LOW Engine Compression

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Apr 17, 2002
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#1
Having 1300 miles on my 1994 KDX 200 I thought is was time to check compression. According to my manual the minimum compression should be 119 psi. After removing the seat, gas tank, warming up the engine(with remainig gas in carb), and securing the new compression checker I found the highest psi to be 90. Are there other things that could cause the compression to be low (ie, inproper back pressure in silencer due to packing material, head gasket leak, etc.)? I was going remove the head, inspect the cyclinder/piston for unusual wear (repair if necessary) and replace the rings. Are KDX rings fairly generic when buying or is there a certain brand/type I should buy. Where is the best/cheapest place to buy rings? Thanks so much for your input,
mlhend1
 

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
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#2
At 1300 miles you must have 50 and 100 hours on the bike so I'm sure it's due for a top end. Just buy a Weisco pistion kit.
 

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
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#4
They are the most common replacement piston & rings. you can find Weisco products Online or at your local shop.

If you shop online, please use a DRN sponsor; www.mxsouth.com
 
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#6
It doesn't matter if it's cold or hot outside. When the engine is cold, turn the choke on and kick it over once, and its started. When warmed up, it takes also one kick, no choke. One of the best starting bikes I have owned!!! I wonder if the new Pressure gauge is defective.
 
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#7
I put a lot more miles on my '94 than that and never had any reason to check compression. I question your compression readings. Some KDXs run 3 times as much as you before replacing anything. Get a reply from others that know more than I, but I would suspect you can get by with just replacing the rings and not the piston, if you find you need to do anything.
 
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#8
You held the throttle wide open while kicking the bike over until it toped out on the gage, right? (Several times.) Also make sure the rubber seal was seated to the plug hole. Everything else you mentioned sounds correct.
Yes the gage could be bad, stranger things have happened.
tom
 
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#10
If you want to cheap out get the stock 1989 stock piston it’s a lot cheaper!!! And weighs 7 grams while the newer style is 7.3 grams. Jeff said the rings are all the same, the 89 piston has a lot smaller intake hole but it will work.

13008-1116 rings 2004-1989
13002-1053-piston pin 2004-1986
13001-1317 pistons for 2004 –1990
13001-1295 pistons for 1989
 

jdbrusch

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#11
Just curious what type compression tester do you have?,does it thread into the spark plug hole or do you just press a tapered rubber plug into the spark plug hole,the most accurate is the thread in type.Make sure the engine is up to temp and you kick the bike over several times while holding the throttle open as suggested by others already,1300 miles is extremely low on a 94 bike,if you suspect the rings and piston are worn take the top end apart and measure all the clearances or if you do not have the tools take them,with your shop manual to a engine shop and ask if they can do it for you.If its just the rings that are worn replace then,the stock piston is known to be a extremely durable unit,I got just over 9000km on my first piston and basicly took the top end apart each winter for kips cleaning and replacement of the rings.It sounds like the bike starts and run good.
Another possibility is the rings maybe gummed up or stuck in the grooves and just need to be freed and cleaned.
 
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#13
I warmed the bike up before testing. I have a $30 dollar unit from Advanced auto with threads, just a little bit shorter than the spark plug, and a rubber seal. I tightend the unit down snuggly. I also kicked the bike over 5 or 6 times with the throttle wide open, releasing the pressure between kicks, until I reach a peak pressure. I made sure the kick starter was under pressure at the top of the kick.
I tried to test my sons motorcycle (Honda XR 100), but the spark plug hole is about a 1/4 of the size of a typical "adult" size motorcycle. Is there another way to a pressure gauge?

Like I said before, the bike starts the very first time everytime, and kicks Ass from top to bottom. Wouldn't I see problems in these areas if the pressure was so low? Could the problem be related to improper silencer packing or something else?
 

jdbrusch

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#15
You mentioned you are releasing the pressure between kicks ???,if you are releasing the pressure on the tester between kicks then this is why you are getting such a low reading,you simply kick the bike several time until the highest pressure is reached and this is your reading,you only release the pressure when you are finished.As for your sons xr you should be able to get adapters that fit onto the gauge for different thread sizes. :thumb: