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Old 08-26-2020, 03:18 PM   #1
Don
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Default Low comp on one. Yl

After rebuilding a 21 stud,doing a valve job !new rings and everything else,,put on engine stand,bad miss!rechecked everything!dist ,wires took off intake ,recheck valve clearance,everything good but no comp on no 7,,
About 100# on everything else,,put some oil in no 7 and got about 70 # ,spin the engine a couple times ,no comp,,,will it get any better or do I take it apart and see what the trouble is!!!
Iím sure the rings are there because I didnít have any left over.i used nos rings that had the spring in behind the ring,..030 over
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Old 08-26-2020, 03:22 PM   #2
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Default Re: Low comp on one. Yl

Maybe a ring upside down?
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Old 08-26-2020, 05:44 PM   #3
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Default Re: Low comp on one. Yl

Yes, take it apart and see what's wrong. If valve clearance is correct and valve action is steady (watch #7 valves from intake area while cranking) and the valves move freely when using a pry bar to open them, then the rings need to be looked at.

Before taking it apart, though, I'd put air pressure in #7 and see where the air goes...intake port-exh port-or crankcase. That will give you a clue. You'll have to keep the piston at tdc and valves both closed to apply the air. Shouldn't take much pressure to hear what's going on.



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Old 08-26-2020, 08:03 PM   #4
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Default Re: Low comp on one. Yl

Did that and hear air in crankcase!!
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Old 08-26-2020, 08:22 PM   #5
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Default Re: Low comp on one. Yl

I had a just barely bent new valve one time, that'll sure do it.
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Old 08-26-2020, 08:22 PM   #6
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Default Re: Low comp on one. Yl

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You need to check it, a ring may have broken on installation.
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Old 08-26-2020, 11:06 PM   #7
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Default Re: Low comp on one. Yl

Quote:
Originally Posted by Don View Post
Did that and hear air in crankcase!!



Only one way to get air in the crankcase with closed valves,and that's past the rings. It won't be just one ring upside down...I think that piston is going to have to come out to see why the rings are not sealing. Stuck in their grooves? Both compression rings broken?



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Old 08-27-2020, 06:28 AM   #8
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Default Re: Low comp on one. Yl

That’s today’s project,take it apart to find problem,oh well keeps me out of the house!!!
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Old 08-27-2020, 06:33 AM   #9
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Default Re: Low comp on one. Yl

Quote:
Originally Posted by Don View Post
That’s today’s project,take it apart to find problem,oh well keeps me out of the house!!!
IMO, given what you have posted, I'd pull every one of the pistons AS you have "ONE" that the rings could be installed wrong, I'd make darn sure that ALL were installed correctly....a few hours of pure 100% assurance. Again JMO
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Old 08-27-2020, 05:02 PM   #10
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Default Re: Low comp on one. Yl

Got piston out a few minutes,second and third ring were stuck in grove,,these are four ring pistons ..top ring was loose ,,,
I was very careful when installing rings. so sure there on correct,
I had to put one new piston in,can’t remember !this might have been the new one,
Now
to get it out with out breaking,then use grove cleaning tool to see what’s going on
Will let you know how it goes!!!
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Old 08-27-2020, 10:58 PM   #11
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Default Re: Low comp on one. Yl

That'll do it! I've had luck heating the piston (propane torch on the crown) to expand it so rings have more clearance if they are stuck. Cross your fingers!


Is it any wonder mechanics get gray hair?



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Old 09-16-2020, 09:32 PM   #12
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Default Re: Low comp on one. Yl

Got engine all back and running after freeing rings in number 7 ,,new piston,,after running a short time started missing !,changed carb same thing,changed dist and cond, same thing,pulled plug wire off and had spark interminent,only thing left was the rotor,,removed dist ,found a original black rotor in junk box,setup dist again ,installed,ran smooth like it should,,,,first time I’ve run into this,I did have the shaft wrapped in high voltage tape...learn something everday
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Old 09-16-2020, 10:11 PM   #13
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Default Re: Low comp on one. Yl

Not sure if it's possible, but I think my hair got whiter just following along with your ordeal. I feel your pain and your joy, and I'll say you did a great job figuring it all out.
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