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Drop in cranking compression.....

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I put a fresh Wiseco top end in my '01 CR250 engine 5 hours ago on the hour meter. At that time, the cranking compression was at 160 psi (~200 psi at sea level) after 15 hot kicks (the kelstr method). Most of the 5 hours since have been idling and light cruise trying to figure out jetting. I measured the cranking compression again today, and it's around 145 psi (~ 180 psi at sea level) after 15 hot kicks. I looked in the exhaust port and the rings look OK; no sign of blow-by. I ran a leak test and the engine dropped way less than 0.25 psi over 20 minutes (it's really well sealed). Anybody know if this is a reasonable drop in cranking compression?

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New VF3 installed at the same time, but I'll pull it and check.

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im only guessing here, but you could try the pressure test again without the airfilter installed, and see what happens... that engine is cursed man... :thumbsup:

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New VF3 installed at the same time, but I'll pull it and check.
Reeds look great. Bottom ring on intake side looks great.

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that engine is cursed man... :cry:
I'm starting to wonder :ride: ..... But it really runs like stink when the jetting is close :thumbsup: .

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Do you feel a noticeable difference in the power or did you just test it because you're a psycho like me..you know..tear the whole bike down just to make sure ..lol

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I'm pretty much leaning toward the psycho approach right now...... I haven't had a chance to ride lately, so the last hour or so of engine time has been in the garage tweaking the jetting.

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I'm pretty much leaning toward the psycho approach right now...... I haven't had a chance to ride lately, so the last hour or so of engine time has been in the garage tweaking the jetting.

psycho approach is good!

I always spray some soapy water around compression tester fitting and hose to make sure it isn't leaking. No white smoke right? Head gasket torqued and OK?

GOOD LUCK! :thumbsup:

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psycho approach is good!

I always spray some soapy water around compression tester fitting and hose to make sure it isn't leaking. No white smoke right? Head gasket torqued and OK?

GOOD LUCK! :thumbsup:

Put a new, greased, rectangular cross section o-ring on my compression tester today to make sure it's sealing well. No white smoke. I just put on a new head gasket with copper spray sealant, so that's tight as a drum.

Has anyone measured the number of hours required for their cranking compression to drop?

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That much blow by wouldnt show on the piston with short hours. You would be able to see it if you took it out and flogged the crap out of it though.

Its leaking past the ring..sorry bud. Unless the exhaust bridge cracked...which happens...Your looking at a 10% drop. Its too much. You need to get the jetting close or perfect and put a new ring in it after a hone job and buzz it up down through the gear box a couple times to seat the ring and you should be good. Did you assemble it completey dry? Not that it matters now.

You could just run it the way it is, but it'll be kinda pipey (soft on the bottom) with the compression that low.

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Shawn has some good points , -- on these little trx 250rs that i have been building for the hill shooter dune guys,

( 370 cc 5mm stroke )

i end up with right at 215 psi on the 370,

and i found i can run 240 on the stock 250 cc ones,------,

the 370 with no power valve will run about 10 hrs and the piston and ring start to leak and it gets down around 185 190 or so ,

the standard 250 one without a power valve will run 15 hrs before it starts to drop ,

on the ones with a power valve tho i have noticed that very quickly my static compression tests become skewed because it only takes alittle leakage on the power valve when its shut to make the compression read low ,

on the cylinders i use that have a power valve ,--i will test the compression with the valve closed and sealed ( hopefully )

and i will test right away again with it open,

generally its like 240 closed ----and 190 open depending on the exhaust port height change ,

that way when the valve leaks alittle on the ft of the piston and i get a lower compression reading --- 215 220 or so --i can open the valve and do it again and still have my 190 and so i know my cylinder , piston and rings are ok----

its just i needed to take more pains to get my exhaust power valve to fit tighter and slam shut better and not leak off around the edges when it gets some burned oil on the dam thing :ride:

you may look into your valve ---it is more than likely you reason for lower compression that soon :thumbsup:

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on the ones with a power valve tho i have noticed that very quickly my static compression tests become skewed because it only takes alittle leakage on the power valve when its shut to make the compression read low ,

on the cylinders i use that have a power valve ,--i will test the compression with the valve closed and sealed ( hopefully )

and i will test right away again with it open,

generally its like 240 closed ----and 190 open depending on the exhaust port height change ,

that way when the valve leaks alittle on the ft of the piston and i get a lower compression reading --- 215 220 or so --i can open the valve and do it again and still have my 190 and so i know my cylinder , piston and rings are ok----

its just i needed to take more pains to get my exhaust power valve to fit tighter and slam shut better and not leak off around the edges when it gets some burned oil on the dam thing :ride:

you may look into your valve ---it is more than likely you reason for lower compression that soon :thumbsup:

Thanks kelstr, now I'm hoping for PV. I recall you mentioning open and closed measurements, and now I know why :ride::cry: . I started out rich with my jetting and have been leaning it out ever since, so there could be lot's of spooge in there. I'll do an open PV measurement today.

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It seems like my PV isn't sealing well when closed. kelstr's ratio of open/closed is approximately 0.8 (190/240). My original "good" closed compression was 160 psi (200 psi sea level) so my "good" open PV compression should be around 160 * 0.8 ~ 125 psi (155 psi sea level) which is exactly what I'm measuring today with the PV open. BTW, I really slammed the PV shut for the closed PV compression tests. So it seems like my rings, piston and bore might be fundamentally OK....... Any ideas on how to make the closed PV seal better??

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re-do the test, but this time hold the throttle wide open. turn the gas off so you dont flood it tho.

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It seems like my PV isn't sealing well when closed. kelstr's ratio of open/closed is approximately 0.8 (190/240). My original "good" closed compression was 160 psi (200 psi sea level) so my "good" open PV compression should be around 160 * 0.8 ~ 125 psi (155 psi sea level) which is exactly what I'm measuring today with the PV open. BTW, I really slammed the PV shut for the closed PV compression tests. So it seems like my rings, piston and bore might be fundamentally OK....... Any ideas on how to make the closed PV seal better??

the pv on most cylinders in stock form just do not fit very well and they are to far away from the piston to really work correctly.

you want that pv to be an extension of the cylinder and when shut it should be just like the cylinder and let the rings run on it and seal,---and the pv needs to be a very good fit on the sides of the cylinder to not let to much pressure leak by on compression.

it can be very hard to get a pv to really work and seal,

the 01 was a better set up than the hpp valves , but still required some welding ,machining and fitting to really get the thing to work,

i found that the motul 8002t 2 stroke oil at 50.1 really did stay cleaner , held better ring seal and kept the power valve from sticking and or leaking .

but to get the initial fit of the valve can take alot of work and playing around with,

some cylinders were better than others, but generally the valve was way to far away from the piston and rings and would hang up easily with very little carbon anywhere in the workings of the valve , :thumbsup:

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the pv on most cylinders in stock form just do not fit very well and they are to far away from the piston to really work correctly.

you want that pv to be an extension of the cylinder and when shut it should be just like the cylinder and let the rings run on it and seal,---and the pv needs to be a very good fit on the sides of the cylinder to not let to much pressure leak by on compression.

I was looking at the PV-to-cylinder wall distance last night. You're right, it's not really close. BTW, I think I found the compression problem..... Will post soon.

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the other thing i always had alittle trouble with was the flat top piston would get the crown edge hot on the exhaust side and slightly detonate alittle piston off and cause a slight drop in compression and even sometimes hang the ring alittle and lose compression,

i generally changed the squish angle to 12 degrees and went to a domed 12 degree piston ,

i found i could really lean down the jetting to make better power before pulling piston on the plug.

i could run more compression and run a tighter clearance and have better durability ------the dome is a stronger piston design and it has more surface area and will take more pressure and heat before it fails :thumbsup:

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