2stroke headache- help needed in diagnosing my DT 175 MX

jank · Mar 24, 2015

Hello 2stroke Buffs, time to use your vast knowledge to help a fellow in need:

Some time ago I bought a 1980 Yamaha DT 175 MX for about 400USD, running, titled, and mostly complete, with 12kkm on the clock, but it was a bit neglected.

The problem is that on the lower revs (until about 4500RPM on any gear) the bike doesn't have any power, is surges, missfires, and the ride is unpleasant. When the tach goes past the 4500RPM, the power comes instantly, the bike rides smoothly, and sounds great.

The bike starts good cold or warm, first, second kick, with a bit of throttle, and needs to be on the throttle a bit until it warms up. It goes down from the revs fast when I let the throttle go.

Things I have done until now:

- Checked the carb for any signs of wear or damage, cleaned it, set it as in the manual.
- Installed a new air filter, checked for air leaks.
- measured the coil resistance- in spec
- checked for any loose connections, and replaced the negative, and positive terminals on the battery.
- replaced the battery
- replaced the tires (not that it matters, just wanted to brag about it)
- replaced the spark plug for a new one, according to the sepck (gap 0,7mm)
- inspected the reed valve- looks fine
- replaced the tranny oil, and the autolube oil
- checked and bled the autolube system
- checked for air leaks in the air, and exhaust systems
- installed new needle valve, and set the correct float height (21mm, just touching the tang, as in the books).
- Checked the carb for any signs of wear or damage, cleaned it, set it as in the manual.
- Replaced the piston, and honed the cylinder (my bad- first time Ive made a huge mistake setting he float height which lead to seizing of the piston).
- Replaced the spark plug cap.

Do you have any experience with similar problems?
Can you help me with some useful tips on diagnosing this problem further?
I would be very grateful for any help, as I planned to use the DT as a daily driver this year :)

XRGRIFF · Mar 24, 2015

Any two stroke that has been sitting around for years, crankshaft oil seals are usually the first suspect and if I recall the smaller DT's had a habit of clogging up their exhausts.

jank · Mar 24, 2015

XRGRIFF- thanks for the tip.

I suspect the oil seals are ok, cause it holds the low revs nicely, doesn't drink tranny oil.

How do I clean the exhaust? The expansion chamber has only 2 small pipes at the ends, and the muffler is not dismantlable.
EDIT: never mind, I've found a blow torch tutorial :) it's a shame, cause I've just painted the exhaust.
Any other ideas before I burn my house down trying to clean the muffler?

MATTY · Mar 24, 2015

I tend to go with xrgriffs suggestion of the crankshaft seals, i have had issues with seals in the DTs in the past and its what you should look at next i believe.

jimroid · Mar 24, 2015

I would pull the pilot jet out and give it a very close eyeball. They cheese up often and easily. Varnished gas you know.

G54B · Mar 24, 2015

A faulty mag side crank seal won't let oil into the crankcase, just air. So the fact that it's not using trans oil doesn't mean squat.
You should replace the crank seals on any older 2-stroke as soon as you buy it anyway whether you think they need doing or not, as it's neither hard nor expensive. Better off spending an hour and $40 now than $XXX rebuilding your engine after an air leak causes a hard seizure.

Spikester300 · Mar 25, 2015

jimroid said: ↑

I would pull the pilot jet out and give it a very close eyeball. They cheese up often and easily. Varnished gas you know.
Click to expand...

+1 On this, the hole is tiny and if it's plugged it won't run right. Hard to find a wire small and stiff enough to poke it out, just buy a new one if nothing else.

jank · Mar 25, 2015

Thanks guys for all the ideas.
I'll replace the crank seals, and clean the exhaust.
If that won't work, i'll search for a new jet pilot.

I'll let you know about the outcome. Thanks!

Wolfshirt · Mar 26, 2015

Reading your list of work, you said it seized due to the float not being set right. I would not think that would cause an engine failure. It would be more like running out of gas. I think the engine is very lean in the lower rpm range and ok at high revs. This is why it needs throttle to run. So options are:
1. plugged pilot jet=not enough fuel from idle to half throttle. The jet may be ok but the passage under the jet may be plugged.
2. crank seal leaking allowing air in and leaking the mixture. It will be the flywheel side because only air = lean.

I would address both issues because if you don't get it right you can ruin another piston.

I had a friend smoke his banshee engine because he thought it was cool to rev it out in 6th gear on the road and then close the throttle and let it decelerate with the throttle closed. 10,000rpm and the fuel mixture for idling was not a good idea. melted one piston and scored the other.

So I guess my point is if you do not address the lean issue at lower revs, its hard to safely slow your engine down without melting stuff.

mudgepondexpress · Mar 27, 2015

Start with the pilot jet, cheapest, easiest and most likely the problem. I had a 78, bit had an 80. They were jetted horribly from the factory! Mine had Boysen reeds but otherwise stock. I remember I was 1 up on pilot jet, 3 down on main and lifted the needle when it ran perfect! Good luck, love the late DT series.

jank · Mar 27, 2015

I' very happy with the number of comments, and ideas, didn't get those in other forums. Go ADV!

The engine holds low revs nicely, even 1000RPM, but when it's warm. After cold start, with the choke on I have to keep the revs a bit higher.

Tommorow i will buy the magneto puller, and will search for the seals:
mag side- SD-25-40-8
clutch side SW-28-40-8

Thanks again for all the feedback!

Scootern29 · Mar 27, 2015

You can get an entire seal kit all day long on Ebay for some good prices. If not there, other sources are available.

Try

Northwest Vintage
Speed and Sport / Yamaha

They have really hard to find parts if you need anything else.

I just saw that you are in Poland which could rule out the places I gave you. But I am leaving them in case you or others are looking for hard to find Yamaha stuff.

jank · Mar 28, 2015

Today I bought the magneto puller, and ordered the seals in a Yamaha dealership.

There is a shop near me with all the simmering seals you can imagine, but they are opened in the same hours as my office, so instead of buing the part there fo 0,5$ I'll pay 4$ for the OEM part. Oh well, as long as the DT is o its way to health, I'm happy :)

jank · Jun 9, 2015

Well guys, now I'm really confused.

After a while I have found some time to replace the crank seals for a new, original Yamaha set.

Now the bike will idle, and run roughly on choke, but it will die without choke on anything more then 2000RPM.

The float height is addjusted properly, and the overflow is not leaking.

Moving the main jet needle up or down will not change anything.

So it's starving for fuel? Clogged main jet? Any ideas?

G54B · Jun 10, 2015

Pull the carb, soak it in a tub of carb cleaner for a few days and scrub everything half to death with an old toothbrush. Make sure everything's spotless, throw it back together with the needle clip and float level exactly as they were when you pulled it apart and see how you go.

How's the plug? Coils, CDI in good shape?

jimroid · Jun 10, 2015

Did you pull the pilot jet out? Did you completely remove the pilot air screw(the adjustable mixture screw)??? Double check all the passages for cheese or dirt.

stevo7706 · Jun 10, 2015

reeds

Gamequeazy · Jun 10, 2015

jimroid said: ↑

Did you pull the pilot jet out? Did you completely remove the pilot air screw(the adjustable mixture screw)??? Double check all the passages for cheese or dirt.
Click to expand...

I'm with jimroid. A bike dying w/o choke unless the throttle is open says clogged pilot to me unless proven otherwise.

third eye · Jun 10, 2015

One suggestion I might add, and this happened to me after buying a bike that sat, two stroker mind you.......Take the fuel petcock completely apart and make sure there's no dirt build up in there. Mine was chocked full of whatever and its a wonder the bike ran at all. I had a fuel starvation problem

jank · Jun 10, 2015

Gamequeazy said: ↑

I'm with jimroid. A bike dying w/o choke unless the throttle is open says clogged pilot to me unless proven otherwise.
Click to expand...

You got it mixxed up- the bike idles fine w/o anny throttle, or choke, when I want to rev it higher that 1500-2000 RPM, then it will die w/o choke, and with choke it will run on revs, but very rough. Maybe the main jet then?

Stevo- could you elaborate on the reeds part? what could be wrong with them? I checked them visualy, they look fine.

G54B- The CDI, coils, and plug are ok.

Between "bike running well apart from a surge at 400RPM" and "bike runnig only on idle, unless with choke on" I have only replaced the crank seals

I ordered a new float bowl gasket today, because the last time i took the carb apart, the old one leaked. I gave it a very gentle, and fine layer of engine sealant, maybe it got into the carb while the bike was waiting for me in the garrage for two months?
As soon as I'll get the gasket, I'll tke the carb totally apart, and clean it once again.

Thank you for all of the ideas, and comments. If you had a similar problem, plese share it with me

some pics from yesterday:

This came out from the drain hole, 200 km ago it was a fresh tranny oil. It could not mix with water, so i susspect its gasoline soaking through old crank seals.

I made a mess on my parking space, I miss my old house with a garage, as a workshop

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