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96 YZ250 Persistent Plug Fouling ...

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I'm not sure this is the correct forum to post this so my apologies if it is not. I have a 96 YZ250 that is really a nice bike, one of the most powerful 2-strokes I've ever owned which I recently bought in seriously mint condition. The issue I'm having is with plug fouling. Standard jetting for this bike is 42 - slow, 175 main.  It has a Boysen reed in it and Boysen recommends 42 - slow and 168 main which I installed, clip is dropped one notch, air/fuel screw 1-3/4 out (have tried backing it out more but still has fouled). Put in a new plug and it runs like a cotton-pickin monster, really well, but then begins to blubber down low after riding for an hour or so and then will completely quit. The weird thing is that when it quits it's almost as if I hit the kill switch which I've never experienced before with all the other 2 strokes I've owned. I've had fouling issues and have always been able to solve them but when they have fouled it has never been in the manner this one does... it's getting very confounding and I'm not sure if I should play with the jetting more or look elsewehere. Compression is not an issue, I believe it was around 190 psi (did it a couple months ago).  While the plug is degrading I will also get backfiring at times when I shut it off (kill switch). Makes me think there could be an electrical problem, a short or grounding somewhere (timing ?), I'm really not sure. The plug is black and sometimes wet though normally not soaking wet. I really enjoy riding this bike but plug fouling is a thing that really gets in my crawl... I want the bike to run well and be relaible like my other 2T's (3 Cr's, KTM, Suzuki TS125). I would appreciate and and all advice, its a beautiful machine..

 

Michael

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Could you be over oiling your air filter?

 

Are you running the proper spark plug? Specs usually give the standard plug and a colder one. Be sure you're not running the colder one.

 

I think a bad coil can display the symptoms you're seeing too.

 

Hows you're muffler packing?

 

I know it may not seem relevant but I'd also rule out the gas cap not breathing.

Edited by Trailryder42

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Thanks guys.

 

 

It sounds a bit like a bad coil. Does it start back up after cooling down for a while?

Thank you.

 

Once the plug fouls it is will not run. When I put a new one in it starts right up and runs like a brute. Guess it's possible that the coil could be degrading with temp to the point that it produces a weaker and weaker spark. The problem really is beginning to feel electrical to me. I did just pull the plug cap/boot off and there was moisture in it. Don't know how long it's been in there but it was a lot of water where I rode Saturday but the problem has been going on since I've owned it. I also pulled the coil off and there was a little corrosion around the mounts but not sure if enough to effect continuity. I ordered a new cap anyway.

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Could you be over oiling your air filter?   Air filter is brand new and came pre-oiled . I don't believe I've over-oiled in the past and I have never fouled a plug on my other bikes thru over-oiling filter so I'm assuming I've been doing it correctly.

 

Are you running the proper spark plug? Specs usually give the standard plug and a colder one. Be sure you're not running the colder one. - I run  8ES but I've also tried 7ES wit hsame results

 

I think a bad coil can display the symptoms you're seeing too.  - I think you may be right

 

Hows you're muffler packing?  - have not checked it but I willl go ahead and change it, great idea.

 

I know it may not seem relevant but I'd also rule out the gas cap not breathing.  - have not considered gas cap howeve it runs so great when it does run except it gets blubbery down low.

 

 

Thanks so much for taking time out to help, much appreciated.

Edited by delawhere

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It's quite possible your crankseal is letting in trans oil, which is fouling the plug.

 

Or, if your stator is going bad, the sparking will stop, fouling the plug, and when you put in a new plug, the stator is cold, and now works fine.

 

 

Both are easy to test for in the service manual (leak down test, stator test)

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Are you running the proper spark plug? Specs usually give the standard plug and a colder one. Be sure you're not running the colder one. - I run  8ES but I've also tried 7ES wit hsame results

 

 

 

 

You realize the 8ES you're running is the colder of the two plugs?

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If it is not smoking badly it could be

your spark plug cap.

You can replace it with a NGK aftermarket

cap for about 5$. I think your stock cap has a resistor

in it and they can fail. This happend to a wr250 I own.

Same symptoms as you. BR8EG is stock and works fine

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quoted from original post : "I will also get backfiring at times when I shut it off"

 

If you mean it "backfires" thru the carburator it would perhaps indicate that the reeds aren't sealing properly ?

In theory, any pressure inside the crankcase forces the reeds shut and seals them tightly, no gases should get past.

 

If you meant it 'backfires" thru the exhaust, never mind my theory.

Edited by mlatour

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It's quite possible your crankseal is letting in trans oil, which is fouling the plug.   Yes it is possible however I don't believe there is a leak

 

Or, if your stator is going bad, the sparking will stop, fouling the plug, and when you put in a new plug, the stator is cold, and now works fine.  - great point

 

 

Both are easy to test for in the service manual (leak down test, stator test) - if the replacement spark plug cap doesn't solve ther isssue I will perform these tests. I believe I still have the pressurizing set -up from a couple years back, I'll have to dig it out... Thanks !

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"I will also get backfiring at times when I shut it off"

 

If you mean it "backfires" thru the carburator it would perhaps indicate that the reeds aren't sealing properly ?

In theory, any pressure inside the crankcase forces the reeds shut and seals them tightly, no gases should get past.

 

If you meant it 'backfires" thru the exhaust, never mind my theory.

 

 

Thank you. It backfires thru the exhaust.

Edited by delawhere

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If it is not smoking badly it could be

your spark plug cap.

You can replace it with a NGK aftermarket

cap for about 5$. I think your stock cap has a resistor

in it and they can fail. This happend to a wr250 I own.

Same symptoms as you. BR8EG is stock and works fine

 

 

 

You're about the 4th person who has pointed a similar issue with the cuplrit beingthe plug cap which leads me to belive it could be the problem. I ordered an "OEM type" replacement whcih I sssume has the resistor in it (thanks for pointing that out I did not know it was part of the design)... would you recommend a generic one or woulfd you yuse same dsigh type?  Thanks a bunch!!

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Quick update:

 

- Checked resistances at ignition coil, primary and secondary coils and all are within spec. Cleaned grounding points on ignition coil

 

- Replaced plug cap.

 

- Replaced exhaust packing. The old stuff was in bad shape and the inner most section was almost completely carbonized solid... had to cut it off

 

- Have not perform leak-down test as of yet

 

 

How would I check kill switch for a short? Check for continuity while wiggling wire?

 

 

Have not had an opportunity to start it and ride it yet, hopefully this weekend

 

Appreciate all the help!

Edited by delawhere

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Update Te Sequel:

 

  • Checked kill switch , it works as it should. Wiring was secured too tightly around the frame (with plastic wire tie) so I re-secured it so it could move more freely but it passed the continuity/wiggle test.
  • Took flywheel off and there was a bit of corrosion on the electrical pickup - it seemed to be enough that it could effect performance. Cleaned the rotor inner (had some light corrosion) surface and the stator electrical surfaces. 

 

I'm looking forward to taking it out and see if there is improvement... I think there will be.

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