2 strokes - really, I'm utterly clueless

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As I've been saying for quite some time (over and over it seems) I'm approaching the "endgame" on my DT rebuild. TBH I've stretched it out over the last couple of months 'cos I couldn't really ride it anyway. Wednesday that changed so time for a good tear about

I've said this before: I've never ridden a 2-stroke before and so I'll probably be repeating myself here but I just want to understand the thing and not fuck it up so apologies in advance.

Okay, so first up: engine braking or lack thereof. I understood this as a concept but actually out on the road is another matter. It's only a 175 so there's little to it and I'm getting a feel for the fact that, if one were to fly down a hill, you could have the wheel driving the motor rather than the other way round. With less throttle there's less oil squirting from the pump so something to think about which wouldn't even enter my mind on a 4-stroke.

On the flip side, changing down a gear doesn't seem to do anything but over rev the engine. So what I've been doing is slowing down with the brakes and getting the gear to match the speed reactively rather than utilising the gears proactively like wot I would do with a 4-stroke. Am I doing it right? I only whizzed about for half an hour but that technique seemed to work.

The other thing, and this is why I put this in the Workshop section, is it does seem to hang the revs if I get it wrong. For example: if I were to do what I'd do on my Rebel and slow down to stop I'd change down while rev matching and use a bit of engine braking and some front/back brake. Of course, switching down too soon (before the engine's had a chance to slow down) just gets the revs jumping back up. If at this point I pull the clutch in it'd just sit there at 4k or whatever. Applying any load, e.g. switch back up a gear and let out the clutch, and the engine settles down again.

"2-strokes hang a bit more on the revs" my father-in-law commented but I don't think this is right Too lean perhaps? The inlet rubber's in good nick and tightened down with an OEM clip but I admit I haven't done the WD40 test as of yet.

I did consider that the spring was a bit light on the slide (the very, very first spring had been cut down to "lighten the throttle" by some previous owner and then the vacuum would literally hold the slide open) so I put in an even beefier one but it's made no odds. I also checked the slide and carb for grit or any reason for it to stick but it seems fine. Throttle cable also moves very freely.

tl;dr 50% "You know nothing, Jon Snow" and 50% probably still not jetted right
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Nobby the Bastard]

2ts have a definitive power band (I prefer purple ones) so you need to rag them to get the best out of them.

Correspondingly you should be changing down a lot earlier to hit that sweet spot.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Kickstart]

Hi

Possibly a bit lean, possible an intake leak, possible main bearings seals leaking.

As to engine braking, less of it but still a bit. With a pump you are still getting 2 stroke oil on a closed throttle, just a bit less . Unlike running premix.

All the best

Katy
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Easy-X]

I suspect there'd be a touch of engine braking if there wasn't the rev hang. I'll try going up a jet size tomorrow, see what happens I knew it wasn't right.

Main bearing seal: I take it this would be more of a problem on the stator side (air) than the clutch side (oil) ?
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Nobby the Bastard]

It's a problem. Both sides.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Kickstart]

Clutch side will give you an air leak, gearbox side maybe a bit if air but a load of smelly exhaust smoke.

But check the carb settings first. Way easier to sort!

All the best

Katy
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

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Had a little tinker today. Checked the head bolts torque and they're still spot on then pulled the carb. The air screw's about 2 1/2 turns out so I don't think it needs a bigger pilot jet so I went up two sizes on the main jet and took it for a trip to Tesco...

Wow! Night and day No longer hangs the revs and there is a bit of engine braking (but obviously doesn't compare to a 4-stroke.) Now I find I can ride it about without over thinking. WIN

And then I pull up to the car park and the clutch cable snaps. BOO

Luckily my father-in-law lives that side of the town so I only needed to push it 500 yards. Wouldn't fancy pushing the Rebel but this one's manageable.

Quick fix later:

https://i.imgur.com/nRlENNO.jpg?1

...and back on the road
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[A100man]

[Easy-X]

Bad news: rode the DT about for a bit today and whilst previously it was fine with the larger jet (until the clutch cable went) today it did the same rev hang thing.

Jets don't suddenly shrink in size but I could imagine ancient rubber doing different things - especially in this recent hot weather. So I'm gonna go with crank seals

The good news is that I found a guide:

https://www.chinonthetank.com/2014/09/replace-yamaha-2-stroke-crank-seals/

...and it doesn't sound half as bad as I thought Especially as the writer keeps banging on about air guns. I don't have one of those but I do have my lovely, lovely Ryobi Li-Ion wrench that just needed to vaguely glance at the front sprocket

Also for some reason months ago I'd already done an impulse buy on the magneto puller! Shame to dump out all the oil for the clutch side but seems a little silly to do one side and not the other?

Any thoughts on whether black gasket sealer is a worthwhile addition on the clutch side?

<addendum> a couple of weeks ago I also did an impulse buy on mechanics' picks. Maybe I should have a stab at the Euro Lotto next
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[A100man]

Save the oil..

Sealant on where, clutch cover?
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[c_dug]

For what it's worth, my little Tuono 125 does pretty much the exact same thing, it'll hang a tiny bit around 3-4krpm and then drop down the last bit maybe 5-10 seconds later if I don't use the clutch to pull them down first.

Other than that it's pretty much perfectly behaved, it makes excellent power at the top end and it responds to the throttle nicely. It's your call to make with your own bike, but I'm in no hurry to start pulling the engine apart for something that has almost no effect whatsoever on my actual riding of the bike, I can't even pull away with the revs that low

I had an air leak on my old TDR250 years back and that was different in that the revs would climb themselves rather than hang on the way down. My Aprilia doesn't do that. The TDR needed new inlet manifold rubbers.
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I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[Easy-X]

[redeem ouzzer]

Don't use sealant on the OD of the seal.

Don't use pattern seals.
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Be a REAL MAN!

[bypass2]

dont no if I have read this right but if the revs were allright then the clutch cable breaks you done the clutch cable and now the revs hang again have you disturbed the throttle cable routing

[Old Git Racing]

could be a worn carb slide.

OGR
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Easy-X]

[Kickstart]

Personally, as if the clutch side goes you will get a LOT of smoke, I would suggest it is probably fine for now. And as pretty much no shared work to do the 2 bearing seals, just do the ignition side for now

All the best

Katy
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[blurredman]

For what it's worth, as above- my MZ does the same..

Hangs at 2k if I put it in neutral before I stop.. If I put the bike in gear the bike tends to hang for a while at 1700 before gradually and slowly going down to 1300 where it should be.

I have no reason for this, as my other mz does it a little too.. But not as bad - and both engines I had rebuild less than 3 years ago!
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S.
Current: 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (bored to 295cc) - 38k, 1990 MZ ETZ251 - 49k, 1979 Suzuki TS185ER - 9k, 1973 MZ ES250/2 - 17k.

[Easy-X]

I had that one day where it performed perfectly normally (before the clutch went) so at least I have a target to aim for.

According to the specs it's meant to have single lipped seals but, on advice from the vintage enduro crowd, I've gone for the double lipped seals from the IT175. And as I have all the correct tools on hand anyway if it's not the seals I can just say I did an upgrade

Got a notification of intended delivery from Royal Mail for tomorrow so I'll start with the magneto side and see how it goes.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[jeffyjeff]

I have not seen the reed valve block mentioned as a possible source of intake leak. If your bike has the original reed valve, it is likely pushing 30 or 40 years old, and probably has lost some of its resilience. You might consider replacing the reed valve.

Also, the needle valve can be raised by moving the circlip down one groove on the needle. This will richen the mixture at midrange throttle positions. Worth a try and costs nothing.

I have a lot of respect for the Yamaha DT175. I rode one to some pretty remote job sites for two years in the 1980's. Never once let me down...not even a flat tyre.
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History shows again and again how nature points out the folly of men - BOC

[Easy-X]

[jeffyjeff]

Here is a little inspiration for you. All the best. Jeff J
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History shows again and again how nature points out the folly of men - BOC

[Easy-X]

Nice! DT1C?

In other news it wasn't the seals that turned up but my emergency cable repair kit or "proper bodge tin" as my wife described it
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[jeffyjeff]

[Easy-X]

Well it's an earlier DT than mine as I see it has the exhaust on the other side But from what I've heard the original DT1 all the way through to the last dualshock DT175s and DT250s are well regarded. The larger ones (e.g. DT400) are loved all the way up to the point the kickback breaks your ankle
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter