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Old 09-06-2012, 07:19 PM   #1
RyanR OP
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Joined: Jan 2010
Oddometer: 113
Question 1978 Yamaha DT125E - Requesting Aid!

Hello,

I have been working on cleaning up a 1978 Yamaha DT125E that I purchased for $100 at a yardsale. The bike had been sitting for years when I got it so I drained all the fluids and refilled, lubed up what I could and took it out camping to see if I could get it running in camp. After about 50,000 attempts at kicking it she fired up and we rode it for maybe 30 miles off and on throughout the weekend. Fun Bike!!

After that weekend of tooling around I decided I wanted to get this baby running properly so I picked up a Clymer manual and set to work. The very first thing I realized was that the oil injection reservoir was missing. Being a complete 2T n00b I thought the system pulled oil from the transmission side since the pump appeared to be submerged in that case. Turns out it is in it's own little pocket and has nothing to do with the transmission oil. So I ran two stroke oil in the transmission and ran straight gas through the motor over that weekend.

I have ordered a new reservoir with oil level sending unit and in the mean time I made my own. I followed the manual and primed the pump and kicked the bike over. And it fired up and even idled! Well it did for about 2 minutes, I could rev it up and all looked good, as soon as I tried to take it for a spin it bogged down and died. If I let the bike sit for a couple hours it will fire right up with or without choke and idle. But as soon as I try to put any load on the engine and give it some gas it bogs and dies. After it dies the engine will not fire again until it has sat for a couple hours then the cycle repeats.

Obervations and things I have done so far:

The oil from the injection system is still not reaching the carb. Everyt time it runs for a bit the oil moves about 1 more centimeter up the tube to the carb but the bike is still running straight gas until that oil hits the carb. I couldn't find any info in the manual on priming the upper tube. It basically said prime the pump and let the bike run for about 5 minutes and it should be good to go.

I put a fresh battery in the bike (6 volt)

I have verified spark

When I pull the plug after the bike dies it is wet, is this an indicator of running rich?

If I pull the plug and clean it, dry it and put it back in the bike still won't fire.

The stock Mikuni carb exhibited the same behavior, I ordered a replica Mikuni off ebay and swapped it in. Other than the build quality on the 40 year old carb being a little better they seem identical and nothing seems to change swapping between them.

The bike leaked a lot of oil so I have replaced all of the case gaskets, new oil plug washer, etc. The bike no longer leaks any fluids but it didn't change anything else. I have not pulled the top end off or the jug and replaced any of those gaskets.

I have found a $10 oil injection pump block off kit... if oil mix is the issue I could just remove the oil injection and run pre-mix.

Short Version: After the bike sits for a couple hours it will fire up and idle. I can rev it in neutral just fine. As soon as I put it in gear and load the engine and give it some gas it will bog down and die. After it dies I cannot get the bike to run again. After it sits for a couple hours it will fire up again and the cycle repeats.


Any advice would be greatly appreciated.
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