2-stroke basics

[Easy-X]

I was gonna add this to my DT thread but I thought it might be more of a general interest topic.

DT175 (1976, 2-stroke) stock exhaust and airbox but a Mikuni VM28 carb over the factory VM24SS. Factory spec #140 main jet, needle at middle position, mixture screw about 2 turns out. New spark plug and air filter.

So far I've got it to the point where it starts and idles fine. I dunno if this is because of the recent mild weather but it starts pretty well from cold with no choke; maybe a little teasing of the throttle.

Symptoms

5 mins of riding it's fine and seems to have plenty of power. After that it seems to splutter in the midrange (>3K)

Ideas

Fuel starvation? Enough of a trickle to keep it idling it seems but not enough to get any real power. Vacuum lock tends to be a time based thing so I'll try popping the fuel cap off, see if it improves. (BTW tank, petcock and pipes are all new, carb recently cleaned.)

Running rich? This would be more noticeable when the engine's warmed up. Quick test would be to pull the air filter and see what happens.

Running lean? I would have thought I'd have a bit more trouble starting it and the reverse: rough from cold but some improvement when warm. A cheeky bit of choke might help but actually it just stalls the engine.

Weakening Spark? Any way to check this with the bike running? Old coils and stators can do funny things once warmed up and corroded contacts can suddenly jump in resistance when warm. Certainly low down on my list of things 'cos... hassle

Timing? I plead ignorance here as to whether it can affect the bike once warmed up. Contact corrosion resistance again I suppose.

Compression? Obviously metal distorts depending on temperature but I'll have to get my hands on a compression tester. Hopefully just some new piston rings required, if compression's down.

Valve Gaps? Only kidding

So that's my Sunday planned out. Anything I've missed?
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Nobby the Bastard]

Plug chop.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[-]

How old are the crank seals? Block gearbox breather, remove fear oil filler and put thumb over it whilst revving. Does the gearbox pressurise?

Any sign of 2t oil in the gear oil?
Is fuel flow good?

Bigger carb with standard airbox is a can of worms.
Just removing the inner trumpet from an airbox can require main jets upping from 230-320 on my TZR’s.

If crank seals are good, i’d Look at fuelling.
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Easy-X]

I'll probably order up a compression tester, seems like a useful thing to have anyway so I'll tag some spare plugs along with that.

I've had the case covers off a few times and they all look new and in good condition. Certainly the minor seals (kick start, oil pump) are still flexible and AFAIK they were all changed at the same time. Not seen any obvious evidence of 2-stroke oil creeping through. Gearbox pressure test is a nice easy one, thanx! Fuel is fresh from the pump BTW.

Comparing the petcock from an original tank to the new one I can't say anything jumps out of me. TBH the design looks more "free flowing" easy check if it comes to it - checking flow rates - although I'd think it'd affect both cold and hot performance if it was an issue.

I agree that the up-rated carb is a bit of a random element but I had almost zero luck with the VM24 copy I have. But then again it is one cast in pure Chinesium

Thanks guys, great suggestions
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[bypass2]

as said could be the bigger carb. try the mixture screw 1 turn out. choke plunger seals worn/damaged. is the oil pump set right. is the throttle slide worn/damaged. do you have pipe coming from petrol cap vent

[WD Forte]

Could be anything

tappets too tight
valves need grinding in
bad fuel injector or ECU needs remapping
turbo vanes stuck
carbs need balancing
Points & condenser need setting/cleaning or replacing
wick in the engine needs trimming
or better quality paraffin

pick one
____________________
bikers smell of wee

[Easy-X]

[Easy-X]

[-]

[-]

[Easy-X]

Yes, that makes more sense It doesn't have an inner trumpet but there is a sorta upside down funnel on the outside of the box. It raises the air inlet to the highest point possible (just under the seat) but also offers some restriction. I was planning to ride it about till it started spluttering and then just whip the side of the airbox off to see what impact that had but the funnel might be a nice, low key alternative.

So, from what you're saying, I should probably go down from the factory spec rather than up, on the main jet Switching to premix would introduce another variable, would it not? I think I'm right in saying you'd up-jet in that scenario to compensate for the slightly thicker fuel.
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[steve the grease]

Mid range is needle. if you open it up and lots of noise but nothing else, it's probabaly weak ,try lifting the needle ( ie , clip in a lower slot.)
____________________
All the above is my personal opinion, you can see my lips move, but I'm talking out of my arse.
I've been riding, and fixing , bikes for 50 years, in that time the more I learn, the less I am absolutely sure of.....

[-]

[Nobby the Bastard]

[redeem ouzzer]

Plug chops don't really show much on modern fuels, especially if the plug is relativley new. If you've exhausted all other options you need to get the thing on the dyno.
____________________
Be a REAL MAN!

[Easy-X]

Ummm... I haven't actually got to fiddle with the bike yet, sorry to keep you in suspense This work from home lark isn't the slack-fest I was promised
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Easy-X]

Can't really go anywhere so I waited until the wife had some bits she wanted from the shops and err... volunteered

Got to one side of the town, no problems, got my shopping, headed back home. Just when I got to within 100 yards of my house it spluttered so first test: vacuum lock. I already had the key in the cap at the ready...

Seemed to work! So I took the cap apart to take a look at how it vents:

https://i.imgur.com/opUMb9t.jpg?1

I get the whole convoluted spiral deal but it all bottlenecks through a little 1mm (if that) hole. Is that enough venting?
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[-]

To maintain atmospheric pressure? It should be.

Even though fuel can flow pretty quick from the hose, the actual rate at which you burn it will be not more than the air that can get through the hole.

If you had a 1mm hole in a full tank, how long until empty, say 2 hours?

How long can the bike drain the tank in when ridden hard, say 2 hours?
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Easy-X]

I'll try again tomorrow if there's some need to be out and about. If not I'll wash the other bike, nice day for it they say
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[mentalboy]

What's with the mess in the centre of the cap? I'm going to assume rough casting but it looks more like someone took a file to it.

Have you got an airline to blow in the hole to make sure there isn't swarf hidden up in there?

Edit: Has the carb float height been messed with and could it do with a new float needle?
____________________
Make mine a Corona.

[Easy-X]

[Nobby the Bastard]

That does sound like a coil issue but with a single I'd expect it to die and and not splutter.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[mentalboy]

The age of the bike could mean it's a bit of everything in the initial list going wrong but because the bike responded well to the opening of the fuel cap I'd still be concentrating on the air-fuel regulation.
For which the earlier suggested removal of air filter for a test run is the easiest next step. Symptoms point to a rich mixture. I'm going to assume that you've already ruled out an air filter lagged up with oily dirt.
____________________
Make mine a Corona.

[-]

What's the flow of fuel like from the fuel hose? Does it start off well butr slow down if you drain off a litre or so?
Is there a tiny gauze filter in the carb inlet for the fuel hose?

It does sound like fuel starvation at the moment.
____________________
TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[mentalboy]

I've just had a look at your DT thread, nice, is that your float needle?

https://i.imgur.com/2GRvuCh.jpg?1
____________________
Make mine a Corona.