85 LC 500xc Low Compression

[Four Digit]

I have been working on a restore for the past year and have run into a problem of low compression.

I purchased a project bike ( 1985 LC 500xc) and began the task of putting it back together. I did get the motor to start and run successfully, albeit a bit rough. Put engine on the back burner and worked on getting suspension, brakes and running gear spiffed up and operational. Finally got back to the motor but couldn't get it to start.

Ran a compression check and found it to be approximately 60-70 psi. Decided to pull the top end down and found that it needed a bore, fresh piston and ring.

Purchased a new Woosner 86.92mm piston, ring, pin, circlips and small rod needle bearning. Sent the cylinder out for boring and honing along with the piston and ring. When completed I installed the top end with a new base gasket and head gasket. Ended up using .013 end ring gap. Torqued everything up and conducted a leak down test. Pumped 5 psi into the motor and in 5 hours it had lost only 1.2 psi. To me that was an acceptable test.

Installed the motor into the frame at got the bike to where I could start it. Only problem I couldn't get it to fire....not one pop in about 45 minutes of trying. Ran a compression test on the new top end and achieved a reading of 60 psi....!!

I have a screw in type compression tester. Throttle position was wide open and I kicked 15 kicks to obtain the reading. I made three separate tests and changed out the dial gauge but got the same or similar results....nothing more than 60 psi.

Dropped a small amount of oil down through the spark plug hole and did some more compression tests but could only bring the compression up to 70 psi.

I've checked the intake reeds and they seem fine, no cracking, chipping or distortion and they readily snap back.

I pulled the exhaust and can see the ring is properly in the ring slot and there is no markings on the intake side of the cylinder except for nice cross hatching from the honing. In addition, photos I took of the rebuild confirm the piston is correctly oriented in the cylinder ( arrow pointing to exhaust).

I am at a loss as to what could be the problem and would appreciate your insight and help with this matter.

Thanks Doug Walker

 

[justintendo]

how are you doing your test? kicking wide open throttle until the needle quits climbing?

 

[Four Digit]

Yes, wide open throttle and kicking until my leg falls off..about 15 kicks..and the needle doesn't momve.

 

[fran...k.]

The spark plug is 14 mm thread which is the most common size. How about trying out your compression tester on a lawn mower or some other device. For me those 500 liquid cooled bikes are hard to start I don't know why I don't have problems with the air cooled 500. If you flooded it and got gasoline into the lower end turn it upside down remove the plug and crank it through or thread a copper tube or similar into the spark plug hole, through a transfer port on the side with the piston low and blow a lot of compressed air or else leave it overnight with the plug out in that position. that is the best I can come up with at this time. Sometimes closing down the spark plug gap will help if it is a febble spark. Generally fresh top ends start up good for me though it might need to seat the ring.

 

[2premo]

Yes, wide open throttle and kicking until my leg falls off..about 15 kicks..and the needle doesn't momve.

question since you own a running 400 how does it feel in comparison the effort to kick, it should be similar if the 500 has the original gears in the kickstart system
one of the difficulties of getting a good compression reading on the 85 500 (not sure about others) is the kick gearing is low and piston speed gives better readings on static compression
now that said I feel the readings are a tad low but a short break in will likely help there
back to basics, did you completely disassemble the engine? if so have you checked the obvious, spark, wet plug etc.
as tough as this sounds, put the plug in the head, put another plug in the plug end and have someone watch for spark, when the plug is in it slows the ignition down dramatically
the reason I asked about complete disassembly was ignition timing, is it correct, did you run a ground wire from the ignition to the coil, is the plug wet from kicking or dry, wet is usually ignition as the dry is usually carburation

 

[Four Digit]

Point of clarification: my 400 motor isn't running. It's sitting on the shelf, so there is nothing to compare the kick effort to other than my 250WR. That being said the kick effort is quite a bit easier than the 360 I put together a few years ago.

After this rebuild I did expect more effort to be able to turn the motor over. That's why I decided to check the compression and was of the impression that anything less the 100 psi would be unacceptable.

No, I did not completely disassemble the engine ( did not split the cases). I just did the top end but I did pull the flywheel and ignition. Currently it is timed at 2.3 degs BTDC. I set the timing with a dial indicator. No, I did not run a ground wire from the ignition to the coil.

The plug is wet from kicking and I am aware that ignitions can perform poorly when the plug is under pressure. Have run into this on two other Huskys'
I am in the process on running a couple of tests, from Vance Smith's Motoplat website, to determine if mine is with specs. Ohm meter reading between the lead Blue and Black wires showed 23 ohms, within the specs of 16-24 ohms that Vance cited. All that aside, the current system does provide a bright and consistent spark. However, I need to check it under load and I like your idea about using the two plug test. That is definitely the next test I will run.

I'll check my compression equipment on my 250 to see if there is a equipment problem.

However, in closing, my impression is that you are not overly concerned about the initial low compression readings, correct?

 

[grouty]

Just as an aside .... I bought a new tester a few months ago. It worked fine the first time I used it. Came to do the same test on the same bike again, and it would read about 50% of what it had before. I removed the schraeder valve in the end that fits the spark plug hole. I did notice that is was not that tight. Cleaned it and refitted. It now works a treat.

Maybe try a different gauge, or check the one you have. It would be nice if your fix was that simple. But......

 

[justintendo]

hmmm, i hope your gauge is broken...should be seeing at least 160 broke in or not...i dont understand how you were able to buy a piston before the machinework was done..dont you have to bore it till its round then see what size youre at? weird its so low on psi...

 

[Four Digit]

The motor was on standard bore. I wanted to use a single ring piston and couldn't find a first over Woosner so I opted for a 2nd over. Felt there was enough cylinder material to accomplish the bore from a standard to 2nd over.

 

[Four Digit]

Ran a compression test on my 250 and my testing equipment works. Ran right up to 130 psi with no problems. Tried the 500 again and still get a reading of 70 psi...!

 

[justintendo]

on the jug maybe a stud isnt threaded in entirely and a nut is bottomed from tightening any more? has a leakdown test been done? odd..on my snap-on gauge thats in great shape i get 160-170psi on the 250-500...my 88 250 is a bit higher, always 210-220psi...either way im not sure whats up with your 500...

 

[Four Digit]

I did a leak down test after I put it together. It only lost 1.2 psi after 5 hours...certainly adequate. I'm really scratching my head on this one..! I am fearful that I got a bad bore job. That being said it was done by an old machinist friend who does this thing for a living....which complicates the situation.

I'm going to do one more leak down test to see if anything has changed.

 

[justintendo]

hard to fault an old timer but even if your gauge is a tad off it should be well over 100...i would be suspect. maybe double check some stuff. your leakdown test results sound awesome

according to my gauge my 250 started running rich at 130 psi...fresh topend and it was 165! this was a wossner single ring replacing a wiseco dual ring

 

[justintendo]

do you still have your old piston? maybe you could check it with the new wossner to make sure the pin is at the same place? maybe its wrong?

 

[Four Digit]

Yes, I still have the old piston. I do remember matching the pin holes up and they looked the same...which is different then measuring them..!
Looking at some of the photos I took during installation the new piston is right up near the deck.

I have run out of possibilities on this so guess I'm going to have to bite the bullet and pull the motor (again) and tear down the top end. Hoping it is a cracked or chipped ring.

Was, and am, hoping someone comes up with something I've overlooked.

 

[2premo]

you have a wet plug did you do the dual plug test yet?
before you pull it remove the carb and reed and try it, it will remove the intake issue entirely if it exists (not likely but to be sure)
next remove the head and inspect the gasket, 70 is a little light but it should start
a leak down test will confirm seals not piston heights or ratios all it says is there are no leaks
mechanically pressurizing is read with a compression gauge
still look at the wet plug as a huge issue, Motoplat ignitions need RPM to generate even if the resistance is good

 

[Four Digit]

I haven't done the dual plug test. Will do it tomorrow.

I pulled the carb and reed this afternoon and performed a compression test. Same results 70 psi max.

 

[fran...k.]

I dont understand how you were able to buy a piston before the machinework was done. Dont you have to bore it till its round then see what size youre at? Weird its so low on psi.

Fixed some capitalization and punctuation keeping in line with the top class intent of the fourm creator's preference.

The half milimeter increments in bore and also 20 thousands for a lot of stuff using inch is enough to get good results. If some calamity occured perhaps not. Even if you send it to a shop that sells the piston chances are they will write on the side of the box the clearance and send the piston in the box out to another business.

I am not sure the honing and the holes in the cylinder are a good match for the abrasive balls on stalks or the spring stone ones normally seen that don't limit how far the stones can go out. The compression should depend on the bore above the ports.

To the origional poster here, It is pretty impossible to put the ring in upside down isn't it? Pressure from the top gets behind the ring and usually there is a little cut out on the inside top of the ring to aid this phenomenon. It sounds like you had the ring off and put it in the bore and checked the end gap. Did you do this in multiple spots. Kind of hard to figure what could be different if you take it apart. I have been able to start engines that had the ring stuck in place. Have somone to pull start it with?

 

[ruwfo]

I haven't done the dual plug test. Will do it tomorrow.

I pulled the carb and reed this afternoon and performed a compression test. Same results 70 psi max.

Take the top end off & measure the gap between the ring & cylinder bore.

Use this procedure: http://twostrokemotocross.com/2012/10/wiseco-2-stroke-tech-tip-piston-ring-gap/

Husky John

 

[Motosportz]

Is your cam timing right? Valve lash correct? Its a fresh piston ring and bore and you seem to have that in order I would look at cam timing, valves and head gasket at this point. This doe snot have a compression release right?