low compression

Alright guys i've been working on this bike for 2 years i've done everything under the sun i can think of to do to it but i just still can't get her to love me. I have cleaned the carbs up and down in and out, rebuilt the starter, bought a new battery. Right now it will not start at all it is getting good spark but it wont even try i checked the spark plugs they are not wet. i took it to a shop last year so it was supposed to be synched but who knows anything that happens in a shop. yesterday i checked my compression it was at about 59 psi and i checked my ignitions coils they had 0 resistance so i i'm pretty sure i need new coils but i need to know if there could be something in the carb that would cause such a low compression or is that the motor?

thanks guys.

 

Re: low compresion

DId you check the compression on all four cylinders? Then you should have four readings.

IS your compression gauge known to be accurate? Have you checked the compression before, with the same gauge, say, when you bought the bike? What were your readings then?

 

first reading. borrowed gauge compression was the same on all 4 between 58 and 60 is where it would stop it never got over 60. he takes pretty good care of his tools so i would imagine its accurate but theres always that possibility.

 

You have more than 60 on all four or you would be barely running if at all. Good battery for a strong cranking? Throttles held open? Clean air filter? Don't forget to unplug your ignition module (TCI).

 

58 to 60 PSI on all four cylinders is very unusual for an engine with valves within specs.

Are yours in specs?

 

First thing to do when checking compression is valve adjustment. If the valves are tight then they can't close fully and hold the compressed gasses. A small leak can drastically affect the compression. Then while cranking with the gauge in place the thottles should be held wide open to allow as much air into the cylinder as possible. Compression of 60 psi is pretty low. They should all be about 150 psi.

 

checked and fixed the valves before i cleaned the carbs. all the clearances check out. the bike wont start at all. did not know the throttle had to be held wide open on check tho. i will have to try and check it again after this 95 degree weather cools down a bit tonight.

 

Were the valves much out of spec when you did the adjustment? If the engine has been over-revved, this could lead to valve float and piston contact, which then leads to bent valves. Bent valves can't close all the way, so should normally, suddenly, have too much clearance.

This will also mean low compression, even after thicker shims have been installed.

If this is what happened, a valve job or a replacement used head will be in order.

 

Throttles held wide open, and a fully charged battery. (And unplug the TCI to prevent damage.)

Readings that close to each other tend to indicate a problem with the testing methodology, or the gauge itself. If you re-test and get similar results, get a "second opinion" with a different gauge. Readings that low are especially questionable if the motor runs.

 

X2

 

More? Damn. I need to look at what I'm typing.

 

Sometimes it takes some "re-iteratin'" to get the point across.

Plus some of us <Fitz> don't always bother to read the last dozen or so postings (especially if at work) of course you were right. Although the questions of gauge accuracy and testing methodology still needed to be raised.

I don't know about you, but I have two compression gauges; one is about 40 years old and the other relatively new. So far, when checked against each other on the same motor, they agree. That being said, any time a wonky reading comes up, the "second opinion" gets deployed.

 

everyone keeps saying a running bike, if you read carefully the bike will not run anymore it will not even try to start that is why i thought it was compression. I just tested it again it's still roughly 58 to 61 all the way across. would it be because the engine can not warm up and start?

 

Your statement that the plugs were not even wet makes me think you have some problem with the carbs, notwithstanding the compression issue. If the bike has not been running, I would question the shop sync. It would seem to me that if you have the enrichment (choke) circuit on and it won't fire at all then if gas is available your plugs would be wet--unless there's not enough compression to even pull fuel out of the carbs, and I don't know how low it would have to be for that to happen.

I agree with the others, something is strange about the compression numbers, especially since they are all so low and so close together.

 

big_rob

I would expect higher readings even on a non-running bike. I'd say more around 95-100 psi on a cold engine. Your readings are very low, no wonder the bike wouldn't atart.

Low compression is not caused by "something in the carbs" to answer one of your first questions. It has more to do with valves or piston rings condition.

At this point, I'd do a wet compression test and compare the readings. It is relatively easy. A wet test is performerd by adding a teaspoon of engine oil in every cylinder (thru the spark plug hole) and then proceed as usual (unplug TCI, wide opened throttle). If your readings are way higher, piston rings are loose. If there is no significant difference, valves are probably damaged (burnt or bent).

 

your numbers are very low
but I suspect your gauge is off
yes Valve Clearances are super important
I had a perfectly running bike last feel before I did all
my work on the bike
put it back together and it would not start
took me 3 days and many dead batteries
it all comes down to simple fire mentality
Fuel - Gas, Air - Airbox , and Spark or Combustion.. if you are
missing any of those things.. you will not have a running bike
my problem turned out to be a replacement fuel filter not letting
enough fuel thru
good luck

 

This ^^^has gotta be the way forward..Good diagnostic tip to eliminate (or not) the compression issue. BTW how many miles on the engine..?

 

alright good advice thanks guys. i will pick up a new gauge and do a wet check with both gauges. There are a little over 15k miles on the bike. The boots were bad so i replaced the boots and airbox and put pods on the carbs. i thank you all for the help, this is my first bike and i am hell bent even after 2 years to never give up on it. let me ask about the coils tho, i am getting a blue spark i've known that to be good but the specs say i should be getting 2.7 ohms +- 20% but i am reading 0 ohms on both coils that is something i can't seem to understand just doesn't make sense. good spark but bad coils..

 

Just a longshot but is your meter set on too high of a setting?
2.7 ohms is a very low resistance. Just make sure your meter is not on k or M ohms

 

Here is the way to test both primary and secondary resistances of the coils:

 

Rob, like Andrew said, lowest range on the meter. then touch the leads together, tight, note the reading. now the coils. clip the leads on, make a good connection and set them down. take your reading without touching them. now subtract the leads only reading from the coil reading and you have it.
if you have a good spark, chances are the coils are good.

 

alright thanks guys and thank you quebecois5 for the pic. the CD Manual does not show any pictures on how to perform tests it just shows the specs, so i was a little left in the dark on how to check it. but i will check my voltmeter and re check the coils first thing tomorrow as well as a compression check.

 

Be careful testing with the caps on, can give some strange results.

 

Actually, it could be interesting to test with the spark plug caps on and then without the caps, specially if the readings are strange with the caps on.

Make sure the caps are screwed tight in before you perform the test.

 

alright guys did a check with the old gauge didn't have time to run to the store and pick up a new gauge but i added a little oil to each cylinder and all 4 of them jumped up to about 90psi. i did the suggested check on the coils and got some pretty weird readings the ohms from the coils checked out just about perfect anywhere from 2.5 to 2.8 right on spec. but from the plugs it was 25 and 18 way out i think there may be something wrong with the plugs or the wires maybe.

 

If your spark plug caps are loose, it could be enough to make the resistance higher than normal. Of course if they are corroded inside, that could lead to higher resistance too. SOmtimes you just need to cut 1/4 inch off of the wire to have better grip for the cap screw.

40 PSI more on a wet compression test compared to your first readings is a huge difference. It tends to confirm that you piston rings are bad. But 90 PSI is still pretty low. It is possible your valves don't close properly.

 

yeah, this bike has been nothings but a 2 year long headache. hell of a learning experience tho. I really don't have the money to pay somebody to fix it and i don't have anywhere i can strip the motor down so i'm kinda sol..

 

90PSI is still rather low. Did you ever answer my questions about over-rev and whether the clearance was too large before adjustment?

 

i had it running for a few months then it started over reving at about 5 to 6k i had to play around with the idle knob a little. but the valves weren't off by much only had to go down 1 size shim on 5 of them i think there was atleast 1 from each cylinder then 2 from the first changed out. no real big clearance issue.

 

well guys it wasn't the other gauge, bought another gauge and got the same results all around 60psi. any idea what could cause such a low compression? maybe a crack somewhere? There may be some confusion the bike ran 2 years ago when i bought it sense then i have been able to get it started but it really has not ran for 2 years pretty much ever sense i joined this site. it ran good for about 2 or 3 months then it just went all to hell and never worked right again. any thoughts or suggestions other than a new bike or a hope and a pray would be great guys thanks.

 

If it was mine, I'd perform what they call a leak test. I don't exactly know how, but it tells you if you have leaky valves.

 

you need a nice expensive tool to perform that test and an air compressor. i don't have the money for any either.

 

So have you checked your valve clearances again during this round of "diagnosis?"

If not, DO SO. Any unexpectedly "loose" valve clearances could indicate one or more stuck or bent valves.

Let's see what you find.

 

Sounds like it needs to come apart and be rebuilt man.
Wish I could sugar coat it for you but most likely rings. Valves could be to blame also.

 

alright i'll check the clearances tomorrow when the rain clears up.

 

I'd guess that all of your valves are leaking, or all of the rings are bad. Perhaps it was run with little or no oil and damaged the rings, pistons, and cylinders. Since all the readings are about 60 I would place my bet on the rings. Bummer

 

Leak down tests are EASY and the equipment is NOT EXPENSIVE.

You need:

Leak-down test kit from Harbor Freight
Air tank from harbor freight

Take the air tank to your local garage and fill it full

Now you have own a tank, and a kit for not much out lay.

The leak down test itself is easy:

pull a plug, insert the tester, hook up the air and turn the pressure on.

Ideally, you won't see the pressure drop too much or too fast, but it will go down. If it goes down fast,you have to LISTEN to three areas:

1. listen for loud hissing at the carbs - indicates intake valves leaking
2. listen for loud hissing through the exhaust - indicates exhaust valve leak
3. listen for hissing in the oil filler - indicates rings leaking

if it's intake or exhaust, you can pinpoint WHICH cyl is leaking too....just listen for which carb or pipe it's coming from.

It doesn't solve the problem but it tells right where it is.

We do this stuff with sleds all the time.

Now after that is all said and done......

1. I'm curious as to what clearances you set things to-- if you set the intakes to .16-.20, that is the wrong range for intake... intake should be .11-.15

2. if you DO have to go into the engine, let me be the first to say YOU CAN DO IT. See that second word ? CAN you CAN do it, don't talk yourself out of it. If you've done the valve shim stuff, you're halfway there already. Just another bunch of bolts and lift the head off.

If its valve work, you don't have to go farther down. If it's rings, well.....the head is already off, the cyl will lift off.

Enough of us have been there, you won't be stranded. We're here to help.
Right next to you.

If you do have to pull things apart, ASK questions at every step if you need to. Again, we're right here.

dave f

 

Good information. But wouldn't we have to make sure that the piston is at TDC on the the ignition stroke to be sure both valves are closed?

 

Yes.

Also, it may not be necessary to actually invest in a leak-down tester. Check the major auto-supply chain stores in your area; see if one of them has a leak-down tester in their "loaner tool" pool.

 

if your compression gauge has a way to take off the guage just put a air source into it and listen for leaks. you don't really need the leakdown part of the leakdown tester.
only take out one plug at a time. when you put air in the cylinder the crank will spin with GREAT force. if your holding it at tdc, hold it tight. it's a good way to get a wrench in the face

 

Good advice. Hadn't thought of the "flying wrench."

 

PostPosted: Tue Oct 01, 2013 3:03 pm Post subject: Re: valve clearance Reply with quote Edit/Delete this post
alright update. thanks to fitz i have a better understanding than what the book told me. so i have some numbers. on the ex side 1-4. 20mm,10mm 15mm and 20 mm specs say 16 -20 so 1 and 4 are good and 2 and 3 need some work.

intake .08mm .08mm .04mm and 13mm specs say 11-15mm

that was from the post when i did the valve clearance check and fix.

but until i can find a garage or a place that is less windy and dusty i really don't feel confident in taking the motor apart, it would be hell trying to get all of the dust and dirt out of the engine flying around in the driveway. i may try and rent a storage building.

 

If those clearances (.10mm on #2 exh; and 3 of 4 intakes horribly tight) are "current" then quit wasting your time with compression tests or trying to make the motor run.

You've got 5 out of 8 too tight. I wouldn't expect anything near good compression numbers until you get the valves in spec.

 

Well of course it has to be on the compression stroke.... You're testing compression..... If there's full compression, you won't have much leak-down......lol

Yeah, some places will have loaner equipment.

I've never had an engine kick over ....

 

the clearances where checked and fixed correctly october of last year i just copied and pasted that from an old post i had on here. but all the valves are or still should be within specs from last october when i did the adjustments.