Boost ports and compression.

[Dale Fisher]

I'm having difficulty getting a good compression reading on a EC boost ported cylinder.  New .80 over piston, rings, and super light crosshatch.  Would the boosts effect anything?  Next trip out is for a new torque wrench, the old one with the bending bar has probably seen it's last days as well.

[firstturn]

Dale,
  You should be able to get a good reading if you have no leaks.  I have probably owned ever kind of ported Sachs cylinder know to mankind and never had a problem getting a reading if everything was sealed tight.

Ron Carbaugh

[454MRW]

Don't forget to open the throttle all the way when doing a compression test. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S

[joe novak]

Please list the compression reading which you received.  What do you expect the compression to be ideally?   The compression on a newly rebuilt top end will not be optimum until after the break-in period.  Anyone want to share their method of proper top-end break-in?

[David Laite]

I had this discussion today with a buddy who works on turbines, getting compression up after a piston engine rebuild. As Joe stated we hone the cylinders in order to aid in breaking in the rings and get a good seal but what if there isn't enough compression for the engine to start initially? A couple of squirts of oil into the cylinder will usually seal it up in order to develop enough compression to start.

A recent post over in the CRF forum where a rebuilt 250R four stroke refused to start, compression readings in the 50's. I recommended this procedure and the Honda started, smoked for 20 minutes, then settled in so it is quite effective.

1973 Penton Six Day
1973 Penton Jackpiner
1982 Yamaha XT200
1982 Yamaha XJ650J Maxim
1984 Husqvarna 400WR
1987 Yamaha YZ490
2005 Honda CRF450R

[Dale Fisher]

I've had up to a 90 p.s.i. reading, throttle open.  Reasonable feel on the kickstart (I've only had the 125 for comparison).  Initial start I let it idle for about 15 minutes.  Been out for a couple of slow 15-20 minute rides around the yard.  No visible signs of leakage around top or bottom gaskets.  Seems pretty snappy (again I know I'm giving up 5 hp and I've only ridden one other 100).  It may be that the tester connection is the culprit.  Will check the bolt torque with a new wrench first and then take a shot at a more secure connection.

[brian kirby]

My 100 has 150psi, 90 is definitely too low.

I break in a new engine by heat cycling it several times. Start it and get it to operating temp then let it cool completely. Do this 3-5 times, then put 15-20 min of some easy riding and it should be ready to go. Right after break-in it had 150psi and after 1-1/2 seasons of getting flogged like a rented mule, it still has 150psi.

Brian

'72 Berkshire

[gooddirt]

Make sure you do not have a 125cc head installed .[:0]

[brian kirby]

Also make sure you dont have the gasket backwards.

Brian

'72 Berkshire

[Dale Fisher]

Brian,
Just when I resigned myself to believe it's the junk Autozone $34.95 compression tester.  Put in on the Cheney 125 and now also on the  Triumph 500.  All three register 90-95 lbs., I'm thinking there's a pattern here.  Haven't wrenched an engine for over 35 years and I apoligize for agonizing over what probably is not an issue.  After another half an hour of putzing and another heat and cool down I opened her up.  She seems to scoot well once up on the pipe.
My belief is the head is correct.
Brian you got me on the gasket though.  Both aligned properly, not sure if head gasket is metal side up or down though if that is what you are referring to.

[firstturn]

Dale,
  Attach your gauge to a known source of PSI like a compressor and see what it reads?  Just try and eliminate any problems with the gauge before you move on.

Ron Carbaugh

[tomale]

So what is the reason for doing a compression test witht the trottle wide open?

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[454MRW]

An engine is in essence an air pump. Any restriction of airflow in or out will minimize the results. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S

[tomale]

I never thought of it like that... thanks

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)