1978 Yamaha DT175

[oisact]

Hi all. I bought a 78 DT178 this summer that's practically new (1400 miles). I really enjoy this bike, and I've put around 500 miles on it already. However, starting recently the engine wants to die once the bike gets hot.

Hopefully these symptoms will ring a bell with you experts. I have to ride the bike pretty hard (winding it out on-road) for a good 5 minutes or so before it acts up, which is what makes me think its temperature related. I can ride it around the yard, wind it out up and down the street, and it's fine until I get out on the road and push it harder for a while.

The problem is the bike will suddenly cut out, or just barely run. If I can keep it running the engine sounds muffled - it doesn't have the normal crisp bright sound. Something else that makes me think it is overheating is today it died about a block from the house, and I couldn't get it started. So I pushed it home, and when I tried it again 30 minutes later it fired right up.

One other thing is I hear a rattling sound that I've not noticed at any other time, that starts up just before it dies out. It sounds like it comes from the combustion area of the engine, but its hard to tell.

The spark plug looks good, and I'm pretty sure it's getting gas (I cleaned the carb a couple times just in case).

Electrical in general seems to be fine, because the headlight stays nice and bright even when it is running like crap (and I don't have a battery).

Oh, one other thing. The first time I noticed this problem was when I was on a long 20 mile off-road ride on a mountain. The last 8 miles was all downhill, so the bike basically idled the whole time, and I'd let the engine hold me back a bit with the RPMs around 4-5k. Well, it didn't like that at all after a while. That's when it first started running bad and dieing. Even if I held the clutch in it still didn't want to idle right. At one point I swear the thing must have started dieseling. If I held the clutch in the RPMs would shoot up to 8-9k, and even after I killed power it kept running (but sounded really muffled) at those RPMs until I let out the clutch and stalled it.

I'm pretty sure this bike has a reed valve, and the rattling makes me wonder if that could have anything to do with it? Can a reed valve get too hot, or get to the point it can't tolerate heat like it should? Would it rattle, and cause the bike to run like this, or am I barking up the wrong tree?

Any help would be greatly appreciated!

Dan East

 

[2strokerfun]

That is a bad sound and a warning you are overheating and probably too lean. Your piston is getting too hot and it is a warning sign you are about to melt down. Sounds like you are running a little lean. What is your gas/oil mixture?? If you keep doing this, you will need to rebuild the top end because--even if you don't melt a hole in the piston--you are wearing it out and will have too much clearance. Check your float level and needle setting in the carb. This started happening to my Elsinore the other day. After it happening twice, I started to pull the carb off to adjust it and noticed no fuel came out of the gas line, even with the petcock turned on. Turned out my gas cap vent was clogged, starving the carb after riding about 25 minutes. Be sure and check the vent on your tank as it will cause your bike to start running lean and do this after a bit of riding.
One other thing to check is for an air leak. The idling problems you noted may indicate a bad stator side seal, which will do exactly what you described. Again, the lean condition causing this can melt your piston. Do you have a little smoke, no smoke or a whole lot of smoke when you ride??

 

[oisact]

It's auto lube, so I don't premix it. The 2 cycle oil pump seems to be working, because I've burned through 3 tanks of oil so far. It has a warning light when the 2-cycle oil is low, and I just filled it 100 miles ago.

I'll check the cap vent. Interesting you mention that, because I thought I had the problem solved yesterday. I was looking for damaged wiring, and noticed the vent hose from the carb had gotten against the exhaust, which melted and sealed the line closed. I figured when I was running with the throttle opened up for a while that a vacuum might happen in the bowl. Then if I let it sit for a while it would gradually equalize. However when I test rode it the same thing happened again (unfortunately I had my first road kill on that little ride - hit a groundhog at 55 with my back tire).

Now the bike runs perfectly until it starts acting up all at once. It idles fine, has plenty of power, etc. Then just like flipping a switch it bogs out.

It smokes like it always has. There's some smoke when starting it up cold, then after warming up there's just a little bit of smoke noticable right at the exhaust.

I tried to take some pics of the spark plug, but I don't have my digital camera and the camera in my phone sucks. Anyway, to me it looks perfectly fine - it's a dark chocolate brown, including the insulator. It doesn't look like a plug that is running too hot. Well, I tried to post pics (however crappy they are) but the forum won't let me.

Dan East

 

[oisact]

I checked the cap vent and it is fine.

I checked the plug again, and it looks absolutely perfect. On this chart:
(unable to post url to chart because of SpamKiller - can someone please flag my account as not being a spammer?)
Well, here's the url with a comma instead of a period, seems SpamKiller will allow that:
dansmc,com/Spark_Plugs/Spark_Plugs_catalog.html

It looks like picture #14 - a nice chocolate brown.

If it were running too hot I'd really expect the spark plug to show it.

Dan East

 

[2strokerfun]

Well it could be some dirt in carb which eventually clogs main, leaning out mixture--but your symptoms seem too consistent for that. I'm guessing you have a leaking seal letting air lean your mixture after motor gets really warmed up. Have the seals ever been changed, to your knowledge?? Be sure and check all connections from carb to cylinder for air leaks too. Spray some WD-40 around carb intake boot while idling and see if there is any change. Good luck, I've been through all of this on a 1976 Elsinore MR250 in the past 9 months. I finally, just split the case, replaced main bearings and seals. If you do this, you'll need a complete gasket set, main bearings and seals. Should set you back about $100 in parts. Although your bike has low miles, it is the 28 years that hurt the seals if they have never been changed, not so much the miles. If you do end up splitting the case, you'll definitely want to inspect and measure the piston and bore while you're at it to insure they are in good shape and within tolerances. Might be that your top end is just getting a little too loose.

 

[lub997]

Do you have rust in your gas tank? Rust will cause a bike to run fine and then all of a sudden cut out like that. When you first fill your tank, the gas is clean and the bike runs great, but once it sits in the tank a while it absorbs the rust from the sides of the tank and turns from gas into rust water, and often there is very little sign of this when you clean the carb because the rust will be absorbed so well into the gasoline that there are no large flakes. It will even go right through a filter in your gas line. The best way to check for this is to remove the gas line at the point where it slips onto the carburetor and let some fuel drain out and see if it's clear as it should be or if it's brown with rust. Also, a plug from a good running engine should be more of a light tan than a chocolate. Spark plug oil fouling will also cause the symptoms you described, but if your plug is brown, I don't think that is your problem, because it was you would have black gooey spark plugs.

 

[oisact]

Now you may be onto something. I was about to post an update to the problem anyway, and what you said fits right in.

I had to ride the bike Sunday, so I pulled the carb and cleaned it, replaced the spark plug, and took a look at the reed valves. Yes, there is a good bit of rust in the tank, and when I cleaned the carb it had just a tiny amount of rust dust in the bowl, mixed in with a small bubble of water. I hopped on the bike, and rode a 50 mile round trip (all on-road), crossing a mountain both ways with a climb of around 1500 feet. I rode the bike hard, and it didn't miss a beat. On the way back I led my Dad (he was on a 1100cc Shadow) and we stopped at the top of the mountain for a minute, where he told me my bike could really move on down the road.

So I figured maybe the plug was bad internally, or that little bit of water somehow was causing the problem.

The next day I topped the tank off and confidently headed down the road, only to find after 2 miles or so that the problem was back - rattling, buzzing sound, then it would bog down. I pulled over and the bike dieseled up to red-line again like it had before.

I rode it really easy the rest of the trip (70 miles), and if I kept the RPMs down the problem wouldn't surface. I cleaned the carb again that day and there was just a bit of rust dust in the bowl that time too, but no water at all.

So I know it wasn't the few drops of water or the plug (new plug looks perfect still). So can the rust cause the rattling noise (I assume it is the reed valves?), and can it cause it to diesel?

If so then that makes perfect sense. When I fueled up it must have broken some more rust loose in the tank.

Dan East

 

[OldTimer]

Maybe your fuel supply can't keep up with demand when you run hard for long distances. Clean out the tank and petcock. Make sure the fuel flows freely from the fuel line and that it isn't occasionally disrupted by trash in the lower portions of the tank.
The high revs sound like an overlean condition to me as well.

 

[oisact]

Well, the problem is more RPM bound than fuel bound. On a straight stretch I held the throttle steady (low throttle, maybe 15% or so) and the bike gradually wound up to around 7k RPM at close to 55 mph. The rattling sound started and the bike started bogging out. I've checked the air filter and it looks like new. I can open the throttle all the way up and it runs fine, as long as I don't run over 6k RPMs for very long.

I'll clean the rust out of the tank this week and see if that helps.

Dan East

 

[OldTimer]

Opening the throttle wide open doesn't use up the fuel in the bowl unless you hold it wide for a long time in which case the RPM's would naturally wind up. It's the long drawn out steady pace that would eventually sap the reserve fuel held in the carburetor float bowl if the supply were somehow restricted.
If the fuel is flowing into the carb at a trickle then it may well keep up with demand until you're tooling down the highway for extended periods.
Clean out the petcock.
And make sure the float valve opens all the way and is unrestricted.