Pre-ignition?

JDXsand00s · Post by **JDXsand00s** » Sat Dec 24, 2005 1:20 pm

I just rebuilt my 500 engine, pressure tested it, and it had new pistons and rings, and the pto side at 200 pounds of compression and 150 mag side. Took it out for a ride, carb set at 1 on the low needle and 1 1/2 on the top one. I was also using champion n3c plugs. half a mile later, blew up the engine again, that is the 3rd engine my 500 has went through. it was 35 degrees out and my cylender 1 plug disintegrated. it also almost blew a hole through the piston. i don't think my seal caused meltdown this time. could it be that my metering lever was set too lean or were my plugs to hot of a heat range? also, is there a point at which opening the high speed needle doesn't let anymore gas in because the metering lever is letting the max amount of gas in? thanks.

400brian · Post by **400brian** » Sun Dec 25, 2005 2:33 am

How do you mean the plug disintegrated? Blew apart, melted, what?

Did you acid treat the jugs before reassembly?

Has this engine been timed?

A half mi to failure sure seems to indicate something very wrong

JDXsand00s · Post by **JDXsand00s** » Sun Dec 25, 2005 7:21 pm

I blew the bottom off of the plug, it was pressure tested, engine timed to .023 btdc, never did anything with acid. I believe that my carb may have been too lean, my plugs too hot, and I might not have been properly breaking it in (too much rpm).

400brian · Post by **400brian** » Mon Dec 26, 2005 1:49 am

He has '73 500 in the title bar.

Most of the guys seem to think NGKs are superior to the original ACs or Champs.

If you have scored a piston, or had even a mild seizure, you most likely have aluminum smeared into the surface of the cylinder liners, this has to be chemically removed, or another seizure will result, aluminum is a very poor lubricant.

A couple of things bother me here. 1) you had a pretty significant difference in your compression readings between cylinders, and 2) you shouldn't blow the plugs apart under any normal conditions.

Again, the fact that the engine failed so quickly seems to indicate something very amiss. Take a close look at the piston and jug on the cylinder that failed. Re-think your plug selection. Make darn sure you start with the carb adjusted on the rich side, and then after break-in, lean it toward optimal. Is the carb clean and functioning properly, would be another issue. What does the plug look like on the cylinder that didn't fail?

JDFanPa · Post by **JDFanPa** » Mon Dec 26, 2005 5:17 am

Like Brian said WOW on the compresion difference. Personally theres no way you would do that damage that quick from a wrong heat range plug. If the other cylinder is in good shape nice plug color etc Id suspect it sucked major air somewhere seal,intake maybe even the crankcase where they split. If the other side looks very lean also the carbs probably got a plugged up high side.

JDXsand00s · Post by **JDXsand00s** » Mon Dec 26, 2005 11:04 am

plug was the lightest brown i've ever seen, also i hone the cylenders to get rid of the aluminum thinking that will clean it sufficiently. also, what is the difference betwen the 72 and 73 440 engine? everything is exactly the except the stator which has the same readings and the fan shroud is bigger. i've never had any luck with carbs, this one was completely rebuilt and all of the ports cleaned, but i can never get the metering lever right, i thought i had it set too rich, the lever was portruding way above and the needle was opening a lot, then i bent it down again and the needle seemed to be opening still but not as much. also, how long should it take for and engine to blow up? it was about 3 miles on my x8, 1 mile on the 1st 500 engine, 2 miles on the 2nd, and now a half of a mile on the 3rd. piston was both scored on exhaust side and top was screwed up this was a nos piston.

harleysportster · Post by **harleysportster** » Mon Dec 26, 2005 12:09 pm

Ryan,
Your picture shows signs of excessive heat, problably from pre-ignition.
Pre-ignition is the igniting of the fuel-air mixture in the combustion chamber before the spark fires.
The most common causes are:
Wrong heat range of spark plug
deposits in the combustion chamber(how clean was the inner dome of the head)
Pre-ignition causes the fuel-air mixture to burn longer than was intended which creates excessive heat in the piston dome. If the situation is not corrected, the piston dome will start to melt and eventually burn through the dome and / or scuff the piston wall.
What 2 stroke oil ratio are you using.

harleysportster · Post by **harleysportster** » Mon Dec 26, 2005 12:39 pm

TakeItToTheLimit,
He mentioned in the first post of pressure testing.
JDXsand00s, did you use the method I outlined in the FAQ section.
Did you replace the crank seals,base gaskets, intake gaskets.

JDXsand00s · Post by **JDXsand00s** » Mon Dec 26, 2005 2:00 pm

All seals and gaskets were new and engine held a vaccuum for quite awhile without leaking. this was the first engine though i have pressure test and i probably should have done all the ones that i've blown up in the past. this is why is it confusing me so much because i knew afterwards on the other engines that blew that they had used bad seals, these were good though and that is why i think it was the plugs and carb. when i put the jugs and head on the last engine i reused the parts from the previously burnt up cylende and just honed the inside. i did not clean the head however, i guess i didn't know it could cause a problem. also, gas mixture is between 40 and 50:1, i try and put 16 oz in 5 gal but i spill an oz here and there, so closer to 14 oz.

harleysportster · Post by **harleysportster** » Mon Dec 26, 2005 2:50 pm

JDXsand00s,
The cylinder that blew, is this the one you mentioned that had 200 psi compression.
Normal engine break-in: 40:1 ratio... 16 onces of 2 stroke oil with 5 gals. gas for the first tankfull. Then 50:1... 16 onces of 2 stroke oil to 6 gals of gas or 12.30 onces of 2 stroke to 5 gals of gas.
Here is a handy chart,
http://www.csgnetwork.com/oilfuelcalc.html

400brian · Post by **400brian** » Mon Dec 26, 2005 4:22 pm

Honing will not sufficiently remove the aluminum from the walls of the jug.
It MUST be chemically cleaned. Back in the day, your JD shop wouldn't mess with this, they would just put in a new jug and piston.

JDXsand00s · Post by **JDXsand00s** » Mon Dec 26, 2005 9:21 pm

There is no problem with the comments, alsoI have "rebuilt" most of my engines: ther is no point in doing a just the top end unless you only blow a head gasket. Then i would just say i changed the head gaskets and wouldn't use the word rebuild. also, locating and correcting the cause seems like a great concept though it seems there was more than one problem at a time with these previous engines and it is hard to diagnose them all. i just wish i would have never blown my 1st engine, put 40 miles on it one day at 40 mph through a foot of snow last year but thought it was too slow as my 400 was doing 55 down the trail. leaned the carb to get better performance and a mile later down the trail the next day it was dead. i also think that my lead foot my have contributed, the sled cruised good for the time it ran at full throttle. also looked at mag side cylinder through plug hole, had rough black covering over half of it and it was nos piston. don''t know if this means anything. i was lucky though the day i blew it up, i finally got my x8 running after i blew up my spitfire and my 400 seemed to lose power so i finally had something with enough balls to haul it home. i did run out of gas though 100 feet from my house.