Detonation, timing, and jetting

[pscook]

I rode my 2002 XC300 in anger for the first time yesterday. Well, maybe not "anger," but "mildly perturbed." Anyway, I spent some time getting the bike warm and tried several needles and clip positions before the race but couldn't get it "perfect." Of course, not knowing exactly what is "perfect" I settled on "not too bad, all things considered."

However, during the race, my bike got properly warmed and the idle went right up. I couldn't lower the idle screw so I had to deal with a high idle on some slick descents. I also noticed some knock on high rev under load, and some knock on middle load with slight closing of throttle. I assumed it was detonation, so I did what I could to avoid those throttle positions and conditions. Otherwise the bike ran great and was super fun to ride on the pipe. Wasn't "grunting" up the hills, I let it breathe. It hit like my old YZ426, little bit of bottom, great mid.

Here is where I am with timing and jetting, at this point:
Timing- 10mm head on 5mm screw with 1mm of gap (1* of advance, per the timing thread)
40 pilot
170 main
N3EW on 1st clip (leanest)
AS at 2 or 2.5 turns (can't remember specifically)
Squish is large, can't remember the number. Probably send to RB after I inspect the piston.

Where I want to go:
Remove 1* of advance, should soften the hit, right?
Raise the needle one clip
Lower the idle screw
Proper squish

Am I on the right path? It's hard for me to sort the jetting under proper conditions as I live in an urban area and they don't take kindly to someone screaming up and down the road. I will pull the head this week to investigate, I really hope I didn't erode the piston. I think I will pull the plug after I have a cup of coffee, need to wake up after the beating I took yesterday.

All of this leads to this question- Does a two-stroke get richer or leaner as it gets hotter?

[bchatt]

Thank you pscook, your "mildly perturbed" brought a smile to my face - just what I need 1st thing Monday morning

[Jakobi]

A lean bike will take a long time to come up to temp. Somewhat cold blooded, but once up to temp it will run hot. You had all the symptoms of being lean. Don't forget to take some pictures for us when you pull it down.

I was under the impression that the older models liked a bit richer needle, particularly in the lower rev range. Maybe try moving up a few sizes on the pilot and main. This needle has 2 tapers with a transition from rich to lean in the middle which is where the 'hit' comes from. The timing should tame it down a bit from what I have noticed.

[MJC]

Originally Posted by Jakobi

A lean bike will take a long time to come up to temp. Somewhat cold blooded, but once up to temp it will run hot. You had all the symptoms of being lean. Don't forget to take some pictures for us when you pull it down.

I was under the impression that the older models liked a bit richer needle, particularly in the lower rev range. Maybe try moving up a few sizes on the pilot and main. This needle has 2 tapers with a transition from rich to lean in the middle which is where the 'hit' comes from. The timing should tame it down a bit from what I have noticed.

X2, With that needle, your pilot and main are lean, unless it's very hot and humid where you're riding.

[pscook]

Wouldn't a rich bike take even longer to come up to temp? More fuel cooling the piston, etc? I am just going to pull the head as the cylinder is a bit of a bugger (and I'm a lazy person who is getting ready to take a well earned vacation) but so far the sparkplug is nice and dark with no added aluminum (or missing electrode).

So a 42 pilot and 172 main with this needle, or just raise the needle and turn in the AS? Original jetting as received from previous owner was 38 pilot, 168 main and a CCK needle on the 2nd from leanest clip, AS at 1.5 turns from seated. However, the original owner raced it at high altitudes so those settings probably wouldn't work too well at my elevations (no higher than 1500').

[MJC]

If you want to use that needle, I would use it in the top slot(#1) with a 45 pilot and a 175 main. The n3ej needle, imo is a litttle more flexable in the 250 than the n3ew. I used a #8 slide with the n3ew and that worked well also.

[andoman]

You mentioned that you thought that you were detonating at certain RPM/loads.
Easy to check for: Pull the exhaust pipe and look at the top of the piston near the edges. May need a small mirror and a good light.
If its pitted, then you have experienced pre-ignition.
If you take out a degree of advance, it will be a move in the direction of less/prevention of detonation, if that is indeed what you are feeling/hearing. I'm doubting there was detonation upon throttle closure. It usually happens with a load on.

As for your squish: if you are thinking about doing it, just do it. Then sort your jetting when you get the head back on. Saves chasing your tail. Sloppy squish makes the power inconsistent thus leading to the chasing of ones tail. It runs soooo much cleaner and consistent with the squish correct, that you won't believe it. Small changes in jetting (one pilot jet, or a clip position) will make a noticeable and consistent change that you can feel, confidently note, and continue to correct from.

[Jakobi]

Everyones got good info. MJC can I get you to pass me some info re your jetting ventures on the 250. I'm doing the top end change on my 300 so wouldn't mind hearing how things worked. I was looking at starting 45 N3CJ#3 178 and dialling in from there. As a 300 I am at 42 N3CJ#2 172.

Back on topic.

As a suggestion go big enough on the pilot with the needle where it is to get some 4 stroking and smoking happening. You'll realy start to get a feel for when the needle comes into effect and the pilot off. You can then down size the pilot to get best response. The main is the same. You can safely go huge and then come back down size by size.

Regardless of what throttle position you are at in the rev range you want zero run on. So when you cose the throttle the revs fall. If it surges or races a moment you're still a bit lean.

Best advice yet is from Ando though. I spent endless hours jetting a stock 300 with a 2.6mm squish. The large squish made the bike so sensitive to altitude, temp, and humidity that it was like jetting a different bike every day. I went to the S3 head after which still ran a big squish. Around 1.8-1.9mm and while an improvement its still not ideal. I can imagine having one set around the magic 1mm mark would make all the difference.

[pscook]

I didn't get any run-on with the throttle closed, the idle was just too high for warm. I'll get some new jets and see about giving it a good wringing when I can. My next ride will probably be the Northwest meet and great in May, so I can have some "experts" help me sort it out. I found that I can spin the carb in the boots with the desert tank installed if I lift the tank a little and remove the carb top. Much faster and easier than tilting the subframe. However, removing the bowl is a bit tricky as the float pin and needle tend to scatter when exposed to the smallest chance at freedom.

I'll post pics and measurements when I get a chance to work on the bike.

[pscook]

Okay, no pics as there is no evidence of corruption or scandal afoot. I measured all that I could and here are the facts:

Squish: 2.2mm

Base gasket stack (from previous measurements): 1.5mm

Top of piston above floor of exhaust port: 1.83mm

This is where I get a conundrum. If I raise the cylinder so the piston is level with the port, I have ~4mm of squish and 3.3mm of base gasket. If I drop the cylinder to achieve a 1.5 squish the piston will be ~2.5mm above the port floor, thereby reducing the size of the exhaust port.

I am not shooting for a race winning machine, just a consistent and easy to jet/adjust trail bike that I wish to learn to ride properly. As of right now there is PLENTY of power. No question in my mind that there is more bike available than talent.

All of that being said, what is a good squish number for pump premium and what disadvantages will I experience with the piston above the port floor?