DT120c 1976

serge · Jul 31, 2009

Hello, this is my first post on this forum!

I bought 1976 Yamaha DT125c last week. It was not registered since 1979 and fell off DMV records. However, I got it registered today and got new CA plate ! Now this is one of few left 2 strokes licensed for the street use in CA :cool: I am afraid though that bicyclists in my college town will start throwing stones at me because of smoke :whoa:

The oil pump was plugged so I put 16:1 ratio mix in a gas tank to make sure it will get enough oil.. Can I use less oil? Is oil pump prone to fail in these bikes? Is there any place to get part besides e-bay? I got pdfs with Clymer and Hanless but I would prefer Yamaha service manual for this model. Is it available somewhere?

Thanks for sharing the info!

BadgerMan · Jul 31, 2009

Cool bike….is it electric start?

Oil pumps are generally very reliable. If you cannot get it fixed, I would mix fully synthetic two stroke oil at 40:1 and not worry about it. Correct me if I am wrong but I think the DT’s will lubricate just fine with premix.

If the parts are still available, any good Yamaha dealer should be able to get them. Otherwise, try bikebandit.com. Good luck with the manual…..you will most likely have to keep an eye on that auction site.

BTW, if a Lance Armstrong wannabe were to throw a rock at me, I would take him out with a brake-check! :laugh:

serge · Jul 31, 2009

Thanks for the idea! I guess if I want to make less smoke and use synthetic oil, the stock pump is useless anyway :cool:

Enduro_Nut · Jul 31, 2009

I have my '76 dt400 I bought new. I disconnected the pump within 1 week of owning it due to failures - even thought it is rare I have seen it happen. I run 50:1 full synthetic in all my bikes(vintage or modern). When new I ran 40:1 with mineral oil(Golden Spectro). With any mixture I would make sure it's jetted correctly.

Mad Norris · Jul 31, 2009

If there are 2 pipes coming from the oil pump (one to the barrel, and one to the crankcase), then you should check to see if the pump is working, and use that instead of premix. The pipe to the crankcase lubricates the left hand main bearing, but it won't get enough oil if you use premix.

There's a good chance that the pump is still working, as many people disconnected oil pumps needlessly (just in case) and ended up having to replace the LH main bearing instead.

serge · Aug 1, 2009

BadgerMan, it is both electric and kick start. After I could not find the starter, I was surprized to learn that it uses generator as electric motor to start the bike. I wonder why it does not work this way on the 4-strokes thumpers... High compression maybe..

Mad Norris, it seems there is only one inlet and one outlet of the pump connected to the carb, so I guess it would lubricate fine with premix. I think the pump would put more oil in then 1:50 with synthetics? From what I found the premix on older yamahas was 1:15?

Unfortunately, my cleaning with rust remover etched through the tank.. so I have a leak now in the upper half.. So I do not know if I should go with JB weld or try something more fancy like POR-15 gas tank sealant.. That stuff is expensive though ..

Another thing I experienced on intersection after first 10 minutes ride at 50 mph was engine running away to 8k rpm continuing for a short time after I hit engine stop. I could not believe it happened and thought I did not pressed the stop correctly. However, now I think about it, it may be dieseling with all carbon in the chamber doing work of ignition.. Is it something which happens on 2-strokes or I am just :coocoo: Where did the air came from when I closed the throttle for these high rpms...

Mad Norris · Aug 1, 2009

Sounds like you have an air leak somewhere. On a '76 bike it's likely that the LH crank seal has perished, but it can be replaced without splitting the crankcases. Just be very careful removing the stator and armature.
It could also be a leak at the carb manifold, or cylinder base gasket, but my money's on the crank seal being blown.

The lean mixture causes the engine to run much hotter than normal, so the running-on is probably caused by red hot carbon on the head. This should stop when you fix the air leak.

sm7482 · Aug 2, 2009

not only that you should decarborize the piston soon, its somthing that needs to be done. alot of people dont and endup with engine problems. also to answer your questions about the starter system i read an article on that, they only used it on small displacement bikes for some reason heres a websight that will help out. http://www.dansmc.com/mc_repaircourse.htm

serge · Aug 8, 2009

oil pump

I can not figure out where I supposed to connect the outlet from the oil pump.. From manual on earlier models it seems that it should go directly to the cylinder block, but there in no banjo bolt on mine.. there are two connection on carb, but one seems to be the vent and another connected to the cranckase, but it does not seem anything to do with the oil pump.. Maybe the cylinder was swapped?

helio lucas · Aug 9, 2009

usualy the oulet tube from oil pump goes here. at least in some late models from 80+ :cool:

Mad Norris · Aug 9, 2009

The crankcase vent hose should not be connected to the carb, it's just meant to be open to the air (or you can get a dust filter for the end). I think the oil feed hose connects to the carb where the crank vent hose is now.

DT120c 1976

serge

BadgerMan

serge

Enduro_Nut

Mad Norris

serge

Mad Norris

sm7482

serge

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