Topic: No compression after recent rebuild

[skishyishy]

Hey all,
           I found several threads with pieces that were useful but nothing that comprehensively helped my problem.  Thanks in advance...

I Recently rebuilt my 78 750k8 (1000 miles ago) and it had been running reasonably well.  I had it machined to fit +.75 over brand new pistons and rings. The first time i put the engine back in and fired it up i developed a bad leak at the head gasket which made me suspect the head may have been warped.  I pulled it out and took it to the machine shop at school and put it on this precision block dealy (not the correct technical name) and checked with a feeler gauge for warp.  It was within tolerance, and the second time with new gasket, i had no leak.  I was fairly tame breaking in the engine though i did open it up occasionallly in steady bursts to try to get the best of all the varied schools of thought on engine break-in that appear here in the discussions.  Recently (last 200 miles or so) I'd begun to see occasional blue smoke out of 1 and 4 when running the engine a little harder.  Then it happened (gasp...).

Running the engine fairly hard at abut 6000rpm on the freeway with the engine fully warmed (i have an oil cooler PO put on) I all of a sudden lost about half the power.  It literally felt like cruising on 2 cylinders, but it wasn't making any odd noises or anything.

I pulled into a friendly neighborhood and went home for my tools on foot.  Heres where I stand now...

I definitely have fuel.  Starter turns over but engine wont catch and pulling the plugs they are wet with gas.  Plenty of gas.

I have spark.  I checked the points and coils and all four plugs will gap to the block when i test them.

Compression for 1-4 is really low. 1: 65psi, 2: 90psi, 3: 90 psi, 4: 90psi.   There is no oil leaking from the head or base gaskets and no oil spots where i park.  there is a little leaking from the valve cover, and some from the tach gear.  SOME OF THIS LEAKING OIL IS A STRANGE (SADLY) METALLIC OR GUNMETAL GREY COLOR.  I also found some of this discolored oil on the compression tester after removing it.  I have only changed the oil twice since rebuilding.  Once after the first 50 miles or so and the second time about 300 miles in.

where did i go wrong?  I was quite careful.  since all four cylinders have low compression is this probably a gasket and not rings?  I intend to pour some oil in the holes and run the compression test again to test the rings but havent yet.  Bike actuallly ran for over a mile with some power after the 'blowout' and it ran pretty smooth so i dont think it is cam chain or timing.  All other electrical works fine. 

ALSO:  when i got the oversize piston rings they were a little different from my old ones.  They were Art and one had a beveled outer edge (unlike the square ones i pulled off pistons) which after much useless help from a clueless rep we decided to put second with the bevel facing down.  Could these have been early model (69-76) rings that messed up my engine?

Sorry about the long post all but i wanted to get all the info in. Thanks again. My homage to all the collected wisdom that appears on this site.

 

[Lumbee]

...first off, did you check the compression immediately after the rebuild, like even before you first started it?  The reason I'm asking is because your compression numbers just don't look right for a freshly rebuilt motor.  They are way to low.  You should have at least 120.  In addition the fact that all four cylinders are low make me think that something just was not done right when the cylinders were machined.  Did you verify what size the cylinders were honed to after the machine work was done?  Sounds to me like maybe the were bored out to large maybe.  Do the leak down test with oil and see if that raises the compression.  Either way I would say the motor is going to have to be gone into again.  If you didn't verify on your own (like with a caliper) what size the cylinders were bored to, that would be the first thing I would check after tearing it apart.

[skishyishy]

yeah, i think the original compression was around 140 after the rebuild.  The guy who did the cyclinders is a real reputable fellow who actually does all the machining for the local (large) bike shop, and I gave him the piston (which he requested) before he did the work.  i did not check with a caliber when it came back however, but the pistons and rings did seem to fit well with the ring gap within tolerance inside the bore.  Can the head or base gasket leak without any oil visible?  Whatever happened seemed to be heat related in some way, but if the cylinders were to big at high temp some compression would return when cold right?  The low compression tests were on a cold bike.

[Lumbee]

...hmm...sounds like you might be OK with the machine work then...if u have faith in u'r guys work, then lets assume the cylinders are the right size.  How well does it hold the compression it does have.  Does it go up, then slowly settle down to 60 or 60?  I guess you should also check out your vavles and timing make sure they are set correctly.  If they were way off I think that could cause you to have low compression.  Also, like you said if you blew a gasket it could effect the compression, but again, not across all 4 like that.

[scondon]

? Whatever happened seemed to be heat related in some way, but if the cylinders were to big at high temp some compression would return when cold right? The low compression tests were on a cold bike.

   Compression #'s will be significantly lower on a cold engine. 90 sounds about right. 65 on #1 sounds low. Have you checked your valve tappet adjusters to see if any have worked loose?

[skishyishy]

hmm, thats a little reassuring.  90's seems about right cold eh?  Does anyone know the correct psi warm?  Any suggestions on why i cant get it to turn over then? i definitely have spark and gas. Could one stuck open valve be enough to keep the engine from turning over?

[TwoTired]

Check to see if you have a broken cam.

Open the tappet adjustment covers and rotate the engine to verify each valve moves up and down when rotating the crankshaft.

Simple check.

Cheers,

[skishyishy]

Great call TT.  Ill check on that and do the leakdown test and report back.  Any idea what healthy compression on a hot engine should be?

[crazypj]

yeah, i think the original compression was around 140 after the rebuild.  The guy who did the cyclinders is a real reputable fellow who actually does all the machining for the local (large) bike shop, and I gave him the piston (which he requested) before he did the work.  i did not check with a caliber when it came back however, but the pistons and rings did seem to fit well with the ring gap within tolerance inside the bore.  Can the head or base gasket leak without any oil visible?  Whatever happened seemed to be heat related in some way, but if the cylinders were to big at high temp some compression would return when cold right?  The low compression tests were on a cold bike.

Checking with a caliper would have been a waste of time. A reputable machinist will be working to nearest +/- 0.0002", less than half a thou or 0.05mm.
the head gasket can leak between two cylinders without oil leak.
90 psi is very low for a rebuilt engine. it should go up not down as rings, etc. bed in
 I would say you need to pull motor and get top end back off, sounds like a ring, valve/cam problem to me.
PJ

[TwoTired]

The shop manual calls for 150-170 PSI, hot engine, choke open, throttle fully open.

However, it really depends on how much the compression gauge aparatus adds to the combustion chamber volume.  These engines are tiny compared to most auto engines where the typical compression guage is intended.
A 750's cylinder displaces 184cc. A 9 to 1 Comp ratio makes the combustion chamber around 20.4 cc or about 4 teaspoons.

A test aparatus with a 3/8 hose, a foot and a half long, can easily reduce the comp ratio significantly, which will yeild low pressure readings.  I usually only worry when the numbers across the bank are not within 10%.

I've not done a leakdown test to a motorcycle engine.  

Cheers,

[Noel]

That still doesn't make any sense to me. It seems to me that adding volume merely increases the number of piston strokes it takes to fill that volume. It shouldn't decrease the ultimate pressure that the system is capable of producing.

IOW, if you attach a compressor motor, capable of producing 100 PSI, to a 5 gallon container, it will attain 100 PSI in a certain period of time. If you attach the same motor to a 10 gallon container, it will still produce 100 PSI. It just takes longer to reach it.

Am I missing something?

[Lumbee]

...I'm with you Noel...I'm not engineer (though I like to act like I am), but I would think it would just mean more kicking to achieve a given PSI.  And even beyond that, 90psi still sounds way low even on a cold engine...I've rebuit 3 motors, and I only check the compression when they were cold (I refuse to burn my self getting that comp. gauge in the 2/3 cylinders  ) and all were 120 or better cold...

[dusterdude]

not an engineer,now thats funny.

[TwoTired]

Am I missing something?

Oh yes.  You are ignoring valve placement and operation.

A compressor's exhaust valve is open on compression stroke and closes immediately after.  A compressor will keep pushing air out the exhaust valve as long the the pressure it creates is higher than what exists on the far side of the exhaust valve.

An IC 4 stoke engine has both valves closed during compression stroke and exhaust opens immediately after.  The peak pressure created is a function of the ratio of cylinder volume difference between BDC and TDC.  Change the ratio, change the pressure.

Milling the head and high compression pistons exploit this fact.

What may be confusing you is that the Compression test apparatus has it own check valve.  The pressure of it's spring must be over come before filling the gauge's Burdon tube and connecting airway volume.  
The cylinder itself gets full pressure on the very first compression stroke.  The check valve only allows part of it to go to the burdon tube. (minus the check valve spring pressure, and the volume of the burdon tube and interface tube.)

The equations get more complex on successive cycles as the building pressure in the gauge reduces the volume entering past the check valve on each successive compression stroke until they approach equalization.

Much depends on compression tester design and where that check valve is placed on the apparatus.  The closer the check valve is to the spark plug hole, the less effect the apparatus has on the chamber volume and pressures achieved in the cylinder.  Not all compression testers are physically constructed the same way.

As important as what you test is how you test it.

If you are still not convinced, then put 10 or 20 feet of compressor line hose on the spark plug hole and place your compression tester at the end of it.  Let us know how many kicks it took to match the pressure readings taken with the tester placed directly at the spark plug hole.  Make sure the check valve moves with the tester.

[Lumbee]

If you are still not convinced, then put 10 or 20 feet of compressor line hose on the spark plug hole and place your compression tester at the end of it.  Let us know how many kicks it took to match the pressure readings taken with the tester placed directly at the spark plug hole.

...OK, I'll play Hondaman vs. TT.  My guess is it will take more kicks to reach the same PSI reading you got with a shorter line.  The question is could you reach the same reading as you did with the shorter line?  Is your contention that you could not?

[TwoTired]

Key is the placement of the tester's check valve.  If the valve is placed at the spark plug hole, then your guess is correct, line length won't matter, rather number of cycles.  If the check valve is placed at the end of the hose, the hose volume adds to the compression ratio equation, lowering the ratio and the peak pressure achieved.

[Noel]

Hmm...

It's hard for me to understand how valve placement and operation affect compression testing. It also seems to me that the check valve in the tester ought to be a constant, i.e. irrelevant to hose volume. However, you have made me realize that hose length will certainly affect CR which in turn should limit ultimate pressure of the system.

Thanks for the explanation.

[skishyishy]

well i really sparked something here huh? For what its worth, the length of hose on my compression tester (which may be for automobiles since it has an adapter at the end, i cant recall where i got it) is about a foot and a half.  i also got the bike running again after carefully checking spark on all four.  the cold compression however has not changed, and when i put some oil in the plug hole, it goes up on nearly all four from 90 to about 120.   

[Serge]

did you keep the gas and choke fully open when cranking the engine for compression measurements?  - serge

[bryanj]

There are three posibilities here, in no particular order:-
(1) You aint doing the testing right

(2) you got it too hot and the pistons nipped in the bore

(3) Running with unfiltered air has worn the pistons badly

Obviously, without being rude i hope its No 1 BUT as you said you saw blue smoke i suspect No 2 UNLESS you aint got an air filter in which case No 3

[Bodi]

Rings:
The oil ring should be pretty obvious. Of the other two, one should have a relatively flat face and one should have a slight bevel across the full face, making a scraper edge all around. The bevel one is the 2nd ring and goes with the bevel facing so that it is scraping down the cylinder and "floating" upwards. If the flat faced one has a single chamfered edge it should go downwards, both edges chamfered equally then it doesn't matter, if there are any letters engraved on the ring they face upwards.
The top ring is the compression ring, 2nd ring is an oil scraper, oil ring keeps the oil moving down. The holes in the piston at the oil ring let oil flow inwards, it gets flung off inside during piston accelerations and either goes down to the crankcase and to lube the wristpin or up to help cool the piston crown.
Did you gap the rings correctly?

[Terry in Australia]

well i really sparked something here huh? For what its worth, the length of hose on my compression tester (which may be for automobiles since it has an adapter at the end, i cant recall where i got it) is about a foot and a half.  i also got the bike running again after carefully checking spark on all four.  the cold compression however has not changed, and when i put some oil in the plug hole, it goes up on nearly all four from 90 to about 120.   

How long (in miles) did you run in the new pistons and rings mate? I think Bryan might win the "closest to the hole" prize with option #2. Whoever bored your cylinders should have checked the piston-bore clearance (about .001) and the ring gaps, so if your pistons have "grabbed" in the bore, there was either insufficient oil, or too much heat, too soon. What's your oil pump like? If your oil has gone metallic, then you've had a meltdown! Cheers, Terry.

[skishyishy]

i ran the compression test with the choke and gas full open.  I do have an air filter which is relatively new.  The bike was definitely hot.  I would say i was running it the hardest and hottest i have yet since the rebuild about 1000 miles ago.  The rings were installed like Bodi described.  If the piston did 'nip' the bore, that might explain why cyl. 1 is lower than the other three but does not fully explain why all four seem low.  I guess it could be that along with a bad oil pump which caused the rings to wear poorly?

[Lumbee]

...I would guess a bad oil pump would cause the topend to seize before it would cause the rings to burn...

[bryanj]

Oil pump, would not cause pistons to sieze , only overheating caused by wrong timing or mixture would cause that