Scoring question

[Bigbill]

I took apart a 390 or engine. I noticed the cylinder has a slight scoring in the area of the reed/transfer ports towards the bottom of the cylinder. The piston and the rest of the cylinder looks ok. Since this is on the cooler intake side I’m thinking either a bad or dirty air cleaner was the cause of the scoring. Or a broken intake boot. What do you think?

I also work on tractors too. I pull the head on the cub cadets to find scored marks in some of the cylinders. I check the air cleaner and it’s very dirty or is rotten with holes in it.

Being a retired engineering lead tech I see things from both sides of the fence I want to know the root cause of the problem why it’s scored.

Something as simple as cleaning your air cleaner could ruin your ride.

 

[jimspac]

If your air cleaner flow is impaired as in a damp paper filter or a filthy foam filter, the engine would be running rich in fuel air mixture. However in the case of a broken down that no longer blocks particle flow that would account for scuffing as well as a possible lean mixture.

A broken air cleaner boot would account for particle intake and a dry rotted rubber manifold would cause an air leak. I am aware the 390 does not have a rubber manifold unless the reed case was changed out at some point.

 

[Bigbill]

The later 390 has a rubber carb manifold and air cleaner rubber boot. I’ve seen the owners put silicone on the cracked rubber to continue to ride. I know none of us would do that but the inexperienced would. Rebuilds are costly.

 

[silverstreakNZ]

Supercars and fighter jets are glued together these days ...
Don't correctly with the right glue I don't have a problem with glueing up a cracked manifold

 

[fran...k.]

For the rubber manifold like on the 420 and air cooled 430 there was no sense on smearing it as it was essentially a Micuni part that could be acquired for around $20. Some of the other ones later on there was a time when a billet manifold that then used the Micuni or generic type was what my parts sources offered. As long as the boot was intact where the carb attached the cracks seemed to be more shrinkage cracks that did not go all the way through, I smeared them. I think the original has been reproduced by now but still like $80, could be off. The black oil resistant silicon stuff is way better than the red or blue.

There are pictures of scoring and causes on the internet if one is curious, I never was an expert understanding it. Had one friend in the aircraft engine repair trades take some bored cylinders to some super measuring equipment, I could not even understand the print out but it sure was not a perfect cylinder.