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

Go Back   SmokStak > SmokStak® Vintage Electrical Equipment > Generators & Electric 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 3159 times

Reply
 
Thread Tools Display Modes
  #21  
Old 02-10-2017, 12:55:53 PM
Andrew Mackey Andrew Mackey is offline
Sponsor
 
Join Date: Nov 2004
Location: Rockaway, New Jersey USA
Posts: 12,754
Thanks: 1,742
Thanked 5,794 Times in 3,772 Posts
Default Re: Ex-Military Spec 6JB compression issue

You have to set the valves at TDC Compression stroke. Could by accident, you set the valves at TDC between exhaust and intake? If so, clearances will not be correct. The fact that liquid is getting past the rings, means there is a mechanical problem. Bad or stuck rings in the ring groove, carbon on the rings, broken rings, worn out compression rings, bad piston. All mean the piston has to be pulled. It may be as simple as freeing and de-carboning the rings, to as complicated as replacing the rings and piston. I would also check that there is no carbon under the valve faces. If you have to replace rings, do both sides, as if one side is worn, then chances the other side will be worn too, even though it is now holding. You need all cylinders to have compression within 10% of value to each other, or you will have a work imbalance that will cause overheating in the good cylinder after the one with poor power, as the good cylinder tries to make up for the power loss. As an example, if the highest cylinder pressure has 100 PSI, the others need to be within 10 PSI. I would do a leakdown test to be sure you are not losing pressure thru the valves. You may have several issues here, both valve seating and ring issues. Personally, I would check both while you have the heads off.

There are several products that state they can free stuck rings, found at auto stores. One here in the US of A is Sea Foam Ring Free. Over on your side of the pond, there may be a comparable product. For myself, I don't take much stock in such products. Only a physical teardown can tell you exactly why you are losing compression or fluids past the rings. Let us know what you find.
Andrew
Reply With Quote
The Following User Says Thank You to Andrew Mackey For This Post:
Sponsored Links
  #22  
Old 02-10-2017, 03:22:37 PM
nothingbutdarts nothingbutdarts is offline
Sponsor
 
Join Date: Apr 2014
Location: Bailey, Colorado, USA
Posts: 2,575
Thanks: 5,691
Thanked 1,245 Times in 822 Posts
Default Re: Ex-Military Spec 6JB compression issue

In reality, since he says one piston is holding the diesel fuel he put in both cylinders and the other cylinder is leaking out I am going to take a wild guess and say he has a cylinder sealing problem and not a leaking valve issue.
However, just to bolt the head back on without adjusting any valves and doing a leakdown test should take less than 30 minutes for peace of mind.
Reply With Quote
The Following User Says Thank You to nothingbutdarts For This Post:
  #23  
Old 02-10-2017, 05:52:03 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

I hadn't touched the engine prior to it losing compression...
im the old skool - if it aint broke dont fix it!

But alas compression had gone from the 1st cylinder, and normal compression on 2nd... cranking by hand gives good resistance but second rotation nothing, ther should be compression on every rotation.

So today the piston came out, and it all looks fine, there is a slight difference in the gap between the open compression rings, 2mm at the most, and the oil seal looks ok too... no damage to bore or to piston...
The top compression ring is polished (shiney) but the second one is dirty in places... Could this be a tell tale?

other than that i cannot see anything bad on the piston, rings or bore!
Reply With Quote
  #24  
Old 02-10-2017, 06:49:47 PM
grif grif is offline
Registered-III
 
Join Date: Oct 2011
Location: Yakima, Washington
Posts: 1,274
Thanks: 454
Thanked 331 Times in 243 Posts
Default Re: Ex-Military Spec 6JB compression issue

Valves are free?
Reply With Quote
  #25  
Old 02-10-2017, 06:56:51 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

yeah thoroughly checked top end before going to the bottom end! valves checked and reseated

and putting diesel in both cylinders to test for leaks, all diesel dissapeared from 1st cylinder, so there is a leak down the side of this piston
but i cannot see any faults
Reply With Quote
  #26  
Old 02-10-2017, 06:58:34 PM
BigBlockChev's Avatar
BigBlockChev BigBlockChev is offline
Subscriber
 
Join Date: Jan 2007
Location: Coquitlam, BC, Canada
Posts: 2,004
Thanks: 356
Thanked 1,543 Times in 907 Posts
Default Re: Ex-Military Spec 6JB compression issue

What is the ring endgap on each ring when they are inserted about 1 inch down the cylinder. 2mm is a huge endgap for rings , about .080, the manual calls for .010 to .020. What do the cylinder walls look like , do they still have a bit of crosshatch. Is there a ridge at the top of the cylinder? What is the taper on the cylinder top to bottom. What is the piston skirt clearance? Are the rings free to move in the ring lands not stuck in the compressed position. To me catastrophic compression failure means , burnt valve, misadjusted valve, broken rings, rings stuck in retracted position, broken ring lands, broke piston skirt, loose head bolts or blown head gasket. The fact that diesel leaks past the rings when it is sitting does not indicate to me that it has enough wear to completely lose compression, these are very low compression engines. I would bolt the head back on and do another compression test but squirt a bit of oil in the cyl to see if that brings up the compression substantially , or do a leakdown test as others have suggested to identify the source of the leakage. Show us some pics so we can see what you have been finding. Cheers Dan
__________________
It's always something simple.
One test is worth a thousand guesses.
If some is good and more is better then too much is just enough
Reply With Quote
The Following 2 Users Say Thank You to BigBlockChev For This Post:
  #27  
Old 02-10-2017, 07:13:18 PM
grif grif is offline
Registered-III
 
Join Date: Oct 2011
Location: Yakima, Washington
Posts: 1,274
Thanks: 454
Thanked 331 Times in 243 Posts
Default Re: Ex-Military Spec 6JB compression issue

If it were me,,, I'd swap plugs and wires after making sure there was spark available for both cylinders.

We're missing something easy here. (I can hear my dad).
Reply With Quote
The Following 2 Users Say Thank You to grif For This Post:
  #28  
Old 02-10-2017, 08:49:27 PM
Motorhead's Avatar
Motorhead Motorhead is offline
Subscriber
 
Join Date: May 2007
Location: Central Coast, California
Posts: 3,708
Thanks: 1,733
Thanked 2,537 Times in 1,325 Posts
Default Re: Ex-Military Spec 6JB compression issue

Fisrt, If the valves were adjusted at the wrong TDC then they would be clattery as all get out running. We had people trying all the time to adjust their own valves on air cooled VWs and had more than one guy come in with the car sounding like it had BB's running through it. You would have to loosen the rocker nut a bit to get any clearance.
Second, if the 6jb is the same as a 7.5jb then it has a dual coil where both plugs fire at the same time, one cylinder getting a waste spark. So I would be inclined to test the spark by cranking the engine with the plug wires hooked to two spark plugs sitting external from the cylinder where you could see if you have spark at both plugs. This set might be a 6JB 50 cycle only running 1200 rpm?
Lastly, Big Block Chev has a good point to check ring gaps. A compression test would have been something I would have done first on both cylinders.
__________________
The Control panel
and gauges of my
7.5JB
Reply With Quote
The Following User Says Thank You to Motorhead For This Post:
  #29  
Old 02-10-2017, 08:56:15 PM
Wayne 440 Wayne 440 is online now
Registered-III
 
Join Date: Nov 2004
Location: Kentucky
Posts: 5,813
Thanks: 354
Thanked 4,889 Times in 2,346 Posts
Default Re: Ex-Military Spec 6JB compression issue

Quote:
Originally Posted by grif View Post
...We're missing something easy here. (I can hear my dad).
Agree. I have been a victim of the "it can't possibly be..." line of thinking more than once.

So to me, the cause of the original problem is probably not that the rings won't hold liquid. I had a Ford 8N with so little compression that it would coast like the Wheel of Fortune when shut it off at idle, but it still pulled about as well as anyone else's. Was easy to start too, just put it in 2nd or 3rd, then reach over and give the back wheel a push.
Reply With Quote
The Following User Says Thank You to Wayne 440 For This Post:
  #30  
Old 02-10-2017, 08:57:07 PM
David C David C is offline
Registered-III
 
Join Date: Mar 2006
Location: Lake Jackson,Texas
Posts: 1,369
Thanks: 1,033
Thanked 855 Times in 557 Posts
Default Re: Ex-Military Spec 6JB compression issue

Really needed to do a compression check before the initial teardown. The next step would have been to put each cylinder at TDC on the compression stroke and apply air to the cylinders and see where the air was escaping.

I can't imagine what would fail in the piston and ring area while shutting down. I have a well neglected and used 2.5LK that had a .250" (yes 1/4") ring gap on the rings and it would still run. It had some blowby and smoked some, but it would start and run fairly well considering the ring gap.

David C.
Reply With Quote
The Following User Says Thank You to David C 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 02:58:12 PM.

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