how much compression?

[DEmily]

Hey all,

Finally got the old girl running and running good!!! It has been a good year and allot of bucks but well worth it. I had a question about compression. Now that I have everything right, no oil running out the exhuast, no failing coils, new valve guides, She seems perfect. I checked the compression with the throttle open last night. I've got 180 of one and four and just over 170 on two and three. I thought the max was suppose to be around 150 or so. Chain timing is right on, valve shims are with in range. Is this something to be concerned about? I know shes tight but thats a big jump past the maximum based on my manual's specs. I thought once the rings seated in it would improve compression, but I don't have enough time on this motor yet for the rings to be seated in so I am just a bit confused.

[Jerry]

The XJ service manual lists the pressures as:

Standard: 142 PSI
Minimum: 128 PSI
Maximum: 156 PSI

All numbers are at sea level. Note: these numbers are the same in both the 11E and XJ manuals, even though there was a compression change (from 9.2:1 to 9.0:1) sometime in the model years. Theoretically the late XS/XJ numbers should be slightly lower.

Testing parameters:

Valves adjusted.
Engine warm. (2-3 minutes at idle.)
Fully charged battery.
Throttle wide open.
All spark plugs removed.

If you are getting into the 170 - 180 range you may have non-stock pistons, a Wiseco kit, (which includes higher compression pistons) or there is an error in your guage or measurement technique.

The manual says 14 psi max between cylinders, at 10 psi difference you are still in spec.

[DEmily]

Jerry,

I followed the proper steps for the test, so I must assume it is a gauge issue. I did run the same test with the throttle closed and it hit right at 150 across the board.
The pistons are standard with new standard sized rings. Hones the cylinders before installing. Had a nice tight slide in the cylinders. Ring gap right on the bottom end of specs. I will look into a new gauge. Maybe that is my deal. I have never questioned the gauge before in use on my street rod projects, but maybe it's time for a new one.

[Geezer]

It's probably a little carbon on the tops of the pistons. I wouldn't worry about it but it'll probably run better on premium.

Geezer

[MartyA]

Probably more important that you have only 10 between hi and lo.
Where's you get new rings? I thought they were as rare as hen's teeth.

[DEmily]

Got the new rings from Zanottis motors. John has always done a real good job of finding the parts if they are available.

[DEmily]

Hey guys,

I just finished my compression check with a new gauge and the readings are still the same. I'll ride it for a while and recheck.

[TopCatGr58]

Hey there Dan,

Well, after I put my 1179cc big bore in and broke it in, I got 175-180psi on mine, at true SEA LEVEL here on the east coast! You sure you didn't shave some off of the head!? Just enjoy the compression while it lasts, but I, too, recommend premium gas!

T.C.

[John]

Hey TC! What was your break in brocedure for the Big Bore Kit? Did you use 30 weight, non detergent oil, or 20w-50 detergent oil? When did you change the break in oil?

[DEmily]

hey Topcat,

No, nothing but standard. I will the the advice about premium and drive the heck out of it. I will of course break it is. What is the break in miles for this beast before the first oil change anyway?

[DEmily]

Sorry about the horible typing, my contacts are bugging out

[Geezer]

Originally posted by DEmily
hey Topcat,

No, nothing but standard. I will the the advice about premium and drive the heck out of it. I will of course break it is. What is the break in miles for this beast before the first oil change anyway?

I don't recommend any special break in. Just ride it but don't abuse it. I'm really picky on oil changes; I use either Castrol of Valvoline 20-50. If it was a full rebuild with new bottom end bearings and a few new transmission gears, I'd say change it at 600 miles. With only rings and valves, I wouldn't do any thing special. The usual 1500~2000 miles between changes will be just fine.

Geezer

[wpc57]

Stock rings are still availible but yes they are hard to find.... Penzoil 20w 50 works well in the changing weather here in Southern IL.

I replaced the rings in my Midnight last summer/fall.... took a while to find...lol

[TopCatGr58]

Hey there John and Dan,

Well, this was several years before having full access to this web site, and I was used to running 20/50, so I used that for the break in, and did it by the book even though I only rebuilt the topend, took it easy with NO FULL THROTTLES for the 1st 500 miles, and I think IIRC I changed the oil around that time as well, but Geezer is probably right, since there really won't be much metal parts being rubbed off, the oil won't get so dirty or full of debris until the usual 1500 miles or so.

The GURUS say to use straight 30 wt during the initial break in period, allows the rings to seat better and quicker and won't get a glaze on the cylinder walls!!! Then change to your usual multi-grade as desired! HTH, and enjoy!!
T.C.

[DEmily]

Thanks guys,

I did by the way replace all crank and con bearings. Those bearing took Zanotti about two months to get in. The standard rings were here in less than a week. Of course all these parts were purchased last year about this time. So I will probably do the chenge around 500. Pretty much the same schedule I have used for a few street rods I have done. I just wasn't sure about the break in on this thing as it has taught me there is a huge difference in the technology used in these bikes as opposed the big block chevys I had worked with. Thanks a ton for all the help. I will keep checking back in and if I can answer any questions other new XS riders have I will do my best to contribute.