detonation

[maxis417]

Guys i'm having a problem with detonation. I installed a new crank and new top end along with getting the cylinder re-sleeved. i had a knock before i did all that thinking that the new crank would fix the problem but.. It didn't. I had a buddy look at it and listen to it and he suggested detonation and told me to try running some race fuel and see if that cleared it up. It very much helped the problem but it still knocks a slight bit. I know my plug and spark is hot.

What can i do to fix this? Is it my CDI box possibly? Ive played with timing but that didn't help anything. Any ideas??
2004 RM250

 

[rmc_olderthandirt]

If you have installed a high compression piston or similar compression raising modifications you may need to run a higher octane race fuel.

If your compression is stock then you might be experiencing pre-ignition, which can be caused by the spark plug getting too hot. You might want to try a cooler plug.

Rod

 

[maxis417]

well the bike is a 04 rm250. it has the plug that it is supposed to have in it a NGK BR8EG i believe? But its what my manual says. I have 110 octane fuel in it. It has 190 - 200 pounds of compression. but even after i ran the fuel in it it still has a lil knock so there is something that is causing that and i need to know what that is.
the bike was knocking before the rebuild and it then before had 125 psi. i know its pre ignition but i need to know what can fix that. i currently have 110 octane race gas in it and that helped it but its still there.

 

[drrm123]

Look at the plug and see if it is running lean. If so richen the mixture. depending on where your getting the pinging at upper or lower rpm's will determine the circuit you adjust. And cooler plug should help also. last option is to check the cooling system to make sure it's working properly and coolant if mixed correctly.

 

[rmrider45]

sounds like your jetting.. or air/ fuel mixture screw is out to far (to lean)

 

[whenfoxforks-ruled]

So, you do not know if its really detonation? Could be a few things. A simple plug check can verify, lean or detonation. Jumping the gun and running race fuel does not help, it may complicate the diagnosis. Is it race fuel from a sealed container? You can run it a little lean on the stand, but I would want no part of my piston splattering off. That knock could even be the wrong sparkplug temperature. Back to the beginning, premium fuel and check the plug. From that check for lean, timing and plug temperature. And see if its detonation, aluminum splatters? Vintage Bob

 

[IndyMX]

It has 190 - 200 pounds of compression.

That's a good indication that your cylinder, piston & rings are in good shape, but doesn't say anything about the compression ratio.

To know that, you need to take measurements.

It seems this is one of those factors that is often confused like the whole rich/lean thing.

 

[maxis417]

So, you do not know if its really detonation? Could be a few things. A simple plug check can verify, lean or detonation. Jumping the gun and running race fuel does not help, it may complicate the diagnosis. Is it race fuel from a sealed container? You can run it a little lean on the stand, but I would want no part of my piston splattering off. That knock could even be the wrong sparkplug temperature. Back to the beginning, premium fuel and check the plug. From that check for lean, timing and plug temperature. And see if its detonation, aluminum splatters? Vintage Bob

what do u mean it splatters? should i see particles on the spark plug?

 

[whenfoxforks-ruled]

Yep. Pinpoint small and real shiny. Well, what does the plug look like? No camera either? Just because you have a tester telling you its pushing 200 psi is no indicator it needs race gas. CC the head, or try premium fuel, and check the plug. There is a lot of things that can cause a sound like a knock in a 2 stroke race engine. Its imperative that you learn to read the plug, or post some really good pics, a lot! Some just sound like that, especially if it has been modded? Vintage Bob

 

[maxis417]

Yep. Pinpoint small and real shiny. Well, what does the plug look like? No camera either? Just because you have a tester telling you its pushing 200 psi is no indicator it needs race gas. CC the head, or try premium fuel, and check the plug. There is a lot of things that can cause a sound like a knock in a 2 stroke race engine. Its imperative that you learn to read the plug, or post some really good pics, a lot! Some just sound like that, especially if it has been modded? Vintage Bob

im sorry no camera... but tomorrow i will drain the gas a put my premium back in it and check the plug. im nervous to do that though beause the thing really sounds like it is fixin to blow apart with the premium gas. it really makes me nervous riding it like that.. but i will and ill read the plug and post again.
thanks for your help so far

 

[whenfoxforks-ruled]

If it IS detonating I would not start the bike! Not 1 word that defined it from you, yet? Vintage Bob

 

[maxis417]

How can i exactly check the timing? I know that the marks on the stator are supposed to be lined up and my manual says that this means the timing is correct but is that the only way to tell? How can i accurately check the timing?

 

[whenfoxforks-ruled]

Reading the plug, or a timing light. So no detonation? Advanced or retarded timing? The plug is wet or dry? Vintage Bob

 

[DEANSFASTWAY]

Does the noise go away when you pull inthe clutch if so it could be noise in the tranny and ive heard quite a few of these RMs with loder tranny noise espescially when guys beat the balls out of them in the sand.A bad CDI box will burn the piston down usually with a hole in the dome but id make sure the base timing is right. Good luck

 

[maxis417]

Does the noise go away when you pull inthe clutch if so it could be noise in the tranny and ive heard quite a few of these RMs with loder tranny noise espescially when guys beat the balls out of them in the sand.A bad CDI box will burn the piston down usually with a hole in the dome but id make sure the base timing is right. Good luck

How can i test the cdi? My base timing is correct and today i ran a cooler plug and that may have helped a little bit but the problem is still there. I checked the plug and it appears to be running a little lean but its not that bad. Im pretty sure its not pinging but i could be wrong.
Yes i do have some tranny noise but it stops when i pull the clutch and its not bad. Im still lost on my problem. I believe its detonation or pre ignition but im not sure on how to exactly diagnose it

 

[maxis417]

Reading the plug, or a timing light. So no detonation? Advanced or retarded timing? The plug is wet or dry? Vintage Bob

Checked the plug today and i didnt see any particles on it. It was a little dry but it wasnt that bad. Im going to take the carb off tomorrow and check again to see what size jets i have but i still feel like its detonation or pre ignition. What would u do? I have a new air filter and i tried a cooler plug and that helped a little bit but its not fixed.
Also the bike is extremely hard to crank when it gets warm. It cranks so much better when cold. Any more ideas?

 

[whenfoxforks-ruled]

If its something still mechanical going on, you are beating your head against a wall. If I had a knock, and no sign of detonation, ovaled bore or the exhaust valve. If the plug appears lean, an air leak, it should have an erratic idle, or erratic throttle response. To test the cdi you need the owners manual and a multi tester. In the mean time: www.dirtrider.net/forums3/showthread.php?t=92818 . If you go to the archives forum, pull down and list all the threads from the beginning. ALL the answers are in there, and no one is hardly ever in there. Vintage Bob

 

[maxis417]

If its something still mechanical going on, you are beating your head against a wall. If I had a knock, and no sign of detonation, ovaled bore or the exhaust valve. If the plug appears lean, an air leak, it should have an erratic idle, or erratic throttle response. To test the cdi you need the owners manual and a multi tester. In the mean time: www.dirtrider.net/forums3/showthread.php?t=92818 . If you go to the archives forum, pull down and list all the threads from the beginning. ALL the answers are in there, and no one is hardly ever in there. Vintage Bob[/QUO

Its not mechanical. Its all new. New crank and bearings and seals, new piston and rings and wrist pin and bearing, cylinder was sent off and resleeved. The exhaust valve was also inspected when it was resleeved and clearances checked out good. I did the entire rebuild myself and strongly stand behind my work. I have tested the cdi and most all check within spec. I have seen one or two post that read i little bit high.. Can i have my cdi sent off and re mapped or must it be replaced

 

[DEANSFASTWAY]

This might sound like a screwy question but i have to ask it because ive seen it before but When you got your cylinder resleeved did they match the ports correctly? because if they just installed a sleeve and didnt do it the bike will have both incorrect port timing and probably too high compression .Reason I ask this is that not many auto machine shops have a right angle grinder and bits and theres a couple local shops by me that send me their stuff after theyve installed sleeves. Also could your power valve have too much preload that is a compression altering device ?? On CDI I dont know if theres a way to check the timing curve to any type of specs but I could be wrong. That might go into the arena of find a buddy ya know find a buddy with the same bike and swap. Usually manuals only shoe how to check for spark.

 

[maxis417]

Yes its matched correctly because i talked to the man on the phone and he said he had to grind out the ports with a special grinder. And i wish i had a buddy but i dont know anybody with that kinda bike. I really have absolutely no idea what the problem is... Not the slightest clue. It appears to be pre ignition or detonation because the race fuel really helped the problem but still didnt exactly fix it bacause it still has a slight knock to some sort. If anybody thinks of anymore ideas please help!!! I need all the help i can get on this one :bang:

 

[whenfoxforks-ruled]

Big end bearing thrust clearances are good? Small end id and bearing okay? Are there any dents in the expansion chamber and fresh packing? Is the core still attached on one end? Am I glad I did not have to bring up the sleeve disorder, Thanks Dean! Double what he said. Was the cylinder and/or head modded? Did you check the piston clearances, and what did you set the rings at? Vintage Bob