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  #41  
Old 4 Weeks Ago
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Ferrous Head Ferrous Head is offline
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I keep thinking of things like oil in the fuel (not two stroke pre-mix in there by mistake ?)
but they all generally will affect both cylinders.
Show us a pic of the rear piston and both plugs.
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  #42  
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Hi,

so pistons were old, they were already solid black when I installed them.

come to think of it, the front exhaust push rod was on the loose side when I was taking it out.

I didn't notice any blueing inside the cylinder, just exhaust when I run it very first time, about 30 centimeters of blue..

Bike started ok, with electric start she would kick in pretty quick with kick start would take longer but then my technique is pretty shit I reckon. still need to figure that out.

I think she idled ok, I couldn't have here idling super slow, like super super slow, but I will post video link here, this was taken only about 10 minutes before she stopped.
I literally hopped on the bike after I finished making video, rode about 10 km at 90 KPH.

https://www.facebook.com/521745373/v...0382186280396/

there was no bang no sound she just stopped like she run out of fuel.
she would not start. but then both front valves were caked in the debri from piston and seized.

I have new pistons coming with pins, rings, gaskets.
Also looking at electronic ignition but I am lost here, can't decide which one I should buy, and would fit into the distributor. If to keep the advance weights in and get the @simpler version or get the more programmable one without weights, but then I don't want to fiddle with computers.
Also single or dual fire coil, 3 or 5 ohm? I run 5 ohm coil now
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  #43  
Old 3 Weeks Ago
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OK. I can see from the video your running what looks like stock XLH distributor. (Timer).

That means dual fire so it highly unlikely the ignition system was at fault.

Nike sounded ok from what I could hear on the video. Didn't sound like it had any major problems.

Based on what I've seen I would have to say this was pre-ignition leading to detonation. The heat generated by this situation would be enough to expand the piston in the bore until it seized.
Seizing is described as an interference fit in this case.

It would account for only one cylinder having the problem.
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  #44  
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So my assumption that I have damaged the piston with my excellent break in procedure is valid?
And it caught up with me few km later?

I should be ok to put new pistons in, pins, rings, seals and get her running with fresh oil and slowly break her in using your procedure?

I will time her as indid before and see what happens. ����
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  #45  
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P re-igmition happens when you have something in the combustion chamber that can act like a glow plug. You have a small point which heats up just like a glow plug in a diesel engine. This can be as simple as a small flake of heavy carbon buildup.

If this was the result of ore-ignition then it was always going to happen. Pre-ignition that leads to detonation will destry an engine in minutes.

With pre0ignition the ignition point of your air/fuel mixture happens way too early. he flame front starts long before it should leading to an increase in pressure inside the combustion chamber long before that should happen. Having the air/fuel mixture alight and expanding the gases inside the combustion chamber too early leads to pressures that are so high the rest of the fuel ignites in an unconstrained explosion rather than a fast burn. This is detonation and it can hammer holes through pistons, bend rods, all bad things.

A light carbon coating on your pistons is not a bad thing. It acts as an insulator to a small extent. It also slightly raises the compression ratio. But a heavy coating of carbon is prone to breaking off small bits leaving sharp edges than can heat and act like a glow plug.

That is the most likely explanation I can think of that could have caused this problem.

But as zI said, heat is always the enemy.

My break in procedure is time consuming and some would day unnecessary. But ou spend a lot of time on money building an engine and not giving some consideration to how machines surfaces "mate" with one another is asking for trouble.

Oh, and it makes more power.
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  #46  
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Thank you so much for your help! Really really appreciate it! Thanks again.
Yeah the old pistons were thick black on top.
Let's get into the rebuild again. Hurry up post!
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  #47  
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Hurry up post!

Take your time. There's never enough time to do things right, but somehow, always enough time to go back and do it again.
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  #48  
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Sorry to hear your dramas. Like Ferrous said, play it safe and heat cycle when you get it back together. When my motor was new it would get super hot in a matter of seconds. I would run it for 30 seconds, shut off, let it cool down for half hour, repeat. Gradually lengthened run time when it didn't heat up so fast. (T&O bored and fit pistons for me, would have been a tight fight for street running). Did this several times before riding around the yard, then up and down the street, etc. It was a pain in the neck but 30k of riding later I feel it was worth it.

I had a BSA B31 seize on me when it was rebuilt, I also got stuck on the side of the road with an XS750 that had overheated (had to wait for it to cool down enough so I could safely ride it home) after a top end rebuild. Both times I did not heat cycle. When you buy a new bike it has already been run and test ridden before you get it and that critical first few minutes of run time has already been done for you, unlike a fresh rebuild.

Some will tell you to ride it like you stole it, good for them. Its your money, don't risk it.
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  #49  
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So, I just read in one of the posts (I can't relocate it so I don't know which one it was) where someone posted that he missed the original pictures in the first post. Well, so did I. I just went back and looked at those pictures. Sorry, I wish I had seen these first off, but late rather than never, here is what I see there.

First, that definitely looks like the head gasket was leaking badly. Note: the few shiny places that seem to have been sealed, compared to the grungy remainder of the gasket surface.

Second, that is typical looking detonation including the buildup on the exhaust valve. So I agree with everyone on that.

Third, I think you said that the major scuffing was on the intake side of the piston? Well, that pile of aluminum on the intake side of the piston as it sits in the jug confirms that the scuffing was caused by debris and that pile is the debris that did it.

Fourth, you confirmed that the exhaust valve adjustment was loose. That definitely raises heat. If the heat can't escape, it will radiate into the adjacent metal and improve existing detonation. That exhaust valve and the aluminum laminated to it was most likely glowing like the red light in Sakahachi alley.

Fifth, I agree that you will need to do a proper break in, but the scuffing was not caused by lack of break in.

Sixth, correct me if I am wrong, didn't everyone rule out the ignition since this is the front jug only? Not being familiar with that ignition, I will agree with those more knowledgeable and they have spoken.

So from everything I can see, this was caused by an air leak at the head intake, head gasket and made extremely worse by the exhaust valve barely opening. That causes backfiring through the carb when the exhaust valve stays almost closed by the way, so you may have heard that. It also shows up on a compression test because the jug with the exhaust not opening will cause the compression to pump up much faster than the good jug will.

Oh, I saw the bike picture too. Nice looking bike. Looks like you spent a lot of time getting it ready and this sucks that you are in this mess, but I think you got it from here.

Also, Ferrous mentioned doing a compression check before ever firing it up. That's a given, but people do get excited near the end of a build so do make sure you do that, write it down for both jugs and always keep a dated log as you do compression every time you are in there to change plugs etc.

Also, you have a lot of cleaning to do. You need to flush the cases and cam chest out good and don't overlook the oil bag. Any of that that got past the piston (and it did get past the piston) needs to be out of there. I know it's only aluminum but that can seize a cam and send you right back here in a heartbeat.
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  #50  
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I just can't see those pictures well enough to see gasket damage on them.

But if it did blow you have to consider why.

If the heads were torqued correctly it won't happen. You now would need to consider decking the heads. Any good machine shop can do that for you. You just want them skimmed to remove any high/low spots.

I lap my heads and cylinders. Slow, Tedious work. But if you lap the heads and the cylinder (base as well) you can run without gaskets. Probably overkill for you, it's just something I do.

But it's one way to be sure your not going to blow head gaskets. Lapping will show any problems.

Exhaust valves get hot when not fully seated. Or with too small a seat area. They take the brunt of the heat from the combustion process and need to be in contact with the head to allow them to cool down a little. If the tappets were loose the valve will spend more time on it's seat. Had they been tight it would have been a different story.

Your actually pretty lucky. A little longer running time and the piston would have holed. That sprays metal down onto your con rods and flywheels. makes the whole job a lot more difficult.

After you've been through the rebuild process and the break in do another compression test. And yes, write the numbers down. You now have a baseline to jusdge how your engines going in the future.

I know a lot of people think some of the things I do are overkill.

But it's all done in the pursuit of making power.

To make power you need ROUND holes in your cylinders to start with. They need to be honed correctly. Not many people can really do that.
The clearances have to be what's called for. Near enough is not good enough here.

A lot of power gets past rings that aren't really seating perfectly. The pressures you see when checking them is nothing compared to what is generated when the engine is running. You will always get some blow by, but to make good power you need to reduce that to a minimum. The better you are at doing that, the more power you will make.
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