DT175 low km carbon buildup

[kiln]

kiln can you give a little more detail we need to know all the facts

what is the bike and its year

this piston...is it new and has never been used?

I humbly appologise Oldgit.It is a 1979 DT 125. Standard barrel and standard piston size,used but rings end gap still within tolerance.Compression tested at 92PSI.As mentioned earlier piston is a Wiseco with halfround open port on piston skirt,previous "new" piston with twin port broke a ring near the exhaust port thus installing the Wiseco that I recieved with the barrel off a purchase made in the US.I have opened the exhaust port a little and polished it with a Dremel.Carb is a new Mikuni replacement from Japan and behaves exactly the same as the previous original Mikuni.Easy starting,no idling and idle screw screwed in fully. Intake rubber new without any leaks, tested with carb cleaner sprayed. Any suggestions or should I strip down the bottom end?

[Airhead]

so it has never successfully run with this piston am i correct?

[kiln]

It has run and when ridden, no problem, 96km/h, Ijust cannot get this setup to idle though. i have tried 22.5 and 25 pilot jet, no success.

[Airhead]

Why did you change the carb?

can you try the old one again which presumably worked well?

[kiln]

I will change back to the original carb and post findings when I have done that.I changed it because i suspected it to be worn too much for it to idle properly, now it is the same with the new carb which is why I suspect the piston design, do you know if it is an "AHRMA" design?

[Airhead]

I will change back to the original carb and post findings when I have done that.I changed it because i suspected it to be worn too much for it to idle properly, now it is the same with the new carb which is why I suspect the piston design, do you know if it is an "AHRMA" design?

i'm getting confused now, first i was suspecting the piston because it was new...but then the carb because it performed well enough, now to find that the piston caused trouble with the old carb idling and subsequent replacement didnt help!

yes so we may be back to the piston now!

go back a few steps...where is the old piston,,,the one you ran with?

[kiln]

Piston got damaged when a bottom ring broke on one of the transfer port edges. I have subsequintly ever so slightly broken the sharp edges of the transfer ports that might have caused the ring to broke. I only had this Wiseco piston to run with, whilst I am waiting on a replacement from Yambits, 20 days delivery. I do not think the porting is an issue though since many recommended it here and on other forums.That is why I suspect that I must live with the no idling untill I can install the proper piston.I thought maybe someone else also had a similar experience.

[Airhead]

Piston got damaged when a bottom ring broke on one of the transfer port edges. I have subsequintly ever so slightly broken the sharp edges of the transfer ports that might have caused the ring to broke. I only had this Wiseco piston to run with, whilst I am waiting on a replacement from Yambits, 20 days delivery. I do not think the porting is an issue though since many recommended it here and on other forums.That is why I suspect that I must live with the no idling untill I can install the proper piston.I thought maybe someone else also had a similar experience.

1 Broken ?...Not slightly dressed with a file?

2 But you still have the old piston that broke a ring?

Compare it with the wiseco, slide the pin into each piston side by side and compare the top, bottom of the piston and the top of each inlet cutaway in each...they should be exactly the same dimebsions

[kiln]

1="broken"=slightly dressed with a file to broke the sharp knife edge.

2=I do have the old piston and they both match except for the transfer ports on the piston skirt.

And so the plot thickens with an answer and weird solution and a question. Allow me to elaborate.I have 3 carbs:A)old dt 125 carb B)new chinese carb and C)new mikuni carb.

A) and C) gave the same results (bogs down and cannot idle even with idle screw fully turned in) but only B)works.

Turns out that carb B)is a Keihin knockoff with an interresting setup: 5G4 Jet needle,O-2 Needle jet, 160 main jet,25pilot jet.This is exactly what is listed in this thread: (Thanks Cynic) listed for a Yamaha DT175A/B. Bike idles easily, can be adjusted and the idling goes up when choke is pulled. Maybe I will go leaner to 140/150 main jet, still need to test ride further. So my thinking is this:Order a 5G4 jet needle and O-2 Needle jet for carbs A) and C) and have 3 working carbs rather than only one chinese carb.

Which brings me to the following question:Where to buy these items from and import them to South Africa?

All suggestions welcome.

[Airhead]

but the link is for a 175 yours is a 125, what jets are in your original 125 carb and what is the id number of it?

[kiln]

Original carb has 125 main jet and 22.5 pilot jet, Y jet needle , 2.0 slide and carb has a 4hc20 stamp on it.

I have found a solution though. I bought this carb (b in previous post) and installed a 25 pilot jet, 160 main jet and 2.0 slide with 5G4-3 jet needle( http://cgi.ebay.com/ebaymotors/_W0QQcmdZViewItemQQ_trksidZp3984Q2em1439Q2el2649QQitemZ180408901679QQsspagenameZSTRKQ3aMEWNXQ3aIT ).

Now the bike can idle easily and even the choke won't bog it down. This Kei Han Taiwan replacement carb is highly reccommended since with a bit of tweaking it can even be used for a DT175(Tested on DT175 and now on DT125) .

Thanks to all that is always willing to assist with advice.

[kiln]

Explanation of 2stroke additives in more detail.Article for R/C Aircraft but still relevant.

http://www.geistware.com/rcmodeling/articles/oil_test/index.htm

Enjoy the read.