Back Cylinder

 

Do you have ACR's fitted (or manual ) ?

Manual CR's and right at that spot, but why the lack of carbon right there

It's almost a hot spot type mark, so either the decomm releases are glowing hot or they leak and let air in which makes a lean hot spot.

Fairly common, probably something to do with air trapped in the small release hole "chamber" causing a lean spot.
Seems a bit more extreme in your setup though.

You have two things happening here, the obvious is the trace left by the decompressor ports acting much like power cells do in indirect injection diesels making a pattern on the piston crown ,interesting but not of much concern unless it's running hot or lean.
I would be more concerned about the condition of your oil rings ,or your ring pack/bores in general as the crown suggests oil bypass .

thank you all for the advice/comments

I am not overly worried about it as the motor only has about another month before a new one is ready to go in. I just wanted to try to fix a few things that weren't working out the way I thought when I built it. If it runs better when I get it back together I may leave it in for a bit longer and leave the new one on the bench.
Just wanting to understand whats going one for future reference as the new motor will have MCR's fitted also.

Mark It runs hot but don't think it is lean. AFR is 13.7 at it's leanest. It runs around 140-145 C out on the road but in traffic or at the lights it climbs real fast to about 160 -165C which is why I pulled the heads to try and fix it with the squish and loose a bit of CP by opening up the chambers a little. I just got sick of the way it heats up and always records a knock event or 2 at times I wouldn't expect. Good spot about the oil, the valve guides/seals are leaking but the oil usage is not measurable on the dipstick at 3000kms I notice it at start up occasionally, nothing from the breathers but the intake ports are oily, exhausts look normal (what ever that is)

Again thank you all for contributing to my continuing education.
cheers
Max

Hi Ozy, I am not looking at the solid carbon on the crown but i am looking at the wash on the od of the crown,,hence rings, not taking other factors into account. If you have a straw or darker colour in the bores its heat.

fucking spammers .

anyway
I pulled 3cc's from the heads and increased the squish, strange that with the lower CP it pinged it's head off. pulled some timing and seems much better, will get it tuned properly again.
Road it 260 odd Km's yesterday and seemed much better mannered and way cooler. Can't actually prove the temp difference but the gauge was way lower than I usually expect and it was a hot day.
Over all happy, lesson learned the hard way as usual.

If you widened the squish it will tend to ping more, even with a little lower compression. What was your cam/compression combo?

well that's what happened alright.
Stuck a .040 under it, was .030 and opened up the chambers 3cc, pinged it's head off first ride down the street. Had me scratching.
pulled a little timing from it, made a few fuel adjustments, and it is manageable.
should be at 9.4:1 now, was at 10.9:1 Woods TW777

Andrew is going to give it the magic touch Friday morning as best he can just to see how it ends up and that's the end of it.
Just patiently waiting on Ollie for the new one to finish machine work. Get some parts ceramic coated and then assemble it. should be ready in a month or so.

WOW 10.9 with a 42 close. That will make it hot/ping etc. By Andrew I guess you mean young Fred. He is a good guy, I am sure you will be happy with what he does for you.

yes mate it was a bitch, I never intended it to be that way but a few issues with the machinist, NOT ollie and the wrong cams sent out. So I have been chasing it for some time. Just never had the hit it should have.
Yes Fred, always very impressed with his work on my stuff and the many people I have sent there over the last few years. One of the real good blokes both as a person and a tuner.
I think we now have it manageable so he should be able to get it acceptably tuned at least, new owner should be happy.

Bring on the well mannered all rounder 124 I say

Yep, 9.4 with a 42 close will work well, you may be surprised how well it does. What are you coating in the new one?

It was originally meant to be set up at 9.6 but after the heads got screwed I just lost interest and put it together thinking I might be able to sort it out, lesson learned, maintain patience and just wait as long at it takes for the best guy to do it. And so here I sit considering the future while the new one takes shape. All good things come to those that wait.

ceramic coating to chambers, valve faces, exhaust ports, piston tops and headers. Friction coating to piston skirts.

Good work, there is about 3 hp alone on a 120hp build. They SOUND different when you start 'em up after that is done.

didn't really consider the extra hp, that's a bonus.
Doing it to keep the heat where it should be or away from where I don't want it

hmm be interesting to have a listen when I get it fired up, hope it's not a OH FUCK WHAT HAVE I DONE sound hahaha wouldn't be the first time

One should consider everything :-). It will need different work on the tune as a result as well. But I am sure you're on top of that. Are you building it or is Fred/Ollie? Cheers

Fred will be tuning it for sure, Dynojet are building an initial tune for me to start with. Ollie is doing the bottom end (actually he has the bottom end together) and I will put the rest together. Maybe another month or so I guess before I get it running. Need to get another clutch and a few bits. Probably going to fit it into a tourer now instead of the dyna so need to talk to both Fred and Ollie about any changes that maybe needed. Currently set at 11:1 with .04"squish but can go to 10.8:1 with .045 squish and 60 closing

So what hapened to the last one when you widened the squish?......