Hi all,

Looking for suggestions on a 'hanging' idle issue. It's a 1980 DT175, stock airbox, carb, reeds and the like. 

When the bike has been running at higher revs it will occasionally stick at about 5500 rpm, even with throttle closed. Regardless, it will always (when its been ridden at higher revs for a bit) drop down to about 2600 rpm and stay there for anywhere from 3 to 15 seconds before quickly dropping right down to about 1500 rpm (where the idle adj is set).

Don't know if a video link will work, but here it is (not the 5500 bit, kinda hard to vid that lol).

drive.google.com/file/d/1j61WtOrYsI92Yod...MgJ/view?usp=sharing

I have limited carb experience, but do have some with lean issues - this doesn't sound like an erratic idle, to me or fluctuate or slowwwwwllly drop down, like other lean issues I have had. A different online site suggested this could be solved with a larger main jet - I've gone up once size, no difference.

What I've checked (doesn't mean I did it right
clean rust free tank
32:1 bypassing the oil pump
new air filter/ oiled
new plug (B9ES)
OEM carb cleaned and new OEM pilot and main jets (22.5/150)
float valve assembly new OEM
float at 21 mm as per spec
needle at third clip as per sec
throttle slider snaps down solidly every time. 
cable no kinks tried turning bars lock to lock stationary no change
sprayed intake / manifold boots carefully with carb clean listening to idle no change

I want my kids to be able to ride it, but worried till I get the high revs without throttle solved.

Any thoughts?

Cheers!

c
 

I've seen that more than once on that model.  Change the ignition side crank seal.  Use the double lip seal from the 77-79 IT175.  Part # 93103-25125

will do and thank you for the suggestion Mark! fec, can't believe I didn't think about the crank seal - sheesh.

on it and will post an update in a more timely manner than my last one lol

appreciated, as well as for the idea of a double lip seal (and part no vs. the single stock one for the DT

cheers!

 

I was fighting that exact same erratic idle problem on my 76 DT175C and the experts here suggested checking the ignition side crank seal. I knew it was bad as soon as I removed the side cover and oil poured out. The seal was actually missing about 30 percent of the lip. So I checked the clutch side seal and it was the same, missing a big portion of the lip.

I would like to confirm that the new seals have solved my idle problem, but I have now lost spark after putting it back together…… I am sure it has something to do with the previous oil bath that the flywheel was spinning thru…….

I will get it resolved……

Sounds quite lean in the video at the fast idle it hangs at.
How's it for comp feel on the kick lever.
If soft on comp you'll end up with idle speed screwed up to get it to hold an idle--speed it finaly slows to--but that makes it hang higher coming off the throttle as shown in the video because you're encouraging a false idle either due to leanness or low comp-or both.
If it wasn't a cdi "80 dt175 i'd say it sounds like an electric start AT/DT125 which is hanging at the point of the governor pulling it back, which it sort of sounds like when it finaly idles.
Without watching it & loosing my typing i think when it comes right down it sounds low on comp to give a nice lumpy sort of idle--possitive idle. Especialy being a 175, should have a good pulsing idle. I guess that doesn't account for hanging at about 5500rpm i think i saw earlier in your post. Could try a heavy throttle spring as an experiment.

Hey SCHU - nuts to hear re oil and flywheel, hopefully you can get it cleaned up quick - seals deteriorated w age maybe? I'm hoping to get a new seal in next couple of days, going to ignore the clutch side for now try to only change one thing at once or I get confused lol

RT325 - I hadn't thought lean due to compression, but it is comparatively fairly soft on the lever - good idea re spring, thank you - I happen to have a heavier one sittin on the bench right now -

If timing and weather works out, I hope to spend a few hours testing things later this week

Cheers!

Not lean "due to compression", that's two separate issues. Compression "lack of" making it sound like a laboring idle. & lean idle mixture just making it sound just 'lean' at the high idle it starts of at in the video. Almost getting to where the slide cutaway would affect it but if low on compression then that's best sorted first. 1980 DT175's by long term memory had rings made of cheese. Think you could tie them in a knot. Some fancy metal but didn't last well although most of ours "yamaha shop where i worked" went to farmers so probably sucking dirt etc.

A 175 of that era is pretty easy to kick over.with not a lot of resistance when in good condition. Easy enough that you can often use your hand on the kickstarter to start it if you really try.

You certainly don't feel "coming up on compression" nearly as easily as you do on the bigger bikes

Make sure, the throttle cable on the oil pump and the cable wheel on the pump spins freely, and returns back in idle position
Just a thought, my 250 had this issue....

Hey Wessi, good idea, I did check after you posted, all good there - cheers!

An update fwiw - I changed the seal (local dealer got it next day), and when thrilling my neighbours with testing it (I live in a residential area) it, the clutch started spinning loose at about 4500 rpm, and no cable adjustment would help.

So the delay was in ordering and waiting for clutch friction plates and new springs. Got those installed today, and its working again. The old ones were in spec, but glazed and with a fair bit of corrosion. couldn't' find replacement steel plates, so I cleaned and reused what I had.

Can't get to the trails for a few days, if it's warm enough I'll trail test it Monday (low 70's in Toronto today, low 50's Monday

A few pics - the seal, with a gentle tug or two after freezing, popped out. 

The clutch side cover bolts needed a manual impact driver for most, a powered one for 2, and I sadly stripped one of those / drilled head off / extracted and steel epoxied the cover, but it was the one inside by the oil pump...

drive.google.com/drive/folders/1TwZPCbIF...Sqv4dtQQ?usp=sharing