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Old 11-02-2018, 12:51 PM   #41
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I just watched the video I made of the valves and I realized one of them is not open and the other 3 are. No compression at all on 3. So that one valve must be bent and slightly open enough to not seat properly.


Next trip I will plan on a head swap. Although I might just try and bring it to someone if they don't want an arm and a leg to swap it.


What made me recheck the video was that when I pulled the cover to check the compression. The same valve bridge was shifted off of the valves again. I'm guessing what's happening is the one valve isn't moving and it's causing the bridge to shift over.


I'll update again on the next trip and thanks to everyone that chimed in.

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Old 11-02-2018, 04:00 PM   #42
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Bummer, but what was suspected to start with. Was hoping you got by ok without any real damage
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Old 11-02-2018, 08:23 PM   #43
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You adjusted the valve bridges too? It's something that takes a hands-on feel thing,
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Old 11-03-2018, 01:24 AM   #44
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Originally Posted by Iceni John View Post
I'm with Gene on this one - the runaway is the cause of your problems. Why exactly did it runaway?
John
Perhaps a bit late to the party, and I have no experience working on diesels, but if memory serves from talking to people who have worked on these old 2-strokes, they have said that the rack not being set right can cause lots of issues, one of them runaway. That the rack can hang and run the engine wide-open. Hope that helps.
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Old 11-03-2018, 11:17 PM   #45
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You adjusted the valve bridges too? It's something that takes a hands-on feel thing,

No, just one of the bridges on number 3 moves more than it should. I'm guessing it due to a sticking valve. When you remove the rocker bridge the valve bridges just slide right off the pins they sit on. This engine does not have spring loaded valve bridges so the only thing holding them on is the pressure from the rocker arms and valve springs.
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Old 11-04-2018, 02:21 AM   #46
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I haven't done valves and injectors on a '71 in over 40 years, but if I recall correctly, you adjust the valve bridge to achieve an equal lash on each valve, then you adjust the valve lash. Not difficult, but definitely a feel kind of thing.
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Old 12-18-2018, 04:38 PM   #47
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Finally, I get to more engine work. Take a look for me and tell me what you think. I look inside the number 3 cylinder with a camera and I think one of the valves might be closed when the other three look open. In any case, it looks like a head swap will be the next step.

I did have someone comment that the rod could be bent on that cylinder causing it to not fire. Any thoughts on that or how to check it without taking off the oil pan?

Thanks for watching and your help as well.

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Old 12-18-2018, 05:28 PM   #48
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Will watch the video a little later.

As for a rod being bent causing it to miss, umm no not happening. If slightly bent it would still have enough compression to fire, if bent a lot the engine would not turn all the way around. It is highly unlikely to have a bent rod, over speed will cause valve float first. With valve float the chances of the piston hitting one become high, and is most likely the cause of the one valve not seating.

The valve spring can only move the valves so fast, once you try to run the engine faster the spring is not strong enough to pull the valve back up in time. This is why hot rodders use heavier then stock valve springs. Same with the cummins 6BT, 3200rpms is considered max with stock springs. there are 4000 rpm valve springs available and a 4000 rpm governor spring to go with it. My Gradall runs a 6BT governed at 4000rpms.

I hope this all makes sense.
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Old 12-18-2018, 06:17 PM   #49
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Will watch the video a little later.

As for a rod being bent causing it to miss, umm no not happening. If slightly bent it would still have enough compression to fire, if bent a lot the engine would not turn all the way around. It is highly unlikely to have a bent rod, over speed will cause valve float first. With valve float the chances of the piston hitting one become high, and is most likely the cause of the one valve not seating.

The valve spring can only move the valves so fast, once you try to run the engine faster the spring is not strong enough to pull the valve back up in time. This is why hot rodders use heavier then stock valve springs. Same with the cummins 6BT, 3200rpms is considered max with stock springs. there are 4000 rpm valve springs available and a 4000 rpm governor spring to go with it. My Gradall runs a 6BT governed at 4000rpms.

I hope this all makes sense.

Absolutely, I didn't think the rod idea sounded right but I wanted to get some other opinions. I am familiar with valve float. I've built a few Mustangs in the past. One was a supercharged four valve with big cams, titanium valves, fully ported heads, and all of the supporting reciprocating parts to make it work without blowing up. It was quick! 7500 RPMs from a V-8!
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Old 12-19-2018, 06:06 AM   #50
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the only way ive seen a bent rod cause a cylinder not to fire is when the bent rod was AGAINST the I-Beam portion of the rod and caused the piston angle to change slightly.. not enough to destroy the cylinder, but enough that the rings failed to seal correctly.. we were getting oil belched in the exhaust manifold in that case so if you arent getting oil smoke id thiunk its more likely a valve issue..



broken rings can also cause low compression... you'll usually see the metal pattern with a bore-scope though.. a peppered look on the head and piston head where pieces of rings got impaled..



if you just bent a valve then a head replacement isnt needed.. just pull the head and get it worked.. that valve, seat, and guide are likely shot.. and with a lot of hours it makes sense to get a complete valve-job and head inspection done,.



ive wasted a few valves over the years.. (comes with hot-rodding.. esp missing 3rd in a muncie!).. ive never destroyed a head because of it..



-Christopher
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Old 12-19-2018, 08:27 AM   #51
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I think he is thinking a reconditioned head to swap to expedite the process in as much as the bus is a long way and an ocean apart from home.

as for the rod being bent the direction you are talking about I can see how on a gas engine this would tip the piston enough to not seal the rings, but on a Detroit the piston is about 5" long, it would be hard to tip it enough to do that without causeing other real damage like cylinder scoreing, cracked or broken piston.
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Old 12-19-2018, 08:55 AM   #52
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wow! ive not seen a detroit in pieces.. that would also be much more tough to bend a rod that way too with a piston that size..

-Christopher
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Old 12-19-2018, 09:59 AM   #53
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Just watched the video. With the angle of the camera, it's hard to say if that bottom valve in the video is open or closed. However, you should be able to tell if the valves are opening or not from the rocker side. That valve not opening wouldn't necessarily cause a miss, it not closing definitely will though.
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Old 12-19-2018, 11:11 AM   #54
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wow! ive not seen a detroit in pieces.. that would also be much more tough to bend a rod that way too with a piston that size..

-Christopher
I have rebuilt a few of them, need to do one (6-71)this winter
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Old 12-19-2018, 09:55 PM   #55
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Just watched the video. With the angle of the camera, it's hard to say if that bottom valve in the video is open or closed. However, you should be able to tell if the valves are opening or not from the rocker side. That valve not opening wouldn't necessarily cause a miss, it not closing definitely will though.
That was my thought as well. It's hard to tell with the camera angle. I think it may be bent and not completely opening and then not closing either.

Ronnie, that's a good point. Those pistons are huge! It would be impossible for them to rock in the cylinder. The cylinder walls actually look really good in the video. I couldn't see any evidence of anything that looked like it had made contact. No dents or dings on the valves or on the crown of the piston either. I didn't put the video in of the piston because its harder to see.
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Old 12-20-2018, 05:24 AM   #56
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I did finally watch the video, and yes the cylinder walls look pretty good to me. The one valve looks different but to hard to tell for sure. The rocker arms not staying put says a lot though.

Gas engines it is very hard to push on the rocker arm or directly on the valve stem and get them to open, just to much spring pressure. However on a diesel that is not true, usually you can because they have a much "weaker" spring. Yours are in a nice place to get a big ole screwdriver and try pushing down on a few that seem to be ok then try the bad one. Just another way to verify proper movement or lack there of.
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Old 12-24-2018, 04:48 PM   #57
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Confirmation on the bent valve. Zero compression on number 3.

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Old 12-24-2018, 05:27 PM   #58
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After skooliepalooza I plan to continue west to California then north. If you are going to be stateside in later January we might be able to meet up and could give you a hand changing the valves or head.
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Old 12-24-2018, 06:39 PM   #59
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That would be awesome! I might be able to make that happen.

How long does it usually take to swap out the head? My only concern was getting the rack and governer back on and tuned. I've been sitting here reading my manual trying to decide if I want to attempt it. Having someone there that has experience would be a no brainer!
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Old 12-24-2018, 08:12 PM   #60
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should be a day , but would plan two just in case of unforeseen issues.
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