74 DT175A won't idle . . .

[Stasch]

Hoping some carb guru's can help me out here.

I have a '74 DT175A I got last fall from a friend, along with a bunch of spare parts he had been buying on Ebay. I posted a question here back in the fall introducing myself and to find out the standard carb settings. Since then I've also come into some PDF manuals. Last fall, I got the bike to start and run, but the original petcock leaked so I wasn't able to check out much.

Having watched the winter go by without touching the bike (daughter's wedding this April) I finally got a new petcock installed on the bike to replace the old leaky one.

The little Yamaha starts and runs at first with choke (enricher) plunger pulled out. While riding it around the yard it runs strong if the revs are kept up (with choke plunger back in) and runs very strong at full throttle. It doesn't run as smooth as I think it should even though it runs strong. Even so, at full throttle in the power band, it scoots right along. It seems as strong as my 79 Honda XR 250.

If I try and turn the idle down below 3000 rpm it will die out within a few seconds. If I rev the bike to keep it going it will stay at the higher rev and doesn't want to come back down right away. Sometimes the revs will hunt up and down a little before they head downward until the bike stalls.

If I pull the choke out, the revs settle down to a pretty smooth and sustainable idle at about 2000, which seems odd to me. Leaving choke plunger out, I can adjust idle even lower with no stalling. Push the choke back in (off) and revs go up and hunt around until they come back down, with bike dying out.

Carb has been dis-assembled, dipped and cleaned with carb cleaner. All passages have been cleaned and blown out. I know my way around carbs fairly well, having rebuilt and cleaned multiple 4 bank carbs for 4 stroke street bikes. But I am missing something here.

At this time there is no battery in the bike since the one that came in it was dead.

I've made sure I have the right carb for this bike and this year (#44361 on the carb body).

The main jet (60) and pilot jet (25) are stock sizes. Floats have been set to 21mm. Needle has clip on 3rd groove (middle groove out of 5)

There is plenty of fuel flow and the gas is good.

The throttle cable is adjusted properly and the throttle slide moves up and down freely. With the bike off, I can hear the slide return back down with a click / snap when I turn and release the throttle.

I've started with 1.25, 1.75 and 2 turns out on the air screw and 2 turns out on the idle screw and tried to adjust from there.

None of this has made any real change.

Any help or advise would be greatly appreciated.

[YPVS TONE]

Sounds like the pilot jet is blocked. Unscrew it and you should be able to just see through it. If not try blowing through it in reverse direction of flow with compressd air or aerosol carb/contact cleaner.

[Stasch]

Thanks for the reply.

I've had pilot jet out twice, including just before I posted and it is clean, including the little holes in the emulsifier tube.

I'm going to look at it again, and also make double sure the passageway that the pilot jet screws into is clear.

Stay tuned . . .

[Airhead]

Thanks for the reply.

I've had pilot jet out twice, including just before I posted and it is clean, including the little holes in the emulsifier tube.I'm going to look at it again, and also make double sure the passageway that the pilot jet screws into is clear.

Stay tuned . . .

The emulsion tube is the needle jet here in UK. It's good you cleaned it out but what you said is a little confusing...it's not the pilot jet.

Heres a mucky needle jet

Pilot jet location ( somehow I feel you know this already)

Another thing to consider is

on some of the mikunis there is a small hole, the air jet. and worth checking its not blocked, this is an air intake that is regulated by the mixture screw

Last but not least there is a tiny passageway which goes between the carb venturi at the bottom just behind the throttle valve...and the cavity around the needle jet (I think)

[Stasch]

You are right, what I said was confusing - actually incorrect.

I've had so much going on that I did get the two confused. The emulsion tube is in the needle jet. I thought about this earlier today and wondered if I'ld gotten them turned around in my brain. (There's a lot of empty space up there, so its easy to do )

You pics clarify what I know the pilot jet to be. I need to get a good look at its passageway, as I did have it out and could see the jet hole was clear.

You pics are great. I will also double check the air jet opening and all the other passages you've pointed out again too.

[Stasch]

I have removed the carb, and cleaned all the areas again, including pilot jet and the small openings as shown in oldgitonabike's post. This involved poking wire and making sure air and light gets through the openings.

Things have improved but I still cannot not get the bike to idle on its own below 2000 rpm's without eventually dying out or get it to stay at a steady idle.

Any suggestions are welcome.

[Airhead]

I am having a hunch about the crank seal left side, is there any trace of fuel behind the points cover?

Also check for splits in the rubber inlet manifold, you can spray WD40 on it when running to see if it affects anything.

[Stasch]

oldgitonabike was right.

I replaced both crankshaft seals and the bike now idles between 1100 - 1500 rpm.

The left crankcase seal was so worn, it practically fell out on its own when I removed the rotor and condensor / points plate.

Started up first kick after reassembly of everything. Runs great. Couldn't buzz around for long. Only 33 degrees F and my eyes kept watering up.

Thanks a bunch to everyone for helping out on this.

Also replaced right engine cover gasket, shifter, shift shaft and kickstart shaft seals, all of which were leaking oil.

Now on to cleaning other stuff up, turn signals, cosmetic stuff, etc.

[Airhead]

Thanks for the update stasch, glad you have it sorted now.

Have fun on the little DT