Author Topic: Two-Stroke Woes...  (Read 745 times)

AZRedhawk44

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Two-Stroke Woes...
« on: March 24, 2023, 09:50:24 PM »
I've got a 1980 Yamaha DT175 dual sport that's acting up.  I'd appreciate any expertise, experience, yarns or ideas.

It was having trouble kicking over.  Plug looked a little fouled but not awful, but I changed it anyways since I had a spare.  No dice.  I took the carb out and cleaned it, still no joy.  It's an old kick start bike and should start without a charged battery, but I did some lead tracing and the feeds off the battery were looking really ratty.  I changed them out with fresh wires and terminals and the bike kicked over right away.

Making progress, right?

I rev it for a bit in the driveway in neutral to warm it up, and head out into the neighborhood street.  Feels really hesitant at about 5k RPM in first gear.  I shift to second and it can barely sustain the load of the bike.  After about 50 yards it sputters and dies.  It won't kick over after dying.  I walk it back home, about 100 yards, in neutral.  In the driveway I kick it over and it starts.  Rinse and repeat; it will pull the bike to about 5k RPM in first, shift to second and it has no power at all.

I've made no tweaks to the carb and the bike has run satisfactorily for a couple years and several trips out on it, though I'll caveat that by saying it's SWMBO riding it as she putters along with me and the little one, him on a Honda 50cc and me on a 250cc four stroke.  It's got an easy life, at least with us for the last 2 years.

This bike has an automatic oil pump that injects 2 stroke oil into the carb rather than having to make your own mix.  It appears to be working fine.  I can disconnect the tube that feeds the carb and get a bead of oil coming out of it.  Fuel filter seems fine.  Air filter gets cleaned every other ride, which is pretty good for how lightly this bike gets ridden.

What are symptoms of blown piston rings or worn valving on a 2-stroke?  Is there anything else I should check before tearing the cylinder open?
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French G.

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Re: Two-Stroke Woes...
« Reply #1 on: March 24, 2023, 10:04:55 PM »
Can you verify the fuel flow? I know you said the filter is good but if you have a trickle from the tank it may fill the filter and bowl but not sustain. Cap not venting also part of that. Make sure no lines rotted.
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Nick1911

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Re: Two-Stroke Woes...
« Reply #2 on: March 24, 2023, 11:13:07 PM »
I haven't done much with two stroke motorcycles, so I'll be curious what you find.

Random thoughts:
Spark ignition engines need compression, fuel and spark to run.  After the bike dies, does it still feel like it has good compression?  Run it until it dies then check for spark.  Could be something titchy in your coil or ignition (CDI?) But my seat-of-the-pants guess is fuel.  Seems like maybe you're running the float bowl dry and have to wait until it refills.

Fuel from the line feeding the carb flow freely when disconnected?  When cleaning, were you sure to got all jets?  Did you pop welch plugs and check for crud in those pasageways?

RoadKingLarry

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Re: Two-Stroke Woes...
« Reply #3 on: March 24, 2023, 11:18:26 PM »
Go back to the basics.
Fuel/air. check for air leaks around the carb boot/gasket
If the fuel lines are original to the bike they should be changed just because.
Spark
timing
compression
What's the ignition of that one? Magneto, points or electronic?
This link says CDI ignition.
https://www.cyclechaos.com/wiki/Yamaha_DT175

When it is running is the alternator charging the battery?
It's been 40 plus years since I messed with Yamaha 2 strokes but I seem to remember that they always kick started easier with a hot battery.

Good luck

This link says CDI ignition.
https://www.cyclechaos.com/wiki/Yamaha_DT175
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Jim147

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Re: Two-Stroke Woes...
« Reply #4 on: March 24, 2023, 11:48:48 PM »
When you say it won't kick over what do you mean? It kicks over easy but doesn't start or you can't kick it over?
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AZRedhawk44

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Re: Two-Stroke Woes...
« Reply #5 on: March 25, 2023, 12:00:53 AM »
When you say it won't kick over what do you mean? It kicks over easy but doesn't start or you can't kick it over?

It's not bound up; I just mean it won't run.
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Tuco

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Re: Two-Stroke Woes...
« Reply #6 on: March 25, 2023, 12:56:29 AM »
1980 DT?  Electronic ignition? Because old 2 strokes with points could get out of time on a gear change.  My IT-250F had solid ignition by '79 but the older DTs (pre-76) were finicky.

Dollars to donuts it's fuel starvation.  If the tank, filter, petcock and fuel line are solid, my money is on a carburetor reassembly error. Your bike has a Mikuni carburetor. Mikuni carbs are rock solid and dead simple - unless it's removed and reassembled wrong.  They still appear to work, have limited function, but the flow in the bowl is compromised-
Don't swap it out, it's likely fine... but check a few things.

There's a rod/needle in the center of the throttle slide. The top of it goes up through the throttle and attaches with a tiny circlip to the slide.  Cant recall the attachment to the return spring, screw cap / snout which then somehow hooks to the throttle cable.  But it's in there.
The bottom of the rod sets down into the main jet/valve in the bowl.  I've seen more than once, after the carb's been removed, and the screw cap and throttle left hanging on the cable, the circlip holding the slide in position on the rod will end up on on the wrong side of the throttle slide. It gets loose as the fuel really starts flowing, sets back into the main jet, and cuts off the fuel into the throttle.

I also recall some interplay between that assembly and the float lever and float valve.   I can't explain why, but the fuel demand isn't linear in the higher gears, like when you're accelerating in 2nd, and the motor gets starved for fuel when it was fine in 1st.

It's been 30 years, pardon my vague recollections, but I maintained a stable of 1970-1984 japanese 2 strokes.  They can run, and run well, with cooked rings or loose and sloppy jugs.  If you're not convinced, screw in a compression gauge before pulling the head.  No need to open that can of worms if you dont need to.
 
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Tuco

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Re: Two-Stroke Woes...
« Reply #7 on: March 25, 2023, 01:00:38 AM »
Doubletap
« Last Edit: March 25, 2023, 09:14:49 AM by Tuco »
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zahc

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Re: Two-Stroke Woes...
« Reply #8 on: March 25, 2023, 02:32:42 AM »
I never trust oil injection.

However it works out, disable it and premix your fuel.
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Cliffh

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Re: Two-Stroke Woes...
« Reply #9 on: March 25, 2023, 05:32:49 PM »
Quote
There's a rod/needle in the center of the throttle slide. The top of it goes up through the throttle and attaches with a tiny circlip to the slide.  Cant recall the attachment to the return spring, screw cap / snout which then somehow hooks to the throttle cable.

IIRC:  The throttle cable goes through the snout, through the cap, through the center of the spring and goes through a keyhole slot in the throttle slide.  A lead "blob" on the end of the cable goes through the keyhole and the pressure from the spring holds everything together.  The needle rests in a hole in the center of the slide - it just sits there, nothing holding it in place except gravity.  The needle has slots cut into it so it can be adjusted up or down as necessary.

The needle shouldn't need to be adjusted.  If it was running fine before the current problems arose, the needle was in the correct position.

A new plug wouldn't hurt.  I've had numerous times with 2 strokes when the plug looks just fine, and the engine will start but not perform to spec.  Never mind, I just re-read the OP & saw you've already changed it.

I ran a 250 2 stroke in a micro sprint for a few years.  It started burning pistons, I'd go through one a night.  Turned out that the CDI had taken a crap and the timing was off.

griz

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Re: Two-Stroke Woes...
« Reply #10 on: March 25, 2023, 08:28:48 PM »
Since it runs Ok at idle and low throttle, I'm guessing it's running out of gas or air.  Usually blockages in the path of the air aren't hard to find, so it's probably gas.  Look for fuel flow, not just that it's got gas in the carb.  Not sure if you checked it, but the petcock should have a screen built into it.  A lot of times, especially in bikes sitting for a while, rust in the tank plugs up things.  You can take the petcock off and see if there's crud in there.
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Cliffh

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Re: Two-Stroke Woes...
« Reply #11 on: March 25, 2023, 08:37:20 PM »
Another thing I just remembered:

Prepping to wash my bike, I put a plastic cover in the air filter box.  Washed it, then got distracted & forgot all about removing the plastic bag.  Next time I tried to run it, it would idle fine & accelerate poorly, then die.

RoadKingLarry

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Re: Two-Stroke Woes...
« Reply #12 on: March 25, 2023, 08:38:05 PM »
This thread reminded me of a problem I had with my old DT125 way back in the '70s. If I got crud in my gas either from crap in the tank or crappy gas at the pump it would bog out and die. The immediate action solution was to pull the plug on the bottom of the carb and drain out the crud and I could be on my way.
If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or your arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that you were our countrymen.

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Cliffh

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Re: Two-Stroke Woes...
« Reply #13 on: March 25, 2023, 08:55:08 PM »
Does your bike use reeds?  Some of that era had a set of reeds in a block between the carb & intake port on the cylinder.  They've been known to go bad.  Sometimes, removing the block & flipping the reeds over* would help. 

Also, does it have a rubber boot connecting the carb to the reed block?  Old rubber can start cracking, causing an air leak.  That'd be more likely to cause high revving instead of bogging though.

*On the ones I've worked on, the reeds were held to the block by two screws (four total, two sets of reeds).  Remove the screws, flip the reeds over, reinsert screws & put it back together.

zahc

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Re: Two-Stroke Woes...
« Reply #14 on: March 26, 2023, 12:30:42 AM »
Don't underestimate stale fuel. My old XR250 wouldn't start on fuel that had been in the float bowl more than a few days! I would have to drain the float bowl before trying to start it. It wouldn't run on fuel that had been in the tank more than a couple weeks either.
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tokugawa

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Re: Two-Stroke Woes...
« Reply #15 on: March 26, 2023, 01:57:52 AM »

What is the plug condition each time?

Will it start and run at idle for a long time, I mean, does it only stop when trying for more speed?

A lot of good suggestions here- it is either running out of gas, or out of spark.
Out of gas- most likely- clogged petcock-line-vent or filter- some carbs have a tiny filter in line INSIDE the fuel line right at the carb. Yank the line off at the carb and see if it pours fuel out in a nice stream.

Check for a clogged vent- see if it runs ok with no gas cap.

 IDK what the ignition system is, sometimes coils can get hot and fail, and condensers also.

Cliffh

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Re: Two-Stroke Woes...
« Reply #16 on: March 26, 2023, 12:40:29 PM »
I'm curious if that era will run* on E10, or does it require non-ethanol?

ETA: *run properly, that is.

charby

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Re: Two-Stroke Woes...
« Reply #17 on: March 26, 2023, 02:18:35 PM »
I'm in the crap in the tank or air leak in a fuel line camp.
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Cliffh

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Re: Two-Stroke Woes...
« Reply #18 on: March 26, 2023, 10:31:43 PM »
The rubber crank seals are another thing to look at.  This video is simplistic, but the info's good.

https://www.youtube.com/watch?v=uLMK9HGnbPM

tokugawa

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Re: Two-Stroke Woes...
« Reply #19 on: March 27, 2023, 12:20:36 AM »
One other thing- some bikes have a On-Prime-Reserve petcock- the on position being vacuum activated with a diaphragm- if that is leaking air the fuel flow will be interrupted- check by seeing if it will run OK on reserve position.

The initial problem cranking over may have nothing to do with the subsequent bogging down- except in a related way- for example if the tank was removed, maybe the vent line got pinched on re-installation. that sort of thing.

AZRedhawk44

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Re: Two-Stroke Woes...
« Reply #20 on: March 27, 2023, 11:33:11 AM »
Thanks for the suggestions, everyone.  I spent Saturday working on my bathroom remodel and I went for a ride Sunday on my Tuareg, so I didn't get any time this weekend to follow up on the suggestions but I think I'll have some time Tuesday afternoon after work.  Sounds like I need to take a good look at the tank.

I think what I'll do is drain what's left in the tank into a gas can, then dismount and disassemble what I can of the tank.  I may look into some rust treatment solutions.  I do see some spots of rust inside it.  I'll be sure to check the petcock flow and might as well get a new inline filter while I'm at it.  I kind of wonder if there's some sort of epoxy or spray film that can be done to the inside of old gas tanks to eliminate the rust potential.

It hasn't had a problem running on 10% ethanol so far.  It does sit for long periods and while I know I'm supposed to drain the carb and tank... it doesn't happen.  Mostly out of hope that the bike gets used more than it actually does.
"But whether the Constitution really be one thing, or another, this much is certain - that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist."
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RoadKingLarry

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Re: Two-Stroke Woes...
« Reply #21 on: March 27, 2023, 12:25:41 PM »
Quote
I kind of wonder if there's some sort of epoxy or spray film that can be done to the inside of old gas tanks to eliminate the rust potential.

https://ripsandrides.com/best-motorcycle-tank-sealers/
If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or your arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that you were our countrymen.

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tokugawa

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Re: Two-Stroke Woes...
« Reply #22 on: March 27, 2023, 09:06:09 PM »
That tiny inline filter I referred too earlier- IIRC, it fits into the carbs gas line inlet. I don't know if they are still in use.

French G.

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Re: Two-Stroke Woes...
« Reply #23 on: March 27, 2023, 10:21:36 PM »
For as little as you are going to run it buy some real gas at an exorbitant price. Sunoco race gas in a five gallon can, or VP. No ethanol. 
AKA Navy Joe   

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MechAg94

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Re: Two-Stroke Woes...
« Reply #24 on: March 27, 2023, 10:53:47 PM »
Buc-ee's down here sells non-ethanol gas.  There ought to be a station somewhere that sells it.  I buy it for my mower.
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