Bore seize why?

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Limp1144
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Bore seize why?

Post by Limp1144 » Mon May 21, 2012 10:17 pm

So I have been building scooters for five years now. First seize. Here are the specs my setup.

48mm big bore
Dio derestricted exhaust 26mm.
Nyc variator stock roller weight
Temp gauge 14mm cht.
78 main
Stock airbox.

On my way to work today after running 260f came to a stop and it turned off. Oil mix is 40:1 cant be a heat seize.
Last edited by Limp1144 on Tue May 22, 2012 9:44 am, edited 1 time in total.

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Re: Bore seize why?

Post by bakaracer » Mon May 21, 2012 11:19 pm

the only way to know is open it up and see.
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Re: Bore seize why?

Post by tru72 » Tue May 22, 2012 3:32 am

LOL cant be seized? hehe
but baka is correct open it up and study the piston, it will tell you pretty much exactly if it was a heat/cold sieze, misalignment..
78 jet seems pretty small

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Re: Bore seize why?

Post by Limp1144 » Tue May 22, 2012 9:43 am

It runs super cold with that 78 main jet. I did not modify the airbox bore never seen over 300f. It was a cheap taiwan bore not ruima. When it locked up it idle down then when I went togive it gas it made a baaaa noise then turned off. The electric start fired it back up. Could it be the stock gears doing about 50mph so I am sure it was high in the rpms

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Re: Bore seize why?

Post by Downsouthdio » Tue May 22, 2012 10:20 am

Just curious.. Did you ever look at the plug? And if so how'd it look
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Re: Bore seize why?

Post by Limp1144 » Tue May 22, 2012 11:40 am

Havent pulled out the plug but the temp gauge elminates that. I drove it around all day to and at 35mph 195f-205F. Top end run almost at 50mph 225F so it is getting cooler on the temps. I will pull the plug out but i would bet it is black from richness

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Re: Bore seize why?

Post by tazland001 » Tue May 22, 2012 12:07 pm

That 78 sounds way to lean for that set up. If your running your stock carb you need at least a 100 main in my opinion.

Also did you break the kit in????

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Re: Bore seize why?

Post by Limp1144 » Tue May 22, 2012 12:48 pm

Where is everyone getting these jet sizes. No bore had 25miles on it. 4 heat cool cycles. Wot temp doesnt go over 260f now. Most bores can go to 375-400 before the seize. 100 jet my temps would be in the low 100f lol

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Re: Bore seize why?

Post by LMH » Tue May 22, 2012 1:15 pm

It can seize even at 200, just pop the head off and look at the top at least or the exhaust and look through the port to see at least exhaust side. Or better yet compression test it.

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Re: Bore seize why?

Post by Bear45-70 » Tue May 22, 2012 1:23 pm

Actually the heat spike that seizes an engine happens so fast that a gauge will never respond that quickly. If you are lucky it will show up afterwards but not always.
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Re: Bore seize why?

Post by tru72 » Tue May 22, 2012 1:32 pm

Limp1144 wrote:Havent pulled out the plug but the temp gauge elminates that. I drove it around all day to and at 35mph 195f-205F. Top end run almost at 50mph 225F so it is getting cooler on the temps. I will pull the plug out but i would bet it is black from richness
wow. you sure about that "temp gauge eliminates that"? More like the opposite. you can run and tune a bike with looking at the plug. Just relying on the temp gauge isnt a good idea. There are a few variables , bad gauge or bad sender.. Also im hoping when you read your plug you dont just go by the color. That way of reading a plug is not really the true way of reading plugs. Mopeds with a stock carb will look nice and clean. try using a fat pilot most times a good tune will still look oily and black over a time. All plugs look tan in color if pulled at the right time. The real way is to cut open a plug and look at the base for the fire ring. I usually know what im looking for so a good bright light source can see down the plug if its new.

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Re: Bore seize why?

Post by Bear45-70 » Tue May 22, 2012 2:04 pm

Or use one of these to see without destroying your plug.

http://www.ebay.com/itm/140755408539?ss ... _823wt_931
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Re: Bore seize why?

Post by Limp1144 » Tue May 22, 2012 2:09 pm

Weds off work will check it out. Gonna I thought the cht gauge would give me the insight on the true temps. do a plug chop since it turned off put about 40 miles so far no issues. Thought the cht was pretty accurate. Getting better gas mileage to 68mpg now

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Re: Bore seize why?

Post by Downsouthdio » Tue May 22, 2012 2:59 pm

Somethings not right somewhere.. Cht is good but cant jus rely on that..good luck man , its frustrating i know
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Re: Bore seize why?

Post by Wheelman-111 » Tue May 22, 2012 3:18 pm

Greetings:

Jetting and apparent temps aren't the only things that can kill a bore.

A bad tank of fuel - either too-low Octane or shrapnel blocking a jet - will cause sudden failure. Chunks-O Hot Metal can cause pre-ignition and the Detonation Death Spiral faster than any gauge could register. Often the Temp That Really Matters happens off-throttle - if the Pilot jet is too small. Bad or insufficient oil can do it too. So can sufficient oil, if it's not where it needs to be.

I'm surprised Baka hasn't mentioned chamfering the edges of the ports and piston skirts - he has many times. The more affordable bore kits have knife-edges that scrape too much of the oil off the cylinder walls and basically the piston/bore interface runs dry of lubricant. It's a short time indeed at 200 strokes/second before the piston goes red-hot. Poof!
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