Not what you want to see.

[Steptoe]

A 1200S was dropped off yesterday evening. It wasn’t fully engaged in 2nd gear as the owner accelerated. It popped into neutral and I assume the revs went into the red. It lost power and sounded like a stricken Stuka.
On the bench this morning for a quick diagnoses, the engine won’t turn over...... removed spark plugs, right plug ok, left plug, oh dear ..

 

[Steptoe]

Dropped off the downpipes to have a look and one valve managed to stay in one piece in spite of being “S” shaped, the other has lost its head.

 

[ChasMill]

That is going to be expensive ...

 

[Mac One]

Disaster....... does the owner know yet, he/she may need counselling.

 

[MikeS]

Wow, so what would cause that then, timing problem?

 

[JohnnyBoxer]

Wow, so what would cause that then, timing problem?

I think Sir Steptoe described it admirably in post 1

Over revving

Comprehension ....it's not what it was

 

[Wapping]

Wow, so what would cause that then, timing problem?

Pixies, it’s always pixies.

 

[Steptoe]

Wow, so what would cause that then, timing problem?

Brexit.

 

[MikeS]

I think Sir Steptoe described it admirably in post 1

Over revving

Comprehension ....it's not what it was

Thanks, I got that, but as a non mechanic I was just trying to understand what actually physically happens, presumably the piston hits the valves? But how does over revving cause that?

 

[DrFarkoff]

Yeah I did the same during an overtake

Missed the change up to next gear on an overtake where I was being followed by 6 or 7 others

and instead of just letting the rev limiter do its thing or shutting the throttle I pulled in the clutch

That allows the momentum of the spinning mass to keep moving rapidly and contact happens as the valve timing is quite neat time wise

Anyway The Result of Mine ( The Net result was a replacement engine)

 

[Engineer]

Thanks, I got that, but as a non mechanic I was just trying to understand what actually physically happens, presumably the piston hits the valves? But how does over revving cause that?

Valve springs can’t shut the valves quickly enough before the piston comes up to meet them

 

[botus]

that's a minor inconvenience, I guess that's not coming out with a stepper motor reset and a carb balance then?

as it has a rev limiter isn't the story closer to: I was doing a 100mph went down 5 gears and let go of the clutch

 

[Steptoe]

as it has a rev limiter isn't the story closer to: I was doing a 100mph went down 5 gears and let go of the clutch

Engine was under no load, hence lots of flailing around ..

 

[JohnnyBoxer]

Brexit.

You are a bad man

 

[Deleted account 210604001]

Wow, so springs can't react quick enough which makes sense. I've hit a few false neutrals using my quickshifter on my Ducati 899 and the revs skyrocket into and past the usual redline (I had one last weekend), does the desmodronic valve design mean that they shouldn't suffer the same as no springs involved ?

Seeking to understand.

RBW.

 

[botus]

Engine was under no load, hence lots of flailing around ..

still a bit surprised at that level of damage, do you think the piston let go and got the valve rather than the other way around? whole engine is bin material I guess? to have taken out a piston and do a rod that badly the bottom end will be too risky to revive?

 

[daytripper!]

I remember missing a gear under acceleration as a callow youff on a Yamaha 350 YR5, a mistake which sounded horrendous (ring,Ding,DING!!!). So just out of curiosity, and to make the numbers easy, lets say 12,000rpm for 6s. That would be 1,200 revolutions. Now say in one revolution one piston travelled 100mm (fcuk knows am guessing) then in 1,200 revs the piston travelled 120,000mm ie 120m. So the pistons (ie x2) travelled about 0.25km during that brief embarrasing moment. Is this all bollox, dunno. Anyways the outcome, well nothing really, a non event, ring,Ding,DING!!!

 

[Delboy777]

Wow, so springs can't react quick enough which makes sense. I've hit a few false neutrals using my quickshifter on my Ducati 899 and the revs skyrocket into and past the usual redline (I had one last weekend), does the desmodronic valve design mean that they shouldn't suffer the same as no springs involved ?

Seeking to understand.

RBW.

Im no expert, but as i understand it a Desmo engine has no valve springs and the valves are closed mechanically. I stand to be corrected by anyone with superior knowledge.


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[NicD]

Im no expert, but as i understand it a Desmo engine has no valve springs and the valves are closed mechanically

that's right. fwiw:
http://shortsleeveandtieclub.com/understanding-the-operation-of-a-desmodromic-valve/

 

[Delboy777]

that's right. fwiw:
http://shortsleeveandtieclub.com/understanding-the-operation-of-a-desmodromic-valve/

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