hotexhaust 1 hot exhaust, 1 cold exhaust

[Steve Sutton]

Apologies for the long post, but I’m including as much info as possible.
I have a 2-Stroke Yamaha TDR250 1KT, same motor as the TZR250 apparently.
I just completed a top-end rebuild. I only replaced the left piston as the right one look good. Probably should have replaced both, but anyway…

My problem is the right cylinder/exhaust gets very hot very quickly and smokes a lot. The exhaust is hot only after 30 seconds of idling. The left remains room temperature and does not smoke.

I don’t want to ride it too much in case it causes irreparable damage. But when I did try to ride her, I can only get a few hundred meters before she cuts out. I have to start again and can not get into 2nd gear before it stalls again. I’d have to limp home at a bicycle’s pace as anything above idle revs kills the engine.
Not sure if the poor running is related to the hot exhaust, or another problem entirely.

I’ve done the following troubleshooting:
- swapped around plugs – no difference
- swapped around plug leads – no difference
- swapped around Carburetors (but not the Jet Needles) – This then made the left cylinder smoke and the right smoked a little, but not as much as previously. The carbs are back on their original sides as they are specifically left and right.
- sprayed carb cleaner onto the carb boots to test for air leaks and that had no effect.
- Raised the Jet Needle in the right carb for a richer mixture – It appears to smoke less (could be my imagination), but it still gets hot quickly.
- Checked the plugs again and the troublesome cylinder’s plug looks to be running very rich (all dirty), so I lowered the needle this time for a leaner mixture. Still no difference.

I removed the exhaust and wiped my finger in the cylinder’s exhaust port. Looks like black paint!

Has anyone ever experienced this and what was the solution?

[William1]

Sounds like the crank seal is gone on the right side. Do a leak down test to confirm.

[Steve Sutton]

6 minutes ago, William1 said:

Do a leak down test to confirm.

OK, will try to get that done. May have to take to to a specialist for that.

[William1]

If memory serves, replacing a crank seal on TZ engine is very easy. Pope off the primary gear, remove the clutch, pop out the seal, replace, reassemble.

You also may simply have a power valve issue.

[ossagp]

When you changed the carburetors the side that smoked changed?  Is that what you are saying?  If so along with checking seals,  I would disassemble and clean each carburetor and make sure that the floats are each set to factory specs and shutting off at the same point.  I would take them apart idividually,  and I would write down the jet sizes in each as I went.  

[Steve Sutton]

On 3/23/2017 at 7:53 PM, ossagp said:

When you changed the carburetors the side that smoked changed?

Hi OssaGP,

Yes. The non-smoking side began to smoke and the smoking side smoked less. I honestly can't say if the cylinder got hot - wouldn't want to swap the carbs again as it's not as simple as it sounds.

Carbs have already been stripped, except for the needle jet on the side with the new piston. But lots carb cleaner was sent though it.

I will recheck the float levels. Wasn't much of a concern because she ran smoothly previously. Getting the the seals tested or replaced will have to wait until I can get a mechanic with the right tools to do that for me. Will keep you posted.

FYI, here is an image of my plugs (NGK BR9ES). The right one is the smokey/hot cylinder.

[ossagp]

54 minutes ago, Steve Sutton said:

Hi OssaGP,

Yes. The non-smoking side began to smoke and the smoking side smoked less. I honestly can't say if the cylinder got hot - wouldn't want to swap the carbs again as it's not as simple as it sounds.

Carbs have already been stripped, except for the needle jet on the side with the new piston. But lots carb cleaner was sent though it.

I will recheck the float levels. Wasn't much of a concern because she ran smoothly previously. Getting the the seals tested or replaced will have to wait until I can get a mechanic with the right tools to do that for me. Will keep you posted.

FYI, here is an image of my plugs (NGK BR9ES). The right one is the smokey/hot cylinder.

How long has it been since it "ran smoothly".   What has happened or changed since that time?

[Steve Sutton]

The bike stood for about 9 months or more. Then I could not get it started because the fuel was old. I thought I'd take the opportunity during the winter to clean her up while I resolve the starting issue. One thing led to another then I ended up replacing a piston and both powervalves. Those plugs pictured are new - only run for a few KMs.

[ossagp]

If new needles and seats were not installed you may want to look and see if the needles are shutting off where they should.  Just takes a piece of fuel line to blow through, and for you to remove the float bowls.  those mikunis have seats that screw in,  and have a gasket between the brass needle and the cast aluminum housing. that gasket goes bad over time,  especially if let to dry out after having time to absorb fuel earlier.   that will make them run really rich.  have you tried starting it and checking to see if it will remain running on the "cold" side alone?

[Steve Sutton]

Just the quick update.

Turns out the major fault was on the cold side! I unplugged the spark plug cap on the cold side and the engine still ran. Did the same on the hot side and it died. So it was cold because it was just not getting any fuel to fire.

I removed the carb and took out the needle jet which I did not do previously. It was full of dirt. Oh, and I also installed the float upside down, but we’re not going to linger on silly mistakes, are we?

Got her fired up and both cylinders ran, got hot and smoked :-) Some progress! Took her for a spin and she cut out after a couple of hundred meters again. SO I think the carbs are just not getting enough fuel. I’ll remove my petcock and check it out. I suspect it’s also full of dirt.

Will concentrate on fine tuning the carbs once I can get to ride her without it cutting out after a short distance. Will keep you posted.

[ossagp]

6 hours ago, Steve Sutton said:

Just the quick update.

Turns out the major fault was on the cold side! I unplugged the spark plug cap on the cold side and the engine still ran. Did the same on the hot side and it died. So it was cold because it was just not getting any fuel to fire.

I removed the carb and took out the needle jet which I did not do previously. It was full of dirt. Oh, and I also installed the float upside down, but we’re not going to linger on silly mistakes, are we?

Got her fired up and both cylinders ran, got hot and smoked :-) Some progress! Took her for a spin and she cut out after a couple of hundred meters again. SO I think the carbs are just not getting enough fuel. I’ll remove my petcock and check it out. I suspect it’s also full of dirt.

Will concentrate on fine tuning the carbs once I can get to ride her without it cutting out after a short distance. Will keep you posted.

I didnt know you ever had any doubt about that.   Heck,  it has to run to MAKE heat.  Lol.  Darn,  just like they have to get run to get hot,  they get black because they are not burning cleanly.  Now,  does that sound like they don't have fuel?   I bet it at least ran stronger while it was fouling out again