Piston Burn-Down Problems, please help

MountainMax

MountainMax

New member
Could you guys give me some opinions on why this burn down Hapened on my 80 SRX440 with race porting, and Aaen Race Pipes using 38/42 TMX Carbs, It's from my 1980 SRX440 modded to the nuts. It burned down both Exhaust sides only,My plugs read fine, nice tan, my egt's were about 1100 degrees, running 110 Octane Race fuel with Dominator Race Oil, Coolant Temp well in the green. Timing is also advanced as per SRX Snow Race Yamaha Manual. Oh, yeah they were New Wiseco Pistons that were coated at Swain Tech with ceramic on top and anti friction race coating on sides. I had good cylinder clearances and ring end gap. I Only got aobut 6 runs down the 660ft track (@75MPH) then i lost power as compression dropped and this is what I found.
 

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whats your piston to cylinder clearance? was the motor broken in properly before flogging down the track? was the motor properly warmed up before running down the track? is there seziure marks or scuffs on the intake side? is there pitting, ash formation, aor anything else in particular on the top of the piston on the exhaust side that would indicate you had detonation?
 
MountainMax said:
Could you guys give me some opinions on why this burn down Hapened on my 80 SRX440 with race porting, and Aaen Race Pipes using 38/42 TMX Carbs, It's from my 1980 SRX440 modded to the nuts. It burned down both Exhaust sides only,My plugs read fine, nice tan, my egt's were about 1100 degrees, running 110 Octane Race fuel with Dominator Race Oil, Coolant Temp well in the green. Timing is also advanced as per SRX Snow Race Yamaha Manual. Oh, yeah they were New Wiseco Pistons that were coated at Swain Tech with ceramic on top and anti friction race coating on sides. I had good cylinder clearances and ring end gap. I Only got aobut 6 runs down the 660ft track (@75MPH) then i lost power as compression dropped and this is what I found.
Click to expand...

It looks like a classic lean burndown. The mph is very low for 660'. If you feel the jetting was good other things may have contributed to the lean condition. Even if you monitor your EGT motor conditions can change in a blink of an eye. I have seen similar burndowns due to over rev conditions. This would explain the low mpg. Once ideal shift rpm is exceed and especially on a race ported motor with a very narrow power band. The motor over rev's drops off onthe back of the power curve and a lean condition occurs. It may have caused the failure. Squish and compression may have played a role but I assume they are within tolerance.

Is there past experience of good performance with the set up you ran or is this all new.

Vmax535
 
Hey Adam, my clearance is within specs, forget actually now what it was but i did measure it last year when i put it in, she was broken in with lots of oil and it was warm when i raced it. there is not marks on the top or intake side of the piston what so ever, mostly the top of the exhaust side only and the ring melted to the piston, she lost power and i shut her down, had 50 psi compression on one cylinder and 100 on the other. Running 410 main jets and 30 mains, at sea level -10 degrees C 20f? app
 
the 75mph speed is run on a snow track, not ice and no studs, and I weigh 255 pounds, lol I was just a ski length behind modified 700's so i though i was running competatively. My squish is around 40 thous and my compression is 160 psi, My timing was set at 1.5mm btdc . This is an all new rebuild/setup and my RPM was engagaing at 6200 and topping out at 10500, aaen calls for 6500 and 10200 so im close, I since ordered a diff spring to get the rpms' up a little on take off and down a little on top end, im gonna retard the timing slightly and go fatter on the main jets, put in new pistons, break her in on 114 octane fuel up from 110 and see what that does for me, think this may cure it??? Also should i leave the theremostat in or out??? i had it out
 
Yeah, that could be it, Im gonna try 114 or 116, think that will be enough? Thats the highest i can find locally
 
KEEP IN MIND

Keep in mind that Forged pistons require more clearance for piston to cylinder wall I believe it is an extra 5 thousandths of an inch but they do require more clearance than cast if not enough they will seize, Wisecos also have to warm up real good first because the fact that if you run a motor with forged pistons before warmiing up real well the piston will expand faster than the cylinder and cold seize. Let me know what you find out.
 
what was the clearance that wiseco recomended on the pistons when you got them. keep in mind you will need more clearance than that becuase its a mod motor but thats a place to start.
 
Wiseco pistons should have 4 to 5 thou piston/cylinder clearence.
110 octane is not too low of octane for a mod motor. Olaf Aaen sugests 105 octane for his mod motors.
timing should be set to 1.8mm btdc Rob, not 1.5mm btdc.
 
Not really, after i rebuilt it i put in some race temp probes and found my egt's were a little high so i upped the jetting I bought new cylinders and pistons, put her back together and she's fine so far, although I ported the new cylinders myself, she don't have as much power as she use to :( im no expert in porting but followed specs exactly to the best of my ability but it's not quite there like it was before, she was also 40 thous over bored also, maybe that made a diff.
 
Here is my take on it. Too lean - the pistons are eroding all across the center area. That area should be the size of a .50 cent piece and should be brown. The "wash" (where the transfers blow across the sides should be shiny aluminum. Go to 1.6 timing/40:1 synthetic/up three jet sizes and then work down. 10,800 is too high for these motors and vibration will do in the clutch and MAY seperate the crank. In the old days ONLY the SSR motor would rev to 10,800 and they worked best at no more than 10,400. Listen to one of the previous guys - once you are above the power band the moyor leans out.

Bob B
 
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