Onan 4K Valve clearance adjustment

[DonTom]

Is the spec. on my Onan 4.0 KY-FA/26100 J for each valve clearance?

There was a break in the rain this afternoon so I did the following:

I removed the spark plug and then I could easily rotate the engine by hand with no tools from the fan. I rotated the engine until the valve clearances were as loose as possible and then I put a long allen wrench up the spark plug hole and made sure the piston was at exactly top dead center. That should put my genny dead center at the maximum possible clearance on both valves.

So then I do the measurements with my feeler gauge.

My intake valve: .010" (.254 mm)
My Exhaust Valve:0.000000000000000" or less (no clearance--it could be pushing in a little).

So it looks like my intake valve has way too much clearance. And not nearly enough clearance on the exhaust valve.

First time I have seen them go in opposite directions in any engine.

I wonder how much this has reduced my genny performance.

I have not yet adjusted the valve clearances, I only checked them. I will do the adjustments soon as long as it is not raining.

-Don- Auburn, CA

 

[DonTom]

I think I found the correct info here. Where it says:

"For both intake and exhaust:
Valve Stem to Rocker Arm Clearance (Valve Lash)
ALLOWABLE LIMIT: 0.0008/0.0032 (0.02/0.08)
All clearances are at 70° F (21° C) room temperature.
All measurements are listed in inches with millimeter measurements in parentheses."


I thought the above was a little confusing, but I assume it means to:

Set the valve clearances on both valves @ 0.0032" (.08mm).

And be sure to be within 25% or .0008" (.02mm) when checking the work after a few engine rotations.

Does that sound correct? Even if not, that has to be better than where it now is!

I am also replacing the carb, but just with another Chinese carb.

It will be pouring nonstop rain here tomorrow during the daylight hours, so I will need to wait until Sunday to do the valve clearance adjustment and put it all back together. Then I will adjust the RPMs and test it here for an hour or two. I will test it at 7,200' in a week or so after I am happy with it here in Auburn.

I assume I will now get more power out of it at any elevation, with both valve clearances being that far out--in different directions- which I find unusual.

-Don- Auburn, CA

 

[Mark_K5LXP]

You're making the assumption it was ever set properly in the first place. Often in production, things are assembled by people that don't want to. I think I still have my honda valve lash feeler gauge. .002 on one end and .003 on the other. Go at .002 and no go at .003 and you're done. The adjusters were anything but precise so it was more a hit or miss than critical adjustment. The whole point of the critical lash is to make up for a more expensive hydraulic lifter. Too tight you burn a valve, too loose and you beat the **** out of the valve train. 10 mils vs 3.2 mils will have zero impact on performance, might rattle a little less during operation. But you kick the can of a mushroomed valve stem or loose rocker down the road a little further, so check that box. The $64K question is what the carb is going to do for you. Despite being "new" I would tear into it to check for oils or debris, check/work the float assembly a bit and maybe give it a blast in all the ports with carb cleaner and compressed air. The other thing I might check while I was in there is any static timing adjustment.

Mark B.
Albuquerque, NM

 

[DonTom]

Often in production, things are assembled by people that don't want to.

Yeah, I noticed that on my 2017 Zero motorcycles. They have all these torque specs for the bolts that the Zero factory does NOT go by when they build the thing. I have broken tools trying to remove some of their very over-torqued bolts. And many others have had the same issue.

The $64K question is what the carb is going to do for you.

Don't know, but to check it I need to remove it anyway, so I then may as well put in another carb since I have a couple of new spares.

The other thing I might check while I was in there is any static timing adjustment.

Yeah, I can do that. I need to find my service manuals and see if I can find the specs and where things are marked for ignition timing. Or do you happen to know? It would be interesting to find that the ignition timing is retarded from what it should be as that would greatly effect the high elevation operation. But is it adjustable in an Onan 4K genny?

-Don- in rainy Auburn, CA

 

[DonTom]

10 mils vs 3.2 mils will have zero impact on performance,

Are you sure about that? I would assume the 3.2 mils of clearance on the intake valve would allow the valve to travel down that much farther and let more air in than 10 mils of clearance. Same for the exhaust valve in letting the exhaust air out.

But if that is enough to make a noticeable difference, I have no idea.

-Don- Auburn, CA

 

[Mark_K5LXP]

if that is enough to make a noticeable difference, I have no idea.

It's probably not directly linear but just in terms of magnitude the cam lift is what, a good quarter of an inch? The cam lift isn't specified but you could watch it as the rocker pushes the valve down. A quarter inch is 250 mils, a increase of ~7 mils I would argue isn't zero, but not a lot in a practical sense. It's a few sheets of printer paper.

Mark B.
Albuquerque, NM

 

[DonTom]

It's probably not directly linear but just in terms of magnitude the cam lift is what, a good quarter of an inch? The cam lift isn't specified but you could watch it as the rocker pushes the valve down. A quarter inch is 250 mils, a increase of ~7 mils I would argue isn't zero, but not a lot in a practical sense. It's a few sheets of printer paper.

Mark B.
Albuquerque, NM

But if my exhaust valve would not close all the way (it had zero clearance or less) that would be a loss of compression and that will reduce performance.

Anyway, I completed the valve clearance today. An easy job. I used a feeler gauge of .003" (0.076mm), the closest I could find to 0.0032" (0.08mm) which would be dead center of the allowable clearance range.

For others who have not done valve clearance before on an Onan 4K--this is what I discovered:

Valve cover bolts are 8 MM.

Only a single nut to adjust on each valve stem, use a 11 mm socket wrench or a 11 mm open end or box wrench.

Can easily turn the engine by hand from the genny fan when the spark plug is removed. Can confirm TDC with a long reach allen wrench up the spark plug hole, but this is not necessary, just find the middle of the range where both valves are fully open (or should be open, as in my case as my exhaust valve had no clearance). Adjust both for .003" (.076mm) and you're done so put it all back together with a new valve cover gasket. My 4.0 KY-FA/26100 J Valve cover gasket Onan Part Number:110-3861


I also changed the carb. The two long carb bolts are 10mm.

I started the genny. A lot quieter than it has ever been! Ran perfectly, I didn't need to make any adjustments, it was at 60 hz & 121 VAC as soon as I started it.

But I noticed something very interesting today that could have been my high elevation problem.

The choke tube was missing!

The Onan stupid design (IMAO) uses the heat from the exhaust to warm up a cheap temperature sensitive coil. The default is full choke such as when when the tube flies off.

I have no idea when I lost this tube. It can happen while driving and will probably not be in with the genny. There is plenty of room for it to fall out by the muffler and then be on the road.

I have no idea when I lost this. It could have been before Flagstaff or after. The tube was not in with my genny so it is on the road somewhere. But would the genny work as well as it did with the choke on? Or perhaps enough heat built up inside the genny to turn off the choke a little later? I have no idea. Fortunately, I had a spare choke tube that I installed today. Seems very flimsy to me-- by design-- so I will soon order a couple more of them.

From now on, every time I look inside the genny for any reason, I will be sure to look at the choke tube! It is easy to miss if not thinking about checking it. It way to the right side of the genny.

I could now remove it and run it for an hour to see how it behaves. Perhaps was missing for more than a year!

I will soon (perhaps tomorrow since I can do this in the rain) run the genny with a loads above 3 KW and see what happens.

But I will not be able to do the high elevation test for a while as too much snow up at the summit. But my gut feeling is that it is all fixed and will now be reasonable. It sure sounds a heck of a lot better even here in Auburn. I didn't know it was making excessive noise (perhaps from that extra loose intake valve clearance) until it was gone. The intake valve clearance was more than three times the clearance spec B4 I adjust it.

-Don- Auburn, CA

 

[Larry N.]

Great- that does sound as if you found the problem, given that it runs better.

 

[Mark_K5LXP]

But if my exhaust valve would not close all the way (it had zero clearance or less) that would be a loss of compression and that will reduce performance.

And a rapidly burned valve and seat going from reduced performance to no performance in fairly short order.

The choke tube was missing!

That explains a lot. Before I discovered my choke problem and it would run ratty, I always found the choke was still engaged, and a quick flick of the linkage resolved it. But it didn't do it all the time and I think part of that was ambient temperature. Thinking that maybe if ambient temperature was warm enough it didn't matter if the riser tube was there, the choke spring was warm enough to hold the choke off. Ultimately I discovered that some hardware around my choke thermostat assembly was missing and after resolving that, along with the static setting at 75F or whatever and it's been perfect ever since. I would have to agree that the foil tube riser setup could be better, but 20 years or so is a pretty good run and could be a strong argument for good enough.

Mark B.
Albuquerque, NM

 

[DonTom]

And a rapidly burned valve and seat going from reduced performance to no performance in fairly short order.

I hope I found it in time before any permanent damage from excessive wear to the valve & seat.

Now that I have done and discovered how simple it is to do in an Onan, I will check the clearances fairly often. The one thing Onan did right, have the valve cover right in front for easy maintenance when the genny is installed in an RV. Easy to reach everything that is required for this job.

-Don- Auburn, CA

 

[DonTom]

Since it is raining here, I am inside and cleaned up the house a little. In the process, I found my Onan Service and other Onan manuals in a closet.

The service manual for my 4.0KY-FA/26100 J, on page 2-1 says I should have set it both clearances to 0.002"(0.05 mm). I set mine to @ 0.0032" (.08mm).

But since it is on the more clearance side, I think I am close enough, especially since it runs quieter than before.

But I will set it to 0.002" (0.05mm) on the next valve adjustment, which probably will be just after a few hours of usage.

-Don- Auburn, CA

 

[DonTom]

The other thing I might check while I was in there is any static timing adjustment.

No reason to bother with that. Spec is "25° BTDC, non-adjustable".

-Don- Auburn, CA

 

[DonTom]

I find the spec of .002" (0.05mm) for the valve clearance in my Onan Service Manual on page 2-1.

I now find an allowable range of clearance on page 3-2 in the same service manual.

On page 3.2, it says:

"Valve Stem to rocker arm clearance:

0.0008 (0.02 mm) min 0.0032" (0.08mm) max."

Dead center of this range is the 0.002" (0.05 mm) as per the spec sheet.

IOW, in my message number two here, I interrupted it incorrectly.

But it makes no difference.

Because . . .

I just happen to set mine at 0.0032" (.08mm) and I think that is what I want anyway. The most clearance within spec. Noise is better than burnt valves if it all starts to change with genny usage.

So I think I have it set perfect as is. Right where I would want it anyway.

-Don- Auburn, CA

 

[Mark_K5LXP]

Since you observed that popping the cover off is readily done it seems if you're curious you could check clearance periodically, say annually, and see how stable it is. I would bet that with a 450 hours service interval it's not going to move much but if you want to ride the outer edge of the spec it only takes a fraction of a mil to be out. Given where you found it you're probably splitting hairs but you retired guys have 7 saturdays a week to tinker with such minutia.

Mark B.
Albuquerque, NM

 

[DonTom]

Since you observed that popping the cover off is readily done it seems if you're curious you could check clearance periodically, say annually, and see how stable it is. I would bet that with a 450 hours service interval it's not going to move much but if you want to ride the outer edge of the spec it only takes a fraction of a mil to be out. Given where you found it you're probably splitting hairs but you retired guys have 7 saturdays a week to tinker with such minutia.

Mark B.
Albuquerque, NM

Yeah, now that I have done it and discovered how easy it is to do in an Onan, with everything in the clear, it will be checked often. At least during every oil change or so. I will check it even more often if I notice the clearance changes a noticeable amount.

Later today, I will adjust the no-load frequency to 63 hz and see is I can then squeeze a few more watts out of it.

-Don- Auburn, CA