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Another Onan 16 BF/MS Oil Question


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13 replies to this topic

#1 dthomp17 ONLINE  

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Posted November 13, 2011 - 04:08 PM

I have a 16 HP Onan BF/MS on a Sears SS Twin. Compression reads about 110-115 psi on each cylinder. The problem is that it seems to be smoking more now than when I first got it running a few months ago. I’ve also recently noticed small drops of oil on my mower deck (apparently coming from the exhaust). There doesn’t seem to be any oil in the air breather. I read in another GTT post that a leaking fuel pump diaphragm can allow gas to leak into the crankcase resulting in diluted oil and possible engine damage. Can’t smell any gas in the crankcase oil but not sure if I would be able to smell gas if it was present either. The fuel pump will not hold a prime after sitting for awhile and this could be an indication of a leaking diaphragm problem. Also, the oil dip stick does not snuggly fit the tube when fully inserted, if this matters. Any suggestions on the approach I should take would be greatly appreciated. Rebuild the fuel pump, consider new piston rings, or complete engine overhaul?

                       

#2 olcowhand ONLINE  

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Posted November 13, 2011 - 06:47 PM

Those are excellent compression readings! You could have diluted oil, but "usually" you will notice higher oil level on the dipstick. Air leaking past the dipstick seal will cause oil to blow into the air filter housing from the crankcase breather, and especially if the oil is diluted & above normal levels. A rebuild for the fuel pump costs nearly as much as an electric fuel pump, and electric is much better. A Mr Gasket model 42S plays well with the carbs on the Onan engines. I have 2 of them on Onans right now & never a problem. You might try just getting a cork that will block the dipstick hole temporarily just to test if that is the main issue or not.
To test the fuel pump, you could temporarily replace the vacuum line with clear vinyl line so you can see if any fuel is passing through the diaphragm.
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#3 smasher OFFLINE  

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Posted November 13, 2011 - 06:58 PM

Id say the oil control ring is gone and or you cylinder is in bad shape, ive seen this before so much oil gets past the rings it seals them up enough to give you good compression readings. Pop a head off and see if the bore is glazed or scored up.
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#4 ducky OFFLINE  

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Posted November 13, 2011 - 07:22 PM

olecowhand know these Onans. Try that first.

#5 dthomp17 ONLINE  

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Posted November 13, 2011 - 09:27 PM

Thanks for the advice, I think I'll try Olcowhand's advice first since it will probably be the easiest and cheapest.

#6 dthomp17 ONLINE  

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Posted November 15, 2011 - 12:32 PM

I followed your suggestions and plugged the oil dipstick tube with a cork, disconnected the vacumn line from the fuel pump and plugged the line, connected a temporary gravity feed fuel tank to the carb, and fired it up. It may be wishful thinking on my part, but I believe it's smoking a lot less. I know it didn't smoke nearly as much as it has in the past when I started it up. However, there's no indication of oil in the air breather as I would expect and there's no indication that the oil level in the crankcase is over full. Before I pull the heads and check the cylinders, I think I'll run it with the oil dipstick tube plugged for awhile and see how it does. If I do have to put in new pistone rings, will I also have to have the cylinders bored and install new pistons? From my limited searching, it seems that new pistons for this engine will be hard to find and if I do find some they will be expensive. Thanks again for your help and I wellcome all suggestions.

#7 olcowhand ONLINE  

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Posted November 15, 2011 - 03:46 PM

If you need to replace the rings, if the bore looks good & is within specs, no need to overbore....just a light honing to break the glaze & provide proper cross-hatch for new ring seating. Even std size rings are sometimes harder to find, but Hastings Manufacturing is a source for ring sets also, as I've ordered std rings for a BF/MS from them myself.

#8 olcowhand ONLINE  

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Posted November 15, 2011 - 03:48 PM

Also, if plugging the dipstick continues to provide smoke relief, either get a good used one or try to find new. A bad seal at the dipstick or faulty crankcase breather reed can cause weird problems.

#9 smasher OFFLINE  

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Posted November 15, 2011 - 03:57 PM

Its been awhile since i seen one but i believe the 16bf/ms had a smaller bore then standard, if so you can punch then out to onan std size (3.250") and use pistons from a p216 etc. But as long as the bore isnt too out of size or scored up you could probably get away with a quick honing and file fit a set of oversized rings. Shoot me the whole model/ serial number and ill see what i can dig up/cross referance for parts
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#10 olcowhand ONLINE  

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Posted November 15, 2011 - 04:18 PM

Yes smasher, the BF/MS has a bore of 3.125 and enough steel liner to bore out to the 3.250 bore & use later style pistons/rings.

#11 dthomp17 ONLINE  

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Posted November 15, 2011 - 10:46 PM

Its been awhile since i seen one but i believe the 16bf/ms had a smaller bore then standard, if so you can punch then out to onan std size (3.250") and use pistons from a p216 etc. But as long as the bore isnt too out of size or scored up you could probably get away with a quick honing and file fit a set of oversized rings. Shoot me the whole model/ serial number and ill see what i can dig up/cross referance for parts


I believe you are right about the bore on this engine. I'm pretty sure it's 3.125", at least that's what the manual says. The model number is BF MS/2379B and the SN is 0373605727. I have a small micrometer but don't believe it will measure inside bore to see if it's in or out or specs. Guess I'd have to take it to a machine shop and get that done. Would the amount of ring groove (if any) at the top of the cylinder give any indication of the amount of cylinder wear? Also, can you re-use cylinder head gaskets? Some say you can and others say don't do it.

#12 coldone ONLINE  

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Posted November 15, 2011 - 10:48 PM

Yep the oil dipstick will cause oil to come up in the breather, at least on mine it did. Didnt know mine was loose, it fit tight but thats was due to the stick itself being bent and binding in the tube. Once I straightened it and cranked the engine, it looked like a bobble head bouncing around. I wrapped a few wraps of electrical tape around around the stopper to make it a tight fit. Finally gave it a true test the other day and mulched the entire yard, no more white smoke!
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#13 smasher OFFLINE  

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Posted November 16, 2011 - 10:08 PM

Well if you dont have much of a ring ridge then you probably dont have alot of cylinder wear (unless its real groovedup). If you want to make a real cheap and effective measuring tool go down to your local hardware store get a couple bolts and nuts and coupler nut, cut the heads off the bolts and round them smooth put one nut on each bolt and thread them into the coupler. You can then use it like an inside mic, once set measure it with one of those cheap digital vernier calipers its accurate within 0.001" or so. As far as head gaskets go if they are in good shape you can use them if they leak its not a big deal to replace em not like you have to worry about coolant or oil leaking.
cheap mic.jpg
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#14 dthomp17 ONLINE  

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Posted November 17, 2011 - 12:44 PM

Well if you dont have much of a ring ridge then you probably dont have alot of cylinder wear (unless its real groovedup). If you want to make a real cheap and effective measuring tool go down to your local hardware store get a couple bolts and nuts and coupler nut, cut the heads off the bolts and round them smooth put one nut on each bolt and thread them into the coupler. You can then use it like an inside mic, once set measure it with one of those cheap digital vernier calipers its accurate within 0.001" or so. As far as head gaskets go if they are in good shape you can use them if they leak its not a big deal to replace em not like you have to worry about coolant or oil leaking.
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Thanks Smasher, that's a great idea for an inside mic. I'm pretty good at coming up with cheap ways to do things but I hadn't thought of that. I put some electrical tape around the oil dipstick tube like Coldone suggested and It seems to be tight now. I want to see how it does before doing any engine overhaul. I can deal with a little smoke as long as it doesn't become a problem with fouled plugs and look like I'm fumigating the neighborhood.