detonation

[Pat Lopez]

Having problems with detonation 3/4 throttle to full. Did a pressure check and all is good. My bike is stock, but with a 38 a/s and a Pro Circuit Platnum Pipe. I run 93 octane fuel with Maxxima K2 oil at 32 to 1. I use a NGK br8es spark plug. I replaced piston at the end of last season. I live 20 minutes from Glamis, and since its the season, I'm missing out. All information would be helpful. Thanks all and keep on riding those 2-strokes!!

[deathman53]

your igntion is avdanced too far, too high compression or too low of fuel octane.

[Pat Lopez]

Okay, I had the compression checked at my local shop, they said with the 93 octane, the bike should run good. They checked the timing and that was okay, didn't need any adjustment. I read on one forum, where someone had a similar problem, and they had said they had a bad stator at high rpm's. If all else fails, I can always rebuild the bottom end. Don't want to spend that much money before christmas!

[trx250rider]

compression is not the problem..the motor is stock so 93 is about the right octane...good luck on the problem

[86 Quad R]

need more info............

what head are you running? if stock, what gasket? what is the actual static compression? are you running a adjustable timing setup? how old is that gas?

[All250R]

3/4 to full throttle is main jet territory. Fatten up that main jet a few sizes and the problem will probably go away. Call a dealer and ask for genuine Keihin jets. Make sure they're order what will have the K logo on it. They will cost more, but you won't have to wonder if the jet has a problem if something still isn't right. You might want to pull your needle clip down a notch as well. I'd also recommend a Br9 for Glamis. You need a pretty cool motor to run an 8 in Glamis. The sand creates a lot of extra heat from holding the throttle open more particularly at slower speeds.

The stock ignition is not adjustable. If yours is aftermarket AND adjustable, put the timing back to the stock position or put the stock ignition back on.

There are a lot of things that could be in combination making your engine not run the way it should. Start with the main jet and plug and once the motor is not detonating see if you are happy with the way it runs. If not, go down the list of items that cause detonation and make sure they are right before leaning the jetting back.

[Pat Lopez]

The head and gasket are both stock. I bought genuine Keihin jets. I went all the way up to a 190! I have stock ignition, so there shouldn't be any adjustment, right? Should I put an aftermarket on it. I will change out the plug to a B9. The gas usually sits for 2 weeks only, if anymore, I put the gas in a 4 poke quad! I want to get the cylinder ported, but I need to get this problem fixed first! Do you guys know of any good shops that do good port work? Thanks for all the info!!!!!!!!!!!!1

[Pat Lopez]

Also, when I put in a new plug. Road the quad, I checked the plug and it was just a little wet, so I went down on the jet 2 sizes to a 180. Put in a new plug, road the quad again, checked the plug, and this time it wasn't as black, but at least it wasn't wet. I want the plug to have a brownish color to it right? What gap should a run the plug at? Right now, its at .028.

[skyhighatv]

Try gapping that plug to .18 and see what that does up top

[Dave83]

I agree with lowering the gap on the plug.Also,look the counterbalance bearing holder over rel good.I fought this on my first R and finally found the holder was cracked and must have been flexing when the engine was wound up.I replaced it with a billet unit and the top end miss went away.

[Pat Lopez]

Finally got a chance to make the changes to my quad, lower needle, 91 gas, fresh! Changed plug to a br9es, gapped at .018, still have the same problem! Can you guys help me out? Thanx!

[LONG-ROD]

3/4 to full?? sounds like the carb is not completly open at WOT. take the air box off and check to see if it is a perfect round hole at WOT. if not make it so it does. It not opening fully will cause the needle to bleed over the main jet and creat a nast popping and sputtering feel. and try checking reeds. and make sure you have the coil stripped good.

[Pat Lopez]

What do you mean by stripped coil? Never heard of that term before. I'll check my reeds, the only thing I can remember about the reed cage, is that it doesn't have those plates that curve outward like on the stock reed cage. I'm thinking about V-3 set up also.

[machwon]

Can you explain a little better what it is that makes it detonation. I've rarely heard it but only once in an R while riding. What did the piston look like that you pulled out. It should have some tell tale signs if it is detonation. I do porting and motor work if interested. Feel free to e-mail and I'll try and help you out on your issue.

[Pat Lopez]

The old piston and a hole in the top of. When I'm at 3/4 to WOT, the top end makes a chatter, or dieseling noise. Only at 3/4 to WOT. I changed my plug to a br9es and gapped it at .018. The only thing I haven't done, is check the reeds for cracks. I put carbon fiber reeds. One thing I noticed on the reed cage, is that it didn't have that metal piece that curves outward from the reeds. I don't know if that would make a difference or not. Just noticed that is the service manual, it shows that part of the cage.

[C-LEIGH RACING]

Set the piston at TDC, ( you may need some help with that) get some soap stone to mark the flywheel at the right spot & on the case as well & then get a timming light to check the ignition timming.
Just a guess, but could be something wrong with the ignition system causing it to spark at to high an advance in higher rpms.
Wrong slide needle can sometimes cause a 2 stroke to do what yours is doing, not letting the proper amount of fuel pass while at the 3/4 throttle setting.

Holes in pistons come from to high advance timming or to low of a octain fuel.

My moneys on the gas though, cause when you pull that nozzle out from the service station pump & fill up your jug, makes no difference if its reg or prem its no telling what kind of crap your buying.

As a test, find you some 110 race gas & see if it changes anything, but you still need to check the timming.
Neil

[Pat Lopez]

What slide needle do you recomend for dune riding, elevation about... I believe to be about 300ft.(Glamis) I didn't see any letters on the slide needle inside my 38 a/s. But I'll keep looking. Will have the timing checked. Thanks.....

[Pat Lopez]

Okay, went riding yesterday at Buttercup and ran 50/50 110 fuel and 91. The bike actually ran better than before. It had a little detonation later on the ride, probably because it got hot? Will still take the bike to a shop and have the timing checked. If the compression is soo high, how can I lower it to run pump gas? Was thinking of putting a cool head and a low compression dome? Thanks guys for all the help!!!!!!

[Pat Lopez]

What slide needle would anyone recomend to ride Glamis? I didn't see any markings on my A/S, so I don't know what it is. I bought it new from Sudco. I'm still having a little detonation on the top end, but it's not as bad as before, some improvement. Thanks for all of the help!!!

[Honda 250r 001]

your gonna have to go bigger on that main jet. are you positive this isint an exhaust leak making you think its detonating?

[bluetrx]

the definition of detonation:
when temperature and pressure cause your fuel charge to ignite before your spark plug does.
if this is happening its very bad and will probably burn a hole in the top of your piston in about 5 minutes. there are only 3 factors in detonation.
1 temperature: dont run head temp above 220F
2 pressure: make sure your mechanic is checking actual cylinder pressure, static compression ratios are inaccurate with two strokes because port sizes and port timing affect pressure.
3 flash point of fuel: the higher the octane the higher the flash point. if you know temp. and press. are fine the only thing left to change is your fuel.

that being said there are a host of other problems that may sound like detonation and many have already been mentioned.
over advanced timing can create the exact same problems, carb adjustments but not as likely, if youve spun a crank bearing that knock sounds alot like detonation, check your head pipe- once i experienced detonation at the dunes, i shut it down and got towed about 3 miles across the sand just to find out my exhaust was rattling when i got back to camp. also detonation shouldnt be affected by throttle position so much as load on the motor, but it can be difficult to distinguish the diffrence.

[Honda 250r 001]

so that means that the gases ingnite and are suppose to explode, but the piston is still going up, so it compresses the gases suppose to be expanding. which means BAD THINGS. heat and stress come quickly.

[wilkin250r]

Originally posted by bluetrx
the definition of detonation:
when temperature and pressure cause your fuel charge to ignite before your spark plug does.

Not that it helps with the current problem, but there is difference between detonation and pre-ignition, if you were in the mood to get technical about it.

Detonation always occurs AFTER the spark plug fires. The spark creates the initial fuel burn and a flame spreads outward from the spark plug (called the flame front), burning and building pressure as it goes. Too much pressure/heat causes the fuel at the outer edges to ignite spontaneously, creating a secondary flame front, and the two flame fronts collide.

Pre-ignition is when your fuel ignites BEFORE your spark plug ever fires, usually caused by a hot spot in the engine.

Depending on how early the detonation occures, and it's location, it can be mild or severe. Some engines can continue to run a long time with mild detonation, but severe detonation can destroy an engine within minutes. Pre-ignition will destroy an engine within seconds.

[wilkin250r]

Now that I've put you to sleep with my boring, know-it-all post, I'd agree with C-LEIGH RACING. You should check the ignition advance, and if possible, compare it to somebody else's that isn't having the same problem.

If you need help with it, let us know, I'm sure we can walk you through it and help you intepret the results. It will obviously be time-consuming, but it shouldn't be too expensive. It will certainly be less expensive than a new piston.

[05LSR250R]

Good information right there!

[wilkin250r]

if it's not a fuel/compression/heat issue, if something IS messed up with your timing, there are only three causes, two of which are rather easy to confirm or eliminate.

1. Your flywheel key is sheared. Pull off the flywheel and have a look.

2. Your pulse generator is out of position. This would either be a loose bolt, broken bracket, or adjustable timing plate.

If either of these possiblities isn't the culprit, the only thing left is your CDI. If you have a friend that has a 250r, try swapping out your CDI and see if the problem goes away.

[Pat Lopez]

The timing was checked and the mechanic said it was fine. It's stock, so I thought it couldn't be a adjustable? I did take off the stator cover, and found what looks like to be small hairline cracks above the flywheel mounting bolts. I tried taking it off without a flywheel puller, but stopped after I thought about. I am in the proccess of purchasing a flywheel puller and a new flywheel. I might just purchase a new cdi box since the quad is 22 yrs. old. Like I said earlier, I have only burned a hole in 1 piston since I've owned the bike. I ran race fuel 50/50 with 91 octane, and the bike ran great. I still had a little problem on the top end. Now that I'm getting more information, it sounds more like the problem is pre-ignition. Thanks again for all of your help. I will borrow a timing light, and check the timing myself. It should be just like a truck, plus I have Clymer's Manual for help.

[05LSR250R]

That crack will throw the timing way off! Just not all the time! I was reading a thread from way back and a guy had very similair problems as you do. Said he found a crack like that and that was the problem! Thought he said the timing would check out fine and then the plate would flex under load and jack the timing up really bad! Just my 2 cents!

[Pat Lopez]

There not mounting bolts, but where the rivets are! Sorry for the confusion.

[Pat Lopez]

I have a question, what type of slide needle should I use for riding out at Glamis? Elevation is 300-500, I was told. I haven't worked on my quad since March. But I did by some items that I hope will help out. Flywheel with no cracks, engine ice, and a new cdi. Hope this is a good start. I will take the cylinder off and check the piston for burn marks or even possibility of a hole. Also will check the compression, just bought a new tester from sears. Thanks everybody for all of your help. Let you know what happens!

[Pat Lopez]

Also, does anyone have any +2 front steel-braided brake lines they might want to sell?

[Pat Lopez]

I know there are alot of threads on jetting a 250r, and I've tried all of them. bought new oem cdi box, ran 50/50 gas at 40/1. Everything is stock except Pro Ciruit pipe and silencer, K&N filter with lid off airfilter box. I'm running 165 main, air screw is 2 turns out.. My spark plug is still a little wet. Reading the other threads, my main should be up in the high 170's and 180's. Stock head and gasket. Anything else I'm missing? There are no marks on the needle so don't know what it is. Don't know what the slow jet is. Needle is 2nd from top! Please help!!

[Pat Lopez]

The slow jet is a 52, clip is on 3rd notch.

[fearlessfred]

have you tryed a differant carb.im thinking it maybe its a fuel delivery problem,and your carb is running out of fuel, at 3/4 throttle your sucking fuel at a pretty fast rate. maybe there is some kind of blockage thats preventing the bowl from filling fast enough

[wilkin250r]

Lopez, after a year, are you still having the same problem?

Reading this thread, we need to get back to basics. Detonation really only has three factors to it. Compression, ignition advance, and octane. That's it. (along with heat, but to a much lesser degree) And your ignition should be pretty much set, so that only leaves compression and octane.

Let's discuss those two issues in separate posts, so you don't get too bored reading it all at once.

[wilkin250r]

OCTANE:

You've probably heard people talk about Race Gas. It's really expensive, so it must be really good, right? One of the main aspects to it is the octane rating. This is why some people tend to think that octane equals power. NOT true.

Octane is just the ability to resist detonation. It's actually a measurement of the STABILITY of the fuel, not the power content. There are some fuels that actually have more power in them (alcohol and nitromethane), but that's a separate topic.

High compression creates power. The more you can compress a fuel before you ignite it, the more power you'll get from it. However, compress it too much, and you'll get detonation. THIS is the real key to race gas. Because it's more stable (higher octane), you can compress it more without detonation. It's the compression that gives you the power, not the octane. You just need the octane to handle it.

[wilkin250r]

COMPRESSION:

Many people don't understand compression. For the most part, it's a ratio of the total volume (246cc for a stock 250r) divided by the volume of the combustion chamber at Top Dead Center. In simple terms, a 250cc engine with a 25cc combustion volume has a compression ratio of 10:1.

Compression ratio is centered around the design of the head, but the head has a lot more into the design than just compression ratio, but we won't confuse you with that aspect here.

Under normal circumstances, ignition isn't really an "explosion" like many people think. It's actually a very controlled burn. The flame starts at the spark plug, and continues outwards (building heat and pressure as it does) until all the fuel is ignited in a precise and controlled manner.

When detonation occurs, if you read my earlier post, is that the building heat and pressure is too much for the fuel to handle. You have your normal flame front spreading out from the spark plug, but somewhere along the line, a spot of fuel ignites on it's own, creating a second burn area and second flame front. This is bad, because it's uncontrolled.

That second burn area and flame front happens because the fuel is under SO much pressure, it ignites on it's own. It's related to compression because that's exactly what compression is, pressure. You're squeezing it tighter and tighter with higher compression. The fuel can only handle so much, so if you're already starting with higher pressure (higher compression), then you're obviously going to reach that critical point much faster.

And that's the real definition of detonation. It's when the fuel at the outer edges ignites on it's own, rather than waiting for the normal flame front. When you think of it this way, you might not think it's all that bad, but believe me, all that extra heat and pressure created from that second uncontrolled burn will cause a lot of damage.

[wilkin250r]

Ignition advance is critical because it takes TIME for that flame front to travel and fuel to burn. So the spark actually fires before the piston reached top dead center, to give time for that fuel to burn, so it will create maximum pressure at the proper piston position when it's most effective for max power.

The piston is traveling downwards during part of the burn. That means that combustion volume is getting bigger, obviously, which will drop the pressure (in reality, it's still burning, so pressure is still increase, but because the piston is traveling downwards, it's not increase as fast as it could). And this is all very controlled and built into the design of the engine.

If you fire the spark too early, the piston hasn't traveled as far down, which means you have more pressure than you should. More pressure equals detonation. That is how ignition advance is related to detonation, because it's burning too early, it creates too much pressure.

[wilkin250r]

So that's most of the background knowledge you need to understand detonation. So with that, let's get back to this issue.

Are you still having this same problem? I bet you're getting real tired of this, let's get this thing solved once and for all.

[fearlessfred]

Originally posted by wilkin250r
OCTANE:

You've probably heard people talk about Race Gas. It's really expensive, so it must be really good, right? One of the main aspects to it is the octane rating. This is why some people tend to think that octane equals power. NOT true.

Octane is just the ability to resist detonation. It's actually a measurement of the STABILITY of the fuel, not the power content. There are some fuels that actually have more power in them (alcohol and nitromethane), but that's a separate topic.

High compression creates power. The more you can compress a fuel before you ignite it, the more power you'll get from it. However, compress it too much, and you'll get detonation. THIS is the real key to race gas. Because it's more stable (higher octane), you can compress it more without detonation. It's the compression that gives you the power, not the octane. You just need the octane to handle it. i totoly agree with everything you just said,but isnt this basicaly a stock motor and hes running 50/50 and he has checked all the other things that could be wrong with ignition and jetting. now shouldnt we look in a different direction. a leaned out motor will cause detonation and put hole in a piston in seconds ( anybody ever run out out of gas and heard the sound the motor makes ) i dont know how repeating everything is going to help. giving other answers, right or wrong,may be of help.

[wilkin250r]

Originally posted by fearlessfred
a leaned out motor will cause detonation and put hole in a piston in seconds

I doubt it's a lean detonation. If he was running on a 50/50 and it ran better than before, I assume he ran it for a while. On a stock motor, with stock compression, you have to be REALLY lean to get detonation. Different fuels can require different jets, but not THAT much different. If he's so lean to get detonation on a stock motor, he would have burnt a hole in his piston when he ran the 50/50 mixture, if he ran it for any length of time.

I only see one of two possibilities. Either his ignition is off (sheared key, or something wrong with his CDI), or this isn't a stock motor.

[ytman]

This may seem silly but have you swapped out the coil and plug wire?

[Pat Lopez]

Wilkin250r a true quad head, I was first an atchead back in the 70's, will never seem any information boring or bla, bla, bla. All information any of you give me is taken seriously. Or why would any of us be on here. To help others, right? I do appreciate all of the knowledge given. You give a more understanding than what I get at my local shops. I think they lose patience with me, the customer, who helps pay there salary! Wijth the information you have given me, and others, I've come to a conclusion: Mix 110 octane with 92, mix 40:1 Maxxima superK, 170 main, 52 slow jet, needle clip 3rd from the top. STock ignition with new oem cdi box. Thanks all!!

[wilkin250r]

And if that setup works for you, then run with it. However, I'm sticking with my position, you shouldn't be having detonation problems with a stock setup. Something is wrong, something is goofy. You shouldn't need to mix race gas, it should run just fine on 93 octane.

[fearlessfred]

Originally posted by wilkin250r
And if that setup works for you, then run with it. However, I'm sticking with my position, you shouldn't be having detonation problems with a stock setup. Something is wrong, something is goofy. You shouldn't need to mix race gas, it should run just fine on 93 octane. x2 definitly agree with that.