YZ250 squish clearance

[rjpjnk]

Hey rjpjnk,

When you talk about the burble, blubber in mellephant's recent video are you talking about the surging "BING BING" when he comes off the throttle (between throttle inputs)? or when he is into the throttle?

No, I'm talking about the sputtery sound just off idle when navigating slow sections.

[rjpjnk]

First off, I'm often guilty of thinking about jetting in terms of RPM, not throttle position. I think this comes from experience tuning EFI systems in automobiles. Bear with me as I try to forget that and think in terms of throttle position. Imagining the physical parts interacting inside the carb is helpful.

I think another good word for the rich blubber at 0 to 1/4 throttle is "sputter" - from being too rich. I usually twist straight past that point, but there are moments in the video where it is audible. Listen during my gentlest throttle applications: Pow-pow-----puh------puh------powpowpow...?

So to correct this 0 to 1/4 sputter I could try one of the following:

(1) #48 or #45 pilot jet instead of the stock #50. I'm starting to see that I should have done this at the same time as the N3EW needle.

(2) Switch back to stock N3EJ needle from the N3EW, for the thicker straight portion (N3EW is thinner, so mix is richer, from 1/8 - 1/4) - if so then keep it at position #3, not stock #2

(3) Cut my #7 slide into a #8 slide

Which would you guys recommend? My initial thought is to try (1) with the #45 first, to make sure I feel and hear a difference. If it is too much then try #48. If neither of those helps much then move on to (2) and (3)

Well jets are cheap and easy to swap, so definitely pick up a few to play with. I bought 40,42,45 for 5 bucks each at local shop just for fun. I am currently running the 42. Still plenty of burble, but no more fouled plugs. So pilot did not make a huge difference in sputtering. What did make a difference was the air screw. I turned the thing out as far as I could without it falling out and the burble was greatly reduced. This is what makes me think what is really needed is more air, hence the no 8 slide. All other components stock now btw.

[mellephants]

I ordered 45, 48, 172, 175 earlier this week. They should be here tomorrow, along with more head O rings and more coolant.

I plan to measure the squish on the eBay head and then send it to Brent Ryman from TDC2strokeperformance.com, partially because he is nearby, and partially because he seems to know what he is doing. He said he'd cut the head using CAD program, and agrees that you make good power with air, not compression.

 

My next parts order is probably going to have the Suzuki needles NECJ, NEDJ, and 40, 42, 168, 170... might as well be prepared to experiment and get it dialed in the best I can. I am hesitant to cut my 7 slide into an 8 until I'm running the final head, although I agree it seems like a reasonable step based on how it runs now.

Edited October 7, 2015 by mellephants

[rjpjnk]

Why are you messing with the main jet? Is it running too rich at full throttle? I wouldn't know, since I rarely ever use full throttle ?

 

I would get pilot circuit perfect before addressing main. There is almost no overlap of these circuits so they can be worked on independently.

[mellephants]

Yes I believe it is running a bit rich at full throttle. I haven't tried any leaner mains because I haven't had any. I agree, jetting the main is simpler than and almost completely independent of the pilot/needle. The 175 and 172 should give me enough range to adjust when I get the spare head cut by TDC2strokeperformance.com, and from what I understand, the 168 and 170 would be needed to go along with the richer Suzuki needles.

Edited October 7, 2015 by mellephants

[rjpjnk]

 from what I understand, the 168 and 170 would be needed to go along with the richer Suzuki needles.

 

Oh yeah. That's what I heard as well. Forgot you were considering a richer needle.

[motoxvet]

Why are you messing with the main jet? Is it running too rich at full throttle? I wouldn't know, since I rarely ever use full throttle ?

 

I would get pilot circuit perfect before addressing main. There is almost no overlap of these circuits so they can be worked on independently.

Is that a rule of thumb to jet "from the bottom up" instead of  "from the top  - main - down"?  Ya know I've seen it recommended it both ways on TT.  Doesn't help the confusion factor!

[mellephants]

It is more rational to jet from the bottom up. The pilot is still feeding fuel at full throttle - but it isn't enough to make much of a difference, so really, do whatever you want ?

[Faded]

I would get pilot circuit perfect before addressing main. There is almost no overlap of these circuits so they can be worked on independently.

 

Close, but there is always some margin of overlap and the closer you get the more apparent it will be.

[numroe]

So to correct this 0 to 1/4 sputter I could try one of the following:

 

(1) #48 or #45 pilot jet instead of the stock #50. I'm starting to see that I should have done this at the same time as the N3EW needle.

(2) Switch back to stock N3EJ needle from the N3EW, for the thicker straight portion (N3EW is thinner, so mix is richer, from 1/8 - 1/4) - if so then keep it at position #3, not stock #2

(3) Cut my #7 slide into a #8 slide

 

Which would you guys recommend? My initial thought is to try (1) with the #45 first, to make sure I feel and hear a difference. If it is too much then try #48. If neither of those helps much then move on to (2) and (3)

The smaller pilot with the either N3Ex Yam needle could introduce (a worse) lean hesitation at 1/4 to 3/8th throttle. That's why most people run such fat pilot jets with the Yam needle.

The leaner slide will obviously help. Or eliminate it (the sputter/burble/misfire at 1/8th).

If you try a #8 slide with the Yam needle, then you could consider going a clip pos richer, to fill in some of the stock/Yam 3/8 lean problem.

The way your motor is running, you can use the #8 slide no matter what the head setup is. I do not find the slide impacts a throttle range as wide as that diagram shows.

Jets are cheap, but so are two new needles. Try the Zook needles. It's well worth it.

As I wrote previously: The amount of fuel at wide open is slightly impacted by clip height of the Yam needle. I agree with that diagram. With the Zook needle, I think WOT is 99% main jet and 1% pilot.

In my experience, finding the main jet size that gives best WOT power is so easy/obvious with: the zook needle + correct squish clearance + Fatty pipe + correct timing.

I'll jet bottom up to get the bike ridable, then go straight to the main, then fine tune needle clip and pilot.

[motoxvet]

numroe, WHEN do you have time for a personal life??  You're either trying to shim an SSS fork or straightening out the jetting after a squish job.  In all seriousness, I don't have SSS forks, although I do follow that thread.  But this squish/jetting one, I'm learning something every day

[Faded]

I'll jet bottom up to get the bike ridable, then go straight to the main, then fine tune needle clip and pilot.

 

 

Amen. A one time pass, either bottom up or top down, is only going to get you so close, say 80% effective. Most fall into this category I refer to as 'Jet and Forget'. The bike is close, it starts relatively easy, idles, runs good and makes good power so forget about jetting, lets ride.

 

Also, you're going to spend about 80% of your total time chasing that last 20%.

[mellephants]

YHup that's me lol

[Snider]

Here'a couple of Squished heads the top one from RB and the bottom from EG... I've only ran the RB.. Although the EG dome was machined there is a huge difference in their approache to the dome shape as you can see from the gauge thats in the RB dome .. it was set in the EG dome... both were ordered to run pump gas

 

[Kinger317]

Maybe that's why Eric doesn't feel the need for a super tight squish with a wide band like that.

[Snider]

Maybe that's why Eric doesn't feel the need for a super tight squish with a wide band like that.

Just to be clear.. the gauge is IN the RB dome... the gauge was shaped from a measurement in the EG dome...

[Kinger317]

Isn't Eric's band a lot wider?

[rjpjnk]

Here'a couple of Squished heads the top one from RB and the bottom from EG... I've only ran the RB.. Although the EG dome was machined there is a huge difference in their approache to the dome shape as you can see from the gauge thats in the RB dome .. it was set in the EG dome... both were ordered to run pump gas

 

 

Wow, that is a really cool tool Snider. Can you send another pic that shows the squish band too. The RB CC is so much larger volume. Definitely looks like the EG band is a lot wider from what I can see on the gauge.

 

Good science man!

 

What is the volume of each? Keep it coming.

[Snider]

Isn't Eric's band a lot wider?

The dome with the most volume is the RB...also I totally rebuilt the engine, every bearing,seal new OEM crank ETC... it only has 190 lbs compression with the RB head domed for pump gas... runs like a dream lots of power and seems to run cooler than before... I'll be trying the EG head soon... Sorry RJ but I dont know how to measure the volume... Liquid with a spark plug in?

Edited October 11, 2015 by Snider

[rjpjnk]

Can you please post another pic of the contour gauge showing the entire head including the squish band (again of the EG head inserted in the RB head). I am curious as to how the squish bands differ. It looks like the EG is much wider, so I assume it was this wide stock. In that case, I wonder why RB designs cut into the squish band when increasing the CC dome volume. It might not be a good idea to cut into the squish band. This will change the velocity of the squish effect since there will be less total squished area. Big wide squish area is good for low end generally I believe. And that EG head has one fat squish band! Do you recall if it looked that way stock? What did you ask EG to do for the head?

 

The volume measurement involves water through the spark plug hole with the head on a glass surface I think, but I  am not really sure. Hopefully someone else will chime in on this one.

Edited October 11, 2015 by rjpjnk