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.

You ran it without oil for a whole weekend? :eek1

I'd check the piston and bore. You can look into the intake and exhaust ports. Isn't the oil controlled by the throttle position? Hold it WOT and kick to get oil up to the intake boot.

I had a 1979 when I was a kid.

The rings/cylinder are probably trashed at this point but you can cross your fingers, dump the gas and put in premix to give it a try.

If it runs on that, you can let it idle along till the injector bleeds itself maybe? I'd try to get that working right. Having more oil won't do it any harm at this point.

Yep, ran it off and on for two days with no oil in the mix. The oil injection pump is indeed controlled by the throttle position. The throttle cable has a Y adapter near the gas tank and one cable goes to carb and one to the injection pump.

I have been kicking it and holding the throttle open but the oil seems to move incredibly slowly. I guess that makes some sense if it's supposed to be pumping roughly 1/30th - 1/50th as fast as the gas?

Thanks Navin, I will try that. I can't find the appropriate ratio for this bike since it is normally fuel injected. Should I error on the side of too much oil and try something like a 32:1 mix?

Before anything, you need to do a good compression test, and possibly a 2 stroke leak down test.

If you dont, you are just pissing time and money into the wind.

The trick to priming an OI pump without the old starter wheels is to let the bike run at idle (oil inj cover off) and pull the oil injection cable only wfo. The pump sounds like it's working but a little premix wouldn't hurt until you sort the pump. Generally they are bulletproof.

If it was smoking and didn't seize while running you might be ok. The old Enduro's were tuned pretty rich from the factory and a lot of them load up the bottom end.

All good advice...run 32 to 1 yamalube. Pull the hose off the carb (plug nipple on card) and let the bike run until you see constant oil out the hose. let me tell you...it isn't much oil but it is consistant. Most of the old DT's were jetted way fat...like 4+ steps fat. That is to your benefit right now.

If you haven't cleaned the carb, you should asap. My 2 78's (DT175 and DT250) start with 1 or 2 kicks and run clean (little to no smoke). A handful of jets, a clean carb and a little time works wonders. The 250 was a pig to start until the rejetting.

As for the running...there is a screen in the bottom of the petcock, the cap screws off with a 10mm wrench. See if it is plugged with debris (rust). This will happen with one that has sat a long time them made run again. Over an hour or so enough fuel passes through the screen to fill the carb, it starts, runs the carb dry, then won't start again until it slowly refills the carb. This is easy and quick. Also, pull the fuel line, turn on the petcock and see how much fuel flows...bet you it is nil.

Kenny

Thanks for all the replies everyone! I am back home so I will throw some pre-mix in the tank and see if I can get the pump to prime the upper hose completely while it idles. Thanks for the tip on the fuel screen too! I have tried to get some penetrating oil into the lever as it is extremely hard to turn so I wouldn't be surprised if there is some gunk and corrosion in there as well.

Ok, so I pulled the little plug on the petcock. Mine was a 12mm by the way. There was a wee bit of gunk in there and after I put it back in the lever turns much easier.

I filled the tank with some 32:1 Yamalube 2T and 91 Octane. I disconnected the oil injection line from the carb and wedged the oil pump wide open.

The bike fired up immediately. It was idling at about 4k so I started adjusting the air screw. Once I got below 2k rpm it started stumbling and I started hearing a bit of clatter from the engine. Is it normal for these motors to idle that high? Strangley the idle at 4k sounds slower than my 4t idling at half that speed.

Symptoms are the same it just bogs down and sputters when I try to gas it. When I ran it before I was in the desert at about 3,500ft. Now I am back around 7,000ft. Is it possible that I am just jetted way too high and the motor is flooding?

Thanks!

I disconnected the fuel line from the carb and turned the petcock on, fuel is flowing very freely. It would probably drain the entire tank in a minute.

If you heard clatter from the engine, I would assume it is smoked from running it without oil in in for 2 days. It won't just be the bore. More than likely is the lower end bearings. Prepare for a complete rebuild.

What's the spark plug look like? Sounds like a huge air leak. Leak down test in order. You haven't looked up the exhaust port yet??

I would not be surprised, I wouldn't expect a new engine to come out without damage let alone a 40 year old engine.

Spark plug comes out wet after it bogs and refuses to start again. Keep in mind I have about 3 hours and counting of experience working with 2 strokes

I have been meaning to pull off the header pipe and replace the gasket. When I have it off can I align the piston with the port and check out the shape of the piston and rings this way? Or am I missing the concept entirely?

I also have a compression tester on the way and will check that out. Low Budget mentioned a 2stroke leak down test, I don't know what that is so I'm off to Google for some reading.

Just watched a couple Youtube videos on the leakdown test. Very cool. All my riding buddies are on 4T bikes so it might take me a while to track down a tester.

Sounds like your crank bearings are toast! Dont bother to mess around with leakdown testing...............remove flywheel cover and checking for main bearing wear is to way to go here. Rebuilding the motor properly shouldnt be that costly, and parts are easy to find.

I would too look up the bearings first as Twin-Shocker mentioned, just take off the ignition cover then try to push and pull flywheel forward and backward, sidewards and up and down.

If there is some play when pushing or pulling the rotor this is still OK, if there is play when you push / pull sideways or up or down then the main bearings has to be exchanged.

While you have the ignition cover off you can test the crank seals, start the engine and then spray some WD 40 or brake cleaner between rotor and engine case and rotor, when the idling will change than you have an air leak.

Just to mention:
The Yamahas have some weird sound expecially the 250cc engines not really healthy compared to other engines, the 125 normally don't produces so much ratteling noise.

Please check you intake manifold for cracks too, also check the reeds that are in the intake, there might be some issues too like corroded plates, ...

Thanks for the info guys. I'll pull the cover off tonight and check it out and report back. I guess if worse comes to worst at least a full top/bottom end rebuild on this thing won't cost much, parts seem to be plentiful and dirt cheap.

OK do it,

btw. when it looks like this you have a problem :