Onan Mill Motor Maintenance

[Andries]

I just read a post by kelLOGg on motor repairs and figured I'd share my experience with an old (1992) WM mill. Some of you will laugh your keesters off at my story, but the new guy has to start somewhere, right?

So, I've operated my LT30 for a while now, but there was often a burp or two in how it ran.



Didn't seem to have the power it should some days, the next couple weeks it'd be running like gangbusters. The really BIG head scratcher was that when it was acting 'tired', the oil in the crankcase would slowly move up the dipstick! Yikes, this made no sense at all - an engine that was making oil? Should I run to the patent office?
Gotta admit, my head was occupied with log handling, aligning the mill, and figuring out some kind of milling technique for urban reclaimed elm, ash and oak. There was a ton of new (to me) things going on, and the mystery Onan and the dipstick reading weren't at the top of the brain for quite a while.

[Andries]

My mechanical skills aren't too good, but I thought that maybe the impulse type of fuel pump by Nikki wasn't up to the job anymore, ( novice thinking: ' hmmmm, runs like its tired - might be not enough fuel delivery ') so I swapped that out for an electric pump.

That wasn't too tough, the hardest part was taking the shroud off so that I could get some small bolts and lock-nuts to mount the pump where the old one had been located.
Great, fired the mill up and all was good.
. . . . until I check the oil level a week later. Hadn't added any, but now the level had gone from half to three quarters. Well, what the hay, it's running good now and I'm thinking that my son might be lengthening the dipstick. We get along well and he'd think that was funny.

[Andries]

I also cleaned up the carburetor 'cause I'd been thinking that the under-powered and rough running symptoms were all related to poor gas delivery. Pulled carb off, opened it up, checked all the posts on the FF on Onans and carbs, sent for a re-build kit and re-assembled the whole thing to specs. Even took photos of the whole deal, because I hate having little spring thingeys left over on the bench after the carburetor is all put back together. Hmmm, thinking I'm one heck of a mechanical marvel at that moment.
Yup, the head slappin' moment hadn't hit yet.
Here it is:


There I was, just looking around while the mill was slowly making its way through some bur oak. Spotted the sawdust on the exhaust pipe. Huh? Sawdust on that pipe, but not on the exhaust pipe on the other side? Double Huh?

[joey7319]

Sounds like you have gas leaking into the crankcase, and possibly running on one cylinder?

[Andries]

The light bulb went on - felt like a hockey puck hitting the shinbone.
That side of the motor is cold, the other side is doing all the work!
Has this happened to you? Its that 'wow, figured it out' feeling and at the same time 'How long did it take me to stop being so stoopid?'
Thinking that I'm going to fix this right now, I got mechanically clever and switched the spark plugs from left to right cylinders. No change ..... hmmmmm, = plugs are okay.
Ahhh, switched the plug wires from left to right, must be a wonky conductor, right? Nope, no change = plug leads are good.
A bit of time sitting on the tailgate with a coffee and talking to Hunter my dog, wondering about the meaning of life . . . .
The only thing left was the doohickey that the plugs and the wires connected to was the coil!

[Andries]

I shoulda phoned you Joey.
The Cummins truck centre in Winnipeg had a coil of the Onan on the shelf. Lucky deal! What's the chances of that, for a twenty year old motor?
Jumped in the truck with Hunter, picked up the replacement coil hooked her up and it was a whole new (milling) world!

Super pleased with myself, and my son gives me a grin and says "yup, surprising the difference when you run a two cylinder motor on two cylinders". He likes to have fun. I took Hunter for a walk around the block.

[joey7319]

I would change the oil and filter, maybe even flush a little extra oil through the crankcase to be sure the gas is all flushed out. You would want to ruin anything in the motor.

[Jeff]

You gotta unhook the tank hose and run the fuel out on that onan when not it use. If you search back, you will find I had the same issue with gas in the crank. if you leave the tank hooked up, and the gas expands in the tank, it will push right past the needle valve and fill your crankcase with gas. It took me 3 oil changes to figure that out. You was lucky finding that coil. I had to pay over $100 for the one I found.  Parts for the onans are not cheap.

[Andries]

Joey, your advice is perfect.
Got the same advice for the Cummins employee, after he stopped laughing about my goofiness with the fuel pump and the carburetor rebuild. It sure didn't hurt to do either, but it wasn't part of the problem of one cylinder not firing. He suggested exactly what you did, indicating that the gas that didn't burn off in the cold cylinder had flooded the crankcase and diluted the oil. (note to self: my son didn't lengthen my dipstick after all - that joker).

[Andries]

Jeff, I'd forgotten that you had an Onan powered mill, there's probably a bunch of us out there.
The motors have a good reputation, so adding a couple of extras seemed worth it.
I put a gas shut-off valve on the hose and replaced the old boat motor gas tank with a properly vented WM tank. Cost a few bucks but it wasn't outrageous - and any of the parts sourced from WM have been really good quality.

[Andries]

So, to bring this full circle to my first post on this thread, kelLOGg - don't be worried about getting your hands greasy or (in my case) admitting that I'm not any kind of mechanic. This stuff can be tricky but it's a great opportunity to learn how our mills run and to pass that on to others on this forum.
Most of the Canadians on the FF are in remote locations compared to most Forumites in the States. Around here a three hour drive to town isn't unusual - so getting good at fixing mill shenanigans is a good investment.

I started putting my tools away but at the back of the box there was an old engine compression tester that my FIL had given me years ago. Sez I to myself ' why not check 'er out just to see?'
Yeah, small questions can start a big project . . . .

[Andries]

One DanG thing leads another, but since I'd started on this I thought I should follow that bunny all the way to the end of the trail . . .
The LT30 was running like a new machine. Who knew, a two cylinder machine really does need to run on two cylinders! That's  my Homer Simpson line.
My mill had been running on a hit 'n miss basis for quite a while and the dunderhead operator (me) had been standing around scratching his head ( and other parts) for too long. Worried that I'd pushed things too long with crankcase oil that was half oil-half gas, and had scored the cylinder walls or worse.
Sure enough, the compression tester showed that the dead cylinder was way down from the other side. Like 50% down. Down like my spirits.
OK, suck it up buttercup, wheel the mill into my son's garage and get ready for some tool time.

[Andries]

 

I got the manual for the Onan on the Web and started taking the air shrouds off the motor. I wasn't sure what I was going to find: an engine rebuild, an engine replacement or what? But having half the compression on one side compared to the other wasn't good at all.



Okay, so far so good. The tin is coming off the motor without a problem, but I better take some pics so that I can replace this stuff in the same order.

[Andries]

The air shroud under the right cylinder (see, I'm lerning stuff, like which is the left/right cylinder!) has a sticker which has been cooked from the engine heat.
 
Here's what the same sticker looks like from the left cylinder, which wasn't cooked 'cause it was on holidays. The coil replacement fixed that.

OK, this is useful info, cause I'll be wanting to put this thing back together at some point, right?

[Andries]

The Onan engine manual is really good, its good for a LOT of information.
.... but the right cylinder looks like this;

and the left one looks like this;


Well I'm no rocket surgeon, but this doesn't look right. The manual pictures look super clean because they use a photo of a new motor and yeah, this cylinder hadn't been working on and off for a long time, but whats up with the sawdust buildup?

[Andries]

A wire brush and a trouble light made it easy to polish up the left cylinder fins, but son of a gun (old saying) there parts missing here.

There's a nut missing on the right hand side! ! Going in, I was worried that the cylinder walls had been scored and that was causing the compression leak. Seeing this, I started thinking maybe a missing head bolt could cause the low compression reading? . . .
The missing nut must've been off for a while because there was plenty of oil sawdust on the floor after cleaning.

[Andries]

I have no idea why or how a nut comes off cylinder head, but leave it at a guess that someone before me had been in here and forgotten to tighten the nuts properly.


Between the cowling stickers about torquing the heads down and these pictures, this might help anyone else with the same engine to do all this.
After a little FF checking, it looks like Jim Rogers, Jeff and Chuck White all have Onan motors on their mills.
Jeff; thanks for the advice on carb and gas tank issues with the Onan motors.

[Andries]

Removing all the head nuts let me pop the heads off both left and right cylinders.
This is the right cylinder head:

I haven't done this level of mechanical repair before and feeling like some explorer in new territory. Also, it feels like I'm doing things that need to be done just right, 'cause doing it badly will result in a short and expensive story!
This is the left cylinder head:

Even a rookie like me can see a big difference between the two - but what do the differences mean?

[beenthere]

Andries
You apparently have been working on this motor project for a long time. When did you start?

Great pics.

[Andries]

Two things came to mind straight away:
There's no scratched up scoring on the left cylinder walls, at least to the touch&eye there isn't. The colour is funny on the piston head, but that's probably because it had been acting like a gasoline pump for a few weeks... or longer.
The best thing I see is that the gasket on the left cylinder seems to have a blown out section, which matches to where most of the oil/sawdust had collected and crusted over. That could explain the lack of compression on that side of the motor. Whooopee!
I may not have to re-build or replace this thing!

[Andries]

Beenthere - last winter was the coldest winter on record for over 100 years. A perfect time for me to be in my son's nice warm shop, figuring this stuff out.
WM sent me a gasket kit for the Onan and I set to replacing both cylinder head gaskets.

Gotta admit that I'm flummoxed by the fact that I can order up this material from WM and Cummins and get supply on short order.
Try that with a twenty year old Detroit product. 

[Andries]

The valves looked like the photos of 'normal' valves on the web,

and the old head gaskets came off easy using a pocket knife.

Replaced both of 'em and then it was time to put the heads back on.

[Andries]

After fitting the heads back on the cylinders with new gaskets, it was time to tighten down the head bolts. That's when I had to head into town to get a torque wrench that could be adjusted for the light settings needed.

Turns out the block and heads are aluminum and its really easy to make a knuckle-headed mistake by putting too much muscle on the wrench.

Also paid off big time to keep the decals on the inside of the cowlings.

[Andries]

Well, it all turned out pretty good. The compression readings were really close to what the manual called for. After replacing the cowlings, changing oil and filters (air and oil), doing an alignment, it was spring and time to get back to milling.

[thecfarm]

A happy ending.