Author Topic: Stuck motor scare (????)  (Read 1130 times)

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Offline faux fiddy

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Stuck motor scare (????)
« on: February 14, 2011, 07:45:46 AM »
Working on getting a 350f on the road. Top end is done, honed, new rings, valves lapped, no problems with spinning it over. I used pre-assembly grease on all moving surfaces I could access journals, lobes, tappets etc. Have not put oil in yet, nor connected a battery, just hand cranking it.

I have started adjusting the valves, and it seems like once some of the pistons are set at TDC and stopped, the motor becomes hard to turn with the kick start, almost as if it's locked up.

Put it in [5th] gear, roll it backwards, roll it fowards, seems fine.

The bike sat for an unknown time period before I worked on it, appears to have been inside storage.

Is there a situation where the kickstart locks up/ or maybe the cam lobes and springs create a TDC 'notch' that it likes to settle in where it is hard to spin out of?  Like if there is enough inertia it just rolls over and if it settles between valve compressions it becomes harder to roll over?

Someone said pull it back apart- perhaps, but no thanks?

Also- what type of feeler guage do people use to snake down  into the tappets??
Is it  an obvious thing that the exhaust tappets further down in the hole and are less accessible than intake  on this motor?
« Last Edit: February 14, 2011, 08:04:58 AM by tree fiddy of industry »
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Offline Old Scrambler

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Re: Stuck motor scare (????)
« Reply #1 on: February 14, 2011, 08:00:05 AM »
Couple of possible causes..........head gasket is 'touched' by the pistons.........at exact TDC the rings are 'flipping' from opposite direction pressure with minimal or no oil in the bore and on the oil-ring lands...............if you shaved the head the plugs may be 'touching' the pistons.......Can you feel something without plugs?  Tell us more about the rebuild....stock? or ????
Dennis in Wisconsin
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Offline faux fiddy

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Re: Stuck motor scare (????)
« Reply #2 on: February 14, 2011, 08:03:18 AM »
Plugs are out.
 
Bored just .001 over to clean up some minor pitting. Used stock sized rings, not NOS, new from Washington.

All the flat surfaces looked okay so no   'shaving' anything down. Used a tiny bit of yamabond II (the old kind).
« Last Edit: February 14, 2011, 08:08:54 AM by tree fiddy of industry »
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Offline fmctm1sw

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Re: Stuck motor scare (????)
« Reply #3 on: February 14, 2011, 09:06:25 AM »
Couple of possible causes..........head gasket is 'touched' by the pistons.........

+1.  Happened on a 175 twin I had.  The head gasket was too small and one of the pistons made contact.
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Offline faux fiddy

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Re: Stuck motor scare (????)
« Reply #4 on: February 14, 2011, 09:26:42 AM »
Couple of possible causes..........head gasket is 'touched' by the pistons.........

+1.  Happened on a 175 twin I had.  The head gasket was too small and one of the pistons made contact.

Could also be a yamabond booger, perhaps? Did use just a small bit, barely any squished out the front of the head gasket.

Will this eventually compress if it (head gasket)  is the case?  Run it?

Open it up and file a bit away? What is the fix for that?

It does seem to spin easy once it is turning, but after it stops it's a bit of a shove to get it rolling over.
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Offline Old Scrambler

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Re: Stuck motor scare (????)
« Reply #5 on: February 14, 2011, 12:12:11 PM »
I am not a user of gasket sealer on the head! I think you will need to inspect the chamber through the plub holes with  pistons below TDC. Look for any color on the pistons.  I would be tempted to fill the oil supply and squirt some Marvel's or similar onto the pistons.  Leave the plugs out and turn the motor with the electric start for a few seconds. Then hand-turn for results. The overbore helps to make the rings flex - especially if dry.  BTW - while spinning the motor check to see that you have constant vaccuum at the carbs..........no back pressure......this will confirm you have the cam installed correctly.
Dennis in Wisconsin
'64 Triumph Cub & '74 Honda CB750 Bonneville Salt Flats AMA Record Holder (6)
CB750 Classic Bonneville Racer thread - http://forums.sohc4.net/index.php/topic,135473.0.html
'63 CL72 Project(s)
'66 CL77 Red
'67 Triumph T100C
'73 750K3 Owned since New
'77 750F2 Cafe Project
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Offline faux fiddy

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Re: Stuck motor scare (????)
« Reply #6 on: February 14, 2011, 02:39:34 PM »
I would be tempted to fill the oil supply and squirt some Marvel's or similar onto the pistons.  Leave the plugs out and turn the motor with the electric start for a few seconds. Then hand-turn for results.

I figure that's next- oil it up  and spin it with the starter to see what it does before I open anything up.  Finish the  adjustments, and chalk  it up as  an aberration that will just go away with use.

 
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