Generators and Motors
[Home] - [HELP] - [Forums] - [Library] - [Photo Gallery] - [Groups] - [Classified Ads] - [Subscribe] - [Links] - [Books] - [Sponsors] -

Go Back   SmokStak > SmokStak® Vintage Electrical Equipment > Generators & Motors General Discussion > Onan Generators
Forgot Password? Join Us!

Notices

Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Ex-Military Spec 6JB compression issue


this thread has 84 replies and has been viewed 2723 times

Reply
 
Thread Tools Display Modes
  #1  
Old 02-09-2017, 12:05:06 PM
MadGutts MadGutts is offline
Registered
 
Join Date: Feb 2017
Location: Witney, Oxon, United Kingdom
Posts: 17
Thanks: 15
Thanked 7 Times in 1 Post
Exclamation Ex-Military Spec 6JB compression issue

Hi Guys,
I have had this genset for about 3 months and it was running superbly for the last 2 months... Yet a couple of days ago i started it in the morning and used it for a couple of hours and then turned it off as normal...
Sometime later about 4pm i sterted it back up but it took some starting and wasn't sounding right... turns out is was only firing on one cylinder - it has only done 455 hours so not due a major overhaul yet... but i switched it off and had a quick look over it...

Turning the engine over by the crank i could get get good compression but on the next rotation i could hear air hissing on the compression stroke, then roated to the next compression was good again... So decided to whip the head off..

So after reading these forums, and people saying the valve seat to be an issue, we cleaned the head which was badly coked up and we reset the valves and re ground in the 1st cylinder... all nice and clean and well seated.

Checked the inside of the cylinder to make sure no damage/scoring etc on the inside... it all looked and felt fine, so removed the coked ring around the top and cleaned it all out, re-assembed the head with the old steel gasket and turned over by hand, still had a hissing on piston 1... so took all apart again and have now put some diesel in the cylinders to see if there is a piston ring out...

Since im guessing this is going to be the fault... is there any one on here who has done this job on their engines?

basically this is a heavy lump and i don't have an engine crane to lift this of the cradle and turn on its side/end while i remove the bottom cover to get access to the crank and the big ends...

So is there an easy way? should i check anything else first?

thanks in advance - oh probably should mention im in the UK so parts are like rocking horse poo over here now
Reply With Quote
Sponsored Links
  #2  
Old 02-09-2017, 12:19:09 PM
Jim Rankin Jim Rankin is offline
Registered-III
 
Join Date: Nov 2004
Location: Faunsdale, Alabama USA
Posts: 4,428
Thanks: 68
Thanked 1,182 Times in 967 Posts
Default Re: Ex-Military Spec 6JB compression issue

I doubt you are missing anything having done the valves, but a good check is to put that piston at TDC of compression stroke and while holding it securely with a wrench, apply compressed air to it through the spark plug hole. The air will leak from the rings and you will want to compare the noise between the good and bad cylinders. But if you have a valve or head gasket problem, you will quickly pick up on it.
Reply With Quote
The Following User Says Thank You to Jim Rankin For This Post:
  #3  
Old 02-09-2017, 02:26:01 PM
nothingbutdarts nothingbutdarts is offline
Sponsor
 
Join Date: Apr 2014
Location: Bailey, Colorado, USA
Posts: 1,765
Thanks: 3,920
Thanked 677 Times in 482 Posts
Default Re: Ex-Military Spec 6JB compression issue

Yes as Jim mentioned, do a leak down test at TDC. Either with a meter if you have access to one or just by putting air pressure in the plug hole with a rubber tip air nozzle. + You will want to listen to the three areas, intake side, exhaust side and pull the dipstick and listen in the crankcase. See where the most hissing is coming from before taking it apart again.
The oil pan has to come off to get the pistons out.
Reply With Quote
  #4  
Old 02-09-2017, 02:42:59 PM
MadGutts MadGutts is offline
Registered
 
Join Date: Feb 2017
Location: Witney, Oxon, United Kingdom
Posts: 17
Thanks: 15
Thanked 7 Times in 1 Post
Question Re: Ex-Military Spec 6JB compression issue

thanks guys,

i have already taken the head off as when turning it over there was compression, then turned it over again after setting the rockers and lost the compression!

So with head off, put some diesel into both cylinders and let it rest for a bit.. an hour later and all diesel in 1st cylinder had gone! so deffo a piston out job now

to get the sump off, it will have to come of the cradle... but do i need to take anything else off to do this? - blower shroud etc? the service manual is not too descriptive!

and if i stand it up on the alternator end, will that be ok on the end while i do this?
Reply With Quote
  #5  
Old 02-09-2017, 03:09:03 PM
Billy J Shafer's Avatar
Billy J Shafer Billy J Shafer is offline
Subscriber
 
Join Date: Nov 2004
Location: Centerville, Texas
Posts: 15,602
Thanks: 7,911
Thanked 12,813 Times in 6,036 Posts
Default Re: Ex-Military Spec 6JB compression issue

I have changed pistons in the field. It will have to come out to where you roll it over.I would remove any shields that might bend. Have two good blocks of wood to lay the block on.
__________________
I don't talk to myself. This early in the morning. Why the hell should I talk to you.
Reply With Quote
The Following User Says Thank You to Billy J Shafer For This Post:
  #6  
Old 02-09-2017, 05:42:25 PM
BRae's Avatar
BRae BRae is offline
Sponsor
 
Join Date: Oct 2013
Location: Escanaba, Michigan
Posts: 603
Thanks: 249
Thanked 331 Times in 190 Posts
Default Re: Ex-Military Spec 6JB compression issue

Very strange, running just fine when shut down only to be running low on compression the next day? Could carbon have gotten into a ring? I think I would try a a good penetrating oil. Maybe that could dissolve some carbon and get the seal back. Did you notice a lot of blow-by in the breather?
Reply With Quote
  #7  
Old 02-09-2017, 06:01:43 PM
MadGutts MadGutts is offline
Registered
 
Join Date: Feb 2017
Location: Witney, Oxon, United Kingdom
Posts: 17
Thanks: 15
Thanked 7 Times in 1 Post
Default Re: Ex-Military Spec 6JB compression issue

Will have to find some good blocks then! got to be easier on its side than stood on the alternator! lol

So next question, im assuming i will need a new set of rings... but what size should i buy? with only 500 hours on the clock, surely its never been honed so an oversize set shouldnt be required should they?
Reply With Quote
  #8  
Old 02-09-2017, 06:08:00 PM
LRomero LRomero is offline
Subscriber
 
Join Date: Jun 2016
Location: New Iberia, Louisiana
Posts: 285
Thanks: 36
Thanked 74 Times in 45 Posts
Default Re: Ex-Military Spec 6JB compression issue

I pulled the pistons out on my 4.0CCK by standing the unit up on the alternator end. I used some wood blocks I have laying around and was able to shim it pretty steady. I used a strap/chain from the ceiling for extra safety to keep it from tipping over too. The strap twisted as I turned the flywheel to get the crank turned. Worked perfectly.
Reply With Quote
  #9  
Old 02-09-2017, 06:46:06 PM
Power Power is offline
Registered-III
 
Join Date: Aug 2011
Location: New Jersey
Posts: 3,731
Thanks: 869
Thanked 2,106 Times in 1,246 Posts
Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by MadGutts View Post
Will have to find some good blocks then! got to be easier on its side than stood on the alternator! lol

So next question, im assuming i will need a new set of rings... but what size should i buy? with only 500 hours on the clock, surely its never been honed so an oversize set shouldnt be required should they?
See what you got first.
To have that big a loss of compression, probably more than just ring.
May have broken land(s)
Very likely if someone used Ether in the past.
Reply With Quote
The Following 3 Users Say Thank You to Power For This Post:
  #10  
Old 02-09-2017, 09:00:50 PM
MadGutts MadGutts is offline
Registered
 
Join Date: Feb 2017
Location: Witney, Oxon, United Kingdom
Posts: 17
Thanks: 15
Thanked 7 Times in 1 Post
Default Re: Ex-Military Spec 6JB compression issue

excuse my ignorance... broken lands?
Reply With Quote
  #11  
Old 02-09-2017, 09:26:10 PM
lokay5's Avatar
lokay5 lokay5 is offline
Subscriber
 
Join Date: May 2007
Location: Merlin, Oregon
Posts: 1,256
Thanks: 3,242
Thanked 734 Times in 461 Posts
Default Re: Ex-Military Spec 6JB compression issue

The "lands" are the portion of the piston between the ring grooves.
Reply With Quote
The Following 2 Users Say Thank You to lokay5 For This Post:
  #12  
Old 02-10-2017, 12:09:47 AM
Jim McIntyre's Avatar
Jim McIntyre Jim McIntyre is offline
Subscriber
 
Join Date: Nov 2004
Location: Mooresville, Indiana, USA
Posts: 3,671
Thanks: 575
Thanked 1,880 Times in 1,115 Posts
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?
Reply With Quote
The Following User Says Thank You to Jim McIntyre For This Post:
  #13  
Old 02-10-2017, 12:23:43 AM
nothingbutdarts nothingbutdarts is offline
Sponsor
 
Join Date: Apr 2014
Location: Bailey, Colorado, USA
Posts: 1,765
Thanks: 3,920
Thanked 677 Times in 482 Posts
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.
Reply With Quote
  #14  
Old 02-10-2017, 12:36:39 AM
Wayne 440 Wayne 440 is online now
Registered-III
 
Join Date: Nov 2004
Location: Kentucky
Posts: 4,617
Thanks: 275
Thanked 3,683 Times in 1,754 Posts
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.
Reply With Quote
The Following User Says Thank You to Wayne 440 For This Post:
  #15  
Old 02-10-2017, 01:05:00 AM
Zeromedic Zeromedic is offline
Subscriber
 
Join Date: May 2005
Location: Michigan, USA
Posts: 304
Thanks: 77
Thanked 259 Times in 136 Posts
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
Reply With Quote
  #16  
Old 02-10-2017, 01:22:31 AM
Jim McIntyre's Avatar
Jim McIntyre Jim McIntyre is offline
Subscriber
 
Join Date: Nov 2004
Location: Mooresville, Indiana, USA
Posts: 3,671
Thanks: 575
Thanked 1,880 Times in 1,115 Posts
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...
Reply With Quote
  #17  
Old 02-10-2017, 05:10:13 AM
MadGutts MadGutts is offline
Registered
 
Join Date: Feb 2017
Location: Witney, Oxon, United Kingdom
Posts: 17
Thanks: 15
Thanked 7 Times in 1 Post
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!
Reply With Quote
  #18  
Old 02-10-2017, 09:44:39 AM
Jim McIntyre's Avatar
Jim McIntyre Jim McIntyre is offline
Subscriber
 
Join Date: Nov 2004
Location: Mooresville, Indiana, USA
Posts: 3,671
Thanks: 575
Thanked 1,880 Times in 1,115 Posts
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?
Reply With Quote
  #19  
Old 02-10-2017, 10:34:17 AM
grif grif is offline
Registered-III
 
Join Date: Oct 2011
Location: Yakima, Washington
Posts: 1,176
Thanks: 373
Thanked 298 Times in 220 Posts
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.
Reply With Quote
The Following 4 Users Say Thank You to grif For This Post:
  #20  
Old 02-10-2017, 12:21:56 PM
len k len k is offline
Registered-III
 
Join Date: Jun 2012
Location: Manchester, New Hampshire
Posts: 13,170
Thanks: 324
Thanked 4,687 Times in 3,426 Posts
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.
Reply With Quote
The Following User Says Thank You to len k For This Post:
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

F o r u m Jump

Similar Threads Chosen at Random
Thread Thread Starter F o r u m Replies Last Post
1 1/2 HP Compression Issue Dylan K Antique Gas Engine Discussion 5 01-01-2016 12:41:19 PM
Mogul 8-16 Compression Release Issue tirediron Antique Steel Wheel Tractors - Old Iron Lugs and Cleats 2 11-08-2015 01:08:02 PM
Onan 4.0 BGE Spec K Compression masterfabr Onan Generators 2 10-23-2012 11:51:53 PM
Honda GX160 crankshaft swap Timing Issue:Zero compression Subzero Small Air Cooled Gasoline Engines 13 09-04-2012 07:18:41 AM
Military issue MEP-003A Unit Merc300SD Military MEP and Aircraft Gen-Sets 17 11-19-2008 10:46:35 PM


Use "Ctrl" mouse wheel to change screen size.
All times are GMT -4. The time now is 12:26:42 AM.

Smokstak and Enginads site search!


All use is subject to our TERMS OF SERVICE
SMOKSTAK® is a Registered Trade Mark - A Community of Antique Engine Enthusiasts
Copyright © 2000 - 2016 by Harry Matthews P.O. Box 5612 - Sarasota, FL 34277