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07 YZ250 - engine issue - detonation?

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This is going to sound absolutely ridiculous but here goes.....

If a normal idle on a 2T is – nin nin nin nin nin nin nin

My idle after a hill climb, sand riding on the pipe, or any other load is – nin nin nin nin dung dung nin dung dung…dung nin nin dung dung nin nin nin nin

Sorry I don’t know how else to explain it, but it always occurs after a load is placed on the engine and I pull in the clutch and let the bike idle, like the top of a hill climb, tight section of single track, etc. It appears mostly at lower altitude but I've had the issue from 2000 feet with 50/50 102 octane/premium pump gas, up to 7000 feet on straight premium.

I will go through some of my gopro footage to see if I can include a sound bite. I’m sure the gopro will do the issue much better justice than what I attempted above. :lol:

I've been dealing with this on and off since last January which was my last new topend (all OEM parts, all new gaskets). At first I thought it might be an air leak at the head so I replaced the head orings with no improvement (I DID NOT replace the base gasket at this time) . As a side note back in 1996 I experienced an airleak behind the flywheel on my 96 CR250 which was totally different, it was a very fast lean type idle which my current issue is not. I’m ready for a fresh top end again so I’m hopeful this issue will disappear but I have my doubts.

I was under the impression I was dealing with detonation so for my latest attempt to cure this issue I ordered a new stock cylinder head to replace my original head that I had shaved to raise the compression. The guy who milled my original head told me I could easily run pump gas 5000 feet and above.

I rode in UT this weekend on pump gas with the new stock compression head and still experienced the same issues as with the higher compression head. I’ve read these bikes detonate at sea level but assumed at 4000-5500 feet in UT with the stock head and premium pump I would be fine. Obviously there is a good chance that this issue is not detonation and is something else.

I feel my jetting is really good, a friend of mine has an 06 and we run almost identical jetting all the time, his bike runs fine......

If any one has some ideas, advice, or specific questions in order to help I would really appreciate your feedback.

Thanks.

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Im curious about your crank or is it knocking in the top end? Only thing i can really think of is jetting but i wonder if since you got your head done maybe you needed to change up your jetting. Maybegive your plug a good check and see what color it is.

Also maybe recheck your squish clearance.

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I think what's happening is a slight lean condition during idle.

Try closing down the air screw half a turn or if that doesn't work try one step up on the pilot jet.

The heat generated from the increased load has artificially leaned out the motor.

Give it a try.

Not deto.

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my bike did the same thing.it was a high idle and i started using race gas.its doesnt do it no more.

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lean surge.probably lean on the pilot circuit.as above,turn in the pilot screw.if out less than a half turn from full in,move to next larger pilot jet.also your main maybe a little lean if you are running wide open alot.you maybe getting a little to hot from a slightly lean main and when the motor is hot it will get a bit of lean surge.

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Thanks for all the input. When I first bought the bike I was trying to eliminate spooge and tried a pilot that was to lean and the bike really idled up fast similar to my 96 CR250 with the airleak. This idle is totally different so I didn't think it was a lean condition.

I'm running a stock pipe and FMF Turbine Core II (normal length, not the longer Q)

In UT this past week I was riding 4000-5500 feet, 55 degrees, jetting was 48/N3EJ #1/172

As everyone knows, stock for sea level is 50/N3EW #2/178. I didn't think I could be lean on the pilot with a 48 and the more restrictive silencer than stock and at almost 5000 feet.

Looking at my JD jetting spreadsheet my UT settings were 6-11% leaner than stock at sea level across the board. The elevation gain was a wash because the temp dropped. I would guess that the Turbine Core silencer calls for slightly leaner setting due to being more restrictive but I doubt it would need 11% leaner, I'm not sure though. Maybe I should have actually used the spreadsheet a little sooner :lol: Oh well...

I'll try richer setting this weekend and see where I end up.

Thanks again.

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Did you happen to check timing? My 06 way off by quite a lot. I THINK this explains why some bikes have the issue and others dont.

I was never able to run position one on the needle. I was also able to do a nice compression bump here at 4500 ft without issues. After setting timing to the stock spec, not where the marks line up.

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Did you happen to check timing? My 06 way off by quite a lot. I THINK this explains why some bikes have the issue and others dont.

I was never able to run position one on the needle. I was also able to do a nice compression bump here at 4500 ft without issues. After setting timing to the stock spec, not where the marks line up.

Good idea. I actually purchased a timing gauge/tool over a year ago but never did anything with it. I'll double check that when I do my rebuild next week.

Thanks :lol:

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The sound you describe sounds to me like the powervalve may be sticking in the OPEN position. It might be all gummed up and when it gets hot it sticks open.

When you experience this condition does it feel like the bike has lost bottom end performance?

I had this happen before and the sound was as if the combustion was leaking into the head pipe area (because it is due to the powervalve being open). The sound at idle and on low end went from the normal ding ding ding...... to a louder pang-pang-pang..... that sounds like it is emanating from the head pipe area.

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Let us know what you find out after the check. From what people have reported back, few of the bikes were set accurately.

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Yes let us know. I own a 2007 YZ250 that seam s to run great but I am no expert. I am going to get the RB head mod done soon thanks to RC. I did not know that some of the YZ's were not set up correctly. What else should we be looking for? I was thinking about checking the reeds too. I do not have reason to believe they are bad but if needed I would replce them with Boysen super stock reeds unless there is reason not to.

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Did you happen to check timing? My 06 way off by quite a lot. I THINK this explains why some bikes have the issue and others dont.

I was never able to run position one on the needle. I was also able to do a nice compression bump here at 4500 ft without issues. After setting timing to the stock spec, not where the marks line up.

When you talk about checking the timing, I just want to be clear. Are you talking about checking your timing like the owner's service manual states with a dial indicator or with a timing light. I never saw any reference to any numbers for using a timing light with any two stokes I ever worked on, like degrees BTDC and RPM. When you said your timing was off were you saying that when you get the timing correct, the marks do not line up. I ask because I plan on checking mine in the next couple days before I send my head out for the RB mod.

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RC is referring to checking your timing with a dial indicator.

The mark's he is referring to are the punch marks on your cases, stator and flywheel/rotor.

There is a great thread in the FAQ on setting your timing, and your service manual also has really good section on how to set your timing to stock. I've currently got issues with my yz running a bit rough and before I tackle jetting I want to double check my timing, although I'm struggling to source a timing gauge that costs under $50.

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Yes, just like Tim said. Once you have the cover off, you'll see the marks on the stator and case. They'll be lined up and is easy to see timing is set perfectly. WRONG!

Using the dial indicator reveals that these marks are off, or mis-punched, or production tolerances makes them inaccurate. Setting timing as per the manual, to the recommended specs, helps everything about the bike. You'll need the dial indicator as a timing light would only allow you to set the marks back to stock.

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Yes, just like Tim said. Once you have the cover off, you'll see the marks on the stator and case. They'll be lined up and is easy to see timing is set perfectly. WRONG!

Using the dial indicator reveals that these marks are off, or mis-punched, or production tolerances makes them inaccurate. Setting timing as per the manual, to the recommended specs, helps everything about the bike. You'll need the dial indicator as a timing light would only allow you to set the marks back to stock.

This is exactly what I thought you were talking about. My manual for my 2007 YZ250 states 0.18 mm BTDC. I am assuming this is correct, right? I wanted to make sure I wasn't missing something. I did not know that the timing marks vs. actual correct timing would be that far off from the factory. I have always used a dial indicator to check the timing and in the old days I put the punch marks where I wanted them. I was just curious because I never heard of using a timing light on a two stroke before. I tried once a super long time ago and could not make any scence of it. The lines were all over the place with a timing light due to the timing changes the CDI made, which is very RPM sensitive.

On a side note though, a dial indicator could be a very very usefull tool to identify top and bottom rod bearing conditions, especially if you don't want to tear en engine top end apart to manually check them. Perhaps the shop manuals talk about this as well but I usually do not read the entire manual so I don't know about this. I usually only refer to the manual for numbers I need or procedures I am not familiar with. Thanks for the reply.

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Mine does weird things at idle too.. Know how I fix it? Open the throttle :) Then I dont have to hear anything but "WAAAAAAAAAAHAHHAAAAAAAAAAAA" :lol:

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