flyday58 Posted April 20, 2012 Posted April 20, 2012 Not too much info to start on this one, just want your gut feelings. Without knowing specifics, what would you think of an air-cooled two-stroke running fine, revving normally, after about 15 minutes of riding at 50 mph, starts fading, losing power, not accelerating with more throttle, then quickly bogging and for all the world sounding like it is seizing up? Luckily I had turned around and headed home; when it quit, I was only a half mile away, downhill. As it slowed from around 35 where it quit, clutch in, it felt hard to downshift to get it into neutral, not at all like when it's running. (This could just be incidental, like that extra info they give my kids in math problems that they don't need!) (The info, not the problems.) After getting home I let it sit a few minutes, then manned up and tried to turn it over. To my surprise, it wasn't froze up, and there were no metallic crunching sounds from within the case. Dropped it in gear and tried rocking to see if the tranny had gone, with predictable results - it wouldn't move. But again, no clunking metal noises. So I tried to start it. Fired right up like nothing untoward had happened. Rode it down to the end of our street, about 3/8's of a mile, it shifted fine thru all the gears, revved to 5000 like normal. Rode back home, no changes, still running fine: no strange, unwanted noises, all is well. Based on this info alone, what would you think could be the cause? I will tell you this was the first run with the replacement Mikuni carb with proper jets/needle/slide installed and 21mm float height. Also removed the suction pipe I installed because it wouldn't rev over about 5500 without cutting out, even in neutral. After removal it would rev to redline and more. Not that I would do that...
nayruf Posted April 20, 2012 Posted April 20, 2012 Hi FD A friend had a similar problem on an old Super dream it turned out that a rag had sucked into the air filter intake trumpet and whilst the engine was running it made the rag block up the intake after a few miles, Also I have heard of similar symptoms happening to a bike when the fuel tank does not allow air in through the cap vent as the fuel is used, I know these are off the wall answers but both could fit your symptoms, ????? Your post reads that you have probably checked all of the obvious reasons why out. AJ
Moderator Airhead Posted April 20, 2012 Moderator Posted April 20, 2012 its happened to me Richard...it's a heat seize, too much heat...piston expands in the bore until theres nowhere to expand into! Carburation too lean......... fit bigger jets all round...pilot and main, in the first instance, try lifting the needle one slot if it doesnt make it bog all in all, more carb fiddling Another less likely cause is ignition firing too far advanced, ideally you set it using a DTI for 2 mm BTDC
Keiran95 Posted April 20, 2012 Posted April 20, 2012 Airheads your man with these bikes, i was going to say the same as Airhead
slice Posted April 20, 2012 Posted April 20, 2012 Airheads right HEAT SEIZE almost certainly, do as he says and you should have no more problems.
Moderator Cynic Posted April 20, 2012 Moderator Posted April 20, 2012 its happened to me Richard...it's a heat seize, too much heat...piston expands in the bore until theres nowhere to expand into! Carburation too lean......... fit bigger jets all round...pilot and main, in the first instance, try lifting the needle one slot if it doesnt make it bog all in all, more carb fiddling Another less likely cause is ignition firing too far advanced, ideally you set it using a DTI for 2 mm BTDC +1 for me too. If it makes you feel any better they are prone to it. Very sensitive to detonation too.
2 Wheels Posted April 20, 2012 Posted April 20, 2012 I'd say you had a bad dream and woke up in shock Go outside and check,, there's prob nothing wrong at all
flyday58 Posted April 20, 2012 Author Posted April 20, 2012 Inclined to go with 2 Wheels' interpretation! Don't I wish... That's what I came up with also: too lean, too hot. I deliberately left out the high heat - I had to test y'alls savvy. Well done, all! So. The carb is stock, has all stock parts. The cylinder and piston are .50 over, so bigger jets nessa? Still can't get it to idle like I want, it's fine around 1800, but one tiny fraction more and the idle drops and keeps dropping. Yeah, yeah, I know. Clean the carb again again again. Will also lookit the gas cap. And I guess I will have to man up and buy a DTI checker-thingy like Paul sez, 'specially with an RT3 derelict in the stable and a CS5 on initial approach... Oh, and Paul, nice signature above your icon, except...
Moderator Airhead Posted April 21, 2012 Moderator Posted April 21, 2012 in the short term try lifting the needle by one position and turn inwards the mixture screw by 1/2 turn, if it doesnt bog leave it there for now and make a note of the current pilot and main jet sizes, i assume the float height was checked prior to fitting?
flyday58 Posted April 22, 2012 Author Posted April 22, 2012 Float height checked and double-checked at 21mm. Just put a new #140 main jet in, pilot jet is also the recommended one. Will lift the needle when I get home Tuesday. Do you think the extra bore is requiring more fuel? Maybe today's petrol is formulated to burn hotter to burn more completely to save our little planet? Apparently this set-up worked in '78 but not '12.
Moderator Airhead Posted April 22, 2012 Moderator Posted April 22, 2012 nahh nothing to do with the rebore consider increasing the pilot jet on a warm idle...screw in the mixture screw slowly until it seats...if the motor doesnt begin to stumble ( too rich), then fit a bigger pilot jet because it should have done you fitted a new 140 main jet....was that in when the seizure happened?...what's standard in the DT175E? dont forget to raise the needle...if its bogged down with it raised put it back, if it's not bogged down leave it there it was too weak anyway in midrange Filter? a new one fitted...alll standard and oiled?
Moderator Cynic Posted April 22, 2012 Moderator Posted April 22, 2012 Maybe today's petrol is formulated to burn hotter to burn more completely to save our little planet? Apparently this set-up worked in '78 but not '12. There could be some truth in that with the etanol they are adding to fuel so its cleaner, modern fancy pants electronics have all sorts of sensors to correct the mixture to a finite degree. Our little DT's don't, so you may well have a valid point.
flyday58 Posted April 22, 2012 Author Posted April 22, 2012 consider increasing the pilot jet Parts places don't seem to offer larger pilot jets. Get one from a different carb? What would you suggest, it has a 22.5 installed. you fitted a new 140 main jet....was that in when the seizure happened?...what's standard in the DT175E? 140 is standard. If the needle lift doesn't work, I'll order a 150. Filter? a new one fitted...alll standard and oiled? Filter is new as of when I installed it, what did I finally figure, 9 or 10 years ago? Doesn't feel crumbly or old, and the plastic support looks and feels good. I checked for a collapsed filter right off. I am currently running it on non-ethanol petrol, 87 octane. But I still think today's formula differs from 30-40 years ago. What time is it over there, shouldn't you people still be in bed? It's 1822 Sunday evening here in Tokyo/Narita...
2 Wheels Posted April 22, 2012 Posted April 22, 2012 We dont sleep in this side of the world,,,,,,,, it is now 10.30am Hope you get your carb sorted
Moderator Airhead Posted April 22, 2012 Moderator Posted April 22, 2012 jets are available here no problem, i just changed mine from a 20 to a 27.5 in order to get a rich enough idle, perhaps they will send to States?, http://www.allensperformance.co.uk/ http://www.motocarb.co.uk/ mukuni VM carbs come with different type pilot jets, this is the type in our's, i expect yours would be the same it's now 10:34am...cold and wet as usual
flyday58 Posted April 22, 2012 Author Posted April 22, 2012 Cold and wet here, too, but Sunday night. Been up about 24 hours now so going horizontal soon. Will look up those two sites when I wake up in about 7 hours, thanks for the links, Paul. 2Wheels, you're my kind of nutcase! I appreciate the humour from the proper side of the Atlantic.
Moderator Cynic Posted April 22, 2012 Moderator Posted April 22, 2012 It's 1822 Sunday evening here in Tokyo/Narita... 2012 here,
flyday58 Posted April 23, 2012 Author Posted April 23, 2012 2012 here, Shoulda knowed better than to try to use 24-hour time... OGOAB, I have the same pilot jet you do, but the web sites show a diff model for my VM24: So now I'm confuseder.
Moderator Airhead Posted April 23, 2012 Moderator Posted April 23, 2012 The website has errors then eh... care to post a link? like i said different mikuni VM carbs have different pilot jets! yours will be an M28/1001...order no 004-041 for #25, 004-596 for #27.5 see this link here...DT175 74 to 81 http://www.marshalldistributing.com/files/tech-info/sudcopilotjet.pdf if you can screw in your mixture screw without the motor stumbling the pilot jet fitted is too small....verify this when you get chance Richard
flyday58 Posted April 23, 2012 Author Posted April 23, 2012 http://www.pjmotorsports.com/mikuni-jets.html An Estados Ununidos site. I have seated the pilot air screw with only slight stumbling and virtually no diff all the way to 4 turns out. I'll verify that tomorrow. ANd I know I have the 1001 jet but couldn't understand this site showing the other one.
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