Compression

baggins1984 · Post by **baggins1984** » 02 Sep 2016, 07:40

Good Morning CRM fans,

Would someone be able to compression test their bike when cold for me? I've done a rebuild (no rebore) because the compression was at 120 but after the rebuild I'm only getting 125?

I don't know what mine was putting out when warm as I replaced the piston due to it not running and it still won't start.

Post by **fallenmikethebike** » 03 Sep 2016, 09:13

Have you turned the fuel on to help seal the bore? I understand you have other issues with the engine.
Is there a decent spark?
Is fuel getting through to the combustion chamber?
Mike

Post by **gregorious77** » 03 Sep 2016, 21:18

Just whipped the plug out of my AR and done some testing for ya.
130-135 psi (Fuel off)
150-155 psi (Fuel on)
Both tests at WOT and engine cold.
Fresh top end (bore,plate, piston, rings, bearing) just run in.

baggins1984 · Post by **baggins1984** » 04 Sep 2016, 07:58

Thanks gents. When the fuel is on I'm getting 125 so seems low compared.

Post by **knackeredMk1** » 04 Sep 2016, 18:27

Why didn't you do a rebore?

baggins1984 · Post by **baggins1984** » 06 Sep 2016, 20:22

There wasn't any scoring so didn't think it would be necessary. Should I have?

Post by **knackeredMk1** » 06 Sep 2016, 20:36

It's not necessarily scoring but wear. You should have measured the bore. BUT to be honest if you are replacing the piston and you have low compression you should have had a rebore.

baggins1984 · Post by **baggins1984** » 06 Sep 2016, 21:04

Yeh I'm wondering whether it's the wear that's causing the low compression. To be fair I did take it to a bike garage and the guy just told me to get the same size piston that I had removed.

baggins1984 · Post by **baggins1984** » 06 Sep 2016, 21:08

One more question, so the compression is about 125 cold. I know that's low but wouldn't it still start but not produce very much power? Mines not even hinting at a start

Post by **gregorious77** » 07 Sep 2016, 17:03

should still start I would say.
Need to check a load of things now:
Fuel getting to cylinder
Spark
Reeds (should still run unless non existent)
Timing (some have had woodruff keys fall out/shear meaning the rotor turns on the shaft)
That's about it really

baggins1984 · Post by **baggins1984** » 07 Sep 2016, 17:27

Yeh it's weird. All of the above are fine. I've fitted a new dep pipe aswell. It replaced an old dep pipe so it shouldn't need rejetting should it?

Post by **gregorious77** » 08 Sep 2016, 21:15

nope. when did it run last? did you try a bump start?

Post by **gregorious77** » 08 Sep 2016, 21:16

how old is fuel?

baggins1984 · Post by **baggins1984** » 10 Sep 2016, 07:15

I stripped it diwn this time last year, painted frame etc,fitted exhaust. Bike started fine but was running badly,bogging down when I tried to rev it. Was trying to sort that when it just cut out and I haven't been able to start it since.
Fuel isn't too old, I put fresh in after I had rebuilt the bike.
That's when I replaced the piston thinking that was the issue but still no joy.
Spark us fine, plug is wet when I do try and start it.reeds are fine.carb is clean as a whistle.

Post by **gregorious77** » 10 Sep 2016, 11:08

crank seals? you should do a leak down test really to check!
https://www.duncanracing.com/TechCenter ... sstest.pdf

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