Piston Failure. Need help identifying cause.

Joined
May 12, 2019
Location
Annapolis, Maryland
With no more than 10 hours on the rebuilt top end of my ported 718, I guess the pistons decided they had had enough... lol I was out riding, after bumping up the pilot jets, not doing anything crazy and the motor started to stumble and then died entirely. Thankfully, a wakeboarding boat came by and was kind enough to give me a tow back to the ramp so I didn't have to swim a mile through are jellyfish infested waters. After tearing the motor down this is what I found. One piston has a large chunk missing from the intake side and both have some erosion around the edges of the crowns. They both also have several sections between the two rings that appear to have broken off from the piston. This is first motor I have ever blown and i'm not really sure what could have caused this. Any thoughts or advice would be greatly appreciated.

Pics in next post.
 
Joined
Mar 30, 2013
Location
Riverside, CA
Most likely the cause of the detonation is your pipe/porting/impeller combo.

Jetting could have been considered normal, but if your pipe/porting/impeller
combo is not correct, it will increase the engines load and normal jetting will be
lean.

Although portions of it appear to be the piston brand and you. It looks like it
seized more than once.


Bill M.

We need more detailed information about your set-up and testing.
 
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Joined
Mar 30, 2013
Location
Riverside, CA
That was my leading hypothesis as well. So if my squish was too tight could that have potentially caused the detonation?

I was right around 0.038" for squish.
That isn't even close to "too tight"


Bill M.

What was the geometric compression ratio?

What kind of riding do you do?

Did the piston pins slide out easy or stiff?
 
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waxhead

wannabe backflipper
Joined
Jan 15, 2006
Location
gold coast
That was my leading hypothesis as well. So if my squish was too tight could that have potentially caused the detonation?

I was right around 0.038" for squish.
Cant deto whats not there. but as WMAZZ said its not an issue. I set my squish so it will hit the head at about 1000 rpm higher than the ski will see
 
Joined
May 12, 2019
Location
Annapolis, Maryland
That isn't even close to "too tight"


Bill M.

What was the geometric compression ratio?

What kind of riding do you do?

Did the piston pins slide out easy or stiff?
I'm not entirely sure what geometric compression ratio is, I haven't heard that term before. I may have

Riding Style: Mostly freeride and boat chasing but the goal is to do more flatwater. I don't believe it could have ingested any significant amount of water since I built it.

I will have to get back to you on the pin, I haven't taken them out yet.

Here are all the details for my setup.

Hull: Carbon XFR with an OEM 155 pump and a Skat Trak E-75 Yamaha 155mm 6/12 Big hub impeller, with a 5mm cutback. Blosion stubby pump cone, 65U 5 deg nozzle (oem bore)

Fuel: 93 octane pump gas

Motor:
701 that was bored to 82mm
Pro X Pistons
ADA Head with 33cc Domes, I was getting about 190psi of compression.
Squish 0.035" - 0.039"
Blowsion Super Base Gasket
The porting was done by me. All I really did was clean up the mismatch between the transfer ports in the sleeve and casting, while doing my best to avoid changing the roof angle of the and making sure not to change any of the port heights in the sleeve. I also epoxied (devcon) and did some medium porting of the intake track in the cases. Here is the link to my build thread where I posted pics of all the porting I did.

http://www.x-h2o.com/index.php?threads/my-first-am-build-rickter-xfr-full-carbon.199245/page-6

Exhaust: Is a Factory B Pipe with a Limited Chamber. Bottom screw is open about 3/4 of a turn and the top is half a turn. Also running a JM waterbox

Intake and Carbs:
OEM manifold
V-Force 2 Reeds
Carbs were dual OEM Mikuni 38's
Rebuilt last winter with all Mikuni parts.
I started out with the same jetting I had been using before rebuilding the top end. 75 Pilot, 140 Main, 1.5 & 65 NS. I was having some midrange bog/hesitation issues.
I assumed I was too lean so I tried swapping over to a 2.0 & 95 NS. Ski still had midrange issues so I went the other way and tried a 1.5 & 80 NS and a 77.5 Pilot. I then found out that while my carbs were synchronized when closed, as they opened they progressively became less synchronized. One was opening more than the other. I fixed that issue and the ski ran a lot better but I was finding I had to have the pilot screw open roughly 2 turns for the ski to idle and have good midrange. I ran about 10 gallons like this. I then swapped back to the 1.5 & 65 NS since that was where she had run the best before the rebuild/porting. At this point I was pretty happy with how the ski was running and I ran maybe 20 gallons through the motor like this. I still had to have my pilot screw at 1.5 turns and I wanted to get this lower so I swapped in 85 pilot jets. It was during my first day of riding with this setup that the motor died on me.

Ignition:
Zeeltronics Total Loss with Zero Lightened Flywheel.
I ran the standard Wax curve as well as a more aggressive custom one which I had made for me when I bought the Zeel. I was only using the more aggressive one when I felt like the ski was close to dialed in. I did notice that whenever I switched to the more aggressive cure there was a noticeable difference in the tone and sound of my exhaust, basically more aggressive sounding. Looking back, now I'm wondering if that was actually a sign of detonation? Is it normal for the exhaust sound to change with the timing curves?
 
Joined
Sep 20, 2007
Location
dfw
Your custom curve on pump gas would create you a set of custom pistons in about 30 seconds. The WAX curve is as hot as I would ever use with pump gas.
 

waxhead

wannabe backflipper
Joined
Jan 15, 2006
Location
gold coast
Your custom curve on pump gas would create you a set of custom pistons in about 30 seconds. The WAX curve is as hot as I would ever use with pump gas.
I will go tougher down low up to 34 degrees but i get it out straight away as the power comes in, but 29 degrees of timing at 6000 and 28 at 6500. Damn dude that poor engine had no chance.
 

waxhead

wannabe backflipper
Joined
Jan 15, 2006
Location
gold coast
Would that still hold true if I had been running race fuel?
Its not a curve I would run on any of the engines I build or setup. That much timing up top would/could cause detonation and the pipe would not get hot enough to allow over rev. The heat would end up in the piston and not the chamber. This stops the pipe from pulling on up-top and It would make the power-band feel very short
 
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