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Discussion Starter #1 (Edited)
Hey there,

So currently I am not getting any compression in the front cylinder and am looking for some advice on what I should be thinking about doing to do a repair.

The history of the bike is this:
-I bought it last winter and it had an oil leak coming from the front cylinder.
-I replaced the head gasket and upgraded the ACCT springs along with the vibration dampers while I was in there
-Put maybe around 1000k on the bike last summer then at the end of the season the rear cylinder lost spark, I probably put about 20k on the bike after this happened.
-Over the winter I replaced the ignition coils and the ICU and did some other general maintenance repairs such as the fork seals and air filters (I did properly oil them however)
-I've just got the bike back on the road and at first the front cylinder was smoking considerably but after about half a tank of gas the smoking stopped (i assumed it was just due to the sea foam I had put into the tank as stabilizer last fall)
-At this point the bike ran fantastic and had ton's of power then today I took a friend out on the bike and within one block the front cylinder cut out
-I checked for spark and there was a strong spark yet compression test showed 0 compression in the front cylinder (like the gauge doesn't even pretend to try and move) The rear cylinder is at around 125 just like the front was a few days ago.

So if anyone has any idea of what might have gone wrong and what I need to do to fix it or properly diagnose the issue I would love to hear it. Right now I am suspecting a burnt exhaust valve though I don't really know why this might have happened.

Also the bike is a 86 750.

Cheers,

Andre
 

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Linkmeister Supreme
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Discussion Starter #3

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Linkmeister Supreme
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That could be a distinct possibility, I'll be tearing the engine down on tuesday so I'll keep an eye open for that. Is that a fairly common failure mode?
Don`t know. I just read a thread yesterday where jthills mentioned a couple of guys on this board who knew about not making sure the rocker arm was in position when reassembling the head,:( and recalled it when I read this one.
 

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That could be a distinct possibility, I'll be tearing the engine down on tuesday so I'll keep an eye open for that. Is that a fairly common failure mode?
I'd be leaning toward a cam chain failure,seems more likely to me,but you will see soon enough if you are tearing into it,keep us posted:smiley_th
 

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Linkmeister Supreme
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Hey there,

So currently I am not getting any compression in the front cylinder and am looking for some advice on what I should be thinking about doing to do a repair.

The history of the bike is this:
-I bought it last winter and it had an oil leak coming from the front cylinder.
-I replaced the head gasket and upgraded the ACCT springs along with the vibration dampers while I was in there
-Put maybe around 1000k on the bike last summer then at the end of the season the rear cylinder lost spark, I probably put about 20k on the bike after this happened.
-Over the winter I replaced the ignition coils and the ICU and did some other general maintenance repairs such as the fork seals and air filters (I did properly oil them however)
-I've just got the bike back on the road and at first the front cylinder was smoking considerably but after about half a tank of gas the smoking stopped (i assumed it was just due to the sea foam I had put into the tank as stabilizer last fall)
-At this point the bike ran fantastic and had ton's of power then today I took a friend out on the bike and within one block the front cylinder cut out
-I checked for spark and there was a strong spark yet compression test showed 0 compression in the front cylinder (like the gauge doesn't even pretend to try and move) The rear cylinder is at around 125 just like the front was a few days ago.
So if anyone has any idea of what might have gone wrong and what I need to do to fix it or properly diagnose the issue I would love to hear it. Right now I am suspecting a burnt exhaust valve though I don't really know why this might have happened.

Also the bike is a 86 750.

Cheers,

Andre
I'd be leaning toward a cam chain failure,seems more likely to me,but you will see soon enough if you are tearing into it,keep us posted:smiley_th
If the cam chain was piled up, the engine would be seized, and he wouldn`t get any spark would he?? :confused:
 

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not necessarily, the timing chain can be broken add fallen out of the way and the engine still crank, the triggers on the coils are activated by the magnets on the rotor for the stator.

I think NetMatrix has experienced this problem recently
http://www.vn750.com/forum/showthread.php?t=18285&highlight=valves+-moving
post # 6
You`re right Denny. I had forgotten some of the details of that thread until I read it completely through at one time again just now.

So it is POSSIBLE for the LOWER cam chain to break and fall out of the way, off the crankshaft and idler gears, into the bottom of the crankcase, and allow the engine to turn over. I will have to file that little bit of mechanical knowledge away in my (not always so reliable :doh:) memory banks.
 

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You`re right Denny. I had forgotten some of the details of that thread until I read it completely through at one time again just now.

So it is POSSIBLE for the LOWER cam chain to break and fall out of the way, off the crankshaft and idler gears, into the bottom of the crankcase, and allow the engine to turn over. I will have to file that little bit of mechanical knowledge away in my (not always so reliable :doh:) memory banks.
I need to free up some more space in mine too Ol Hoss! I cleared some of the cobwebs today,with about 180 miles of Eastern KY Backroads,Ijust got in and I am tired but I feel a lot better.:smiley_th
 

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Discussion Starter #11
Looks like OlHoss was bang on. I dropped the engine and took a look inside today. One of the intake and one of the exhaust rockers had come off. Unfortunately when the exhaust rocker came off it somehow managed to dislodge the valve keepers. So the exhaust valve had fallen into the engine, getting hit by the piston before getting pushed up and lodged in the head.

The cams are also showing excessive wear where they were in contact with the dislodged rockers so I'm thinking I need to find someone with a complete head that I can purchase off ebay.

Does anyone know what years the 86 engine should be compatible with? IE is a 92 the exact same engine?

Cheers, and I hope to get some photos up soon.
 

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Looks like OlHoss was bang on. I dropped the engine and took a look inside today. One of the intake and one of the exhaust rockers had come off. Unfortunately when the exhaust rocker came off it somehow managed to dislodge the valve keepers. So the exhaust valve had fallen into the engine, getting hit by the piston before getting pushed up and lodged in the head.

The cams are also showing excessive wear where they were in contact with the dislodged rockers so I'm thinking I need to find someone with a complete head that I can purchase off ebay.

Does anyone know what years the 86 engine should be compatible with? IE is a 92 the exact same engine?

Cheers, and I hope to get some photos up soon.
The heads from any model year are interchangable. Even the 1985 vn700 I think. The bore is smaller, but the heads are identical.
 

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Are the front and back cylinder heads interchangeable?

Cheers
Mmmmm....Not sure about that. Let me do a little research.

Ok, I will give a definite I think so. LOL

The only differences I can find in the Clymer manual is that
1.--the rear head has a heat shield and the front does not, and
2.--the camshafts have a radial groove on the face of the gear in one head to identify them if both heads are pulled off at the same time.

Hopefully someone else can give an unqualified yes or no.
 

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I just checked my heads side by side on the workbench. Even though I think the castings are the same I'd say they are NOT interchangeable.

The upper cam chain tensioner spring holder is cut into the opposite sides on each head. There is also a coolant (I think) line plugged on the rear head that has an inlet on the front head.
 

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Thanks for clearing that up Roach. :smiley_th
It`s much more conclusive to compare the parts than just research the book.
 

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Discussion Starter #18
That's great, thanks alot guys! I'll be ordering up a new head on ebay in the next couple of days so hopefully it won't be too much longer before I get the bike back on the road. Though I think I'll take this time to lap the valves and replace the valve oil seals.

I'm definitely going to be much more attentive to those rockers when I put things back together this time!

One thing that blows my mind is how well the bike ran given that one of the intake and one of the exhaust valves were not properly connected to the rockers. It's going to be a lot of fun to take it out, after getting a new head, to see the real potential of the bike!
 

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Wareagle1970
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Do they have compression testers as loaner tools at local car shops?
 
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