Lower compression when hot?

K

KX_OUTLAW

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Feb 7, 2010
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I have a KX85 with a 100 big bore/port kit.
The first cylinder I put on it got externally damaged so I ordered one with the same specs. With the 1st cly it had a gallon of fuel run through it, I just took a cold comp test before pulling it, it was 180 psi.

The replacement cyl was done to the same spec, and I measured everything also, used the same head, same piston, new rings, new oem gaskets.

1st cly I broke in as recommended which was just warm it up, take it easy up to 3rd, don't go wot for about 30 minutes, let it cool and it's done.

When I took that apart there were about five .125(inch) wide lines that wore the cross hatch off, the piston looked new.

The new cylinder I broke it in using this method:

Now it has about a gallon of gas run through it, it's always had more of a ping sound to the pipe (the normal 2 stroke sound), and it runs stronger.

Only thing is it has about 150 psi hot with 10 kicks, maybe 165 cold.

Same oil, same cold temp.

Normally I wouldn't have even checked since it runs so good, but I read to much I think.

Nothing to worry about?
 
whenfoxforks-ruled

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
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Merrillville,Indiana
It sure sounds to me like your top end is toast. The 2nd cylinder, you checked the ring gap, the skirt clearance, and if it has an exhaust bridge you checked that clearance? When you seen the rub marks in the new piston, you looked up her dress into the exhaust, or pulled the top end? If you pulled the top end, you put new gaskets and rings in her? Then it should be okay, but its not. All the psi readings on the first cylinder are irrelevant. What it was after break in, and as soon a drop is noticed, the rings are toast. It was set up for pump gas? Vintage Bob
 
K

KX_OUTLAW

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Feb 7, 2010
19
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can't reply, says spam killer detects something in my post. It's text only, there are no links


Let me clarify.
The first cylinder was bored, ported, and plated. It was bought in kit form and the clearances were all set by the builder, and I double checked. New ring end gap was .014 iirc.
It ran fine.
It had to be replaced due to external damage. It had enough run time to burn 1 gal of fuel.
I did a comp check on it and pulled it.
There were about five 1/8" wide up and down wear marks in the bore. The piston looked brand new still, no rub on the piston.

Both set up for pump gas, high rpm, and it does not ping (aka detonate/preignition, etc)

New cylinder, same builder ported, bored, plated it. Used the same piston as advised. Replaced rings, and gaskets with same as before except new. Ring gap .012.
Same tank of gas to break it in, as of now it's burned approx a gallon.
Compression test is slightly lower but it runs noticably better.

I could see maybe the oil viscosity would be thicker when cold and produce more compression during testing cold.

I always let it warm up before riding.

I guess I just read too much. Maybe nevermind I don't know.
 
whenfoxforks-ruled

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
8,129
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Merrillville,Indiana
If the port heights are not EXACTLY the same, and most of the time they are not, yes you could see a noticeable difference in the way it runs, and the compression. Forward Motion does offer 2 EXACTLY matching cylinder service, they WILL run the same. Another Eric first! Vintage Bob
 
K

KX_OUTLAW

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Feb 7, 2010
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How does that apply to my situation?
 

ellandoh

dismount art student
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sounds like an explanation to the psi difference between the two cyl. which was one of your original concerns or at least thats the way it comes across in type
 
K

KX_OUTLAW

Member
Feb 7, 2010
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Yes, how do you know Eric did anything?
The wear marks had to have come from the rings, although they aren't torn up. This is why I did the second break in the way I did, who ever wrote that said that the rings need to heat cycle to better conform to the bore if something wasn't perfectly round. First time I've heard of that but I figured it wouldn't hurt to try, the guy said he's done it like that for 35 years. I'll see if I can find out where I got that info.

ellandoh, that is correct, I'm just curious why the compression is different.

I measured port heights, and checked for head surface or base machining. All were the same.

Most of all I don't understand why it sounds stronger and runs better.
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
8,129
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Merrillville,Indiana
You did say the psi reading was lower on the second cylinder. It is going to run noticeably different. Unless the 2 cylinders was set up by Eric Gorr, to run EXACTLY the same, its never going to happen. There are a lot more variables than port position in the bore. The volume and angles count also. You can have a 4 corner seizure, that usually gets the piston. a cold seizure may get just the rings, its early, maybe? No clear pics of the bore and piston? Vintage Bob
 
K

KX_OUTLAW

Member
Feb 7, 2010
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I was told they were set up EXACTY the same. Personally I don't see how since one was upstock. Meaning it was already on the shelf.

As far as cc'ing everything I didn't. I doubt anyone did.

I've seen three of these set up EXACTY the same yet there are obvious differences just by looking at them. A few examples would be a narrower exhaust bridge on the 2nd cyl, a 10mm cut out at the bottom of the center sleeve, and a deeper taper at the top of it.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Eric Gorr has a service for making cylinders the same. You have to specify it, I would think he needs both at the same time, otherwise, it will be different. You never specifically said Mr. Gorr did it, could have been Eric anybody? Did Eric do them at separate times? If you take any 3 new dirt bikes, they will run differently. As always, if its an issue, call Eric during cst business hours. Vintage Bob
 
K

KX_OUTLAW

Member
Feb 7, 2010
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He said they were the same, my question was about compression, so it's a little lower, it runs great, thanks for your time. I don't want this thread to make a 90 off into a ditch somewhere. Bottom line, it runs better, I'll note compression and reference that as a baseline for future engine condition. Thanks for your time Bob. And the others as well.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Remember the air cooled bikes? They lost measurable horsepower when hot. Thats how we ended up with water cooled bikes. Past that, this thread confuses me! That is a first! If Eric set me up 2 cylinders to run the same, I would be up in the land of milk , cheese, and big women! Vintage Bob
 
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