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Hey everyone, 

Today I bought my first motorcycle, a 1978 Yamaha DT175. Not having ridden one before, the guy I bought it from rode it around the block and cycled through the gears. Everything worked fine - it idled in neutral, went through the gears, etc. Back home I decided to ride it in the alley beside my house. It started up on maybe the second kick with the choke all the way out, but when I rolled off the throttle all the way it would die pretty quickly. I started it again and it would stay running just fine if I held the throttle slightly open, with both choke in and out. I rode it up and down the driveway a few times, went through the gears, but sometimes when in first with the clutch all the way out it would die again. 

 

I then parked it for about half an hour and then went to start it again but couldn't. It was in neutral, the clutch was all the way in, the fuel was on, and the bike was in the "run" setting. I probably kicked it 40 times and couldn't get it to think about starting. Did I screw something up earlier? I left the fuel pump on which might have been a mistake. Also, does this sound normal? I don't know two stroke engines well and am having trouble figuring out why it would seem so fine earlier and now not even start. Any advice would be great! Plans right now are to try again in a little bit and if that doesn't work start learning about how to clean carburetors.

Thanks!

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Sounds good, gonna learn how to do that tomorrow. Any idea why it would have been running fine and then not at all though? It makes me think that it was something I screwed up by not knowing how two strokes work. 

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38 minutes ago, mhasson said:

Sounds good, gonna learn how to do that tomorrow. Any idea why it would have been running fine and then not at all though? It makes me think that it was something I screwed up by not knowing how two strokes work. 

If it runs, it runs, but a bike that old is aboslutely certain to have issues with old parts, dirt, corrosion, etc.

 

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I fought this exact problem with a TY-175 Yamaha for a whole trials season.  I cleaned the carb out I don't know how many times and it would be running fine here at home, but when we went out to ride an event the stupid thing wouldn't idle or run at low rpm.  The pilot jets, heck all of the jets in those 175 carbs are so small that the slightest bit of dirt or debris will screw them up.  I don't know about your DT, but the TY had a steel gas tank on it and the eventual solution to the problem was to put an in-line fuel filter between the gas tank and the carb.

Edited by GlennRay

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So a little update: today I checked the spark plug (zapped me, so we're good on that front haha), cleaned the petcock, replaced a cracked fuel line, cleaned the carb and everything inside it, cleaned the air filter, and now it's running much better. I got it to a point where it will idle with the choke pushed in at ~1300 rpm and choke pulled out around 15-1600. However, it still dies during low-speed riding like tight u-turns. Also, it seems to run much smoother/dies a lot less with the choke out... any idea what the next step would be on this? I feel like I made significant progress but it still doesn't feel like I could ride it around town and not worry about getting home. 

Here's a link to a vide I took of the bike running with the choke pulled out at first and then the choke in to see the difference in case this gives anyone any more insight: 

 

 

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From my past experience with the CT-175 and the TY-175 I would remove and replace that pilot jet.  No matter how good you think you have cleaned that carb, the pilot is so small that it can be clogged up by a piece of lint left in the float bowl or a bit of debries coming from the gas tank.  The other issue might be not enough slide cutaway on the throttle slide.  By the way, I used my little brother for the "is it getting spark test"!

Edited by GlennRay

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Sounds like a good use for a little brother! Sounds good about the pilot jet, I'll order one this evening. I'm still using the stock oil injection system... from what I've read it mixes the oil based on how you're riding rather than just having one mix ratio like you would if you did it yourself. Did you pull yours or keep the Yamaha system? 

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My CT-175 came to me from my uncle with the Yamaha oiling system already removed.  It was my very first dirt bike.  I loved it very much.  The TY-175 that I was trying to help out with didn't have the oiling system in the first place, so my experience with two stroke Yamaha 175's is from a premixed fuel prospective.  I will now deliver my standard two stroke carb. lecture which you are free to ignore, laugh at or hopefully use.

In most of the motorcycle carbs I have dealt with over the years the common thread is the following.  The pilot jet and the throttle slide cutaway take care of the 0 to 1/4 throttle range of operation.  The needle jet and the jet needle take care of the 1/4 to 3/4 portions of throttle opening and the main jet takes care of the 3/4 to wide open throttle (WOT) operation.  These are not hard and fast lines, there are overlaps between each zone, but this will get you pretty close to figuring out what is wrong.

The second thing to remember is that "bogging or stumbling" can be caused by either an overly lean air/fuel mixture as well as on overly rich air/fuel mixture.  You have to look at the sparke plug to see which is which.

So lets start with step one.  Start the bike up using the enriching circuit if necessary and let it warm up until it will idle without the enriching circuit.  Then let it idle for three minutes or so and kill it with the kill switch or the ignition switch.  Pull the spark plug and look at the color of the ceramic surrounding the center electrode.  If it is light to medium brown then the idling circuits of the carb are working properly.  If it is white the engine is not getting enough fuel.  It it is black the engine is getting too much fuel.  We will continue this discussion after you check this out.

One more thing I forgot about, there are two adjustment screws on the left hand side of that carb as you sit on the bike.  One is fairly large and has a visible spring behind it.  This is the idle speed adjustment screw and what it controls is how far down your throttle slide can go.  The other screw is much smaller and is recessed into the car housing ahead of the throttle slide bore.  This screw controls the amount of fuel your bike is drawing at idle.  You might want to back this screw out 1/2 a turn or so to see if it makes it run better.  The way we always set these on our dirt bikes was to bring the bike up on a "fast" idle, say 1200 rpm or so and then screw the mixture screw in until the rpms started to drop, then while counting the turns screw it out until the rpms started to drop again and then screw it back into half the number of counted revolutions.  Pretty scientific eh?

Edited by GlennRay

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So I did everything you said and also rode it around for a little while. It got to a point where it would idle just fine without the choke in around ~1400ish. After everything, the ceramic on the spark plug is almost completely white still. Given that it's just sitting above the cylinder, I'm sort of having a hard time understanding why it would turn brown. Wouldn't there have to be some kind of leak to cause that? Sorry for the newbieness haha. 

Also, now that I've got it to a point where it's running pretty consistently I'm noticing that the throttle is pretty delayed in the first 1/4 of the throttle or so and it tends to run much rougher at low rpms (2.5k-ish and below) than at higher rpms. Does that sound like a pilot jet issue to you? The other thing that seems a little funky is that it will idle really high (3000 or so) when I first start it and then slowly drop down to 1400 with the idle screw in the same position. Not sure if that's relevant or not but I figured I'd mention it. 

The more I read about disconnecting the oil pump the better idea it sounds. To do this, would I just remove the oil delivery hose to the carb and plug it/kink the hose over? I'm curious if you remember how it was set up on yours. 

 

Edit: I'm an idiot. You meant the ceramic on the part of the spark plug that screws in. That part is black/brown. I'm including a pic of it for reference: 

IMG_3147.JPG

IMG_3148.JPG

Edited by mhasson
Made a mistake in original.

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Looking at that spark plug I would say that your carb is running a little rich.  You might try backing the idle mixture screw out another 1/2 turn.  As far as the oil delivery system goes that was 45 years ago, I can't remember what I had for breakfast last week!  The general idea was as you said, stop the delivery of oil from the oil tank, cap off the oil input to the fuel system in some manner and mix the oil in with the gas directly.  I like(d) this because as long as the bike was getting fuel it was getting oil.  With the Yamalube system if you forget to keep up with the oil level in the oil take it would be possible to run out of oil and damage the engine before you realized it.  On those old CT-175s the throttle cable had a device in the middle that split it into two cables, on that went on to the carb and the other that went to the oil pump to control the amount of oil being delivered to the engine.  As I recall it was a simple matter to disconnect and remove the oil pump cable from the system as well as removing the oil pump itself.  A flat plate made out of aluminum was bolted on in place of the oil pump.  You could, and I did remove the oil tank itself at that point.  Then all you had to do was seal off the oil delivery spigot and I cannot remember how that was done.  The running condition you are describing and the condition of the spark plug suggest that the cutaway of the throttle slide needs to be increased, that the engine is not getting enough air at low throttle openings.  That would be kind of weird because they jetted those bikes for operation at sealevel as a base line.

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