oldbikerehab Posted May 10, 2012 Posted May 10, 2012 The weather today was about 74 and sunny so I decided that it was time to get my DT 175 out of town for the first time and take a cruise through the local state park. After about 30 min of making my way through some winding roads, up and down hills, and screaming through some straights. All on asphalt and all the while maintaining an even purr, buzzing along around 5000 revs. Thinking, "Man, this bike is fantastic!" as the revs held steady and smooth through all the gears. And then I found myself with a giant school bus right on my tail in a series of pretty steep hills where the speed limit was 55mph. I made it up one hill and was on my way down another steep one at 50 mph and 5000 revs when I started to hear a flapping sound from the cylinder. I eased off the gas and it stopped, so I downshifted and just wanted to make the hill and get off the road when the flapping started again. Just as I made the top of the hill . . . http://i1054.photobucket.com/albums/s481/interestedineverything/piston-burnout.jpg"]http:// 15 miles from home and no cell service I started pushing, glad that I didn't wind up over the handlebars and that my bike only weighs 250lbs, but super !@#$%^& pissed! Luckily, an old friend noticed my long hippy hair while driving by, happened to be in his pickup truck with his ramps, on his way to pick up his yard tractor to take to his farm so he got me and the bike home in short order. After opening a beer and breaking out the tools for a diagnosis, I sat down to figure this out. From what I can gather the most common reasons for this type of failure are, air leak at the magneto-side crankshaft seal (I doubt it), too lean carb jetting (possible), too far advanced ignition timing, or faulty CDI (I doubt it), too-hot spark plug range (NGK B8ES - so nope), too high compression ratio (I doubt it), or a too low octane fuel (my guess). Before my ride, I decided to top off the fuel and grabbed what remained in a 5gal can of gas that I filled last year and added Stabil fuel stabilizer to. There's 2 stroke oil in the line to the carb so I think I just basically filled my tank with ethanol without thinking and killed my piston. Any other votes? Will I need to split the cases now since there's probably junk that fell into the lower chamber? Any advice, jokes, or lessons welcome.
wild foamy Posted May 10, 2012 Posted May 10, 2012 wow, that's impressive... did you do a plug chop to check the mixture?, that'd be my first port of call... as for the fuel, petrol (or "Gasoline" for you guys) has a typical shelf life of 6-8 weeks, filling a jerry can a year ago and then using it in your bike probably wasnt the best thing to do, you've already discounted the spark plug range so you are left with a leaking magneto seal (possible but unlikely), too lean carb mixture is more probable, ignition timing is also worth checking seeing as your bike is a 74 and probably running on points (check your manual for correct settings), faulty CDI is unlikey, compression ratio shouldnt be too much of an issue assuming that the head gasket was changed when the barrel was last off. having said that looks like you've done your homework on symptoms, definately check the mixture and ignition timing though, and as for splitting the cases it would be advisable seeing as all thoser little metallic fragments could now be dry-humping your bearings and bushes
oldbikerehab Posted May 10, 2012 Author Posted May 10, 2012 I've never done an official plug chop with a steady rpm, kill it and coast, but I've been checking the plug and it's consistently an even cocoa brown with a couple of dark threads. The bike's a 78 actually, so no points. I would be most suspicious of the after market carb jetting if it weren't for this bonehead gas move on my part. I think I will look into rebuilding my Mikuni though. I was afraid the votes would start coming in for splitting the case.
Moderator Cynic Posted May 10, 2012 Moderator Posted May 10, 2012 From personal exp that is either bad plugs or lean mixture at higher rpm. Done it myself more than once. In my experience all the crap goes out the pipe. But its your motor so rinsing the cases may be worthwhile? New piston, lift the needle a clip and away you go.
oldbikerehab Posted May 10, 2012 Author Posted May 10, 2012 I'm now wondering if I might have also had an air leak at the carb. I'd noticed that it had a slow leak thing going on and planned to tighten it next week when I put my new cable on. Does this make sense, that a leak at the float bowl would have been leaning the mix?
Moderator Cynic Posted May 10, 2012 Moderator Posted May 10, 2012 Nah, leaky float bowl won't kill the motor, just wasting liquid gold thats all. If you were paying our prices you wouldn't waste a drop.
oldbikerehab Posted May 10, 2012 Author Posted May 10, 2012 No doubt about your prices, Cynic. When I was working in Scotland back in 2001 I thought you guys were getting boned at 78p/Litre but you're probably double that by now with no real rise in wages either. People will start freaking the !@#$ out a $4/gal here. You were at about $10/gal last time I heard.
Moderator Cynic Posted May 10, 2012 Moderator Posted May 10, 2012 How does £1.38 a litre grab you, or around £6.20 for a UK gallon. That works out at over a tenner to fill the DT from reserve.
oldbikerehab Posted May 10, 2012 Author Posted May 10, 2012 That's insane! You'd think the U.S. would cut you guys in on our global oil thievery, err I mean oil surplus derived through nation building.
jeddy Posted May 10, 2012 Posted May 10, 2012 The oil is cheap enough its the thieving government taxes.... That's insane! You'd think the U.S. would cut you guys in on our global oil thievery, err I mean oil surplus derived through nation building.
Tommy10bikes Posted May 10, 2012 Posted May 10, 2012 Hi Jeddy I have had also the same issue with my DT360 and the only faults i can find is that there is a gap formed under the reeds and an impression into the read block where the reeds have sat over the years, this would allow fuel & air to be drawn in all the time and irratic running i would think. While you have the carb off it may be worth just looking and seeing if there are any gap issues under the reed valve. Any ideas chaps.
2 Wheels Posted May 10, 2012 Posted May 10, 2012 Very lean mixture me thinks. And mabey plug,,,, do you need a colder plug ? What altitude is it there where you live ?
AndyDtr08 Posted May 10, 2012 Posted May 10, 2012 1.46 a litre I've seen down here in recent weeks. It's just dropped to below the 1.40 for the time being. When I'm cleaning parts in the garage with petrol I actually start thinking, there goes 50p!
oldbikerehab Posted May 10, 2012 Author Posted May 10, 2012 I think you're right about the lean mix, 2wheels. This bike's never really smoked a lot except if I wind it out when warming it up cold. My needle's in the middle position, but a cheap carb + junk fuel, and then climbing a steep hill was the final nail in the coffin. The plug is actually cooler than the one recommended in the original manual and my elevation varied around 400-800 ft. You can bet when I get this thing back together I'll be on it with the WD40 looking for leaks though. That reminds me, I'd better order new gaskets. Hopefully I can just have the cylinder honed at its 3rd over and I won't have to have it bored to the 4th over. Thanks for the advice!
flyday58 Posted May 12, 2012 Posted May 12, 2012 Per Old Git's advice, I went to bigger main and pilot jets. Had to lower the needle as this was too rich, but she idles like a dream now. BUT I'm gonna pop the plug out and check the top of the piston, given my sudden stoppage of recent note and my current noise and missing oil thread... Funny, on the RT1 project, I used pretty old premix from my chainsaw supply to start it. Guess I'll dump that crap on the 70 or 80 fire ant mounds in my yard and put good, non-ethanol petrol in the tank next time I start it, which won't be for a couple weeks. Funny-er, I don't remember buring up any pistons back in the 70s when I was beating these things up and down the substantial hills of Tallahassee, Florida. Could it be that things have changed since those halcyon days of bra-less ladies and Mary Jane?
Moderator Airhead Posted May 12, 2012 Moderator Posted May 12, 2012 by the way 'oldbikerehab' 3rd over +0.75 seems hard to find for some strange reason, 4th over +1.00 is the last OEM size. Aftermarket pattern pistons available at +1.5mm and +2.0mm
flyday58 Posted May 13, 2012 Posted May 13, 2012 by the way 'oldbikerehab' 3rd over +0.75 seems hard to find for some strange reason, 4th over +1.00 is the last OEM size. Aftermarket pattern pistons available at +1.5mm and +2.0mm Brings up a q I have. If it's too much I can start a new post. The guy who did my rebore bought a Wiseco piston to save me some money, which I don't think it really did. The Wiseco piston doesn't have the intake ports in the piston that OEM pistons do. Do it make any diff? I've been wondering ever since I got the thing back together.
Moderator Airhead Posted May 13, 2012 Moderator Posted May 13, 2012 If you think about it, when the piston is on the up-stroke it is causing a negative pressre in the crankcase. as the holes in the piston uncover the inlet port in the cylinder bore, this crankcase negative pressure starts to draw fuel/air through the holes in the piston. Eventually the port is uncovered by the piston skirt and fuel/air continues to be drawn in until TDC is reached. so yes...without the holes in the piston you are delaying the fuel air charge and the vacuum developed would be greater...I wouldnt be happy. I wonder if this could account for the 'flapping' noise you reported?
flyday58 Posted May 13, 2012 Posted May 13, 2012 Hmm. Might troll ebay for a 3rd-over NOS piston... OBRehab, still running that junk carb? I'm MUCH happier with the Mikuni VM24, just wish I knew before I spent the money on the knockoff. Idles great, as I keep blahblahblah-ing about.
Moderator Airhead Posted May 13, 2012 Moderator Posted May 13, 2012 Hmm. Might troll ebay for a 3rd-over NOS piston... ............. http://www.ebay.co.uk/itm/Yamaha-DT175-MX175-Piston-Kolben-Ubermas-0-75mm-Original-Neu-/261001116720?pt=Motorrad_Kraftradteile&hash=item3cc4e0fc30
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