Help...Disparate compression readings

[guido4198]

The bike is a 1985 FXRS with a 1999 "Crate motor" I bought new from a dealer when the original needed to go. She runs great. The '99 replacement motor has 110,000 mi on it now. Just got back from a 2,000 mi RT to do Rolling Thunder. No real issues, except that for some time now she's been using more oil than she should. On this trip, I was adding a quart every 400 miles or so. Now that I'm home, I thought it might be a good idea to check the cylinder compression.
Front: 140 psi
Rear: 90 psi.
Obviously, the difference is telling me there's something going on in the rear cylinder that ain't good so I thought I'd post this here and see what the next step ought to be.
Thanks.

[rredneckn2]

With 110k on the clock it's time for a bore job fresh pistons and a valve job assuming your rod bearings are in spec.

[Hossamania]

That's pretty much it. Probably new lifters, cam bearing, and possibly a new cam, to be checked.

[Y2KRKNG]

My Evo kicked the wrist pin out towards the carb side on the rear cylinder when the rod bearing were shot. Gouged the Hell out of cylinder wall and burned a "Potty mouth" ton of oil. It still ran like a raped ape though. Only saying it because if you find this, you either need to do a full rebuild or use Teflon wrist pin plugs for a temp patch job.

[guido4198]

Thanks for the comments. I'm gonna have to pull it down and find out why the rear is so much lower than the front. If the compression readings were a lot closer together I'd be OK with it for a while longer but it's the big difference that has me concerned about something more than just normal wear is going on.
The bike runs great...Almost hate to do anything, but I don't want to wait until something in there "grenades".

[Pirsch Fire Wagon]

Leak-down test will provide you good information for a way forward.

[gordonr]

Leak-down test will provide you good information for a way forward.

Diagnostics. Interesting 

[codyshop]

Remove rear spark plug and squirt a little oil in.  I compression goes up, rings/bore are bad.  If it stays the same, valves are bad.   But as said above, with this kind of mileage, do everything up top. 

[JW113]

Guid, the thing has 110K on it. Time for a refresh, what's the problem? My '91 had 120K on it, and similar symtomps as you describe. Ran great, uses oil, low static compression. After opening it up, found that that it had broken compression rings on both pistions. That certainly explains the oil consumption and loss of cyl pressure. But amazingly, it ran great.

-JW

[smoserx1]

I always heard that gas pressure from combustion forcing the rings outward from behind was what really effected the sealing pressure and wonder if this is why a low static compression test situation could still make relatively good power?

[rredneckn2]

With 110 k miles on it Time to refresh period.

[JW113]

I always heard that gas pressure from combustion forcing the rings outward from behind was what really effected the sealing pressure and wonder if this is why a low static compression test situation could still make relatively good power?

I think a big factor is that a static compression test is done at, what, 100rpm? At 3000 rpm, who knows how much leakdown is affecting the normal operation of the motor. Drag racing, where every ounce of power is important, sure. But just riding down the highway, tooling along at light throttle? Probably does not mean that much really. But what the hell, every motor wears out in time and needs attention.  I think it's awesome that Guido got 110K out of his stock motor, but now it's needing some TLC. Don't you think?

-JW

[guido4198]

She's gone in for the rebuild/refresh. Thanks for your thoughts. I wish I had done Codyshop's diagnostic dropping her off at my local Indy shop. Wouldn't change much, but I'm a scientist and I enjoy "Data".
I have resisted the urge to go for "performance mods". Keeping things as close to stock as possible. The only performance related mods on the bike are the S&S shorty "E" carb, Paughco pipes and a KN air cleaner. As long as she'll take me, and the Old Lady DuJour(whomever she is at the moment...) over Monteagle mountain at 70 mph without bogging down...she's just fine. 
That's my personal performance standard. 

[smoserx1]

I think it's awesome that Guido got 110K out of his stock motor, but now it's needing some TLC. Don't you think?

Yes I do think it's refresh time.  This is kind of an interesting thread to me as I have never had a vehicle with a significant compression difference but once and that was due to a severely burned exhaust valve on a 4 cyl car (the valve face looked like pac-man).  Anyway I have always wondered what a compression test was really telling you because you are just compressing air.  It is a cumulative test though as it takes several compression strokes to reach the maximum PSI reading.  I am sure the PSI inside a cylinder from actual combustion is far greater, but I don't know by how much.

I refreshed the top end of my flht at about 178000 miles  It was using a fair amount of oil by then but the last compression test I did at about 150K showed pretty much the same results as new.  The refresh (which was actually a 95 inch job) did not result in much seat-of-the-pants low gear acceleration increase but did yield a noticeable roll on improvement at 70 MPH in 5th gear.  Great discussion.