305 pistons blown!

[ash]

Hello!

I have found a 69 superhawk 305cc for sale that had its engine just rebuilt.

The owner mounted the engine to the bike, set it up, and started it up. It ran rough, and he rode it for about 1/2 and the PISTONS blew. Both pistons have holes in them.

He is selling the bike with 2 new pistons (uninstalled).

Would anyone know what would cause this? Could something else in the engine be damaged?

The bike is for sale for $1000.00 and looks very sharp. It needs the pistons installed and 2 exhaust pipes.

Should I avoid the impulse to buy this beauty?

[Gun]

1000 bucks ain't bad for a rust free bike. :P

take into account that the whole engine may need some serious $$$$ and attention.
If you're up to it, I'd do it.

[LOUD MOUSE]

What sort of holes? Burn through? Hitting Valves?. .......lm

Hello!

I have found a 69 superhawk 305cc for sale that had its engine just rebuilt.

The owner mounted the engine to the bike, set it up, and started it up. It ran rough, and he rode it for about 1/2 and the PISTONS blew. Both pistons have holes in them.

He is selling the bike with 2 new pistons (uninstalled).

Would anyone know what would cause this? Could something else in the engine be damaged?

The bike is for sale for $1000.00 and looks very sharp. It needs the pistons installed and 2 exhaust pipes.

Should I avoid the impulse to buy this beauty?

[ash]

He said the holes were due to detonation because of incorrect timing.

[LOUD MOUSE]

I recommend ya buy the bike but split the engine and do a good cleaning and trans check for parts ya may need to replace. ..........lm

He said the holes were due to detonation because of incorrect timing.

[e3steve]

Ash: buy, play and enjoy! I'd say that's a stinking-cheap bike! You have the replacement pistons (and new rings?), and I doubt that the motor would've sustained any other damage in such a short running time -- exhaust valve seat burning occurs over a longer period of running and inlet valves don't get that hot as their seats get cooled by the incoming charge. When the charge is too lean (and this must've been really lean to hole pistons in such a short run) then the combustion is fierce owing to the presence of loads of oxygen (add more O² to a gas welder and see the ferocity of the flame -- which is why it cuts through metal). The internal combustion became too hot for the piston crowns here, definitely as a result of too lean a mixture and/or over-advanced ignition timing and possibly because of wrong (too hot) spark plugs as well. Timing refers to the relationship of the position of the piston and when the spark occurs. If the spark occurs too soon it can actually push against the piston travelling up the cylinder as it is compressing the fuel/air mixture; this causes pinking (detonation), loss of power, fatally high combustion temperatures, back-popping out the carburettors and early internal parts failures. If the spark occurs too late, the ignition of your fuel/air mixture occurs after the piston is travelling down the cylinder. This results in wasted energy, unburned fuel, high emissions, and lost power.


Follow Ed's (LM's) advice; there's nothing he doesn't know about these bikes!

Go for it, and good luck,

Steve

[e3steve]

Observations on the bike itself:

• 1. There's no such thing as a true '69 -- these were manufactured from '61 to '67, but some would've been registered (titled?) in '68. Am I right, Ed?
  2. It's got a speedo/tacho used on models pre '66
  3. The seat is pre '67
  4. Front forks do confirm it as a '67 model

Also, we can't see the tail light; '67s had a larger tail light than previous models (see pics). But, at the price, who cares?! Exhaust systems are expensive, but the end result will probably be worth the money & effort.