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Old 11-04-2008, 08:37 AM   #1
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Default Pre-Detonation problems NEED u experts opinion!!

Im running a Picco Evo 2 .21 motor on O'donnells 30% race fuel with a non-turbo McCoy MC-9 plug which is a cold plug with a JP3 pipe. Well im getting pitting on top of the piston and inside the head button which i have read that that from pre-detonation. Well i pulled the motor apart and seen it only has one shim and im assuming that its a .01mm shim and its paper thin. Should i be running how many shims and should i invest in a turbo head button? And will pre-detonation cause a motor VERY difficult to tune because im having one hella of a time of getting it tuned and running right. So is the wrong shimming my problem?
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Old 11-04-2008, 09:05 AM   #2
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Thats most likely the problem. I have 3 head shims in my Jammin 21 and running on 30% and it is fine. When you have the higher nitro content it is causing it to light off before it gets all the way up the compression stroke. If I recall correctly the manual for the picco Evo 2 says to run 20-25% nitro. I ran five gallons of Byrons 20% through the Evo 2 I had and never had a problem out of it.
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Old 11-04-2008, 09:58 AM   #3
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Your motor should have 6 to 8 thousandths clearance in the squish area for every 10% of nitro. So you should have .018 to .024 clearance. You can determine the correct amount of shims two ways.

The easiest way is to use a curved piece of solder, insert it into the glow plug hole against the side of the sleeve. Position the solder over the wrist pin for a more accurate reading. Roll the piston over top dead center and measure the end of the solder with a caliper or slide caliper.

The other way requires disassembly. Measure from the top of the piston to the top of the sleeve with the piston at top dead center (e.g. .040. Record the measurement. Next measure how far the button or head extends into the sleeve with shims removed. Measure the shims and subtract from the button measurement (e.g. .030 -.006 == .024). Subtract this measure from the previous recorded sleeve measurement. For example, .040 - .024 = .016., them you will need to add another shim.

Or you could just guess and ad a shim.
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Old 11-04-2008, 11:19 AM   #4
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Originally Posted by TXRC8racer View Post
Im running a Picco Evo 2 .21 motor on O'donnells 30% race fuel with a non-turbo McCoy MC-9 plug which is a cold plug with a JP3 pipe. Well im getting pitting on top of the piston and inside the head button which i have read that that from pre-detonation. Well i pulled the motor apart and seen it only has one shim and im assuming that its a .01mm shim and its paper thin. Should i be running how many shims and should i invest in a turbo head button? And will pre-detonation cause a motor VERY difficult to tune because im having one hella of a time of getting it tuned and running right. So is the wrong shimming my problem?
Be sure to look very close, sometimes the thick shims are aluminum and blend into the head button. Give a little tug near the bottom of the head button to make sure you dont have an aluminum shim stuck there. One thin shim seems a bit to little for that motor. Pitting on the head button and piston can also mean dirt... Double check the rest of the motor and flush it just in case. Also if you did pull a little dirt, the tune goes out the door really quick! Good luck.
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Old 11-04-2008, 01:32 PM   #5
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I have tore the motor down twice just to make sure there was no dirt or debri in the motor.

I also scheck the head button for anyother shims and there wasn't just that only little .01 shim.

And i ordered 2-.01mm shims and 2-.02 shims. So as soon as they get here because my LHS does not have any i will try to add 2 of the .01mm shims so that will be a total of .03mm.

And i also noticed that when i get the motor to TDC it has a hard time of coming back down it seems like its almost getting stuck at TDC or the tolorance between the sleave and the piston tightens up is this just because there is no lubrication when i have the motor apart or should i slightly hone out the sleave?
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Old 11-04-2008, 01:48 PM   #6
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Don't ever hone, polish, sand, anything the sleeve or sides of the piston. This will take life off if not ruin the engine. They make their compression with the fit. If it is tight, it just needs to wear in more by being run. How new is the engine? Usually if you get dirt or something in there like suggested above you will pop plugs within half a tank as well as get the pitting.
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Old 11-04-2008, 02:28 PM   #7
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I have not sanded or honed the sleave at all. I bought the motor used from my LHS and they did not know how much fuel has been thru the motor so im assuming there has been a couple of gallons thru it. But i have smothed out the top of the piston so the pitting is not as bad along with the head button.

And at first i was poping plugs pretty quick but im on the same plug for the last 2 or 3 tanks but. Im thinking the pre-detonation problem was poping my plugs because they were deformed and all.

So i guess everyone agree's that the wrong shimming is my tunning problem? and pitting problem?
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Old 11-04-2008, 02:35 PM   #8
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Sounds like the culprit.
Bad shimming (to low) can cause all the problems you say you have, pitting, hard tuning, going thru plugs.
Tell us how it works when you get the shims in it. Should be easier to tune as well.
You can smooth out the piston top and button like you did without any problems.
Good luck, and write back if the shims help.
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Old 11-04-2008, 02:40 PM   #9
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Quote:
But i have smothed out the top of the piston so the pitting is not as bad along with the head button.
While not the exact same as adding another shim, you have decreased the compression by adding to the volume at the top of the cylinder. Not the best way to have solved the problem but what is done is done.

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Old 11-04-2008, 04:10 PM   #10
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While not the exact same as adding another shim, you have decreased the compression by adding to the volume at the top of the cylinder. Not the best way to have solved the problem but what is done is done.

Ed M.
I agree but i bet i did not even take .01mm off because i did not remove all the pitting. So i will add .02mm shims in a few days and iw ill let you all know how it works.
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Old 11-04-2008, 09:58 PM   #11
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im having a somewhat similar problem with my vspec- got it used and it only has 1 shim, going through a plug every 6 or so tanks. my new shims are in the mail...
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Old 11-08-2008, 09:18 PM   #12
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Well i added a .02 shim with the .01 shim today and it seems running better i actually go to run it up and down the street and i just need to fine tune, its real slugish on bottom end i think its lean because when i pinch the fuel line the motor dies pretty fast, i've been told it needs to idle up a little and run for 4-5 seconds before it dies. But i think the detonation prblem is gone.

Also one more question the idle speed screw clock wise to idle up and counter closck wise to idle down is this correct?
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Old 11-09-2008, 08:18 AM   #13
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Quote:
Originally Posted by xyzyx00 View Post
im having a somewhat similar problem with my vspec- got it used and it only has 1 shim, going through a plug every 6 or so tanks. my new shims are in the mail...
Thought my v spec only had one shim also, but the aluminum shim had actually stuck on the head button & blended in. Took a razor knife on the edge & peeled it up. Mine also was eating plugs & the problem was a worn crank pin. Bad bearings will do it too.
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