burned both pistons

[fabricated]

I burned both my pistons this weekend at glamis and I think it was an octane problem. I just got done milling .035 off the head and doing a dune port on the cylinders. I also Installed a 4 degree timing key and a set of 1/2 inch reed spacers. The bike really had a lot of torque and hauled ass on top end but the last day on the flats I was in 5th gear just at the beginning of the power band and the thing died. I was checking my plugs all weekend and they were dark brown. When I chacked them after it died they were still dark brown but had small fragments of aluminum on them and I knew it was from the pistons. My question is that I was running pump gas in the bike and I think I should have been running at least 50/50 with some vp. Does this sound right. I have toomey t5, 30 pilot jets, 340 mains, and the clip was on the 4th setting from the top on a dynojet needle.

Edited November 28, 2005 by fabricated

[sheefreak]

Sounds like you are definately rich enough for you mods unless you have an airleak. Detonation is very possible, not only did you shave the head, but you advanced the timing. Little late now but you should have checked the compression after the mods. Your port job should have alleviated some of the compression, but you don't know for sure until you check.

[banshee04le]

When you get it back together, be sure to check your squish clearance.

Just insert a piece of thin solder through the spark plug hole until it is against the

cylider wall and then turn the engine over 1 time by hand with the kicker.

Remove the solder and measure the flattened end with a mic or compare to a feeler gauge. It should be like .025-.050. Check it a couple of times in a couple different spots around the cylinder. If it is less than .030 you may need to have the dome recut in the head. Let us know what your compression is when you get a chance.

[TheBansheeMan86]

same kind of thing happened to me. riding at sea level at oregon dunes on the flats. i had ported pistons that were machined for advanced exhaust timing, +4 timing, .030 milled head, t5s, spacers, carbon tech reads, k&n clamp ons, running 360 mains with toomey needles on clip 4 of 6 from top. plenty rich, but blew 1 piston bout 1/2 way through powerband. stumped the shit out of me. i was running 92 octane pump gas, as i always do. only explanation was that everything else was advanced but my fuel. my ignition timing was advanced, my exhaust timing was advanced, my compression was upped, and there was twice as much fuel flowing through my pistons. i (along with several other engine builders) couldnt figure it out. many ignorant retards would instantly say "leaned it out" but my plugs were perfect, and my bike was actually running a little too rich that whole trip. long story short, i have no idea wtf happened.

[Shee-Male]

the plugs may look fine but it dosn't take much to melt down a slug, .030 +4 deg of timing your running borderline on pump gas, if you've gone .035 that could do it

detonation is hard to hear on a 2-stroke and it aint gonna ping forever.

[BigRed350x]

It goes a little something like this...

 

ping ping ping BOOM!

[fabricated]

Would everyone agree that a 50/50 of premium and vp c12 would be enough to aleviate the detonation problem.

[hittintrees srh]

Would everyone agree that a 50/50 of premium and vp c12 would be enough to aleviate the detonation problem.

443731[/snapback]

 

Yep-that would solve it for sure.

[Malott 1]

Would everyone agree that a 50/50 of premium and vp c12 would be enough to aleviate the detonation problem.

443731[/snapback]

Did you ever notice your sparkplugs backing out on their own? That is a sign of detonation.

[boonman]

When you smoke the top end, you usually have one of two things happen. You either blow a hole in the piston, or you melt the piston, near the exhaust port, and it smears aluminum all down the cylinder wall and all over the piston. When you blow a hole in the piston, it is due to detonation. 99% of the time. This can be due to tooooo much advance, too much compression, and not enough octane to combat the problem. With the melting on the exhaust side of the piston, now that would be due to a lean condition. Sounds like you didn't have enough octane for your mods. Or, some slightly old fuel. It all depends..... Compression reading won't mean a whole lot, the squish will tell you more. You can run pump fuel through a motor with 175 PSI with the right squish, or a motor with 140 PSI and the wrong squish and you will blow that mutha sky high!!

[FIRST BANSHEE]

When you smoke the top end, you usually have one of two things happen.  You either blow a hole in the piston, or you melt the piston, near the exhaust port, and it smears aluminum all down the cylinder wall and all over the piston.  When you blow a hole in the piston, it is due to detonation.  99% of the time.  This can be due to tooooo much advance, too much compression, and not enough octane to combat the problem.  With the melting on the exhaust side of the piston, now that would be due to a lean condition.  Sounds like you didn't have enough octane for your mods.  Or, some slightly old fuel.  It all depends.....  Compression reading won't mean a whole lot, the squish will tell you more.  You can run pump fuel through a motor with 175 PSI with the right squish, or a motor with 140 PSI and the wrong squish and you will blow that mutha sky high!!

444624[/snapback]

Now I don't have to type as much... I completely agree.

[PassionRE]

As in any high perf 2-stroke, squish clearance is critical, especially when you have a design goal in mind as far as the type of power you are trying to produce and when you are designing a particular engine to refrain form detonation and piston breakage.. Generally accepted maximum squish velocity(rate of velocity of gases when squished based at engine peak rpm, given compression ratio,exhaust port height, actual squish clearance and actual squish band width) is 15-30 meters per second.15 m/s being a road race type engine or high perf snow-mobile engine, 30m/s being a high torque MX style or watercraft engine. Lower squish velocity's make best power at or near peak rpm and higher velocity's make best power at or near peak torque. Choosing your rate depends on what your after in style of power and your pipes and porting should definately be taken into consideration when trying to decide on this number. Experimenting can be deadly and expensive. Here are a few examples...REMEMBER, ANYTHING OVER 30M/S IS CONSIDERED RISKY BEHAVIOUR.

 

Stock ported Shee with pipes that rev to 8K rpm on STOCK HEAD WITH MODIFIED SQUISH CLEARANCE.

 

SQUISH CLEARANCE SQUISH VELOCITY

 

.030" = 42.4M/S

.040" = 33.5M/S

.050" = 27.3M/S

 

Stock ported Shee with pipes that rev to 10k rpm on STOCK HEAD WITH MODIFIED SQUISH CLEARANCE

 

SQUISH CLEARANCE SQUISH VELOCITY

 

 

.030" = 52.9M/S

.040" = 41.8M/S

.050" = 34.1M/S

.060" = 28.3M/S

As you see, pipes DO make a difference if they yield more peak rpm. In this case, with the higher revving pipes, squish clearance needed to be increased with the given squish band width.

 

Keep in mind that narrowing the squish band width with a given squish clearance will drop the squish velocity. This is why builders modify squish band widths when milling stock heads. Increasing squish clearances will also lower squish velocity's as well. Likewise, wider band widths increase velocity with a given squish clearance, and decreased squish clearance will raise velocity with a given band width...hope that makes sense....Jim

Edited December 6, 2005 by PassionRE

[FIRST BANSHEE]

As in any high perf 2-stroke, squish clearance is critical, especially when you have a design goal in mind as far as the type of power you are trying to produce and when you are designing a particular engine to refrain form detonation and piston breakage.. Generally accepted maximum squish velocity(rate of velocity of gases when squished based at engine peak rpm, given compression ratio,exhaust port height, actual squish clearance and  actual squish band width)  is  15-30 meters per second.15 m/s being a road race type engine or high perf snow-mobile engine, 30m/s being a high torque MX style or watercraft engine. Lower squish velocity's make best power at or near peak rpm and higher velocity's make best power at or near peak torque. Choosing your rate depends on what your after in style of power and your pipes and porting should definately be taken into consideration when trying to decide on this number. Experimenting can be deadly and expensive. Here are a few examples...REMEMBER, ANYTHING OVER 30M/S IS CONSIDERED RISKY BEHAVIOUR.

 

Stock ported Shee with pipes that rev to 8K rpm on STOCK HEAD WITH MODIFIED SQUISH CLEARANCE.

 

SQUISH CLEARANCE                    SQUISH VELOCITY

 

.030"  =                                            42.4M/S

.040"    =                                          33.5M/S

.050"    =                                          27.3M/S

 

Stock ported Shee with pipes that rev to 10k rpm on STOCK HEAD WITH MODIFIED SQUISH CLEARANCE

 

SQUISH CLEARANCE                      SQUISH VELOCITY

.030"      =                                          52.9M/S

.040"        =                                        41.8M/S

.050"        =                                        34.1M/S

.060"        =                                        28.3M/S   

  As you see, pipes DO make a difference if they yield more peak rpm.  In this case, with the higher revving pipes, squish clearance needed to be increased with the given squish band width.     

 

Keep in mind that narrowing the squish band width with a given squish clearance will drop the squish velocity. This is why builders modify squish band widths when milling stock heads. Increasing squish clearances will also lower squish velocity's as well. Likewise, wider band widths increase velocity with a given squish clearance, and decreased squish clearance will raise velocity with a given band width...hope that makes sense....Jim

445531[/snapback]

Is there really a differance between engine builders...

[fabricated]

okay, I pulled the motor apart and both pistons had a crater in the center of the domes. the right piston had a slightly larger crater and the burned material flowed toward the exhaust side of the piston and a little flowed down the exhaust side of the piston. there was a tiny bit of aluminum on the cylinder but it will come off with a hone. there was no scoring and it was hard for me to determine why the bike died because the pistons did not burn completely through and the piston that had a little aluminum flow down the side of it did not change the shape of the piston. The motor did not lock up because it kicked over as easy as it did running. I have this theory that because there was a bit of aluminum on the plugs that it may have fouled one before the motor completely burned through. Well I just bought some new wisecos and will put those in and try running straight c12 through it for the next trip. I am also going to leave the 30 pilots and clip location at the 4th groove but put 350 mains for the change in fuel. If it blows again I know it is not due to detonation.any feedback will help

Edited December 8, 2005 by fabricated

[GREATDEAL]

what ever you do don't race gas with pump gas I knew a chemist through my machine shop that I use for my car he told me that when you mix the two together you end up with a lower octane level that will be more prone to detanation and that is defanatly what you don't want I know there are some of you that think that I am full of shit for saying this but if you want to save your motor don't go cheap just spend the extra $10 and not have to think twice about it Just my $0.2 .