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Ex-Military Spec 6JB compression issue


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  #11  
Old 02-09-2017, 09:26:10 PM
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Default Re: Ex-Military Spec 6JB compression issue

The "lands" are the portion of the piston between the ring grooves.
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Old 02-10-2017, 12:09:47 AM
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Default Re: Ex-Military Spec 6JB compression issue

I'm probably wrong, but I guess I'm a little surprised that a little loss of compression would cause total misfire on one cylinder.

First, what does the compression actually measure? - I could be convinced this is the issue, but need some evidence.

Second - did you swap plugs and plug wires to make sure the 'low compression' cylinder remained the non-firing one?

Ignition issues are common. Low compression, especially all-of-a-sudden, not so common.

Lastly, any other signs of low compression i.e. crankcase blowby overwhelming the breather, or oil blowing out anywhere?
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Old 02-10-2017, 12:23:43 AM
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Default Re: Ex-Military Spec 6JB compression issue

Diesels are just a compression/ combustion engine, if compression gets low it will not get hot enough in the cylinder to fire the atomized diesel if it is a djb, I do not know which one it is.
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Old 02-10-2017, 12:36:39 AM
Wayne 440 Wayne 440 is offline
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Default Re: Ex-Military Spec 6JB compression issue

I am seeing "6JB" in the title- to me that is a spark ignition engine.

---------- Post added at 10:36:39 PM ---------- Previous post was at 10:25:45 PM ----------

To me, this unusual-

Quote:
Originally Posted by MadGutts View Post
...when turning it over there was compression, then turned it over again after setting the rockers and lost the compression!...
I have done more than a few valve clearance adjustments (I'm assuming that "setting the rockers" is a term for same) and have never had a cylinder suddenly lose compression due to the process.
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Old 02-10-2017, 01:05:00 AM
Zeromedic Zeromedic is offline
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Default Re: Ex-Military Spec 6JB compression issue

Maybe no compression on next rev after adjusting valves because that cylinder now coming to
TDC on exhaust stroke?

Steve
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Old 02-10-2017, 01:22:31 AM
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by Wayne 440 View Post
I am seeing "6JB" in the title- to me that is a spark ignition engine.....
Me too. Ignore my post if this is a Diesel...
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Old 02-10-2017, 05:10:13 AM
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Default Re: Ex-Military Spec 6JB compression issue

Morning guys,

Yes its a petrol fired engine... sorry for the confusion - i put diesel in the cylincders to test the rings, low and behold the 2nd cylinder is still full, nothing in the first, so obviously its leaking somewhere..

There are no marks on the inside of the cylinder and its all freely moving, so next job is piston out to investigate further

I hope its not the piston that has had it as parts are not easy to find in the UK!
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Old 02-10-2017, 09:44:39 AM
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by MadGutts View Post
...2nd cylinder is still full, nothing in the first, so obviously its leaking somewhere...
Agreed - just not convinced that this leakage is the cause of that cylinder not firing. Did you swap plugs and plug wires to make sure the 'low compression' cylinder remained the non-firing one?
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Old 02-10-2017, 10:34:17 AM
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Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by Jim McIntyre View Post
Agreed - just not convinced that this leakage is the cause of that cylinder not firing. Did you swap plugs and plug wires to make sure the 'low compression' cylinder remained the non-firing one?
I'm an old ford guy, unless these thing are a lot higher performance than a ford 6 (not likely ;-) it should run just fine with enough blowby to soak everything with oil closer than 10 feet, and sound like it's running fine.

I'm with Jim. (and my dad). Simple stuff first. And breaking into a block was the very last thing he wanted to do.
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Old 02-10-2017, 12:21:56 PM
len k len k is offline
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Default Re: Ex-Military Spec 6JB compression issue

Are you sure you were at TDC when you adjusted the valves. Doesn't take much open valve to lose compression. On my single cylinder 10 hp snow blower ~0.005 inch always open valve will drop compression to ~ 50 psi and it won't even start.
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