, from working on phase-change cooling (i.e. commercial electronics versions of air conditioning evaporators).

 

It’s still misbehaving, albeit very intermittently. Think it’s time to buy new coils? Both header tubes are hot, but I’d expect that until one completely dies. Both throw spark when cold-cranking, but again expected if they’re intermittent. Not sure what else to try, at this point.