Yamaha 1993 DT175 Jetting??

[Qwerty1234]

Bought a 1993 DT175 for $500, it runs and it rides (rough) so I figure not a bad deal. Most of the important bits are there and working its just that everything needs some TLC. I plan on doing a full strip (minus bottom end if I can help it) and rebuilding it right

At the moment i'm tackling the jetting and i'm having a hell of a time finding information. There are plenty of DT175 models and i'm not familiar with them so don't know how interchangeable bits are. In addition to that, there seems to be a few different carby combinations. And none of them match what I have

Most (all?) seem to run a Mikuni VM24SS but with different combinations of bits, and a different code stamped on the carby. I can find a lot on 3U500 but this one is stamped 18L00 and a google search yields nobody with that.
The pilot jet is #20
The main jet is unknown, lot of corrosion on top of it and cannot be determined, best I could decipher is a '0'. Could be a 160? but is a wild guess
The needle is stamped 4L6 whereas everyone else seems to have 4J6? sitting on middle (3rd from top)
Throttle slide is marked 1.5

When I got the bike it would not run without the choke on, given how much neglect the bike had I figured yep blocked carby, sure enough the pilot jet was completely blocked. I stuck a couple bits of copper wire through and cleaned it best I could, can see daylight now. Blasted the rest of the carby with brake cleaner and a dremel to loosen all the **** off. Also replaced the spark plug. It will now idle with the choke off but it's still a lot happier with it on. I've wound the air screw all the way in (richest) but it sounds very lean, blipping the throttle the revs will hang and slowly drop, blip it too quick ( read as anything faster than slow) and it'll hesitate. Choke on it'll idle better, won't hang as much, more willing to rev quickly. If I start screwing the air screw out, 1 turn its not happy and will eventually stall

I havn't jetted a 2 stroke umm....... ever. I'm happy to replace both jets and needle but i'm not sure what jets to get. At the moment the bike is stock intake with stock exhaust but restrictor removed from it.
I plan on possibly replacing the intake with a pod filter however at this stage I want to fully rebuild and rego the thing so i'm happy to tackle that later if its easier to work with - and jet for - a stock airbox

This bike post-mixes the oil with fuel at the engine side of the carby via a small tube, i'm under the assumption its working ok though I guess if its pumping too much oil (no idea how to check it? but it is pumping) it could be leaning out the mixture. Nevertheless, anyone have any experience with these or can recommend jets OR a new carby if thats going to perform better?

 

[my67xr]

i found these specs on the early (1981 DT175) carby, matches up with what you have.

Mikuni VM24SS, Number 3U500
Standard Set up with factory porting, standard exhaust and air filter
Throttle Slide cutaway 1.5
Needle, 4j6 Clip position 3 from top
Needle Jet, 267 0-8
Main Jet, 160



Pilot Jet, 20
Reed side diameter, 35mm
Filter side diameter, 43mm
Length, 85mm
Float Height, 21mm



i would pull the carby down and re-clean the jets,
make sure the hole where the Pilot Jet screws into, down at the bottom of the thread in the carby base is open, and that you can see light through it inside into the carby bore.

undo and remove the Needle Jet, the tube that the main jet screws into, check that it isn't dirty inside the carby, and the holes across it aren't blocked up

while the jets are out, blow some compressed air into the air filter side of the carby through the 3x ? small ports.
check you can feel air coming out on the bottom jet holes of the carby.
also remove the Mixture Screw too when you blow it out, you could have a blockage between that and the Pilot Jet



is the air box modified at all ? are there extra holes in it ?

check your carbys' rubber manifold for any splits.


if you can give me the frame number from the head tube i might be able to find a service manual

 

[my67xr]

any luck with it ?
it is strange why they fitted a 4L6 being that it is leaner ?
is the battery in it ok ? showing 12.6v or more

post up some pics too.

cheers, craig

 

[Qwerty1234]

i found these specs on the early (1981 DT175) carby, matches up with what you have.

Mikuni VM24SS, Number 3U500
Standard Set up with factory porting, standard exhaust and air filter
Throttle Slide cutaway 1.5
Needle, 4j6 Clip position 3 from top
Needle Jet, 267 0-8
Main Jet, 160



Pilot Jet, 20
Reed side diameter, 35mm
Filter side diameter, 43mm
Length, 85mm
Float Height, 21mm

if you can give me the frame number from the head tube i might be able to find a service manual

Actually it doesn't quite match up. Needle is NOT 4J6 it's actually 4L6 (yes I double checked its definitely oriented as an L not a J) and the carby is stamped 18L00 on the right side (small square block), as is the cylinder head near where the carby meets.

Here's a pic, main looks a bit blocked in the photo but thats just something in the background, both are perfectly clear

Here's another in an attempt to find out the main jet size, and no its not much clearer in real life

That said I did forget to pull out the air screw entirely when I cleaned it. So I took it out and yes there was some crud on it and in the hole. Blew it out and then fit my new super lightweight fuel tank (patent pending) and had a quick test

It's not perfect but its better, still prefers to idle with choke on and there's still some slight hesitation. That said I havn't had a chance to give it a real good blast yet. The sprockets are so worn it skips teeth, I need to wait for the new chain and sprocket set. But, it feels good through the midrange

Can't check battery yet, multimeter stopped working

I noticed a bit of a discrepancy with stamped numbers.
2 numbers on sticker say
1EJ-14610
3FJ-2118G-?? (corner worn off)

Yet the engine is marked
1EJ-034475
The cylinder has 'Y-2' on it as well

Is it possible this is not a DT175 engine and/or cylinder head and is indeed something different? it actually seems to go quite well and is a bit 'powerbandy' for a little farm bike, maybe an IT175 engine? could be wishful thinking though...

And I just noticed... what the heck is going on here?

Looks like that pipe going into the corner of the engine has been dremel or hacksawed off?

 

[Qwerty1234]

P.S. airbox does has small holes drilled in it but they were covered with tape (so essentially stock still). As noted I believe the bike has been derestricted to some extent. I've cleaned the air filter which is still in reasonable condition, good enough to re-use

also checked for splits or cracks in the rubber, nothing that I can see it looks quite good. The intake from the airbox is also fine

 

[my67xr]

the dremel cut off tube at the front is for the tacho cable to connect to.
so it's not an IT engine.

the bikes were imported,
can you read the frame numbers that is stamped into the head tube, not the compliance/vin plate



i will have a look into the numbers after tea, see what i can find.

 

[Qwerty1234]

The only thing on the head stem is the VIN, along with the compliance plate

First 8 digits are JYA1EJT0 (last digit looks like an 'O' on the compliance plate, but looks much more like a '0' on the head stem etching)

 

[my67xr]

this is the closest one i can find to your bike- 1992 Yamaha DT175 Service Manual

might pay to check your timing too,
i have heard of a couple of early 90's DT175's shearing the keyway for the flywheel

 

[Qwerty1234]

nice work, that one matches up. Guess its a 170 main. Thank you, that manual is also a lot better than the one I got which was from a Yamaha website (The DT175E) - which doesn't explain much at all

Will check the autolube system. I have a feeling its too rich and that will effectively lean out the bike. Not sure how to do timing, I don't have a timing light. Any other way?

If I was to adjust the jetting what sort of increments for a pilot/main? +/- 2.5/5 respectively or is it likely to be a larger jump at a time? again - havn't ever tuned a 2 stroke

Given the bike can actually idle without choke now i'm going to leave it for the time being. Still have to fully strip it down to a bare frame and build it back up again. May not pop back in for a couple weeks
When its looking the goods i'll throw some pics up, or if I get stuck again

Thanks again

 

[my67xr]

the Main Jet's go up in 10's so #160, #170, #180 etc
Pilot Jet's in 5's


i didn't look through that manual,
but if it's anything like my IT200 timing, it is set at something like 2mm from TDC
you rotate the engine with the spark plug out till it is close on tdc, place a bit of dowel down the spark plug hole and sit it on the piston,turn the engine a bit more till the dowel starts moving back down the bore then turn the flywheel back to tdc, when the dowel doesn't move.
then adjust the stator plate so the marking sits 2mm away from the tdc marking.
so wont really need a timing light.


also give the expansion chamber a good burn out with a gas torch, and some compressed air though the pipe.
it will make a huge difference once it is cleaned of all the oil and carbon.
and repack the muffler too, they need a bit of back pressure

 

[my67xr]

any updates for the DT yet ?

 

[Qwerty1234]

won't be for a little while. I'm tackling this with a friend as a full overhaul project so we have other things to attend to before reassembling

 

[my67xr]

ok, no probs

 

[Qwerty1234]

Front end done, rear end done, drivetrain done, fluids done. Wiring still a mess but thats the next job

Started it up in the meantime and holy crap it runs just fine!
I suspect my super lightweight fuel tank. Which didn't take into account the vacuum line so it was sucking extra air. Runs great now though, albeit very smokey so i'll have to wind the screw out and maybe lean out the needle.
Should hopefully have all the wiring redone with all the RWC gear on by next week, with the rest just being cleaning, touch up paint and polish

 

[my67xr]

the smoky start up and running after having it sit for a while is a 2 stroke thing
because when you turn it off there will still be oil/petrol in the engine, when the fuel evaporates there will still be oil, in the bottom of the cases, rings, exhaust etc etc

so it wont take long to burn it out with a short ride, then if it is still smoky i would look at the mix ratio
my IT clears up after about 50 metres

 

[Qwerty1234]

I've had 2 strokes before just never tuned them. I keep thinking its premix but its got an auto lube system, will need to check and adjust that as i'm positive its using too much oil and yes very smokey, lot of spooge too though I still havn't been able to really hold it tapped to clear the system out. I know well what its like to have a gunked up 2 stroke