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Author Topic: Hole in piston  (Read 2745 times)

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Offline coastn1

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Hole in piston
« on: August 13, 2010, 09:23:48 AM »
A few days ago I reported on locking up my 75 RD350. I pulled apart the top end last night to find a nickel sized hole through the top of my right piston and some scoring on the sides. The spark plug was basically melted too. The left piston was perfect. Soooo I'm guesing this is not a jetting issue but a timing issue with the right side? Does this mean its too advanced or retarded? I used a dail indicator to set the timing. I would post pics but cant figure out how to. I'm sure you guys have seen it before anyway. Any thoughts?
Thanks guys
« Last Edit: August 13, 2010, 09:24:33 AM by coastn1 »

Offline SoCal250

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Re: Hole in piston
« Reply #1 on: August 13, 2010, 09:58:55 AM »
Your description matches exactly to what happened to my brother's 400 during a high speed rip a few years ago. Right side, holed piston, mild skirt siezure. Left side was perfect. His was caused by timimg set way too advanced. I don't recall the exact number, but it was something like 3.xx mm. His lesson learned from that experience is to always double-check timing on newly purchased bikes.
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Offline Yamaha179

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Re: Hole in piston
« Reply #2 on: August 14, 2010, 05:24:27 AM »
Detonation caused by advanced timing.  That can happen with points because the points plate slipped and timing changed.  It is possible that you made an error in setting the timing, but if you used a dial indicator that is unlikely.  What I used to do is set the clearance on the points, then set the timing for each set of points at 1.9mm BTDC, then I used an auto timing light to check the timing like you would do on a car.  Start the engine and check each cylinder at idle and then increase the engine speed as high as the timing light would track the spark.  (My timing light is an inductive pick-up and would track pretty well to 3K RPM then it didn't do so well.  More problem with the light than the pick up, I think.)  It is easier to do this if you mark the rotor with white-out and then rig a short piece of wire - like a coat hanger - to compare it with.  Trying to do it in the side cover is a little difficult.
Lyn Garland

Offline washer man

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Re: Hole in piston
« Reply #3 on: August 14, 2010, 07:14:25 AM »
When setting points gap do you set them to be fully open at 1.9 or just starting to open at 1.9?
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Offline nojibski

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Re: Hole in piston
« Reply #4 on: August 14, 2010, 08:04:33 AM »
i will be corrected if i am wrong;
at 1.9mm you want the points to open, startng to open doesnt matter, the spark charge is created when the coil loses its power- when the points break(open)
it is the charged coil losing power that creats the high voltage when the magnetic field colapses
i am pretty sure this happens when the points break

is bad news about your piston, hang in there

Offline coastn1

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Re: Hole in piston
« Reply #5 on: August 16, 2010, 07:04:12 AM »
When setting points gap do you set them to be fully open at 1.9 or just starting to open at 1.9?

I set the points with feeler gauges and check to see where they are when the piston is at 1.8mm before TDC. The points are open, (not just open or open all the way) somewhere in between I guess. I know the manual says they need to be just opening when the piston is at 1.8MM before TDC but how do I do this while still keeping the recommended gap in the points? I also read about using a light or buzz box to tell when the points are just opening. Didn't do this before when I set points and timing. Could this be the reason the timing was too advanced?

Offline Premiumjo

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Re: Hole in piston
« Reply #6 on: August 16, 2010, 07:35:56 AM »
You need to set the point gap first by moving the points on the bracket when the cam is at it's highest point. Then adjust the bracket until the points break at 1.9mm btdc or whatever you are shooting for.

Offline Vintage Smoke

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Re: Hole in piston
« Reply #7 on: August 16, 2010, 08:09:48 AM »
I cut and pasted this from a page I include in the instructions for my Dyna ignition kit. It's a basic explanation of how a points style ignition works.

The breaker point ignition used in RD400s sold in the USA uses what is called a “collapsing field” ignition. Very simply the ignition switch provides power to the ignition coil and the breaker points provide the coil with a path to ground. When the breaker points are closed, electricity flows from the battery, through the coil, and back to ground through the closed breaker points completing the circuit. This creates a magnetic field in the primary side of the coil.
When the breaker points open and electricity stops flowing through the primary side of the coil. The magnetic field created in the primary side of the coil now “collapses”. This creates a surge of current in the secondary side of the coil which is connected to the spark plugs. The surge of current is enough to jump the gap in the spark plug and light the air fuel mixture. The condenser is simply an “add on” that absorbs any current that attempts to flow back to the coil through the points.

I'll add this as well...
 - Always set the point gap FIRST. When you change the point gap, you change where the little tab that rides on the cam sits. This affects the timing. In other words...if you change your point gap AFTER you have set your timing, then you have also changed your timing.
 - Always set your timing when the points just START to open. The coil will fire at the very moment that the points just begin to open.
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Offline 202wagon

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Re: Hole in piston
« Reply #8 on: August 16, 2010, 01:15:40 PM »
You should also check ALL of your jets and emulsion tube (nj)...if you clogged or partially clogged a pilot or main and ran it all out, you'll surely hole a piston.
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Offline washer man

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Re: Hole in piston
« Reply #9 on: August 16, 2010, 06:13:24 PM »
Thanks for the info guys,makes sense now.Coastin you can use A digital volt meter or even A test light to show you just when points open.
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Offline coastn1

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Re: Hole in piston
« Reply #10 on: September 03, 2010, 10:53:24 AM »
Do I leave the orange and grey wires connected when I use a light or buzz box to test them or do I need to take them off so I just have the wire itself to put my alligator clip on? I tried to make my own buzz box but as soon as I clamp it to a ground and the orange wire it buzzes the whole time whether the point is open or closed. I think I made the buzz box wrong.

Offline vtrumble

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Re: Hole in piston
« Reply #11 on: September 03, 2010, 11:33:28 AM »
I had holes in both of mine when I got it, but I dun fixed em'. Can't wait to get 'er runnin.

Offline GSXRDman

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Re: Hole in piston
« Reply #12 on: September 03, 2010, 11:53:10 AM »
retarded to 1.75mm BTDC. <---Thanks to Lyn G.

140 miles (Estacada to Detroit Lake) and 80+ on the straights yesterday.  I told the FZ6 (this guy took a 15 @ 50mph (paramedics waiting by this corner) and railed on through..tight ass corner and still had about 1/8" chicken strip when i checked..amazing) to keep the speeds down to 80 i was riding w/, so i could catch him on the LONG straights!!!. 

i've holed a few pistons and can tell you, you ain't got shit down low anyway.  might as well retard it and have your power up top and add a little protection for the motor.
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Offline coastn1

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Re: Hole in piston
« Reply #13 on: September 03, 2010, 01:01:28 PM »


i've holed a few pistons and can tell you, you ain't got shit down low anyway.  might as well retard it and have your power up top and add a little protection for the motor.

Shit, I'm not worried about power up top or down low or wherever. I just wanna ride the damn thing for awhile without worrying its gonna blow up. I wasnt even aware I had to move the points plate to get the timing set right. I was just setting the points gap and going. Now that I understand you need to move the whole plate for timing I can see I was way advanced when I holed my piston. I didnt measure it but my points were pretty much the whole way open when my piston was 1.8mm before TDC. I think I'm finally starting to get the hang of this whole "Timing" thing. I now set my points to be just opening when my piston is 1.8mm before TDC. I did this by eyesight only. I dont have a buzz box or light. We'll see I guess. I'm planning on having her running again tonight or tomorrow.
« Last Edit: September 03, 2010, 01:05:33 PM by coastn1 »

Offline nojibski

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Re: Hole in piston
« Reply #14 on: September 03, 2010, 01:41:26 PM »
Do I leave the orange and grey wires connected when I use a light or buzz box to test them or do I need to take them off so I just have the wire itself to put my alligator clip on? I tried to make my own buzz box but as soon as I clamp it to a ground and the orange wire it buzzes the whole time whether the point is open or closed. I think I made the buzz box wrong.
yes, i remove both wires(coil and condensor) from the termanal on the point assembly