uh oh....compression numbers prior to getting valves in spec

OK while waiting for my bucket tool from Len I decided to do a compression test so I could compare before and after numbers. sooo....

here they are:
#1 140
#2 115
#3 132
#4 140

After putting a tablespoon of oil down #2 the compression increased to 170 PSI. so now I'm worried about rings or cylinder wall problems. Not going to do anything until I get the valve cover off but is this for sure a ring/cylinder problem? I Will follow this thread with pics as I proceed but any advice on what to look for (once I get the cover off) as it pertains to #2 would be appreciated!

 

Howdy, When you put oil in a cylinder, you are temporarily causing the rings to seal which is why you got such a high reading. The oil has no effect on the valves. When you get your valve cover off, make sure the cam lobe is 180 degrees away from the shim, and slide a feeler gauge between the cam and the shim. When you get one to slide thru with just a little bit of drag then read the gauge, that will be your valve clearance. If you thinnest gauge won't slide thru you have a valve that too tight and you need to put in a thinner shim. As for your readings, I'd say that the #2 rings are worn out or you have rings that are stuck in the ring groove due to carbon fouling. The rings, have to be free to expand and contract to do their job. You can try to free the rings by putting a solvent in that cylinder, let it soak for a while then use compressed air to dry it out, the try another comp test. Do you have a lot of blow by from the crankcase breather? this is another indicator of bad ring sealing. I have the same problem with my Maxim 750, so I am planning to pull the head and cylinder block to check my rings. I wouldn't worry about the cylinder walls, they are too thick to crack except under a mechanical stress, like a rod coming loose.

 

Re: uh oh....compression numbers prior to getting valves in

Let's get some expert advice on this one because I was wondering about this myself. I had a VW and it burnt the rings on the #3 cylinder, but the other 3 cyl were fine. I was taking a flight attendant to Dallas, it was 102 out and I was driving too fast! But then I was distracted.

Here's my question. The mechanic who did the work for me told me that VW's are known to have problems on this cylinder because it get's the least amount of air. He told me that rings almost always fail in multiple cylinders or they are close to failing if one does. Does this sound right?

With those good compression numbers on the other cylinders isn't it likely that he just has a stuck ring? Probably caused by the bike sitting for too long? I am just guessing here. Plus is 115 really low-obviously it is much lower than 140. Ok engine builders chime in.

 

Knowing something about air cooled VW's, I can tell you that #3 cylinder also has to bear extra heat from the oil cooler, cool air from the "doghouse" also known as the fan shroud, blows the air through the oil cooler and it discharges right on top of #3 cylinder; inexperienced mechanics often don't take this into account when building an air cooled VW engine; the ring gap is greater for this piston when built correctly. So the mechanic was not quite right, #3 does get enough air, it's just hotter.

 

thanks guys, going to try to rent a leak down tester to see if I can narrow down the problem a little, from my understanding it's either leaking through the intake valve, exhaust valve, or past the piston rings. One question, how do I tell when the cylinder is at "top dead center" ?

 

Stick your finger or something soft that'll fit in the spark plug hole and feel it come up.

Do this while tunring it over manually with a wrench. If it goes back down you went too far.

 

If you are going to turn it over with a wrench, that means you already have the left side crank cover off. You will see the "reluctor", that's the part that rotates when the engine is running, you should also see a fixed pointer, line up the "T" on the reluctor with the pointer and you will have #1 and #4 pistons at TDC. But you won't know which one is at TDC under compression unless you have the valve cover off, then just look at the cam lobes. Hope this helps!

 

You need to have the valve cover off, because the pistons pass TDC twice in a complete "cycle" once on compression (both valves closed) and once on exhaust, transistioning to intake (exhaust valve open but closing, intake valve about to open.)

You want to be on the stroke with both valves closed.

Then you can look down the spark plug hole with a flashlight and watch the piston come up. When it gets close, then you simply align the "T" mark as explained above. Or, at that point, you can drop a plastic "straw" from a spray can down the hole and watch it closely for the point at which it stops going up and starts back down.

Check your PMs.

 

OK results are in, the bucket tool came and thanks to Ftiz's writeup (awesome) I had no trouble checking all of the clearances. Here are the results.

INTAKE: .11 - .15
#1 .10 * (290 installed => 285)
#2 .13
#3 .15
#4 .07 * (280 installed => 275)

EXHAUST: .16 - .20
#1 .18
#2 .13 * (270 installed => 265)
#3 .18
#4 .15 * (265 installed => 260)

New shims on the way, will install, recheck clearances, and get back with new compression numbers. Really hoping #2 is due to tight exhaust valve.

 

OK new shims installed as per above. All valves in spec. New compression numbers with engine slightly warm:

#1 150
#2 120
#3 140
#4 140

not sure what to do now, the difference between #1 & #2 is 30 PSI - more than twice the recommended max range (of 14 PSI). bike runs well, tempted to just leave it ?

 

<You need to have the valve cover off, because the pistons pass TDC twice in a complete "cycle" once on compression (both valves closed) and once on exhaust, transistioning to intake (exhaust valve open but closing, intake valve about to open.)>

When I was growing up.....or maybe I should say "when my parents were trying to get me to grow up, not just grow older", we were always taught the the compression stroke is at TDC - top dead center, and the exhaust stroke is BTDC - Bottom top dead center, all that to say that if someone else learned it that way, then the pistons only pass TDC once.

probably a "I say poe-tay-toe, you say puh-tah-toe type of thing.

Dave F

 

Re: uh oh....compression numbers prior to getting valves in

So it looks like you have three really good ones and one that's not perfect. Have you rechecked your clearances after you buttoned up and ran the motor? Just to be sure of course. It does look like you have some wear on that #2 ring, but 120 is not so bad that your not still generating power out of that cylinder or experiencing bad blow by. Looks like to get your engine top shape will require surgery for #2. Your main problem is the difference not the 120. You could try running a heavier weight oil/or motorcycle oil stabilizer and see if that help things any.

You need advice from someone who's tried running a bike with compression out like that, and whether or not it will be a significant problem. I would think that if you don't rag on it or push it to the max you should be able to ride. I drove a Nissan 2.4L that had 2 iffy rings and it was fine (after I added oil stabilizer in place of 1 quart)but of course much bigger motor and a car.

 

What I would do now is "fog" #2 with Kroil, (or add a tablespoon or two of MMO) and wait two days. Then spin the motor a few revs with the plug out, then run the motor for a few miles, and check again. You may just have a stuck ring.

Dave, a four stroke is called a "four stroke" because it takes FOUR STROKES to complete one combustion cycle. "Bottom dead center" simply refers to a piston at the bottom of its stroke. The piston in a four stroke motor passes TDC twice during a combustion cycle. (Only once in a two stroke motor.)

 

Fitz would PB Blaster penetrating catalyst work?? I don't have kroil handy.

 

also I am running 10W40 Valvoline motorcycle oil. MercuryMan may have something, I could switch to a 20w50 weight, would that be likely to improve compression (of course it would probably increase the compression across the board and I still have the same issue ?)

 

I would use MMO then. (Marvel Mystery Oil.)

 

Re: uh oh....compression numbers prior to getting valves in

Hopefully bigfitz's idea about a stuck ring is the problem since it's potentially and easy fix. But if you have a slightly worn ring and you put in heavier oil or oil stabilizer it could raise compression on just that one cylinder. Your three that are higher will not go higher - the rings are doing their job and it won't get any better but on the weak one if the tolerance is very close the viscosity difference might be enough to get you there.

When you try to free the ring with MMO or Kroil warm up the motor before you put in your catalyst and you could also do a little force feeding by applying air pressure (50psi) and a vacuum alternatively to force the catalyst around. EDIT - oh and it is critical that you give it time to work. days better than hours.

 

Right. So don't bother with the air pressure and all that since at that point it wouldn't have had TIME to work.

And DON'T warm up the motor first, we don't want to immediately burn off our penetrant as soon as it goes in.

Again, I speak from experience. And there's no guarantee this will work, but it just might, I've SEEN it work before.

Try what I said and let's see what happens.

 

Re: uh oh....compression numbers prior to getting valves in

To be specific. I didn't mean warm up the engine and put in penetrant while engine is still blazing hot. Just Death Valley hot like say 140 degrees, expansion/contraction aiding the process. Thermal Coefficient applies.

 

No one likes to have a cylinder with lower compression than the others, but I'm curious what difference it really makes. Are you going to notice any change in power, get noticable improvment in mileage or burn less oil?
Sure - a cylinder that is pumping 60 lbs will no doubt cause some or all of the above, but I'm not sure 120 lbs is something to worry about. - but I'm no mechanic.

 

no I agree Rick, even if it is leaking past the rings or valves or whatever the bike runs too well to start dismantling it over this. I do appreciate the "stuck ring" theory as that fix is certainly worth trying. I'm colortuned and vac synched, and it idles great and runs great across all power bands. I'm not going to attempt to bring the compression up in #2 at this point with either a valve job or new rings. I don't like a 40 PSI difference between Hi and Lo but I certainly can't tell when I'm riding it. My plan is to RIDE ON!

 

Re: uh oh....compression numbers prior to getting valves in

Now im not a mechanic either but I do know, when something is broke, fix it. Not saying that your bike is broken but its clearly out of spec.

Its getting to that time of the year where you put your bike away for the winter, (unless your one of those lucky bastards that dosen't have to deal with snow or -40 ((Celsius)) winters. ) might make a good winter project.

 

That would be my advice after you try fitz's free the stuck ring idea.

120psi is low. More importantly it's low relative to the others. An engine, like a chain is only as strong as it's weakest link. You're balancing all your carbs to the low cylinder.

Will it run reasonably well, most likely, and you say it does.

Can you tune it? Yes but you're down on power versus an engine with better, more even compression.

Will it run as well as an engine with cylinders closer together compression wise? No.

Long term, it depends on your plans. If this is a cross country hop on and ride anywhere any time bike for you then I would get it fixed.

If it's more of an around town bike then I vote ride it until it dies and deal with it then.

It will not get better on it's own.

 

It can get a lot better if it's just a stuck ring and can be freed up. I'm not expecting it to immediately match the others, although if freed up, in time it might. The motor is at a really low mileage; the cylinder walls are barely broken in.

 

Re: uh oh....compression numbers prior to getting valves in

That's why I said AFTER you try Fitz's stuck ring idea. :?

 

After a Compression Test done Dry and Wet with the results pointing to RINGS as the cause for an abnormal reading, ... you are left with determining if there is:
A Stuck Ring
A Cracked Ring

Using a Syringe and some Battery Breather Hose, ... add 15 mL of Lighter Fluid (Naphtha) or Camp Stove Fuel into the Cylinder and move the Piston Up and Down several times.
Wait 10 minutes and conduct a DRY Test.
See if there is any improvement in Compression.

If you are lucky and a Ring was Stuck, ... the Value should improve.
If the Value is the same or worse, ... you have Ring Damage or a Scored Cylinder Wall.

Ear Syringe ::::

 

Kroil or MMO have a better chance of working than Naptha or lighter fluid.

 

OK did exactly as Fitz suggested. After two days with the MMO in there I turned the motor over by hand several revolutions, then reinstalled plugs and she started right up (smoking quite a bit). Rode it around for a few miles, rechecked compression, and same result - about 115 this time. well I do have some time this winter for a project but worth it ? - I'm not sure.

 

If the Bike is clean and you have less than 7,000 Miles on it, ...
WORTH IT!!!

You'll get YEARS of enjoyment after making it right.

The economy will be ripe for you to Barter getting help doing the job provided you can offer your skills and expertise in exchange.

Replace every fastener made of soft steel for quality hardware treated to prevent seizure down the road.

There are hundreds of hours of fun to be had after you return it to "Panel All Green" and "Ready to answer all bells!"

 

Yeah, I know why 4-strokes are called 4- strokes ( as in all my bikes (except one) and all my cars and truck), and why 2-strokes are called 2-strokes (as in all of my sleds) and how they work, and what the strokes are called.

I'm just saying the term BTDC as it was taught to me, though it never made sense to me since I only worried about timing at the combustion end of things.

SO, I decided that I'd take a quick google search... Well........my shop teacher too many years ago taught is something wrong! BTDC apparently actually meant BEFORE top-dead-center, not Bottom-top-dead center.

Now things make sense. I never could figure out how you could have a spark advance adjustment, and have a different "exhaust advance" adjustment, but he said that's what it was. Who was I to argue with a teacher when I wasn't a mechanic, other than bicycles at the time?

So, now I am one more degree BTDC smarter.

Dave F

 

If you are trying to determine the severity of suspected Cracked Ring Blow-by, ... as stated in earlier post ... using anything which would inhibit the Compressed Air from escaping past the Rings would not be wise.

Naphtha or Lighter Fluid suits the purpose of the TEST magnificently.
One of the best known cleaning solvents on the Planet.
Cheap; too.
Thoroughly cleans; then, conveniently ... disappears.

Keep a can in your Toolbox.

 

Re: uh oh....compression numbers prior to getting valves in

Oh, sorry, Rick. You misunderstood. The suggested use of Kroil or MMO was to try to FREE a stuck ring, not test for severity of blow-by.

There are two courses of action here: Keep riding the bike, and "tune" around the soggy pot;

or break it down and fix it right so it'll last a fine long time.

The good news in regard to fixing it right is that you'll have no reason to worry about new rings except on that one cylinder; and if you're lucky and they're just stuck, the only parts you're gonna need is base and head gaskets and some o-rings and sealing washers.

Depending upon how this particular bike sat, it's possible that the offending cylinder was the one sitting with its valves open, and that one cylinder got rusty. When the bike got fired up again after 20-some odd years, the piston very nicely cleaned all the rust off the inside, and it got all packed into the rings so now they're stuck.

That's one possibility.

The other is a cracked ring; less likely based on the low mileage and obvious care afforded the machine in its early life.

As nice as the bike is otherwise, especially with the low miles, I'd fix it right. It'll run "ok" as it is, but it won't get better and eventually something bad will happen.

 

Look at the results of your TESTING.
Your 2-Hole went to 170 wet.
You ... need ... Rings.

Right now, ... with the ability to make 170 wet, ...
Mitigate.

Go after it like you have Cracked a Ring and treat the problem before doing any damage that would require Machining to rectify.

Being in Chicago puts you near a facility where you could find an extremely talented student to help you get the work done.

Visit Universal Tech Institute.
Put-up a flier.

http://www.uti.edu/programs/motorcycle

 

Rick, if that set of rings is packed with a half a pound of fine rust "filings" it would behave similarly. I've seen this more than once.

Yes, it could be a cracked ring. It could also be nothing more than a ring set (or just the upper for that matter) loaded up with rust. Hence my recommendation to use Kroil or MMO. That failed.

HOWEVER: The response should be the same. Pull it down, and fix it. Like you said, before it does any damage that would require machining to rectify; IF it's going to do any damage that would require, etc., etc.

Unfortunately, there's no way to tell without tearing it down.

If not taken down to fix, it very well could cause damage that would make it more economical to replace the motor than fix. OR, it could just be a matter of a thorough cleaning. Either way, it would be cheaper to fix now than later.

I'm reminded of Clint Eastwood as Dirty Harry at this point: "do ya feel lucky, punk? Well, do ya?"

I'd tear it down over the winter and fix it. Especially with how nice the rest of the bike is and the really low mileage.

 

all right, so I am seriously considering attacking this problem over the winter. If I am taking off the the head (do I need to take the jugs off to change out the rings on #2 ?? If so does it makes sense to have all the cylinders honed out - not made bigger, just cross hatched like I have read about to improve compression?

 

Yes, you need to pull the 'jugs' to get at the rings.

I'm willing to bet that when you pull it down, the cross-hatching will still be apparent in all of the cylinders (maybe not #2 depending on what's wrong.) If everything checks out OK, I would NOT hone the other cylinders. Honing is to break the "glaze" and allow NEW rings to have a new surface to break in to. As few miles as your motor has, the other three cylinders should need little or no attention. Since you won't be fitting new rings, honing shouldn't be necessary (unless #2 gets new rings, then it should get honed.)

 

Here's what I'd do if that Bike was mine and I was in your shoes.

Acquire a Factory Service Manual. Specific to your Bike, alone.

Visit the Campus of Universal Technical Institute / Motorcycle Department
UTI Chicago
601 Regency Drive
Glendale Heights
Illinois 60139

Speak with the Motorcycle Technical Department Head.
Present your ==> REAL World Problem to him.
Bring a Photograph of the Bike.
The Factory Book with Red Inked Warnings and Cautions on possibly seized fasteners.
Test Results
Diagnosis
ALL KNOWN Complicating Factors and Obstacles faced during disassembling the parts and plant.

Would there be an interest in using your Bike for Training and Educational purposes with its repairs entrusted to UTI?

SALES Photograph
Highly Detailed Information
White space
Factory Book

"Help you save a Classic"

 

Re: uh oh....compression numbers prior to getting valves in

Very thoughtful and constructive advice Rick. We have utilized technical schools around here before with great results. Several pieces of furniture recovered for material cost only and my 74 VW body/frame repaired and beautifully painted with 4 coats and clearcoat for cost. And no doubt that's a highly preservable bike with the low mileage and good care it has had. Those schools are often looking for projects to teach from.

 

thanks boys, I'll dig this thread up when I make some progress and try to take pics of the problem (if visibly evident). should be interesting.

 

Regarding "Complicating Issues and Obstacles" that should be emphasized to you or anyone offering to help you.

Unfortunately, the Fasteners used on the build of these Bikes are not of the best quality.

Specifically, ...
Intake Manifold Cap Screws and Exhaust Header Securing Studs are Soft Steel.
Those Fasteners were installed untreated to prevent seizure.

Because they are soft; they twist and are easily broken-off.
It can become a nightmare.