I think my compression numbers are alright. Double-check me

Just came inside from a compression test. I think these are on the low side, but acceptable. Please double-check me:

1983 XJ650, 53,796 miles

Cyl 1: 115

Cyl 2: 120

Cyl 3: 125

Cyl 4: 120

 

115 is below Min. Spex.
120 isn't anything to write home about.

Did you have the Throttles wide-open?
All the Plugs, ... Out?
Fresh-charged Battery?

Add a Tablespoon of Oil to the 115 & 120 Hole.
See it the PSI rebounds.

Low Comp on adjacent holes can be a Gasket problem, ... too!

 

Throttle open. I knew I forgot something! I'll do it again later, but right this time. Will having the throttle open tend to increase or decrease these numbers? I just did the valves, so if I end up with low compression, it's the rings.

 

Difference?

There ought to be a difference. Yes.

Having the throttles closes is akin to taking a deep breath with clenched teeth.

 

Good. I was hoping you weren't going to say they'll be lower. I'll update this with the new numbers when I get the test done. Thank you for your help and advice.

 

Also remember:

For optimum results, the motor should be slightly warm, a couple of minutes running is fine.

The readings you get aren't as important as the comparison between cylinders (10% is the general rule.) That being said, any time you do get wonky numbers, it's a good idea to re-test; then get a different gauge and get a "second opinion."

To confirm a ring issue (you could also very well just need a valve job, not just adjustment, at that mileage) do a "wet" test after your initial set of dry tests.

 

What's does a valve job, not just an adjustment, entail?

 

Removing the head and disassembling it;

Inspecting the valves, checking their wear in relation to the valve guides, measuring the springs, etc., and replacing any unserviceable parts;

Decarbonizing the head and inspecting the valve seats; re-cutting any that are too worn or pitted to simply be lapped;

Lapping the valves (done by hand and I think it's fun!)

Replacing the valve stem seals, and reassembling the head.

If you're not comfortable with that depth of work, most good automotive/foreign car machine shops do it; although I'd supply the necessary parts. If you "farm it out" expect around $150 (if you get it done for less be suspicious or be sure of who's doing it) PLUS whatever parts (valve stem seals for sure) might be needed. If the head's not horribly worn, that IS about all that you should need parts-wise. Maybe a couple of valve retainers if any are mushroomed.

It's a good idea to do this on ANY of these motors when the head comes off; they're 30+ years old and valve stem seals do dry out.

It's still easier and less invasive than rings and cylinder honing, etc. But that isn't all that difficult either; just a lot of careful precise work. And something you only want to do when necessary.

 

Typically, ....

You pull the Head off and Remove the Valve Buckets and Shims, ... then, either Ship it or Deliver it to a Machine Shop for:


Compressing the Springs and releasing the Valve Stems.
Removing the Valves.
Precision Machining New "Faces" on Valves and Seats.
Lapping-in Newly Faced Surfaces to be Gas-tight.

Possibly replacing a Valve or two.
Evaluating Springs.
Installing New Valve Seals.

::: Attending to Studs and Complicating Factors -- + 1.5 Hrs Labor. :::


Typical Machine Shop Services and Estimates:
• Will vary widely depending on locale.

http://aperaceparts.com/machining.html

 

Or .... your compression gauge is reading a bit low. A common problem depending on what style of guage you're using. Your readings - cylinder to cylinder are within the usual tolerences. I'd try another guage before getting to concerned.

 

Valve job......you CAN do it yourself. I've NEVER done one before, and I am in the middle of one right now. It's NOT difficult if you take your time. To make it more interesting, I'm doing it on a maximX.

Dave F

 

NOT saying that I am a "know-it-all" type of guy, but I've learned to do all this on my own so I think if anyone on here is at all serious about this type of work, they too can do it. Being a young pup at 25 years of age, I've learned SO much about these bikes by tearing into my 650 maxim engine. Before owning anything own my own, I was a typical teenager just helping dad around with car oil changes and small car work repair. Long story short, I've learned quite a bit of maintenance procedures on the XJ motorcycles.
I've got a long thread somewhere titled "Checking Cylinder Compression" on this website when I did an upper engine rebuild.

Sabre, please consider all your options first. Definitely check into another gauge instrument. Also think about keeping your eye open for another spare engine/bike and possibly switching engines should you decide the compression numbers are too low for what you would like to see. It would be more cost-effective if you find a cheap bike on craigslist if you seriously think the compression is too low.

Speaking of which, I feel like your compression numbers are alright. Yeah I'm gonna get JUDGED & possibly criticized by this but that's how I feel. You start dealing with any type of engine repair/modifying & it adds up in a hurry, especially NOS standard piston rings. Because I'm still young its really hard to balance a healthy budget saving for a house, paying back student loans, paying for rent, and all the other stuff in normal-everyday-life. That was the hardest part for me to deal with when I did my upper engine rebuild. Not saying I made a bad decision to do so but I could've thought about all the other possibilities before doing the upper engine rebuild.

-just my 2 cents...

 

I just re-did the test with the throttle open this time. The battery was charged, I'm sure, because I just got home from work on the bike about an hour before the test. All the plugs were out for all of the tests. Here are the results:

Cyl 1: 125

Cyl 2: 130

Cyl 3: 140

Cyl 4: 130

A dollop of oil poured into cylinder #1 raised the pressure to 280 8O . So correct me if I'm wrong, but all my cylinders are within spec. #1 is a little low, since 120 is the minimum spec, and the results of the wet test indicate the problem there is a worn piston ring. It's not worn so bad that I need to start thinking about a rebuild, though.

Looking at the plugs I was concerned I was running a little lean. I took a pic. Plugs are from cylinders 1-4, left to right. Am i right that this is a touch lean, but not dangerously so?

 

With those numbers. I would not get overly concerned about a rebuild just yet. That engine has plenty of life left in it. Regular oil changes will be the key to keeping that engine alive.

The plugs look only slightly lean. But not to the point of damging the engine.
Does the bike have pods, Free flow air filter, or a modified exhaust on it?

Just to let you know. I am only a half hour ride away from you.
If you need help with anything. Shoot me a PM.

Ghost

 

Thank you for that offer!

This bike has a stock airbox (not sure about the filter) and stock exhaust.

 

Those plugs look pretty old and worn--that red color I think I'm seeing is the sign that makes me think that. I know mine had working plugs that were that old, and when I put new plugs in, they immediately showed me I was *very* lean, bone white. They had just been stained.

New plugs are cheap, and upper cylinder lubrication is a big deal if you don't want your numbers to go down.

 

Looking at those plugs and comp numbers, bolt a new set of plugs in and go do another 100,000km.... then worry about what the engine is doing. That engine looks pretty clean and normal to me.

I just pulled a set of plugs on my GTR1000, (you guys call them a Concours C10 over there) and my plugs were similar. My GTR has 120,000km on the clock and I expect another 100,000km before any major work..... oh, my GTR does chair duty too.... it has a HRD 1.5 hanging off the side for the kiddies...

 

Good idea on the new plugs. Is there something special to ensuring upper cylinder lubrication, other than keeping up with regular oil / filter changes, and making sure the oil level stays at the top of the window?

 

No, there isn't.

And those plugs aren't "lean" but they do look a tad old.

Fit a new set of properly gapped plugs and ride the bike. Let's see the new plugs after 500 miles.

 

Again, your compression numbers are fine, but for future reference:

You add a very small amount of oil to the cylinder to do the wet test, and you do it to all four to compare. Oil in the cylinder almost always raises the compression (even if the cylinder is healthy) but if you had one low cylinder, and with oil, the good ones all jump by 10 psi, and the low one meets it, then it says that the rings are the issue.

Adding too much oil takes up VOLUME in the cylinder, seriously increasing your compression ratio (and making the test moot), as oil is not compressible. It's really only a couple of drops that you add.

 

Good to know. The instructions with my tester said a tablespoon.

Roger on the plugs. I planned on getting new ones, so I'll expedite that and take a new pic after I get some more miles on.

 

Yeah that amount is probably meant for a car. You're got a 650/4 cylinders is 162.5cc. With a compression ratio of about 9:1, the total volume at TDC is about 18 cc. 1 tbsp = 15mL or 15 cc.
EDIT: I got the math wrong. Displacement = V_bdc-Vtdc and CR =V_bdc/V_tdc. So solve the first equation for V_bdc and substitute into the second and you get a V_tdc=20cc, still mostly oil. A teaspoon would be more appropriate

 

Run it just like that.
Those Plugs look like the were due for replacement.

Use Anti-Seize on the Threads.
Stick-in a set of New Sparklers.
Peek at them several times over the course of the next couple-hundred miles.

You are right in the Ballpark.
You might need to TWEAK some Richness to 1, 2 & 3 if the New Plugs show they need some more color.

1, 2, & 3 are Burning at a "Performance" Mixture.
4 - Is what some folks worship. Cucumber "Cool".
Run "Flat-out" all-day and night without a worry.

ALL - 4 are within a TWEAK of "Dialed-in"
"Tweak" --> a minute (my-noot) amount.

Stopwatch --> 60 Seconds --> 1 Turn - 360-degrees.

The "Tweaking" you can do is:
1.)
Lean 4 by a teenie-tiny --> 2 0r 2.5 "SECONDS" (Almost nothing!)
That will put all 4 in a Performance Tune.
• FAST Acceleration
• Moderately HEAVY Engine Braking.
• Frequent Monitoring recommended.

2.)
Rich 1, 2, & 3 by a teenie-tiny --> 2 0r 2.5 "SECONDS" (Almost nothing!)
That will put all 4 in a Cruising & Long-Haul "Comfort" Tune.
• Smooth acceleration
• Coasting Mode
• Cooler Burn and Hands-off Idle.
• Safer than Performance Burn. Lower Heat.

You make those changes by tweaking the Mixture Screws a "Hair".
Smidgeon.
About the width of a Nickel.

Half this distance:

 

Re: I think my compression numbers are alright. Double-check

Reading this thread raises a general question for me about reading spark plugs. I have often seen doing plug chops as the recommended way to assess mixture at various engine speeds/carb circuits. Folks seem to recommend plug chops because the spark plug condition only reflects the last few moments of running, which will typically be idle before the engine is stopped. But these plugs are being used to assess a broad engine condition. Is there some degree of plug appearance that stays through varied speeds? Are there different things to look for that are longer term and more general vs. the short term/plug chop condition? Trying to learn always makes me more confused first. Sometimes second too.

 

Thank you, everybody. I will get new plugs, run them for a while, then see where the color is at.

Rick,

I was reading about the mixture screws and I can't quite understand something: I know where they are, but am I correct that they are covered by a threaded brass cap? I read in another thread where you were advising somebody to drill a hole in the cap to reach the mixture screws. Why not just unscrew and remove the cap? You have me a bit confused.

As it is, I'm just going to run it like it is for a while. I don't want to have to be constantly monitoring the color of my plugs, though, and I want this engine to last, so I may eventually take your advice about enriching the mixture slightly.

EDIT: Alright, I read that the mixture screws are covered by an anti-tamper plug. I think mine have already been removed (thank you, previous owner!) They are the small brass screws that are located in a small depression right next to, and I don't know the technical term for this, but the rod that moves up and down with the choke, right? The screws right next to the pin that the choke rod moves in and out of each carb? Sorry for my less-than-technical jargon...

 

Yes.

The "pins that the choke rod moves" are your enrichment plungers. Those screws in the top of the carbs right there are the mixture screws.

The caps you were asking about having to drill out were pressed in, not threaded. Hence the need to drill into them to remove them. Finding them already gone is more common than not.

Constantly monitoring your plugs isn't necessary; like I said, ride the bike. Then let's see the new plugs after 500 miles or so. Based on the condition of your raggedy old plugs, I wouldn't adjust anything just yet.

 

Enrichment plungers! I was one letter off...

Thank you. That's exactly what I'm going to do.