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D troop 3/5 CAV ,162 AHC
 
afterburn 549's Avatar
 
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Bent valves from lean mixture or back fire?

bumble bees do hit sound barriers.

Last edited by afterburn 549; 09-15-2018 at 06:15 PM..
Old 08-21-2018, 02:27 PM
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If it was popping, perhaps it is running lean and burnt it? Can't imagine bending it. Compression is pretty low and there is no mechanical interference in question, right?
Old 08-21-2018, 02:30 PM
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Bent mechanic, coming up with a story like that ?
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Old 08-21-2018, 02:37 PM
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Cannot think of any possible way to bend a valve other than contact .
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Old 08-21-2018, 02:38 PM
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Makes no sense to me. I can’t see a bent valve coming from anything but contact.
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Old 08-21-2018, 04:01 PM
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Burnt > lean
Bent > valve float/contact
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Old 08-21-2018, 04:08 PM
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Well, if it. did hold it open from a backfire, it would still be contact
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Old 08-21-2018, 04:35 PM
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Over-rev due to a downshift?

How are they sure it didn't contact?
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Old 08-22-2018, 04:22 AM
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Only from mechanical contact. But I have seen broken stem / failed keepers where then the valve drops and in that short frame of time, backfires through the carb as the carnage took place.
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Old 08-22-2018, 04:35 AM
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In my experience there had to have been a close encounter of some kind.
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Old 08-22-2018, 04:52 AM
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In a back fire the flame front starts at the valve and pushes out the intake, so the pressure wave is going the wrong direction to load the valve. (Referring back to some research in the engine lab when I was in Engineering school decades ago....)

Certainly a sticky valve guide or a foreign object holding the valve open would cause the bent valve but valve to piston contact is pretty much the only cause..
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Old 08-22-2018, 10:01 AM
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Quote:
Originally Posted by afterburn 549 View Post
I am voting a guide problem.
I would have to agree. I have yet to go into a M8, but I highly doubt in stock form HD would have valve to piston clearance that close. Has to pass Fed smog. With that said, if it already has higher compression pistons in it, and of course a larger cam, it is possible with severe valve float, or a cam installed out of time.


Backfires don't cause bent valves. Contact does. If they were trying to tune it for more HP, and it went very lean, it will easily stick a guide or tulip a valve. Ask if the pushrod was damaged.
Old 08-22-2018, 05:21 PM
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Doesn’t a bent valve cause a backfire, not the other way around??

Nut falling into the intake?
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Old 08-22-2018, 05:57 PM
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