Case 648 update. We start talking it apart

I posted about bringing this little tractor home last week. Today
we started looking it over.

I had some things to do in the shop so son went out and started
cleaning some of the gunk off of it. Hes pretty excited to work
on it!

This is an Oman B48M engine we our working with. The story
we have is that it ran when parked, and its been sitting for 10
years. The first thing we noticed is a missing air cleaner screw
right by the throat of the carburetor. Fortunately it appears to
just be missing and didnt make it into the engine. I see a lot of
corrosion on the butterfly in the carb so Im figuring on getting a
gasket set and opening it up. Im sure it needs cleaned.

Son got the air filter off while I took the plugs out.

Heres a picture of them. If you can read plugs please tell me
what there saying!


This has the usual funky wiring a lot of old forgotten machines
end up with. Its also missing the battery ground cable for some
reason. If we can get it running it will get a rewire for sure. It
needs a set of points as well. The engine doesnt appear to have
much compression so we have the cylinders filled with ATF for
now I can see the intake valves moving and the engine turns
free so Im hoping it just has some sticky rings. The carburetor
bolts run up from the bottom of the intake so it was easier to
take the intake and carburetor of as a unit. We found 1 intake
gasket was missing as well. Thats where we stand on day one.
Pretty typical items so far for a machine that has been sitting for
years. On the plus side the gas tank look pretty clean with just
some rusty loose stuff in the bottom, someone added an
electric fuel pump and it still works! And the engine oil and
hydraulic oil look surprisingly good. If you have any tips tricks
thoughts or ideas wed love to hear them. Thanks!

A couple more pictures

Please keep us posted. I doubt the missing choke screw is the cause of the low compression. On an engine that small would a screw even get past the intake valve? A choke screw went into a 630 Deere I was cultivating with and it did make a noticeable knock. It knocked for a couple minutes then an exhaust valve stuck upen and the knock went away. I didnt know what caused the knock till several years later when I had the carb off and saw a choke screw was missing.

Everything looks pretty good for sitting out for so long.

It's normal for the compression to be down, the rings dry out, not necessarily stuck especially if there is no evidence of water in anything.

On the fuel system, I would replace all the fuel lines. They may look good but be flaking on the inside. There may be crusty sediment in the bottom of the carb bowl. You can probably just ease it apart, clean it out. Pay close attention to thoroughly cleaning the main jet. Scrape it out with a torch tip cleaner or a stiff wire. Once there is gas in the tank and the lines are in place, turn the pump on and flush the pump and lines before making the final connection to the carb.

Hard to say on the plugs. Nothing bad about them, a little dark maybe a little rich or just not worked hard enough. No sign of oil fouling or ash deposits I can see.

Did you check the hydraulic tank and gear cases for water? Might crack the drain plugs loose before starting it. And look for the unexpected, mouse nests in the intake and exhaust system, chewed wiring.

Looks good though, I think it will live again!

Looking at the spark plugs, I would say that the one on the left looks like it has a bit of residue on it but not real bad. It is hard to tell without feeling of the end of the plug if it is fuel residue or oil. If the residue is fuel it will be pretty much dry and feel like powered soot, if it is from oil it will feel wet and slick. The plug on the right looks pretty good but may be running a little lean due to an air leak from that funky two piece manifold or the missing manifold to block gasket. While you have the manifold off look it over carefully for air leaks and for good measure put a small bead of sealer on all of the seams. That funky two piece manifold was not one of Onan's greatest ideas. The left plug might be slightly fouled from running a richer mixture used to help compensate for an air leak on the other side.

Be prepared for a shock when you price parts as they are quite high. Some parts like coils and point sets are available from Amazon and the auto part houses. I bought a coil for mine from Amazon last month. Some important words of caution--- DO NOT check for spark on one plug wire with the other not grounded or hooked to the plug as you can burn up the coil doing this. This is a waste spark firing system because both of the cylinders fire at the same time with one on the power stroke and the other one on the exhaust stroke, thus the name waste spark ignition. I have an 8Kw belt driven generator with an NH series Onan that had been on a tire service truck air compressor before I got it and I have used it for more than 30 years myself. It has been very dependable and almost trouble free other than a carburetor kit, spark plugs, points and finally last month a coil. I have dealt with Onans for many years and most are good but a few were bad news. Some guy on this YT site recently took one out of a Case skid steer and replaced it with a kubota diesel.

Gene
Really great info. This is our first time working on an Onan thats the stuff we need to know. Thank you.

I have a 646 which has the 16hp Onan, I'm going to repower it with a different engine due to a broken rod, it was running at idle when it broke. If there are any engine parts that may interchange on yours let me know.

Steve,
The low compression is the only thing making me nervous right now, Im hoping your correct. I dont really see any evidence of water getting into it

Your on the money about the gas lines they are hard as a brick! To check the pump we ran some line into my gas can with mixed gas I use in my chainsaws I took a couple of seconds to prime but it pulled a good flow of gas through. I was surprised!

I completely forgot to crack the plugs and check for water in the cases! Im writing that down right now so I dont forget again!
I see a big nest right next to the hydraulic tank. I think well take the front grill off to get at it.

Thank you for the advice, we really appreciate it!
Glad you got your Fairbanks Morse up and running by the way!

Fixerupper
I think your right, I dont think that screw went into the engine.
Ill update with new posts as we make progress.
Thanks!

J Hamilton
This is our first time working on an Onan so I have no idea what might interchange, but I will definitely be in touch if we run into something. I dont think were to far away from each other. Id love to see pictures of your 646, I assume its similar to the 648. What engine are you putting in?

I haven't been around many Onan engines to know what parts may interchange, but I'm sure people here would know. I haven't began looking for an engine yet, I will probably get a Briggs or Honda to use in it. As far as differences in these models, I know there was a 644 14hp Kohler, 646 16 hp Onan, 648 18hp Onan. Physical differance in these models, I don't know. I do know someone I can contact who does have a lot of knowledge on these machines and has given me some very good advise. I'm located in Southern Illinois very close to Rend Lake

Strongly recommend that you contact this guy:
He is has a wealth of Onan knowledge, used parts, new parts, complete engines etc. And fair prices. Once you price Onan parts you will appreciate the source at decent prices! Call and discuss your situation and see what is available. FWIW, it is very likely that you will need rings at a minimum. But, Onans don't typically wear the cylinders much at all.

No interest in his business, just have purchased parts in the past and seen his helpful comments on Case garden tractors on the net.

I would not change to another engine as the Onan is a solid performer, smooth running and lots of torque. Lot to be said for not having to make changes to fit another type. Exhaust, pump mount etc can get tricky in the 600 series chassis. If you want to keep the front belt PTO it is even trickier.

I have a project 644 that has been a project for years. Maybe this is the year it gets completed

Oh, before working it hard, open the differential and check the 4 bolts that hold the diff case together. Better yet, order a set and replace them as they are one of the few weak spots in the Case garden tractor line.

Some words of wisdom when dealing with Onan engines: proceed slowly and look carefully as they are quite expensive to replace parts on. As my father used to tell me: Think twice, measure ten times and then cut once. I usually approach Onan engines with ths mind set. They are a little different from the Briggs and other engines. Most of my dealings has been with the CCK engines that powered generators and welders plus the NH model that I personally own. Occasaionally a few of the ones like you are dealing with on a few high end lawn tractors. One problem I had found on several of the CCK series and on my own NH series engine was that in the breaker point box there is a rubber seal thingy at the top of the the push rod hole and sometimes it acts like a pump to draw oily vapor up into the point box and gets the oily vapors on the points. When that happens remove and discard the little rubber seal looking thing on the push rod. It does as much harm as good sometimes. I also ran into a situation one time with own my engine that I could not get enough lift from the push rod to properly gap the ignition points and finally determined that the push rod had worn off at the end and was a little too short. I made a new push rod that was 1/8 longer from a brass rod and it has run well for several years now. The real Guru for Onans is a guy in Texas by the name of Billy Shafer who ocassionally will post on this board. He has forgotten more than most of us will ever know about the Onans especially the generators. He is slso active on the SmokStak forum.

Looks like a fun little project.

I would either fully lower the loader and bucket or else make up some cylinder stops for it if you need to keep it raised for moving the machine around.

Too easy to get a hand or arm caught in a pinch point if someone accidently bumps or leans on a lever or a hose lets go.

A piece of angle iron for each side cut to the same length as the exposed portion of the cylinder rod can be clamped on with a couple of hose clamps to prevent the loader from coming down unexpectedly.

Determined
I had son lower the bucket to the ground before we got going on
it. I really appreciate you pointing out a safety concern. Its to
easy to have an accident from an overlooked safety item. Im
glad you and other here look out for those things. Thank you!

kencombs
I dont see the name or contact information for the person your talking about. Can you check and post the info please? Sounds like the guy I need to get on the phone with! Thank you!

J Hamilton
I reread this and realized I didnt say what part of the world Im in! We a little over a hour north of St. Louis close to Clarksville Mo. so not too far away. Thanks again!

http://boomersonanparts.com/

Sorry, forgot to paste before hitting enter.

No problem. Thanks for posting it.