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295 new cylinder and piston- low psi before first start up

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Hello as some of you know from my other thread- I just had my 05 295 cylinder re plated and put in a new piston.

When I did a compression test before i took it apart I had 220 psi with a chipped piece of plating that scores a .0010" gouge in the piston.

With the replated cyl and new piston now I have 90 psi of compression. Now I haven't started it up yet to break it in. I just wanted to check before I broke it in. The ring ends are in the locator grooves correctly. I have perfect ring end gaps also.

Is this low comp reading from the rings not being broken in and seated? I always use the wet rebuild as I don't believe in the dry method, but I don't want to get into that discussion again. I hope I have enough compression to fire it up.

Thanks in advance

Ryan

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I've never seen that big of a difference in compression on a new build.  I assemble with plenty of 2 stroke oil on the piston/rings/cylinder. I always would see compression pick up after an hour of running, but I am talking 10-15 psi.

 

Didn't leave a rag in the airbox, or plugged exhaust or something, did you?

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It wouldn't be the first time he cocked up the bore diameter for one of those kits.  Send it back.

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The piston to cylinder clearance is dead on as I measured it when they both arrived. Replated 295 cylinder with new 295 piston. I always assemble with plenty of 2 stroke oil, as I don't believe in the dry method. I also dont use rags in the air box or exhaust- that way there no chance of it happening.

It's got me stumped. Ring end gaps are set to spec and aligned in the locating grooves. The gauge is a recent snap on gauge that I use professionally at work and is reliable.

No porting changes- still wide powerband. I did have the head cut for pump gas as it was race gas previously, I can see that lowering comp by 10-20 psi but not this much!

The only thing I can think of is the rings need to seat, but I don't know if it will even fire with 90 psi. I will find out tomorrow when I can put it the rest of the way together.

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You would know the difference between 220psi and 90psi by kicking it over........does the difference feel that extreme?

 

Any gauge, be it Snap-On or whatever, can act up. Pull the little schrader valve and blow it out.  A little chunk of carbon can give funny readings.

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Yeah what does it feel like at the kickstarter?  Can you cycle it by hand?  With the leverage available, you should be able to at actual 90psi.

 

How did you measure piston/bore clearance?

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pull it down, check ring gaps, head gasket, dowell interference, reeds, base gasket, bore for ovality, reassemble dry, and see what you have, there's no magic here, my guess is something holding a reed open. 

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I tried to start it last night and it was a no go. I will start disassembly tonight.

I measured piston to cylinder clearance with a cylinder bore gauge and a digital caliper.

I also tried a spare Mac compression gauge and a borrowed one from work and they all read 90 as well.

It does feel low going by the kickstarter. I can kick through 2 compression strokes in one kick.

I also noticed a weak spark when kicking over. with the lights off at night it is barely visible and takes good kicking speed to spark when grounded to the head. I will take home my multimeter and start checking the stator, Cdi unit and coil by the manual. It sounds like 2 problems- low compression and weak spark.

Does anyone have a spare stator, cdi box or coil I could purchase if I need them?

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I think with 90psi the engine will not fire.

When I got my 260cc overbore cylinder I filed the PV but forgot the extra boost ports. Cost me a piston :( and the engine would not fire neither.

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The Cdi box does have a mark on it, like if someone crashed and the forks somehow hit the box- the bike does have aftermarket triple on it that do come close to touching the box. I will do my resistance checks tonight.

I just did a parts search for my 05 yz250 - the Cdi unit box could be from a 04 or 05.

The coil fits-2005-2011 yz250

The stator says it only fits a 05.

If anyone has any extras Let me know and I will buy.

Thanks

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If you checked piston clearance, and it is between .002-.0035, You either have a broken ring, or a compression leak. Check head gasket seal (you did use a new headgasket?) I always paint my headgaskets, both sides with rattle can paint of some sort, and assemble while it is still dripping. If you only have 90 psi, you won't get it started by kicking, but, if you tow it about 15mph in 2nd gear, it will probaly fire right up. You may have a weak spark, but the real problem is, you have low compression. A cracked (or missing) reed, will not cause a loss of compression.

 good luck....

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Well i did a compression test tonight and im up to 170 psi now, probably from all the kicking yesterday with 3 different compression testers. Id add oil in through the plug hole before every test. it may just be from the new rings seating to the new hone in the new cylinder. thats all i can think of- nothing has changed since.

I verified the intake tract is open and the exhaust was clear- I even burned it out. Powervalve is in the resting position.

i used a new dual layer head gasket torqued at 18 lb ft as well as a new oem base gasket at 30 lb ft per the manual with a snap on torque wrench. I used New vforce3 reeds with new reed gaskets as well, but i did swap back to the old reeds out of curiosity but no change.

Piston to cylinder clearance measured exactly at .0022" which is what eric stated as well as the piston box says.

Now I believe my compression will keep rising but I still have the weak spark issue. I tested the kill switch coil and stater per the manual and my resistance is within spec. Also tested the continuity of the wire harness and found no breaks.

I can't rule out the Cdi box as they don't give you a spec or any means of testing. With the lights off and dark outside you can see a visible spark when grounded to the head but its pretty dim. The Cdi box does have a mark on it from maybe a accident or testing these triple clamps by the previous owner.

Now I'm looking for a Cdi box now. That's all that's left

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Throwing this in here- It will fire with 90psi.  First 125 I owned was on a stock bore and piston/rings with a good amount of hours.  Compression test showed 55psi, and it was a one kicker.

 

Through all these compression tests, you've been putting 2-stroke oil in the plug hole, you may have saturated your plug.  Sorry if I missed it, but I haven't seen where you've tried a new plug.  Forget about compression test until after you've broke it in.  If it's still that low (i'm guessing it will increase significantly as you just stated), Eric should stand behind it.

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Throwing this in here- It will fire with 90psi.  First 125 I owned was on a stock bore and piston/rings with a good amount of hours.  Compression test showed 55psi, and it was a one kicker.

Every type of engine is different. Bore, stroke, porting, intake and exhaust differences, etc, will all play a role in dictating compression test numbers. 90 psi might be marginally ok on a 125 that might only do 145 psi in great shape, but is really bad on a 250 that should be putting up 220 psi or more.

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Every type of engine is different. Bore, stroke, porting, intake and exhaust differences, etc, will all play a role in dictating compression test numbers. 90 psi might be marginally ok on a 125 that might only do 145 psi in great shape, but is really bad on a 250 that should be putting up 220 psi or more.

Agreed, but to say it wouldn't fire imo is stretching it. I've never done a compression test before break in, and don't know anyone who has. But is it stretching it to say that compression numbers could increase 100-150% after break in? He's increased 90% and hasn't even had it running.

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Agreed, but to say it wouldn't fire imo is stretching it. I've never done a compression test before break in, and don't know anyone who has. But is it stretching it to say that compression numbers could increase 100-150% after break in? He's increased 90% and hasn't even had it running.

 

I always comp test before firing up, more out of interest than anything. Typically new and well oiled my bike would do 240-250 psi. After about an hour of riding it would be 245-255 psi. I think my biggest before/after break-in jump was 242 psi new to 254 psi after running.

 

I see the same at work. Assemble a new engine and then run it in and compression is generally the same, might go from 175 psi to 180 psi. Leak down typically seals up a bit better, maybe from 4% to 2-3%.

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I always comp test before firing up, more out of interest than anything. Typically new and well oiled my bike would do 240-250 psi. After about an hour of riding it would be 245-255 psi. I think my biggest before/after break-in jump was 242 psi new to 254 psi after running.

I see the same at work. Assemble a new engine and then run it in and compression is generally the same, might go from 175 psi to 180 psi. Leak down typically seals up a bit better, maybe from 4% to 2-3%.

Do you see these numbers whether it's a fresh cylinder/hone or deglazed?

I have my cylinder coming back from millennium any day now, I will have to comp test before it's ran. But I never perform an interim test. Just at 40 hours when a new piston is going in to give me an indication of how things are wearing.

I will find it surprising if numbers are near 240 before break in. As I've always paid a lot of respect to ring/cylinder seal. If the number is only a few percentage increase, it would debunk all the different break in methods.

The leak down test you speak of is only combustion chamber, not including crankcase correct?

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Looking back at my spreadsheet.

 

Only 1 top end was on a fresh plating, it did 245 psi oiled up with zero run time. At 1 hr 31 min (1 ride) it was at 252 psi.

 

The other top ends were with just some scotch-brite deglazing action, and varied from 242-247 psi with no run time.

 

The compression test is just a tool, and not a full picture of everything happening in an engine. As I said, my "new" tests are done with a well oiled cylinder, which will help seal things up. And the conditions in the cylinder are far different in a compression test vs actual running, where cylinder pressures are going to be probably 1200-1500 psi, and temperatures just a smidge higher than ambient (a BIG smidge). So a ring that seals up great in a compression test could allow a fair amount of blow-by under running conditions. I know I've pulled pistons blackened with blow by that still did around 250 psi on a compression check.

 

 

The leak down I was talking about engines at work is 4 strokes, so we are checking ring and valve sealing. Lets you know if you are loosing pressure out the intake valve or exhaust valve, or past the rings. Again, just another tool, and not an all-encompassing answer.

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