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Makotosun

Oil dumping from exhaust flange (DT360)

  • Barra
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Last year I got my dad's DT360 when he retired and downsized. I used to ride this bike all the time back in high school but the last time it actually ran was about 20 years ago.  It has always had an exhaust leak at the flange where the lower stud snapped off. When I picked up the bike it had what looked like some oil seeping from the flange area. I recently rebuild the carb, swapped out the rotten air filter, derusted the inside of the tank, and swapped in a new petcock since the old one was completely frozen with corrosion. Changed the crankcase oil, dumped out the 20 year old 2 stroke oil and refilled it with fresh Yamalube 2S.

Surprisingly to me I got it to fire up on the second kick but then it spat out bunch of oil from that exhaust flange, maybe 10cc or so and it smoked like hell. I ran it for about a minute or two.

At this point I don't think I'm going to be able to get away with not pulling the motor apart but I'm just trying to theorize where that oil could be coming from.  I don't think it would be from bad piston rings. I was thinking that maybe over the past 20 years as it wasn't getting any fuel at all from the bad petcock that maybe the autolube system dumped some oil in the chamber on multiple startup attempts and it just collected on the piston? I have the original manual and looking at the motor cross section didn't give me too many ideas.  I see the decompression valve has a passageway right onto the top of the exhaust port but I don't know how oil could get up in there.

Just curious what all your thoughts might be on this.
Last edit: 10 Dec 2021 10:07 by Barra. Reason: capitalization
10 Dec 2021 08:19 #1

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Replied by Pedalcrazy on topic Oil dumping from exhaust flange (DT360)

With 2 stroke oil in the tank and not being ridden then oil could have worked its way past the check ball in the pump and flooded the carb and cylinder with it. I think you could ride it and if that’s it then it will clear up. Others may say the right side crank seal has failed and it is sucking in tranny fluid. These are only things I have heard. If it is my first thought then riding will clear it up. Better too much oil than no oil at this point.
1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
1968 DT1 (3)
10 Dec 2021 08:29 #2

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Replied by MarkT on topic Oil dumping from exhaust flange (DT360)

Very common on a bike that has been sitting.  Autolube seeps into engine very slowly.  I've had rescued bikes that have been sitting for years have empty oil tanks, the entire crankcase filled with oil, and even the exhaust bend full of oil.  

Don't tear it down because of this.  It's definitely not the rings.  Cure is just to take it for a good ride and blow out all the excess oil.  Cleaning the pipe and muffler afterwards wouldn't be bad but might not be needed. 

Note that the exhaust stud is a stepped stud...  6mm in the cylinder block but 8mm nut.  Common to break them off. 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
10 Dec 2021 08:36 #3

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Replied by msavitt on topic Oil dumping from exhaust flange (DT360)

agree with Pedalcrazy - go for a nice long ride!
10 Dec 2021 08:37 #4

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Replied by Barra on topic Oil dumping from exhaust flange (DT360)

Sounds like a plan. I'll try taking it out maybe tomorrow and blow/burn the oil out. My dad snapped that stud off ages ago (probably 40 yrs). He tried drilling it out but the drill bit snapped inside it.

My goal is to get it running well enough to ride with my kids. They are both getting Yamaha TTRs for Christmas, a 125 for my 12yr old and a 110 for my 8 yr old. After they get good enough and are riding around by themselves I might try building the DT back up a bit more. At that point I'll see if I can get that stud to back out.
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10 Dec 2021 10:06 #5

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Replied by Barra on topic Oil dumping from exhaust flange (DT360)

Thanks for the encouragement! It seems to have cleared out. I ran it at idle for a bit but couldn't get it to hold after the choke was shut. Plug was pretty wet so I aired it out a bit then went back at it. This time it stayed running and I ripped around the yard a bit. I thought the clutch was slipping but then realized the rear tire was just spinning and ripping up the field. I'm shocked how little it took to get it to run strong again after sitting so long.
The following user(s) Liked this Post: MarkT, Bigdog302, RT325, Ht1kid, Pedalcrazy, adguy2112
13 Dec 2021 14:58 #6

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