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Onan BG-MS (Sears SS-18)

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Blehst

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Discussion starter · #1 · (Edited)
Good afternoon everyone, kind of new to this forum but I have been browsing for the last little while trying to get as much information and help possible on this new tractor I bought at a decent deal.

Got the Sears SS18 with the mower deck for under 350 canadian and at the time the owner knew there was an issue with the carburetor "flooding" the engine as he called it, so when I got home I boosted the battery with the car to get it to crank and would get a lot of back pressure and gas blowing out of the carb so I pulled it apart and cleaned it up and seemed to get it to run with 3/4 choke on but still with gas spitting out the top of the carb...

After a bit of tinkering and looking around I opened up the valve covers with the right side looking fine but I noticed a small sheet metal looking screw sitting in between the intake valve and the seat on the left cylinder (or front cylinder), immediately I knew why there was blow back through the carb so I was able to pull out the screw and it seems like the valve operates normally now.

So to start, here's what I've done:

-I have rebuilt the carb with new gaskets, needles, float, float needle.

-I have gapped the valves

-gapped and filed the points

-compression tested both cylinders @ 105psi (Left side) and 108psi (right side) respectively with a cold engine and 113psi (left/front) 115psi (right side) when warm.

-changed the spark plugs

- tested the coil with a light tester (lights up when key on with light on positive terminal, lights up when cranking and light on negative terminal of coil)

up until now I have had to play around with the carb to get the fuel delivery correct until I noticed sputtering at certain points (turning in the main fuel needle almost to the end) where I hadn't even noticed that the Left/Front cylinder had not been firing but now would fire randomly, pulled out the plugs and the left/front cylinder had black powdery residue which would indicate too rich but the right cylinder has a dry greyish brown residue if anything on it. So it seems the right cylinder is firing perfectly normal but the left/front cylinder runs too rich and only fires when the carb is almost letting no gas in.

I have inspected the spark plug wires, pulled one plug and let the other run but it seems if I have the right cylinder plug wire off and I'm holding the other plug wire onto the other spark plug I easily get shocked through the iinsulated cable, do you think there's a problem with the spark plug wires? coil?

I haven't pulled off the front cylinder head to inspect in there yet but I suspect maybe that screw caused some unknown damage that the compression test didn't reveal?

I have re-used the original gaskets in re mounting all of the parts but maybe there's a vacuum leak somewhere?

I have attached a couple of images for better reference, I'd like to get this nice machine up and running and ready for work when possible! seems like there's only a few hundred hours on the engine but the hour meter was disconnected at some point.

Thanks!
 

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#2 ·
I'd get one of the inexpensive inline spark testers. It's simply a much better test and avoids those "uncomfortable feelings". Unless you like getting zapped.
You may show spark when grounding the plug, but not when under actual compression.

Could you please post the Sears 917.xxxxx number? I don't seem to have that model in my growing database.
The only SS 18 I have is a 917.25961, but no engine listed.
 
Discussion starter · #5 ·
Just as an update, I tested the spark plugs with a test light and it seems to be getting spark as per normal or what appears to be normal. Not sure where to go from here other than removing the head.
 
#6 ·
If the compression is normal no need to remove the head just yet. I would check to make sure you do not have a leaking intake manifold either at the manifold gasket to the head or the gasket between the two halves of the manifold. If it's sucking in air on that side it's not going to run correctly. Did you pull the manifold when you redid the carburetor and did you replace the intake manifold gaskets to the heads?
 
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Discussion starter · #7 ·
Good point, I did not replace the gaskets when I pulled the manifold off and just used some liquid gasket sealer stuff. I'm beginning to wonder if it's the rear cylinder that may be causing an issue, I set the main fuel needle to factory specs but I need to tur n it open 2 more turns for the rear cylinder to to run... I may just order a set of intake gaskets and replace them to see what happens. I've attached a video of the spark plug test and carb test.

https://youtu.be/QYD7INAdDNA
 
#8 ·
Sounds like fuel starvation. Have you checked the fuel flow output from the pump after filter before it goes into the carburetor to rule out fuel supply issue?

Keep in mind my suggestions are just my best guesses. I'm not a pro nor make money as a mechanic so take my suggestions with a grain of salt :tango_face_wink:

I'm sure you'll get a suggestion eventually to point you in the right direction.
 
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#9 ·
If you can set the main jet to where the 2nd cylinder "almost" runs and then take a propane torch around the intake, you might "find" a vacuum leak that way when it sucks in an extra bit of fuel?

I'm a bit suspect about the spark plug wires possibly arcing across to various metal grounded parts.
It could be something odd like at certain RPM's, the vibrations cause the wire to move a bit and cause an intermittent ground.
Often, just run the engine in the dark and you can see plug wires arcing.
 
Discussion starter · #10 ·
I didn't think of that! As you can notice on the video the spark tester doesn't light up all that well but there is spark, do you think there could be a spark timing issue?

I'll see if I can find a propane torch to test the leak though and I'll update.
 
Discussion starter · #11 ·
So I tested for any vacuum leaks along the intake and on the rear cylinder there's a very noticeable leak, when using carb cleaner it stalls the engine out with minor spray so there's definitely too much air passing through there. (it's right at the intake/block gasket) so I've ordered a new gasket set and will see how that holds up when I get them in. I guess my liquid gasket job didn't hold up. I'd assume this is what would be causing the issue.
 
Discussion starter · #14 ·
Hey guys, was doing the new gasket install on the valve covers and noticed something, not sure if I should be concerned about it or not but in the picture you'll see what almost looks like cracking, not sure if that's normal after years of wear or if this is a bad sign.

Any ideas?
 

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#15 ·
Looks like it may be from when the block was molded idk others may have a better explanation of why those are there near the valve cover bolt hole.
 
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Discussion starter · #17 ·
Appreciate it. I have noticed that the rear cylinder has more wear than the front one, which may explain why the front cylinder doesn't function at the same pace (pulses when carb needle allow more fuel but other cylinder runs fine)

This mainly happens at higher rpm. I'm assuming this may be a valve thing. I've adjusted the valves to the correct spacing but haven't opened up the cylinder heads yet (I do have the gaskets) but I'm not entirely willing to pull the engine, I may just pull the front grill and check the front cylinder head, clean the valves and maybe lap the valves to ensure good seat contact.
 
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Blehst
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