Detonation - cause?
 

I just purchased a fellow GGer's 07 EC250. I should be slapped for not looking at the jetting before even starting it, considering the approximately 500 ft elevation difference between where the bike formerly lived and here. Anyhow, I think the jetting is in the ballpark.

When I started the bike, I heard the same noise that my 250 with a 300 top end makes when cold (and more pronounced when external temps are below 32F). It is much louder on the 07 250. It is nothing like any of the sounds posted to youtube ... definitely not power valve from what I can tell from listening to posts. Those sounds are more rattly. This is a loud single knock when gently revving the engine above idle when the engine is cold. The sound is low, like a single whack of the engine case with a hammer. Detonation, right? It goes away when the engine is warm.

The bike has an LTR powervalve cover, and an RB Designs head, and had a new top end a while ago - about 5-10 hours on it. Before starting it again, I am going to check the jetting. My 300 did this fairly frequently when cold, and I just ignored it, and the bike ran flawlessly when warm.

I saw that Glenn (GMP) commented that this could be timing - and noted that octane should not make a difference.

Is this anything to worry about at all?

Thanks!

What ignition do you have? The 2K-3 is timed with a locating insert, the 2K-2 has to be positioned correctly. I have the same bike, also with RBD head and LTR PV cover. Whats your elevation? At 0 - 1500', I run 180 main, JD Blue needle #3, #7slide, 45 pilot(summer) 48 pilot (winter). This is spot on. You can substitute a DDK for the JD Blue and be very close as well.

Have you checked your PV as discussed here numerous times? That sound can be low, Youtube does not do it justice. I think that whatever is making noise stops when warm because clearances are taken up. Detonation usually occurs under a load. What else would raise a flag with me is the problem occured after a recent top end job. I'd check assembly, squish and compression myself. If all else is good, perhaps clearances are a little loose and its just some excessive piston slap.

Yarrp. +1 Detonation under load. and +1 piston slap on a cold engine makes sense. As the piston heats and expands the clearances tighten up and no more slap.

Thanks guys - and appreciate the jetting tips Glenn. I will see what is in there. My elevation is 1050, so your settings should be in the ball park. OK, I take back the detonation. I originally thought that the noise was piston slap, but saw talk of detonation, and thought maybe that was it. Piston slap makes more sense.

Is piston slap necessarily a problem if it only occurs when cold? And, here's an embarrassing question - how should I be warming the bike up? I always hear everyone on the gas when they start their two smokers. Way back when, I'd start my bike, and let it idle on the choke until it hesitated, killed the choke and revved it. Should I be letting the bike idle, or am I going to foul plugs left and right?

Thanks!

I warm my bike up by start with only choke put my helmet on,take off the choke, ride by short shifting before it gets into the powerband - after about 1km the bike is dying to be opened up and I usually cannot hold off any longer.
I have heard bad things about revving a cold 2t your method is fine but u run the chance of fouling a plug before you get going

I do the same as Matt. Start on choke and let it idle until the revs fall down. Then take it off choke and ride under low load low revs until things feel good.

Quick way to check for detionation is to pull the expansion chamber and look st the piston crown for pock marks.

whats a pock?

The easiest way to check for detonation is: if the engine is running you have got detonation, if it isn't running - you haven't! PRE detonation is of course an entirely different matter. To be more serious, the way you describe the noise doesn't really sound like piston rattle, maybe something more like big end knock, or maybe powervalve rattle. I have never found operating temp to make much difference to a piston type rattle - it's an OK theory but I think with watercooled engines, tolerances aren't changing that much as they warm up..just my thoughts.

Wish I had some info on your rattle, I'm struggling to get comfortable with the noises in my own 300.

Theory says it is a temperature related concern, temp certainly has the largest impact on the piston. You said you have a new top end, there are so many variables, what kind of piston? some pistons by design carry a greater clearance cold that others to accomodate the planned expansion. If there is any info you can follow up on in that regard I would check with the manufacturer and find the rec. cold clearance, but then you'll have to pull the jug to check it. Your concerns are warranted, it is hard to enjoy a machine when you are spending your time worried about fixing it if you grenade it.

yes, noticeable cold piston slap is a long term problem, unpredictable time for the same result, a broken skirt.

All that said, another "temp" related noise potential in a two stroke is a fluid knock, most common on older machines of questionable maint, but the issue I've seen is fuel and/or trans oil leaking into the crank chamber and effectively creating a fluid compression condition that can honestly have a pretty freakin deep single knock noise, after a short run time it abates, not becuase of temp, but because it's cleared its self out.


Warm up, I like the fact my gasgas has a thermostat, I fire mine up and wait until I feel some warmth in the upper rad tanks, with a thermostat that would mean it is opening enough to move coolant and such, the thermostat is opening and the jug itself is to operating temp.