How perfectly should my Onan run?

[Straylight]

Hi all! I've lurked here for AGES to fix all my tech issues; you're all very helpful. I'm looking for some info about my Onan Microlite 4KY Spec H generator.

The gist of my question is this: When it's running correctly, should I NEVER hear a surge and should my hz + volts be nearly perfectly stable?

The generator surged endlessly and did not run when I bought the RV. I have never heard an Onan generator running, so I have no idea if my interventions have made it "good as it'll get" or if I should still tinker with it.

Here's the history:

• Clogged carburetor from lack of prior owner use: Took it apart, cleaned it, took it to a small engine shop for an ultrasonic bath.
  
• Current gas tank: Regular gas, a can of Seafoam, and most of a can of Berrymans B12.
  
• Replaced carburetor gaskets, plastic float doohickey, float pin, float hinge. (Onan carb repair kit)
  
• Melted circuit board: Diagnosed at Onan shop; I put a new OEM part in.
  
• Bad fuel pump wiring/Prior owner had bypassed fuel pump/Fuel pump was trash: Long story short, I replaced the fuel pump with an aftermarket Airtex pump and wired + connected it correctly.
  
• Replaced fuel filter with very nice new inline filter before the pump.
  
• Replaced spark plug with new Champion plug. Correctly gapped.
  
• Changed oil.
  
• General wipe-down, inspection, WD-40 on the governor and choke moving parts.
  
• Most recently: Dialed in speed screw, governor sensitivity screw, and idle stop screw according to (edit: I can't put in links, but I used user hndymn's guide posted on this forum on a topic called "Onan generator frequency adjustment?" to adjust these settings).

My resources:

• Local Onan shop is full of super friendly guys who seem to like me. I've ordered some parts from them. I'm already $600+ into the shop for the control board diagnostic and the control board itself; I'd rather not spend too much more cash. Plus, I like learning about my genny!
  
• Onan Master Service Manual, plus the service manual for my genset.
  
• All the correct tools and easy access to the generator.
  
• A Kill-A-Watt that I plug into an extension cord that is plugged into an interior outlet on my rig.
  
• An air conditioner that pulls about 2000W and a microwave that pulls about 1000W. (My "load bank"!)

Here's where we stand now:
I'm getting consistent hertz readings between 61-63 at idle and 58-61 under 2000W load (call it half load, from the air conditioner). My voltage is generally between 110 and 130, up towards 127 at idle and down towards 118 under 2000W load. My voltage is never perfectly steady: When it's running at its best, it goes up and down a volt or two. The generator will run forever if I let it; it does not die. It no longer coughs or sputters. When I hold the throttle tang against the stop screw, I drop down to 54-56 hertz and it will stabilize at that speed (usually 54.7 or so) and will not die.

However, every minute or so under 2000W load it will slow down and then surge once; once in a while, it will hunt up and down for a few seconds. When idling (that is, with a nominal load, when the rig is plugged in but everything I can turn off is off), it will surge slowly but rhythmically.

I have been able to adjust it so it runs perfectly at idle, but when I do so, it's running at 66 hz. I can also adjust it so it runs a little better than previously described under 2000W load, but when I do THAT, it runs worse at idle.

When I load it suddenly to 2000W, it will hunt up and down about five times before it settles on the correct speed.

Is this pretty much as good as I should expect it to get? Should I keep tinkering with the speed and governor sensitivity? Is there something I've missed?

Some final thoughts:

• The only things I haven't done are replace the air filter and put a brand-new carburetor in it.
  
• My Spec H carb has a tamper-proof idle mixture screw. There is a vertically-mounted, very small screw directly below the throttle stop screw; I have not adjusted this screw, other than to remove it and clean it. There is an altitude adjustment, which I have at its lowest setting (I'm in Virginia), and directly below the altitude adjustment is a large screw that I think is the main carb adjust; I have removed that and cleaned it, and it is currently about as far in as it will go without putting a whole bunch of force on it.

I'd love to hear your expertise!

[Chum_lee]

I don't mean to be a smart *ss but are you having any problems running equipment with the genny? Any problems starting it, reaching maximum output, excess fuel/oil consumption, exhaust smoke, overheating, noise, spark plug fouling, etc? I have the same unit and I can't hear yours, but, mine does change speeds/voltage slightly depending on load, altitude, temperature, etc. It has never failed to do what I ask it to do. Years ago I learned not to try to fix problems I don't have and I spend my time resolving the ones I do. Could you be expecting too much? Just asking. You have a very popular genny. It might be worthwhile to listen to others and see what they sound like before you condemn yours.

Chum lee

[bounder39zman]

have cleaned/overhauled hundreds of these "tamper-proof" carbs, with
about 50% success rate. Very difficult to restore to perfect running condition, and getting balance on governor speed, sensitivity, idle, etc is a real art. Surging is usually air mixture problem, and "hunting" is a governor issue, but symptoms are similiar. The altitude compensating screw is the main mixture screw, you can pry the cap off carefully, dont bend the screw. When these units are running right, you should see a few speed fluctuations when load changes, but should steady out. Voltage will vary slightly, but should stay 115-125. A new carb would probably provide smoother operation, but sounds like it is acceptable the way it is.

[maillemaker]

Tried everything to clean my carb. Took it totally apart, carb cleaner, compressed air and nothing worked.

From what I have read, cleaning solutions and even compressed air are unlikely to unclog the tiny clogged passages. You have to get a fine copper wire and thread it through the ports to clean them without damaging them.

Once the jets/passages are plugged only mechanical action seems to be the sure-fire way to clean them out. But you have to be very careful as it is evidently easy to wallow out the hole.

Steve

[azrving]

Read the plug.....

[wa8yxm]

Continous surging (Hunting it is called) indicates an improper fuel/air ratio. I believe that is too rich. Stuck choke, or mal-adjusted carb are the most likely causes.

IT should be fairly Hz will wiggle a bit when a BIG load (A/C. Water heater. Microvave) kicks in/out but overall should be very stable.. "Purr like a kitten" is the applicable phrase if it's running right. A fairly big kitten.

[azrving]

Drill a very small hole in the plug then gently insert a small screw and use pliers to pull the plug

[j-d]

Straylight wrote:
...I cannot adjust the idle screw: There's a metal plug on top of it...
...new governor spring and a new governor linkage spring, see if I can spend twenty bucks instead of two hundred on a carb...
...Is there a failure point that would change depending on the temperature of the generator?

Metal plug is an indicator of the problem with the ONAN carbs. Sealed up to be non-tinkerable. That said, that 90* "corner" one of the techs told me about is probably adjacent to the screw you can't get to.

Governor Spring simply can't hurt. There's also a tiny spring or two designed to keep any lost motion out of the linkages on top of the carb (throttle and/or choke) so I'd guess it'd run sloppily if one was missing or disconnected.

Well, it has an automatic choke (thermostatic coil) to richen it for starting and warmup. So sure, Mixture does change with outdoor temperature and probably the operating temperature of your genny.

[Straylight]

Double post: ...can I just pull that tamper proof plug off the idle screw? With a metal pick or something?

[Straylight]

K Charles wrote:
The OP says he has not removed or adjusted the idle screw and the "main carb adjust" is screwed in as far as it will go. How did the OP clean the idle screw if he never took it out. I would first adjust the idle and main jet.

Correct, one of my worries is that the idle screw area in there is particularly bad; I couldn't get at it with my can of Birkebile and I of course can't determine what effect the ultrasonic cleaning had on it. Note that I cannot adjust the idle screw: There's a metal plug on top of it.

Thanks for all the other comments. I think I'll have my Onan guys put in for a new governor spring and a new governor linkage spring, see if I can spend twenty bucks instead of two hundred on a carb. I'm sort of resined (get it? because of the sinusoidal surging...) to the new carb, though.

Another symptom popped up: I had adjusted all of the screws when the genset was nice and hot. I would describe it as running "acceptably" (not immaculately, per the comment about the tooth fairy above). Later, after it cooled, I went back and turned it on and the surging was very bad again.

Is that an important detail? Is there a failure point that would change depending on the temperature of the generator?

[j-d]

From what I remember from the ONAN Manual, the Voltages and Frequencies you state, are within "normal." How much fluctuation within those ranges is another matter. To a degree, they'll "hunt" for the right throttle position to stay in range when the load changes. Definitely shouldn't "surge" over a wider range, particularly at idle. Remember, "idle" is still 3600 RPM. It's running at that RPM under no load and therefore a different throttle position and different carb passages than at full load.

Ours definitely reacts when A/C kicks on/off, microwave the same, I don't notice it on smaller loads like coffee maker. It probably DOES react, just not noticeable. Is yours disturbing, or something you've gotten interested in, and sit there monitoring?

I mentioned different carb passages. According to an ONAN tech a few years ago, the 4KY carb has a square internal corner in the idle circuit. Very hard to clean. You can try what some of us have done successfully: Make up a tempory "tank" with a gallon or two of gas that's VERY rich with your Berryman. Connect that to the genny and let'er run. Shut off and allow the rich Berryman to soak in. Run again. Do this over a couple days. Just "might" get the last of the goo out.

Many of us put Stabilizer in EVERY DROP of gas that goes into the main RV tank that also feeds the generator. I'm one of those guys, haven't had trouble since I lucked into a clean used carb off a junked 4KY. Stabil is the one everybody thinks of, but Startron works at a fraction of the price. Big bottle from Amazon.

With an old ONAN BFA, I had the carb cleaned and rebuilt by a fuel system shop. Ran FAR better than before but still had a "surge" and a new Governor Spring solved it. A few dollars very well spent.

See what the ONAN guys say when you're there for a new Spring. If Spring doesn't help, try Berryman.

[K_Charles]

The OP says he has not removed or adjusted the idle screw and the "main carb adjust" is screwed in as far as it will go. How did the OP clean the idle screw if he never took it out. I would first adjust the idle and main jet.

[MEXICOWANDERER]

I've seen too many micro Onans to believe in the tooth fairy. If the slight surge does not worsen and electrical output values do not escape the acceptable window I wouldn't worry about it. Woodward electronic governors are extremely expensive and the only small Onan I have seen run flawlessly was a 22 horsepower 4 cyl powering a 12,000 watt generator and it was on LPG. It had it's own problems with engine overheating > 9,500 watts. The old saturated field 6.5 NH was notoriously fickle but it was near bulletproof.
Trying a brand new carburetor is going to be pricey. I hope it works for you.

[Old_Man]

All this is all the more reason I'll ditch the Onan and replace it with a Honda. Onans are stinky and noisy.

[azrving]

It sounds like you have the fuel delivery in order so it's probably not lean. I'd be sure the governor is set correctly. The gov is a balancing act between the flyweights and the gov spring. Any slop in the linkage or incorrect adjustment can cause that problem. Some linkages will have a very fine spring covering them. The spring connects to the arms that the linkage is in to take up the slop.

But being borderline lean could be an issue. What does the plug look like? Black, brown, white?

[DennisVR]

Tried everything to clean my carb. Took it totally apart, carb cleaner, compressed air and nothing worked. Found a new carb on EBAY for $200 and replaced it and it runs great. Mine was the Onan 5500.