Low compression, or do I?

[jimspeed]

Ok bear with me I'm used to nice simple car engines..
In my quest to get this GPZ400R running right I have compression tested it , I only have the 500r manual but am thinking its not going to be far different.
I got with a hot motor and open throttles
70 psi across all of them
now the manual states that the 500 should be between 105-145psi at 500RPM
I'm guessing that the starter is not going to give that sort or rpm tho?
they are all the same and oil does not effect the readings,
she runs really nice now on light throttle but still will not go on anything over 1/4 throttle just runs at 4k with the throttle wide open .
I'm thinking that the cams may be different in the 400r to the 500r which may give a lower copmression reading (in cars if you fit a "race cam" you often find the compression readings get lower due to bigger overlap etc).
starts on the button and runs tidy even from stone cold which is kinda making me think the compression is ok
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[Frost]

So on a little throttle is will rev to the rev line and pull just fine, but on 1/4 throttle it bogs down?
Or Won't rev past 4k no matter what?

The starter motor will easily get it to 500rpm and beyond if you take the other plug out as then it's only having to fight the compression on one cylinder.

70psi is low, on the verge of not running kinda low so it might explain things. It might also indicate that the compression tester isn't a good fit or seal. Try greasing up the thread to help it seal.

[jimspeed]

she will rev to about 10k on light throttle mate but as soon as it starts to lift the slides up it only goes to 4k, 4 cyl all the plugs were out i'm thinking it may be the tester as I borrowed it as our's does not have 12mm fittings,
bloody cvk carbs I'm thinking now as it will rev if put a hand over each carb while its running so it sucks some fuel out of the bowl into the inlet, it will rev on full throttle for a second while it uses it up.
she was a non runner when i got her so its getting better.
picking another set of carbs up tomorrow so will try again.
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[Ariel Badger]

You comp tested with the starter, did you have the throttle open wide?
A closed throttle will restrict the amount of air that gets to the cylinder and thus lower the ratio.
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Bikers make great organ donors, get 115 on your licence today.

[jimspeed]

throttle wide open, car battery connected as the bike one is dead and I'm not buying one till I know it is fine,
it really does run too well to be that far down on compression tho (most modern car engines will missfire with 100psi, VW's suffer sticky valves done loads now)
Thinking alout it has to be too lean on the mains somehow (can't see any no's on the jets or adjustment on the needles) she revs if I "flood" all 4 carbs , I'll vid it tomorrow
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[jimspeed]

Ok just had another trawy thru the interwebs and I found that the needle adjustment is done via a shim under the needle (typical now I'm home and can't check it) I guess tomorrow lunch times job is to have a look, make something fit to try it
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[Frost]

Idle = Pilot jets
Full throttle = main jets
Everything inbetween is the needles.

So check the needle sizes, heights, diaphrams etc. Also just the mixture screws along with carb syncronisation.

If the bike is a known non runner i'd drain the tank and carbs, give them a good scrubbing clean.

It's possible the problem could be top end timing out of sync letting a valve open early.

[timsmith]

Don't forget that with CV type carbs the slide will stay down when you test compression even though the throttle is wide open. Have you checked the diaphragms for any splits / holes as this can give the problem you have?
Good luck
Tim

[jimspeed]

got another set of carbs today so I can check jet sizes etc, she will rev if I really flood it by putting my hand over the carbs to suck fuel from the chambers,
cam timing is correct and its running from a gallon can at the mo as seat and tank are off it.
____________________
Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[TQ]

[Frost]

Actually the overlap between them is very big, but i quoted the extremes as they are what you usually use for fault finding. If the bike runs fine on full or no throttle, but messes up inbetween then it's needles as a problem with the pilots or mains would give a noticeable problem at one of the extremes.

[jimspeed]

sod it its going on ebay as it is I really wanted to get it running nice first and mot'd but I don't really have the time to spend on her,
carbs are clean and balanced ,the floats are spot on and i've fixed everything else that was wrong with it so just going to put the tank and panels back on and stick on the bay (I may just try it to see if its any better under load tho).
If anyone fancy's a cheap GPZ mail me
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[prawny1]

Are you trying to run the bike up with the airbox on or off?

Cv carbs wont operate correctly without an airbox fitted unless you change slide springs and emultion tubes etc

They run exactly as you describe if you try full throttle with the airbox off. You can try popping a couple of matchsticks in the mainjet airholes to force the carbs to run richer with the box off just make sure you don't break them off
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hellkat wrote:

Some people are only alive cos its against the law to kill them.

[jimspeed]

Tried both the airbox on and off mate, But the airbox has a broken top and I think this may not be helping. I have found someone who has a top and its on its way to me now then I'm going to try it up the road and see if its any better or not.

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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"