Misc two-stroke jetting and CR250 questions

[James]

I know excessive smoke is considered a good indicator that you are running rich, but what I wonder is if it is also a sign of running too lean. Is it possible that excessive smoke when running lean means you are not only burning the normal oil, but also the oil that should be on the piston/bearings??

Reason I ask is that I seized my engine Sunday. I was running it wide open on some open farmland and it ran REAL STRONG all day (about 30 minutes straight) right til the point where it locked up. I knew it was jetted perfect for the MX track, and it didn't seem unrealistic that I locked it up as I never run it that hard or long on the track, but the curious thing is that it wasn't surging or hesitating whatsoever. I did notice the smoke and thought to myself that I KNOW this bike is not running rich nor is it residual oil in the pipe/silencer. Piston had a 3 point seizure (exhaust port right side was the worse), hot spot on the bottom, the crown was real clean, and spark plug color looked pretty good to me. I did think I heard pinging the weekend before in WV though.

Also, I am curious about main jet sizes. why would two CR250s with the same bore dimensions, carb, max rpm, silencer, air filter, and almost identical pipe use noticeably different main jets. My theory is that the same quantity of air is being pumped through the same carb, so the velocity should be the same? What would create the difference?

Examples:

2000/2001 CR250, using same Keihin on both, 2001 seems to require bigger main jet. My 2000 had been run exactly the same on the same farm even at colder temps and never locked up with the mainjet I was using with the 01. I put my 00 Keihin (jetting unchanged) in the 01 after reading a few posts on DRN.

OR 2002/2003 CR250, 2002 uses 370/380 and 2003 I have seen many people using 400/410 even 420. Also, the 01 uses a 400+ in the Mikuni so why does the 02 use a 380?

I wondered if maybe an air leak started and I have also been inclined to blame it on old gas/insufficient octane but I don't know exactly. I was mixing 32:1, radiators were full and in good shape, stock plug and heat range. Opinions??

 

[kxmike]

it sounds like your top end wasnt getting enough lubrication. you were running wfo for 30 min without letting off??!!

 

[James]

To clarify, wide open 3rd-5th for about 1/4-1/3 mile let off to turn, wide open 3rd-4th 1/8th mile, turn, wide open 3-5th 1/4 mile, turn, repeat. About 1/2 hour of that.

I had to let off for the turns, and I think there are 12-15 turns linked by 1/8-1/2 mile fast sections. But lack of lubrication was something I didn't really think about (other than I thought the oil was being burned away). Should I mix 26:1 (example) for that kind of riding?

 

[bedell99]

Hey how warm was it out that day. What was your jetting spec's

Erik

 

[James]

45 pilot, 175 main, needle second from top, 900-1000 ft elevation, 85 degrees+/-, 14/48 gearing, stock reeds, stock pipe, Turbine Core SA.

 

[bikepilot]

I seriously doubt that your jetting was the cause of the seizure. When riding in sand duens and desert stuff I'm wide open most of the time. Perhaps you had a head gasket go bad which leaked coolant into the cylinder (would explain the clean piston crown). Just an idea, but something to think about. In my experence, if the main jet is too lean, you will notice the power fading after a bit of hard running (I usually noticed it on the second lap in harescrambles - 2.5hr race, 7 mile laps). Even with it lean enough for the power to fade and the plug to be very light it didn't lock up.

what did your plug look like? any indication of being lean?

32:1 should be fine, I ran it (yamalube @ 32:1)for a month of duen riding without trouble:)

good luck

 

[James]

Plug was black on the threaded part and slightly oily. The insulator was brown and pretty clean at the tip. Plug looked good to me but had been in there a while (sunce rebuild) so is it good for a reading at that point? Piston crown had some black on it but no buildup, very smooth, no debris, and I could still clearly read all of the letters that had been stamped into it. The head was perfectly clean, no deposits. I think the top end had about 8-12 hours on it.

Power definitely did not fade.

Thanks

 

[bedell99]

Wow, that seems like a nice jetting setup. Did you check the cooling and make sure you added premix. I know this is real obvious but you never know.

Erik

 

[sunnyboy]

hum---from what you and erik are saying two things come to mind.first i have seen 2-strokes sieze due to running not wide open but just cracked back a touch so the needle is just hanging into the main a no no for a 2-stroke this melted the piston in 10seconds maybe little more and left a nice brown plug with traces of aluminum on it, that is always the case with a lean sieze it will leave traces on the plug.since you have not mentioned that and are saying the piston was fractured not melted,ibelive your malfunction was caused by broken ring that got caught on the ports and fractured the piston.even new parts can fail.

 

[James]

Erik,

I am 99% certain I put oil (and enough of it to get 32:1) in the gas and the cooling system seemed fine.

Sunny,

The piston was melted some where the seizures happened, and the piston is not fractured, just grooved from where the aluminum stuck to the cylinder walls and then dug into the piston those last few strokes. I am pretty sure there isn't any aluminum on the plug but I'll check it again. I frequently have to back off the throttle just a touch and there would be many occassions where the throttle would be cracked back but I would still be on the gas. So following up on this theory, how would I get around that particular situation?

I can probably get some pictures if anybody is interested. I just found this to be an interesting thing to look into as I thought I was doing everything right and this is the first motor I have messed up. Good (but expensive) learning experience if nothing else.

 

[dirt bike dave]

My '99 CR was well jetted for 'normal' on and off the throttle riding, but I suffered a big end failure after rolling off the throttle after a sustained 2 mile blast of top gear WFO on a road. The bike was running perfectly all day right up until it went boom.

My theory is that sustained high rpm/heavy load running is geneating much more heat than normal. When you roll of the throttle and don't pull in the clutch, the very hot motor is still turning 8,000+ rpm, but now has no lubrication, and can fail very quickly. You are much more likely to do this riding roads or farms than on an MX track or normal trail.

 

[James]

Dave, that makes perfect sense and was kind of the conclusion I was coming to. I think I am pretty good about pulling the clutch in for all of the turns but can't say for sure if I always pull it in before rolling off the gas. This happened about 3/4s of the way down the longest straight out there and now that I think about it, I could have been getting ready to slow down (or had started slowing down) for the turn and rolled back on the gas instead of clutching. If so, that will teach me to not get lazy.

I was just searching the web for a definitive answer on proper technique.

So to go fast, you are either accelerating or braking AND to save your 2 stroke engine, you are either on the gas or clutching?? I must have been lucky that I didn't do this sooner riding out there.

EDIT: taking this into account, should I be overly cautious with the bottom end? It all appears fine but I would expect that it didn't get enough oil as well.

Thanks for the input!

 

[dirt bike dave]

James - there is a reason factory produced road going two strokes were oil injected (oil pump tied to rpm). ;)

FWIW, I have dual sported a few KDXs and never had a problem on road rides.

My guess is the CR might just be more vulnerable than the KDX to lack of lube after sustained WFO. More power = more heat, so that makes sense.

When I killed mine by rolling off the throttle, the bike was working like designed, so I attributed the failure to rider error and lack of knowledge. Expensive lesson - besides a new top end, I needed a new crank assembly and right side crank case. The irony is I rolled off the gas to 'take it easy' on the motor. I am convinced if I had kept going WFO or pulled the clutch in, the failure would not have occurred.

BTW, there are two old desert racer's tricks to cool a hot two stoke without stopping One is to activate the choke (hard to do at speed unless you have a handlebar mounted choke). #2 is to hit the kill button while keeping the throttle open (don't pull in the clutch). Lots of cooling fuel and lubricating oil will be force fed into the cylinder.

A thought on the smoking: If the motor gets super heated, you can start burning residues in the pipe and silencer. These would not be making smoke under normal temps.

 

[parkerCR]

This is an interesting topic. I have a 2001 CR250 and grenaded the motor at the sand dunes. I didn't know about the pulling in the clutch on deceleration. I will definitely try that on my next dune trip in July.

Is this just a two stroke issue or will four stroke motors under heavy load experience similar consequences?

I have E.Gorr mo betta porting and am not sure if it would need richer or leaner jetting. I switched to the keihin off of a 2000 and have been trying to rid myself of the spooge problem.
I think I am running:
40:1
172 main
45 pilot


Thanks for the info

 

[James]

No aluminum on the plug and I am pretty sure the crank is ok. Seems to be a concensus with Mike, Sunny, and Dave all pointing to a rolling off of the throttle and/or lack of lubrication. Also, after further investigation, everything does seem pretty dry in there. There is oil in the bottom, but a light film at best.

We did MANY miles of road riding in West Virginia and there were no problems from rolling off the throttle and I KNOW I did it up there. But I also know I was giving it all it had on Sunday and everything must have been just right for me to seize it that day. Glad it was only a half mile walk to the truck as opposed to having to push it up and down those hills in WV to save it.

Thanks all!!

 

[truespode]

Glad it was only a half mile walk to the truck as opposed to having to push it up and down those hills in WV to save it.

Honda's are disposable. You can just leave it in the mountains and grab another from the garage ;)

Ivan