Don Terrill’s Speed-Talk

A guy I know had an air cooled bike engine running poorly...It's a Guzzi V twin...He pulled a cylinder and the piston was badly scored on the side facing the exhaust valve...The valve and seat looked bad...He claims this is not uncommon when an exhaust valve goes bad...

Truckedup wrote: ↑Thu Mar 26, 2020 6:26 pm A guy I know had an air cooled bike engine running poorly...It's a Guzzi V twin...He pulled a cylinder and the piston was badly scored on the side facing the exhaust valve...The valve and seat looked bad...He claims this is not uncommon when an exhaust valve goes bad...

Well, let's think through what is happening when an exhaust valve isn't sealing perfectly. Compression is lost, some exhaust might be pulled in on intake. Unless the exhaust valve is burned clear through, as a few I've seen, is a less than perfect exhaust valve seal going to score a piston? If he's seen it happen, then it happens, but why?

Too loose, too tight, too rich, or too lean....to paraphrase Moe Mills.

I think it's within the realm of possibility. We know that an exhaust valve relies on it's duration on the valve seat to help transfer heat from the valve so its logical to think that any imperfection in the seat and valve will not cool the valve as well as it should. Also exhaust pulses coming back up the exhaust pipe would have a path into the chamber-more heat! All that combined could certainly kill a piston on the exhaust side..

One sided galling "could" be fuel wash.

He's got it backwards.

Leaking valve is a hot valve,
a hot valve expands to.... Zero valve lash=hot exhaust valve=pre-ignition=overheated piston

Did the leak come first? or did the lash close first? that's hard to know...

modok wrote: ↑Sat Mar 28, 2020 3:36 am He's got it backwards.

Leaking valve is a hot valve,
a hot valve expands to.... Zero valve lash=hot exhaust valve=pre-ignition=overheated piston

Did the leak come first? or did the lash close first? that's hard to know...

Doesn't preignition instantly punch a hole in the piston ?

no, it's the same as over advancing the timing.

Pre-ignition FEELs like overadvanced timing, and same kind of damage.
If bad enough....will cause detonation, but not necessarily cause detonation until it gets severe enough. Same as timing.

The damage will depend on how bad it gets, and probably gets worse the longer it goes on.

Aircooled engines conditions can change FAST, but on a cycle you aren't usually ON it for long periods.
Actual damage could have occurred in only a 15-30 second period. If he stayed on it LONGER, the piston would have probably melted a lot worse.

I thought the exhaust valve/s is/are in line with the pin in a Guzzi?, so i would expect there is no tendency for the cylinder to run hotter on the on or off thrust faces.
If the piston is smearing on the on thrust side only that can be oil film breakdown, as stated earlier fuel wash, but detonation will give a similar result.

Cheers.

Yes the valves are lined up with the wrist pin...

There might be some odd characteristic of that particular engine.....for instance overheating causes it to distort in odd shapes, or something, which I have no idea about.
Scuffing the side of the piston is an odd thing to happen.

modok wrote: ↑Sat Mar 28, 2020 4:33 pm Pre-ignition FEELs like overadvanced timing, and same kind of damage.
If bad enough....will cause detonation, but not necessarily cause detonation until it gets severe enough. Same as timing.

The damage will depend on how bad it gets, and probably gets worse the longer it goes on.

Aircooled engines conditions can change FAST, but on a cycle you aren't usually ON it for long periods.
Actual damage could have occurred in only a 15-30 second period. If he stayed on it LONGER, the piston would have probably melted a lot worse.

With 2 and 4 stroke air cooled bikes, pri ignition most often happens in seconds...There may be no warning signs unless the engine is running poorly and the rider pushes it...It always punches a clean hole in the piston from the heat and pressure,often without causing other damage like skirt galling..
Detonation causes a slower death , depending on durability, it may erode the piston and break rings and or cause a piston seizure. Yes, detonation can lead to pri ignition but generally they are two distinct events..
This is a near new engine rebuild...The rider ran it on year old fuel, the moment he opened the throttle wide, it holes a piston...

What do YOu think will happen when the exhaust valve becomes a "glow plug"?

modok wrote: ↑Tue Mar 31, 2020 3:03 am What do YOu think will happen when the exhaust valve becomes a "glow plug"?

Do you mean the Guzzi in the OP?