I hope this 'professional mechanic' isn't going to be the guy who does your repair work.

 

your mechanic is correct, this can occur - has happened to a mate.

lean = hot = burnt valve/ valve seat = dropped valve = explosion!

but other things can also create the end result, just that burnt valve is more common. Oh and main jet size may not be all important because you burn em out around the mid range so its your pilot + needle settings

My 351 arrives soon and Ill have this mod running on a perfectly built engine to prove its not this mod!. The piston valve clearance would be the same on the 250 to the 350 so my guess is incorrect valve clearances, timming setup, improper torque settings and technique...etc etc etc. Its not hard with the right tools and workshop manual.

 

There is a million things that can cause this to happen.... Looking at the carbon/black crap in the combustion chamber i doubt that it was so lean that it caused the valve to fail. I havent found a guy working on bikes that is an actual "mechanic" yet. As my dad the 40 year ASE certified mechanic says "they are all just parts changers" Good for replacing parts, but to actually figure out a problem, good luck. I knew more than the guy that worked on my bike.

 

Not too lean, Mechanical failure.

When the head was put on last time, a torque wrench was not used.

OP also claimed he shifted with out letting off the throttle, just clutch in and out. This is abuse in my book. The rev limiter should have stopped the engine past 10,500 but who knows.

My vote your mechanic is full of bull.

Just MY opinion

David

I do feel for your loss....

 

I don't buy it.

 

I think the manner in which the work was done on the bike AND the manner in which he was riding the bike was a combination of factors that would mean the end for any engine .
The bike wasn´t really taken care off properly by the previous owner , a look at the wiring "inventions" sais it all .
Seeing his piston and his head one cannot really tell why the top of the piston is so clean but the inside of the head is full carbon deposits , was the head not clean when it was installed ?
Of course , after the engine has been damaged , it´s a little difficult ( and late ) to tell how the engine was running , because you get oil and gasoline in the combustion chamber changing the color of the old deposits and adding new ones that would not be there had the engine not failed .
Looking at the intake valves ( the exhausts are dead ) they look a little recessed to me , that would mean that the valve it´s not seating properly on its seat and it requires a machine shop to redo the seats . The valves transfer a lot of their heat trough the seat . Overheating a valve would block it in its stem . The exhaust valves do heat up a lot more than the intake valves , they are in contact with hot exhaust gases not with the much cooler air fuel mixture that comes from the carburetor . That means the exhaust valves are more likely to stick if there are problems with the seats , problems with the guides or problems with the mixture .
A lean mixture , burning fuel in excess of oxygen ( like in a steel forge ), may increase the combustion temperatures a few hundred degrees . If someone sees the exhaust pipe glowing red that means two things : the exhaust valves are almost as hot ( they get exposed to higher temperatures but they are cooled by the stem and the seat ) ; they need to stop riding and fix the bike . Guiding yourself on how hot you are burning by watching the radiator fan is foolish ; the majority of heat from a internal combustion engine escapes trough the exhaust gases . In a cool day you may kill an engine ( exhaust valves or piston seized ) just by going 50-60 MPH with a very lean mixture and your radiator fan will not turn on even once .
I agree with David , no torque wrench ? what´s up with that ? I´ve tightened hundreds of heads , NEVER , but never ever I had one come back . I ALLWAYS tighten the heads with my calibrated torque wrench , for the small engines I use a small 3/8 Snap On , change the head bolts if they are the strech type or change the regular bolts if they have been tightened 2 times allready or are rusty etc.
BEFORE MOUNTING A HEAD HAVE IT INSPECTED AT A HEAD / MACHINE SHOP .
YOU DO NOT HAVE THE KNOLEDGE , TOOLS AND INSTRUMENTS TO SUCCESSFULLY DO A HEAD JOB .
MOST OF US HAVE INCRASED POWER FROMM OUR BIKES , DO NOT GO WITH THE BARE MINIMUM VALVE CLEARANCE LISTED IN THE MANUAL !
DO YOUR CLEARANCE AT MID VALUE : 0,15 mm FOR INTAKE AND 0,20 mm FOR EXHAUST . That is milimeters not inches
Shifting like Rossi or Doohan is possible if you have a bike like his : On a racing bike / car there is a device that will cut your spark or injector signal ( or both ) the second you move that shifter more than a milimeter or two . You do not have such a device so you will rev the engine to the 7th and a half sky causing the valves to FLOAT . A floating valve is a dead valve .
If you want a lot of revs change the valves to racing valves ( titanium ) , change the springs to racing springs ( much harder ) . Ahh and you will need to do maintenance style racing , tear up after every race and change after every two - three . When the E46 BMW´s M3 first came out , a lot off the 6 speed version were bending or breaking valves , the 5 speed version ... nothing . There is a conection : People miss shifted from 6th ( at over 200 KPH to 3rd resulting in the egine spinning way over 10000 RPM causing sometimes the failure on the road back from the dealer to home .
Doing this kind of shifting/mis shifting play will upset any engine , and it will live you stranded in a blink of an eye . Also do not give long "handfulls" and not rev the " jesus " out of a bike that is not going / not in gear , with no load on the engine . It takes a fraction of a second for the rev limiter to set the condition and to come in to action , but all it takes for a valve to go bad is a fraction of a second .

 

What Jhoffy doesn't make clear however i suspect to be true is:
The bike was modded before he got it so it was lean for a long while and fatigued well in advance of HIS mods. Jhoffy took for granted that the engine was strong when he made it BIG BORE, but the valves were very close to failure. Please confirm or refute my opinion Jhoffy.
Did you get the bike with the pipe on it and air box mods??? was it jetted properly before you tore into it?
I believe it COULD have been a lean condition and a fatigued valve PRIOR to his mods. I don't think HE did anything wrong other than relying on a use engine to start with.
Just sayin'

 

The only engine mod that I am aware of when I bought the bike is that it had the HMF slip on pipe. Everything else was untouched. As far as I know it had stock jetting. There were two owners of the bike before me, and I contacted the one that I bought it from and he told me that all he was aware of was the slip on exhaust. I have no idea the status of the valves before the big bore, as I didn't check clearances or anything of that nature. The combination of speed shifting, failing to use a torque wrench(first time through) and the upgrade to the 351 along with the alleged already beat up valves and lean condition of the bike did me in. I'll never be able to determine exactly what the cause was. Too man what ifs.

 

Could have been as simple as a valve with not enough clearance. (tight exhaust valve kissed the piston) Then down hill from there.

Were the valves ever adjusted on that bike?

David

 

Not that I'm aware of. I never adjusted them. But I never had any problems when I had just the 250.