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78 DT175 Heat seize


flyday58
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:crying: Had another heat seize last night. Temps around 16*C, had gone about 12 miles, consistently around 4000 rpm. Going up a slight long incline, started to hear what sounded like the reeds flapping. When I realized what it was, I closed the throttle, clutched in and it immediately quit.

After some testy moments with a friendly pit bull, I pushed it into a local power station lot, and after about 5 minutes gave the kicker a try. Was happy it turned over, so key on, kick again and it started normally. Rode gently the last 2 miles back home.

I have already upgraded to bigger jets in both the main and pilot jets. I had the needle dropped all the way down but before the ride decided to lift it one notch. In addition, the day before when I rode it after about 2 months of sitting there was a distinct varnish odor. THAT got me really :madflame: cos the gas was fresh in June. So I drained the tank and carb, raised the needle one notch, installed fresh gazolene and went for my ride.

Having a memory like /your fave non-rememberer's name HERE/ and not having taken any pics of the motor during assembly, I decided to remove the head, cylinder, and piston when I got home. I had an idea the piston the guy who bored my cylinder bought for it didn't have the reed holes in the piston skirt. Well, I was wrong about that score, the holes are there. The piston showed minor scoring on both sides where the piston pin goes through, otherwise no damage, top shows no sign of beginning to hole.

Now. Do I up-jet again? One thing I did :shakeno: which I'm thinking could be a part of the prob: I've read a lot about peoples' propensity for the powder-coating route. NO I DID NOT. But I did paint the cylinder and head with hi-temp paint. Same difference? Maybe preventing the heat from dissipating properly or even trapping some heat?

The guy who bored the cylinder has been doing it for over 30 years, so I don't think it's a q of the rebore being bodged up. Cylinder looks nice and no burns on the piston sides. But the rod end bearing is bluish-purple and the pin has blue rings round it, which to me suggests the high temps. Took pics but they are currently on the other side of my "Japan door" so can't post them.

So, what do y'all :eusa_think: ?

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Richard get yourself a temperature indicator fitted like I did!

When io went out on the 2 stroke weekend a couple of weeks ago we ended up on a bloody dual carriageway and yes bit by bit I saw the temperature rise to an all time high of 264C...I wasnt too pleased but pt didnt heat seize and at least it makes you aware of whats going on

as far as jetting goes I would do this

First oil your filter well...I use 120 viscosity oil on mine I'm not keen on using thin oil here!

Find a main jet thats too big and runs rough on full throttle, then fit whichever smaller one stops the rough (4 stroking) running

once this is done do the same with the needle clip position, ie clip to the lowest clip position (highest needle posn) that it will run sweet with

check your ignition timing is bang on and if you find it is then dont be afraid of setting it a bit more retarded...ie 1.5 mm BTDC

Rub marks on the piston either side of the pin are classic heat seize

Have you ever done any jetting tests and set up? mine was standard at 130 and is now fitted with a 155 , may even go to 160 yet if it doesnt cock it up...(I do have a gianelli pipe on though)

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I can't help the feeling that your siezure issues are more to do with oil.

Do you get any smoke, if its about right running in heavy traffic and trafic lights should cause a 'love puff' as you power away onto open roads. Smoke for the first mile or two on start up too.

The blueing on the pin is pretty normal TBH just in my exp heat siezure generally scores the piston front and rear not the sides. Its not a load area unless there is some movement on the mains/ big end.

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I've got a 150 main jet in there now. I was looking at installing a temp indicator but got side-tracked with all the issues with the CS5, now it's on my must-have list. So you don't see the paint as an issue?

I will check timing when I get home, ran out of time last night. Send me a link to your temp gauge, if you would. I could go find the post but, ya know, lazy git.

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Flyday " am with Cynic on this, :eusa_think: i think its a lubrication problem,, i would premix for a few runs,, see if its better,

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I can't help the feeling that your siezure issues are more to do with oil.

Do you get any smoke, if its about right running in heavy traffic and trafic lights should cause a 'love puff' as you power away onto open roads. Smoke for the first mile or two on start up too.

I installed a clear plastic oil line to keep an eye on my Yamalube, so far no probs. Also made sure the pump stroke is set correctly with the right shims. But I will take a closer look at the system while running; I did have an issue once where the engine seemed to suck the oil out at higher rpm but haven't seen it lately.

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Flyday " am with Cynic on this, :eusa_think: i think its a lubrication problem,, i would premix for a few runs,, see if its better,

I think I will give that a try, thanks.

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I can't help the feeling that your siezure issues are more to do with oil.

Do you get any smoke, if its about right running in heavy traffic and trafic lights should cause a 'love puff' as you power away onto open roads. Smoke for the first mile or two on start up too.

The blueing on the pin is pretty normal TBH just in my exp heat siezure generally scores the piston front and rear not the sides. Its not a load area unless there is some movement on the mains/ big end.

its not a load area but it IS the area with the most mass and therefore the biggest expansion with heat, thats where the heat seizes leave their mark

:hah: four corner seizure....must be an American term!

http://www.smellofde..._diag_guide.htm

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Sorry gents first thing I would do is check the bore sounds like a oval bore to me,, wrong clamps/ torque plates used when boring. Not saying the guy dosen't know what he's doing but s happens.

The gas issue is normal now dam crap only last 30 days if your lucky, finding that if you run a gas that starts with s and ends in hell it's worse for varnishing,,just got back to riding my wr after 3 week's of no ride'n and had to clean the carb,that i just cleaned a month ago for the sake of it, and yellow varnish in the bowl even after draining it before parking it in the garrage. Guess I didn't drain all of it out

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There is the greatest mass but it will heat up slower because of that mass so will still be smaller than the skirt. As well as the steel pin being a pretty good heatsink slowing it further.

The front skirt runs against the hardest under power. The sides should float with the little sideways play on the pin and have little contact with the bore.

I still feel its got a lube issue. (matron)

The other part making me think oil is that i have had some shocjing carb cock ups over the years, crsnk seals, no filter even rode back from work with the front of the exhaust at the motor having a kin great hole from corroded metal failing.

Know what, no sieze. Hanging on the throttle for miles on a fast road. Yes heat sieze. So fly is either ridung flat ooot or there is a lube issue.

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If your running a wiseco piston did you drill the 1/32 holes in the piston as per the instructions to allow extra lubeing of the cylinder?

Also a problem I have seen is the wrist pin hole in the piston is too small and the lube can't get in under high temps.Is the wrist pin totaly free to move?

As Cynic pointed out the wrist pin area should be of least contact with the cylinder wall hence my thinking poor bore job, you have it appart anyway so have the bore checked with a bore gauge to be cretian that its got the proper clearence. All the lube in the world won't help if it can't get to where it's needed.

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The main changes I've made since I burned a hole just like this are to run the whole mix rich, for starters. After this last rebuild, I asked my local bike mechanic to give the bike a look. Timing (which was most people's guess as the culprit initially) checked out fine, compression was tight, no leaks, etc. Their end suggestion was to run my mix rich by raising the needle and shimming the oil pump stroke because a lean mix of junk gas was what probably did me in. Their argument stemmed from the Gordon Jennings article I think I linked in one of my threads about how 2strokes get more horsepower and and varnish less with more oil. I also went back to a 16 tooth sprocket on the front instead of the 18 I was running.

So far, I've not repeated the trip I blew a hole on but I've run the bike for maybe 2 miles at a time around 5000revs. Mostly tearing it up around town lately. I've been using the same NGK B8ES plug since my rebuild 500+ miles ago btw and I switched to Yamalube 2 stroke oil. I wasn't really even smoking with the mix I was running when I blew a hole but now I will fog the block when I leave in the morning but after that, just a puff when I take off from the light for the rest of my ride. So far, so good. I'm also running the same cheap imitation carb as you flyday58, and realized all the o rings were torn and bad like yours so I replaced them. I haven't tried messing with the jets since nothing is stamped on the generic thing so I wouldn't know where to begin.

Question for those who've been riding DTs and working on them for years and years: What is the safe redline for revs over a long distance on an old DT? (Assuming that timing, etc. is set correctly, mix has bike smoking a bit, idles good, and all seems well with the way it runs.)

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As an experienced dt abuser i run 1k under max as a cruise limit and it works ok. I will go to the red and beyond with a blast but not for more than a few moments.

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I'm with Cynic on this one short blast at red in top gear, and never on a cold motor,holding it there for any prolonged time is just asking for it to blow.These motors were never ment to be ridden on the road realy.mostly just blasting aroung a track where top gear is seldom ever reached

Drilling a 1/32 hole in the piston 1 cm down from the bottom ring in the 4 corners of the piston alows more oil to reach the load areas of the piston just make sure your not drilling where the ports are. I know it sounds weird but the holes work like a pump and it's been proven to work. I couldn't keep pistons in my go cart on the long tracks when I switched to alcohol class until I started doing it, after a old timer felt sorry for me and told me the trick.

I would still loosen the bore up a bit with a few passes with a hone ,,,,,every piston grows/expands at a different rate and this sounds like it's growing faster than the cylinder

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Well riding up Garrowby hill on the Squires ride-out will expose a poorly set-up 2stroke, :eusa_think: our Dt"s were flat out , chopping down gears to reach the summit, B)

Every year their are casualty"s ,= yellow Rd250 , with holed piston,

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