Today's Whodunnit ...

I only use super unleaded in everything. From bikes, equipment, cars.

Did you ever tracked down the seize? Wonder if the Elex ignition is also generating more heat?

I am really not sure what caused the seize. I thought I did, but I was wrong. Maybe just unlucky ! I was with a friend today who has a brand new "Maico Only" 250 race bike. It had been run in properly, and promptly seized in the second race ! Confused dot com !

Don't think the Electrex would generate any more heat than the Mototwat.

I am thinking that maybe a bit more ignition advance may help. Trouble is, I need to run both 390's in before the 28th !
I have richened the mixture on both bikes. Let's see how it pans out.

Yeah , I'm interested in how this pans out!

Not as in! Panning for gold!

Forged pistons require more clearance over cast pistons.

Another thought? I had a friend that kept seizing pistons after a bore on an IT 175 yamaha.
He kept checking tolerances and honing larger. But still did it. Drove him nuts cause he tried everything.
Jetting, changing oils etc.... Come to find out that is was bored slightly crooked. Not sure how he figured it out but they did?

Maybe , you could talk to the shop who machined it and explain in person and get their thoughts on this and possibly get some in depth knowledge on the breaking in procedure . If you have'nt done so already. Would retarding the timing and shorter break in runs be of help?

Do you have the capability to use a timing light to physically check your timing ? Maybe your new ignition is off a bit ?

I have already spoken to the shop that did the boring. The issue is, I think excess heat. The caliper temperature strips stuck on one of the head fins showed around 178 degrees C. That is very hot ! My thought's were that advancing the ignition a tiny bit may help reduce the heat. More transferred out the exhaust, rather than soaking into the head and barrel. The ignition is Electrex World" set at the Husky spec of 2.2mm BTDC. The timing is set at full advance as per the Electrex instructions. It has a built in retard system for easier starting. So maybe I will try it at 2.3mm or 2.4mm BTDC.
I did also speak to Andy at HVA Factory, he also said to try a timing light (which I don't have).

I must admit to having a timing light but I have not used it, my 420 runs very hot so maybe I should

That seize mark is not in a normal position.
Normally front to back with more on the exhaust side [ front].
That seize is not on the largest part of the piston but is where a cylinder bolt sits.
Cylinder out of round ?
One bolt over tightened ?

Show us the rest of the piston.

I just posted about how the oil mix ratio we use changes as the RPM goes up. What amount of lube we have at the middle of the power band leans out at the higher rpms.
We have less lubrication at the top of the power band.

So .... new piston, started well and seems to run ok. Mixture richened slightly and timing advanced a tiny bit.
This was the first proper ride, so taking it very steady. Just as well as it was raining, and very very slippy. Some interesting downhill descents on wet grass !!

This is the first instalment, and there is an interesting twist in one of the later ones which has led me to a thought as to why I had the seize issue in the first place.

Sounds good.

Does it sound like like it's ring ding dinging from the middle down to idle? Maybe a bit lean there?

It does a tiny bit. But I have already gone a 1/4 turn richer on the metering rod. There is a small amount of wetness at the tail pipe. I reckon if I go another 1/4 turn it will be too rich. I may try it though. I have gone a full turn richer on the powerjet too.

I will upload part 2 of the video sometime later.

Here is part 2.

The first hill climb is at about 3 min 27, runs and sounds lovely, the second time round is at about 8 min 40, not so good.
Felt it getting tighter just before it stopped. Fired up again straight away without any issues and sounded ok. A gentle run back to the pits ! I used the other 390WR for the rest of the day.

If that was an Amal or Mikuni I would be going up a main jet for the WFO and lower the needle for the mid range. Also I can not remember if it's been mentioned is there a spark arrestor and if so is it blocked? Back pressure at full chat will make for lots of heat.

The equivalent on the Lectron is the "Power Jet" Steve. Already richened that by a full turn, so it really should be blubbering at WFO. But it's not. I have an idea what may be the cause, but need some time to test it before I report the findings :-)

Grouty, If you have the Lectron set up for very rich and you are still running lean then there must be a restriction in the fuel line or inline filter, or fuel valve or the tank cap is not venting. That is if there is not an air leak in the motor somewhere. Have you done a leak check on the motor?

Marty