Checkmate Posted December 9, 2003 Report Share Posted December 9, 2003 For the Second time now i've had some serious detonation and keep putting holes in the piston (one side). Both times it happened i've been running in the snow...turning high RPMS on a wide open road. Both times...same piston. I am running vito's forged power pistons, a D&D head with 20CC domes and stock carbs with vitos needles and TORS kit. FMF's, advanced timing (stator plate). I am running 94 sunocco and Motul Oil mixed at 40:1. Jets are 300m/30p. K&N with airbox, no snorkel, and a few 1" holes in lid. After the first meldown (which also destroyed my crank and dome due to debri), i thought i found the problem. That side carb had an air leak. fixed the leak, stepped up a size in jets, and rebuilt the whole motor. This Meltdown, i noticed that the hole in the piston is in the same side, but also....the other side started to burn through the surface of the piston. I also noticed that this time the hole in the piston wasn't as large as the prior meltdown, and my 20CC dome is OK (must have shot most out the exhaust). So obviously i must be way low in jetting to be running in the 10 degree temps spinning high RPMS. (When i did plug chops it was in the 80 degree summer, on the street). I haven't torn the bottom end apart yet, but my crank seems to be spinning free and the rod play is in tolerance. My jugs did get a few scrapes, so they will need a skim. Questions: Is there anyway to flush the bottom of the crankcase of the tiny metal particals without splitting the cases and pulling the crank? Should i be running higher octane? larger jets? Should i go to a single carb setup for more reliability/less aggravation? Will cast pistons do any better? Sorry for the long post, but i am gettin frustrated with this thing!! and i don't want to have to go to a Thumper due to my frustrations.......(As i was broke down in the middle of the street a CRAPTOR and a 400ex went thumpin by me ) Quote Link to comment Share on other sites More sharing options...
txblueshee Posted December 9, 2003 Report Share Posted December 9, 2003 Sounds like detonation to me.... do a google search on your elevation. w/ that +4 timing advance and 20cc domes, you should be AT LEAST 1000ft. above sea level on 93 octane. I run 20ccdomes on 93 at 850ft....... One piston has a hole in it.... the other one is pretty bad off right?? Prolly wouldn't hurt to step up a main size or 2. Mike Quote Link to comment Share on other sites More sharing options...
Checkmate Posted December 9, 2003 Author Report Share Posted December 9, 2003 Sounds like detonation to me.... do a google search on your elevation. w/ that +4 timing advance and 20cc domes, you should be AT LEAST 1000ft. above sea level on 93 octane. I run 20ccdomes on 93 at 850ft....... One piston has a hole in it.... the other one is pretty bad off right?? Prolly wouldn't hurt to step up a main size or 2. Mike Yea....the other side was also starting tp burn. I just did a search on my elevation, and it says i'm 100ft. above sea level. Quote Link to comment Share on other sites More sharing options...
txblueshee Posted December 9, 2003 Report Share Posted December 9, 2003 Sounds like detonation to me.... do a google search on your elevation. w/ that +4 timing advance and 20cc domes, you should be AT LEAST 1000ft. above sea level on 93 octane. I run 20ccdomes on 93 at 850ft....... One piston has a hole in it.... the other one is pretty bad off right?? Prolly wouldn't hurt to step up a main size or 2. Mike Yea....the other side was also starting tp burn. I just did a search on my elevation, and it says i'm 100ft. above sea level. OH DAMD!!! I detonated once b/c of 18cc domes on 93 at my elevation....100ft!! w/ the +4, 20cc domes and the 94 octane=KABOOM!!. That combo puts you about 180psi(maybe higher I don't know too much about the timing advance).... the breaking point for 93/94 octane is 157-165psi. That's about the psi I detonated at. If you want to keep your setup then run at least 100octane... if you still want to run 94octane then drop down to a 22ccdome. Mike Quote Link to comment Share on other sites More sharing options...
boonman Posted December 9, 2003 Report Share Posted December 9, 2003 Being that you noticed it in both cylinders, this can be attributed to too low of octane for the compression. Do you own a compression guage? If not, now would be an excellent time to purchase a good one. I was plagued by problems a year ago. I soked the top end 3 times and the lower end once. i am going to tell you this as well. Split the cases. Clean the crank THOROUGHLY. if you don't, you will throw a rod bearing most likely. I did. I tried to clean the crank without splitting the cases. it doesn't work. i am guessing that the air leak you found on the side last time accelerated the problem and that's why it appeared so quickly on that side. At your elevation, you ought to be at about 320 on your mains at that temp as well. There is no need to go to a thumper. However, there is a need to either switch domes, or go to a higher octane fuel. Also, it may suffice for you to visit BenBB's jettingFAQ page. When you get this motor cleaned out, cylinders honed, and you put new pistons in it, (no need to switch to cast by the way) make sure you do a leakdown test on it to be sure it has no leaks. No need to switch to a single carb, unless you want to..... Quote Link to comment Share on other sites More sharing options...
BenBB Posted December 9, 2003 Report Share Posted December 9, 2003 I agree with tx & boonman...the combination of 20cc domes, advanced timing, low elevation, and power pistons (don't they have a taller crown?) was too much for 94 octane. If you were jetted for 80 degrees you REALLY need to jet the mains higher for 10 degrees too... Someone correct me if I'm wrong, but isn't holes in the middle of the piston usually detonation, and melting on the exhaust side usually lean jetting? Quote Link to comment Share on other sites More sharing options...
txblueshee Posted December 9, 2003 Report Share Posted December 9, 2003 Ben, Holes in the middle means detonation(know from experience) but mine also like melted, gauged, smeared down the front of the pistons. Mike Quote Link to comment Share on other sites More sharing options...
Checkmate Posted December 9, 2003 Author Report Share Posted December 9, 2003 Do you own a compression guage? i am going to tell you this as well. Split the cases. Clean the crank THOROUGHLY. if you don't, you will throw a rod bearing most likely. I did. I do have a gauge......I think i was aroound 180 when i checked compression. At what compression should you stop running pump gas?? Damn.....I was hoping to not have to pull the lower out and split the cases again. Last time i had problems during re-assembly with the clutch. I guess you cant get one of those small 3/8" computer style vacuum tubes in there?? Quote Link to comment Share on other sites More sharing options...
Checkmate Posted December 9, 2003 Author Report Share Posted December 9, 2003 I agree with tx & boonman...the combination of 20cc domes, advanced timing, low elevation, and power pistons (don't they have a taller crown?) was too much for 94 octane. If you were jetted for 80 degrees you REALLY need to jet the mains higher for 10 degrees too... Someone correct me if I'm wrong, but isn't holes in the middle of the piston usually detonation, and melting on the exhaust side usually lean jetting? Not sure about a taller crown, but they have the cut out in the front to advance the exhaust out quicker. My holes ARE buring thru more towards the front exhaust port. So that means not enough fuel (jetting) ??? What is the difference between a lean condition due to jetting, and detonation ??? I thought they were kind of the same thing?? Quote Link to comment Share on other sites More sharing options...
txblueshee Posted December 9, 2003 Report Share Posted December 9, 2003 Originally posted by TXBLUESHEE the breaking point for 93/94 octane is 157-165psi. You will NEED to split your cases, I got away w/ out it b/c when detonated my piston skirt did not break nor did my piston shatter. Seeing as how you have detonated TWICE you will need to split the cases. Quote Link to comment Share on other sites More sharing options...
Checkmate Posted December 9, 2003 Author Report Share Posted December 9, 2003 Originally posted by TXBLUESHEEthe breaking point for 93/94 octane is 157-165psi. You will NEED to split your cases, I got away w/ out it b/c when detonated my piston skirt did not break nor did my piston shatter. Seeing as how you have detonated TWICE you will need to split the cases. sorry....didn't catch your other reply with the octane figures. What did yours look like when you detonated?? How did you get away with not splitting the cases??? my skirt didn't break or shattered piston the piston either...just looks like it was almost sandblasting the top of the piston till it wore thru. I have very fine metal shavings. Oh.....and the last time it happened i did split the cases and replaced the crank while i was in there. Quote Link to comment Share on other sites More sharing options...
txblueshee Posted December 9, 2003 Report Share Posted December 9, 2003 Looked like someone shot the piston w/ a 12 gauge slug/bb shot combo. One big DENT w/ little pits all around the piston(greater towards the exhaust port). The ring holders/rings were ground/melted smooth together on the front (exhuast side) of the piston. You MIGHT (same chance I took) be alright w/ just draining the fluid twice b4 start up and installing a magnetic drain plug(catches debri). Then changine it again during and after break in. Mike Quote Link to comment Share on other sites More sharing options...
Checkmate Posted December 9, 2003 Author Report Share Posted December 9, 2003 Is there a drain plug for the combustion chamber (crank area) ??? that would help me alot!!!!! I thought there was only one for the gearcase. As far as i remember they were not connected right?? If there IS a drain for the Crank, then i may try that.....flush the shit out of it with kerosene. If the smoke the crank, then i have the excuse to stroke it. Quote Link to comment Share on other sites More sharing options...
. Posted December 9, 2003 Report Share Posted December 9, 2003 nope, no drain plug for the crankcase. Your gonna have to pull the bottom end to flush it with kerosene anyway. I'm not positive but i heard someone on here splitting the cases upside down and you didnt have to pull the rest of the engine apart. But since yours is already apart. Pull the top case and get to cleaning. Make sure its really clean, you dont want metal shavings getting into the crank bearings. Especially if its a new crank. Quote Link to comment Share on other sites More sharing options...
boonman Posted December 10, 2003 Report Share Posted December 10, 2003 My holes ARE buring thru more towards the front exhaust port. So that means not enough fuel (jetting) ??? What is the difference between a lean condition due to jetting, and detonation ??? Any hole burned through a piston can almost always be attributed to detonation, or pre-ignition, or both. A strictly lean jetting condition will cause extremely high EGT (Exhaust Gas Temp.) and melt the piston as the exhaust exits the cylinder leving you a tril of melted aluminum down the side of the piston and cylinder on the exhaust side. In your case, I think you definitely had a problem with detonation, being that you were at 180. I am at 195 on a guage, stock porting, and I can squeak by with a 50-50 mix of 94 and VP C-12. Attributing to your problem was the fact that you did not re-jet for the colder temps. This heated things up inside the motor, and when it got hot enough, the detonation starting taking it's toll, and you blew a hole through the wickedly hot piston. Now, even though you didn't crack the skirt off, (uncommon on a Forged piston, common on a Cast) you still have the debris from the melted piston crown floating around in the case. And I don't know where TX was going with the changin oil theory, your tranny oil is just that, tranny oil. It doesn't contact the crankcase like a 4-stroker.... Pull the motor and clean it out good, take your time putting it back together, and if you have any more questions, someone here should be able to help.... Otherwise, I have a strong feeling that you will be needing a new crank... Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.