Compression in cylinders

Hello all. I am new to the website and new to owning a xj. Just bought the bike for 100 bucks. Person i bought it from said he hadnt had the bike running in some time. Thats all good, im up for a little bit of a challenge. I talked to a bike shop here in calgary and they said do a compression test first.

I just did a dry test and there was 90 to 92 psi all through the 4 cylinders. I havnt done a wet test yet.

What should the compression be around on a 84' with 64000 k?

thanks all

 

Did you do the test with all the Plugs OUT and the Throttle held WIDE OPEN?

Those numbers are Low.
I think the Low spec is around 110 or so.
I'll have to look it up if someone else does not.

Do a wet test and hope for the same values.
Then, you'll probably get-away with a Valve Job and avoid having to service the Rings.

 

Nope... i did each test with just taking that plug out and not holding the throttle open. Does it matter if the gas tank was on or off? How ever now that i think of it they were all loose.

The valve cover was loose as well, would that make a difference?

 

Huge difference.
Pull all the Plugs. Tie the throttle Wide Open. Retest.
You'll be amazed at the difference in your readings.

If you scored 90's fighting Compression and Closed Throttles ... you should register gains, across the board, that will put you right within specs.

Well, let's do the retest and see.
Fully charged-up battery.
ALL Plugs out.
Wide Open throttles.
New test.

Good luck. I have a feeling that you'll be OK! : )

Rick

 

Disconnect the TCI or coils before you do the test so you don't risk damaging a coil.

 

sweet!!!! thanks for all the input, this sounds like this group will be helpfull. Can't wait to get riding this spring.


THanks

James

 

were in Canada r u

 

If you've got compression.. even 90 psi under the conditions you described ..Work on getting it running.
As that's enough to run the thing.. and it's not likely that you will be rebuilding to get the compression up.. not when you discover parts Pricings.

 

Thanks again for all the advice... to answer the one question i am in Calgary AB Cananda. I conducted a wet test on the bike with the throttle open all the way and all 4 plugs out. 120 psi across the board.

Now i heard that when you do a compression test and your readings go up that usually you need a ring job. How bad do you think the cyclinders are?

I also conducted a spark test on each apk plg wire and to tell you the truth i could barely see the spark. It was there... just not really strong. Any idea why the spark may be weak?

Other than that I am going to get it all to gether and see if I can get it running this weekend.

I dont have the manuel yet and was wondering what type of battery it takes. I was using my father inlaws battery off of his 82' 650 suzuki.

thanks all

 

The way to test them is to test dry. Then if they are low, test wet. A significant improvement indicates you are leaking past the pistons.

If they stay pretty much the same then the problem is likely valves or head gasket.

120 wet is real low (you used about 1tsp motor oil?). If this reading is accurate you either have really worn pistons/cylinders or leaking at the head (head gasket or valves).

 

It might be late for you but for someone else my experience might help.
At 75k mi I sucked a valve and an engine that old, a full overhaul was in
order. Alas this would cost more than the bluebook on the bike, so I opted
for a wrecking yard special. 8k mi but it hadnt run in a while.

After the transplant I found myself with almost no compression.
Cylinders arent totally vertical. Sticky rings will be slow to seal past a
widespot fast enough to prevent blowby thus costing you compression.

I put a quarter cup of Marvel Mystery Oil in each cylinder.
I cranked the engine by HAND several turns every day for a month,
renewing the Marvel every couple days.

Cylinders coated, I was able to do an electric crank.
Each try I got better and better compression.
The bummer was that once I actually fired the plugs, I had to run it a long
time to burn out the excess oil. I also soon had to replace plugs as the
oil fouled them.

The engine refused to start until I broke 150 lbs.

Notice that I 1st cranked by hand until the cylinders were well lubed and
also the slow hand crank exercised the rings.

I also really let it spin on electric crank before installing plugs to cough out excess oil still in the cylinders.

By the way the last time I ran the engine was Summer 1996.
It ended up in storage and last weekend, I rerpeated the process and it
ran again after being stored for 12 years without prepping in advance.

You can substitute very light oile for Marvell Mystery Oil if you cant find
it. I have heard of Auto Trans fluid being used too. I like the cleaning qualities of Marvel the best though.

Ill be changing oil next week to check for metal shavings and to get
rid of stale oil.

I rambled a bit but I hope some of this has been helpful.

The Phoenix rose from the ashes this week.