701 Piston Bomb

[TacomarK]

First time posting

Ran out to do some testing before WaveDaze in the marsh (probably two weeks ago at this point). About 5 minutes in while WFO, I lost the orings on my dasa head and had a piston suddenly hand in its resignation papers. Went from ~180 psi to 0 in a blink and had to be towed back in once I realized it wasn’t grass stuck in the pump.

Rebuilt over that weekend, running by Wednesday with a new ADA griddle, still made it down south in time and had a blast while breaking in the new top end

701* w/ head + 82mm piston

 

[kevbo]

It got too hot for your fuel. Where is your ignition timing set? Always slow the WFO rpm down a little with the top carb adjusters and stay rich of peak. A perfectly set carb can still do this if a little air gets in the fuel line. I always use a second pump feeding a small header tank {air separator}, or at least a full tank to prevent meltdowns under long full throttle runs.

 

[TacomarK]

It got too hot for your fuel. Where is your ignition timing set? Always slow the WFO rpm down a little with the top carb adjusters and stay rich of peak. A perfectly set carb can still do this if a little air gets in the fuel line. I always use a second pump feeding a small header tank {air separator}, or at least a full tank to prevent meltdowns under long full throttle runs.

stock ignition with maybe a hair of timing advance, ran great no issues for the last year and a half

 

[kevbo]

Open the top screw on the rear carb. make the rear plug a little darker than the front at full throttle.

 

[Flash-FX]

Looks like a traditional rear crank seal leak seizure. Pressure testing the crankcase (& looking for leaks) would be a good idea when you have a piston looking like this.

Also running out of fuel like Kevbo mentioned is something to keep an eye on. Take a look at those 2 little basket filters (under the fuel pump covers), they can create issues when partially clogged.

Freestyling the ignition timing and running WOT is also a perfect recipe for a meltdown.

 

[TacomarK]

Looks like a traditional rear crank seal leak seizure. Pressure testing the crankcase (& looking for leaks) would be a good idea when you have a piston looking like this.

Also running out of fuel like Kevbo mentioned is something to keep an eye on. Take a look at those 2 little basket filters (under the fuel pump covers), they can create issues when partially clogged.

Freestyling the ignition timing and running WOT is also a perfect recipe for a meltdown.

A multitude of possibilities lie in the mix, but the slap together rebuild worked enough for the ride. Gonna go deep this winter and refresh everything I didn’t have time for if I can

 

[TacomarK]

Update: Stopped trying to find a reason as to what happened, knocked a few things off the to do list and she’s running like a top again. Passed the WFO test with flying colors

 

[fastgtfairlane]

It's always good practice to do a leak down test before you do any tear down on a blown engine. Once you break any seals from teardown, you won't be able to verify there were no leaks. Also recommended to do on any freshly assembled engine. You can remove the carbs and headpipe, and either use block off plates or adjustable freeze plugs. Use a small hand held pump and connect to the pulse line, pump up to 8psi and let it sit for 10 minutes. You can spray seals and sealed surfaces with windex or similar to check for any leaks. I recommend removing the carbs from the plates and bolting them back on the intake to check the top intake gasket. I've had them leak on the rear cylinder before.

Your engine looks like the rear cylinder got the combustion temps very hot which could've been due to a small air leak on the rear cylinder or lean carb tune. Front cylinder looks great which leans more towards air leak on rear cylinder.

 

[Smokey72013]

All very good points made by people I would say are always in the mix helping with good information. If it's still holding he got her sealed up good enough for him..... Fingers crossed you stumbled on the leak at disassembly and re assembly. Could have been some trash in the carb or bad batch of fuel that day

 

[bird]

WOT for 5 minutes at 180psi....you're running 91+ octane right?

New griddle head lol? Your head before was a non girdled head?

 

[Ryan Declark]

It's always good practice to do a leak down test before you do any tear down on a blown engine. Once you break any seals from teardown, you won't be able to verify there were no leaks. Also recommended to do on any freshly assembled engine. You can remove the carbs and headpipe, and either use block off plates or adjustable freeze plugs. Use a small hand held pump and connect to the pulse line, pump up to 8psi and let it sit for 10 minutes. You can spray seals and sealed surfaces with windex or similar to check for any leaks. I recommend removing the carbs from the plates and bolting them back on the intake to check the top intake gasket. I've had them leak on the rear cylinder before.

Your engine looks like the rear cylinder got the combustion temps very hot which could've been due to a small air leak on the rear cylinder or lean carb tune. Front cylinder looks great which leans more towards air leak on rear cylinder.

this should be on the how to thread.