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Discussion Starter · #1 ·
Hi guys I got this foreman 350 in a trade deal with my cousin, it ran but burnt oil so badly. Like a plume of smoke and had an exhaust leak so it would be right in your face ha. It got so bad recently when I would briefly fire it up to move it that oil was actually shooting out of the exhaust pipe where the header had a hole in it. I've rebuilt a few vintage Honda motorcycle engines so I'm familiar with most of this and I'm aware that the typical causes for oil burning is rings or valve seals, people usually say, although I'd argue valve guides. I started tearing the engine down and I have not taken the jug off yet I'll do it tomorrow but the top of this piston is insanely clean they've usually got a pile of carbon build up. I'm thinking maybe either someone rebuilt this thing and just messed something up or perhaps it's been running super rich and that's washed the cylinder as I did find fiberglass stuffed up in the intake tee...like someone did it on purpose. I'm gonna measure the piston this may even be an overbore piston who knows but it's turning into quite a mystery. If anyone of you have any advice or potential bets on what is going on with this thing and caused it to smoke that bad let me know
 

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Nice 87 Foreman
I think the piston is clean because there's too much oil in the cylinder to burn it all, yikes!
Hard to say where it's all coming from. Does the cylinder show severe scoring or was it slapping that you could tell?
Are you planning to just do top end or cylinder too?
 

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Discussion Starter · #3 · (Edited)
Nice 87 Foreman
I think the piston is clean because there's too much oil in the cylinder to burn it all, yikes!
Hard to say where it's all coming from. Does the cylinder show severe scoring or was it slapping that you could tell?
Are you planning to just do top end or cylinder too?


Yeah that's a good possibility the oil could've cleaned the carbon. No slapping from the piston, only noise from this engine was valve lash/tappets very out of adjustment. Yes I'm gonna pull the cylinder off later today and I ordered new rings as well. I know someone with a bore guage I would like to have them measure the cylinder to make sure it's within spec hopefully. Definitely no scoring in the cylinder it actually looks pretty good, can even still see a couple cross hatch marks
 

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I'm guessing that a bore size for a 350 would be .00225 minimum and .0035 maximum. A quick way to check is this way.Put a feeler gauge between the piston skirt and the middle of the stroke in the cylinder. This is accurate enough to check. It is reliable to within .00025 in my opinion
 

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Congrats on getting a nice 87 350D!

Along with the usual suspects (bore/piston worn out, rings worn/stuck, valve guides shot, head gasket blown, head cracked, etc.) I'd look at the crankcase vent hoses pretty closely for plugging. With the motor blowing as much oil through the cylinder and out of the exhaust as you explained, it should have been venting a lot of oil out of those vent hoses too.

Welcome to the forums!
 

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Oohh, its blue! You Guys have no idea what I'd give for blue fenders :laugh:
 

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I can see ring gap between piston and cylinder. not sure why you ordered rings before you measured cylinder ( mic'd ) ?. the cylinder could be 2 sizes over for all you know ?, and you ordered the wrong size rings ?, ALWAYS MIC FIRST, BUY ONCE ! :).
 

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worse case ?, you will need to have cylinder bored over ( if its within factory specs ? ), new piston, new rings, pretty much a top end rebuild.
 

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Discussion Starter · #9 ·
Just wanted to give an update on the smokey engine. Once I got the jug off the piston the problem was obvious. The oil rings have no spring left to them even though they are free in the piston groove they have a smaller diameter than the piston itself! So the oil rings were not doing their job at all. The cylinder and piston appear in very very good shape as well as the bottom end of the motor I will bore guage it though
 

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Thanks for the update!

I can see in one of your photos that the chain & sprocket are shot too. You can get a D.I.D or OEM chain no problem, but as far as I know the sprocket is obsoleted. You'll probably have to scrounge for a good used sprocket that isn't worn on the outer 1/3 of the teeth like yours is. Once they get heavily worn like yours they eat new chains pretty fast. Wrap a new chain all the way around the sprocket while it is in your hand, you'll see what I mean...

Looks like a good STD bore & piston though! I imagine someone had it apart & assembled the oil ring wrong?
 
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