Runaway Engine?

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Toadmund

Toadmund

New Member
Jan 19, 2012
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Saint John, New Brunswick, Canada
This is something that happened last summer.
My brother and I just got back from a decent ride, and we got to my house and my brothers engine was revving really high and would not shut off, so we pulled the spark plug wire and it still ran!
It ran as fast as it could.
He had to clutch kill it.

My theory is that since he installed the SBP expansion chamber and despite trying different jets on an RT carb he just installed as well, he was still running a bit too lean and inside his cylinder got so hot that is was self combusting. It wouldn't take much fuel (throttle was in idle position) to make an engine do this so long as there is an abundance of air in the cylinder mix?

Good theory?

Or air leak?

I would sure like some decent imput on this as the bike was just put away because we never got to fixing it and will be brought out in spring.
 
maniac57

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
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A carbon buildup can lead to self ignition. Pull the head and clean the combustion chamber.
 
S

Shobiz1492

New Member
Feb 13, 2013
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Milwaukee
You can clean off the head and top of the piston with high grit sand paper right?? To keep it from scoring the piston, also to remove build up???
 
Bikeguy Joe

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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No! That will scratch up the aluminum, and carbon will form easier. I always use a BRASS wire brush and a sharpened hardwood dowel.
 
2door

2door

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Staff member
Sep 15, 2008
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Littleton, Colorado
Just make sure you clean out all the crud from the edges of the piston. Failing to clean it after you brush/clean the top of the piston might leave enough grit to damage the cylinder wall. The gritty stuff can get between the piston and cylinder just above the top piston ring. Compressed air will work or a good shop vac. Whatever, get it clean before reassembly.

Tom
 
Bikeguy Joe

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,837
252
63
up north now
Fo'Sho'.

I would suggest removing the jug to properly clean carbon.
Stuff rags around the rod to prevent crud from getting inside the case.
 
Toadmund

Toadmund

New Member
Jan 19, 2012
792
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Saint John, New Brunswick, Canada
Thanks everyone.
I asked my brother to look at this thread, wether he does or not I don't know.
When is he going to register here? I don't know.

Going to check for carbon, air leaks and try a wider jet hole.
Going to be one two or all three of these things at once.
 
Toadmund

Toadmund

New Member
Jan 19, 2012
792
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Saint John, New Brunswick, Canada
OK, I took a few pictures of my brothers bike, well, at least a part of the fuel line that I keep bringing up as a possible part of the problem package.

He says "IT'S NOT THAT!", Then I tell him, it could cause problems, but he insists the fuel flow is fine, nothing wrong with it.
I disagree, and I tell him it's so easy to fix to at least see if that can cause problems.
I think that it is possible that when he opens the throttle, because of this fuel line bend, it could cause a fuel starvation leading to a lean condition.

This however does not rule out the other possibilities mentioned previously.

What is your opinion?
 

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2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
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Littleton, Colorado
Tell your brother to snip those zip-ties on the filter and get the kink out of the fuel line.
Better yet, you do it when he's not looking and go ride the bike. That line is nearly closed off due to the kink. Fuel starvation is a real possibility in this case.

We can't see the rest of the fuel line in your pictures. Any other kinks like the one we can see? If so, get rid of them too.

Suggestion: Cut off about half the length of the barb fitting. You'll still have enough barb to hold the fuel line on and it will help eliminate the kink.

Tom
 
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Toadmund

Toadmund

New Member
Jan 19, 2012
792
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Saint John, New Brunswick, Canada
All he has to do is move back the fuel filter and put a longer fuel line length between it and the tank, simple as that.
But he says with a raised voice, "IT'S NOT THAT!, I'M TELLING YA!"

"OK?!" I say. (while I am thinking, just fix it, and I'll shut up)
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
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Littleton, Colorado
Toadmund said:
All he has to do is move back the fuel filter and put a longer fuel line length between it and the tank, simple as that.
But he says with a raised voice, "IT'S NOT THAT!, I'M TELLING YA!"

"OK?!" I say. (while I am thinking, just fix it, and I'll shut up)
Click to expand...
Is he bigger than you? :)

Tom
 
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