 |
PM |  | |
Quote | | |
Reply | | |
Full Topic | |
| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
| Posts: | 573 |
| Status: |
Offline
|
Was nothing else done?... just a valve job? A good compression engine became a no compression engine after nothing but a valve job?
To do a proper compression test, the engine should be brought up to full normal operating temperature then shut down, remove all spark plugs except for the cylinder you are testing, hold the throttles wide open and crank the engine at a speed of at least 200 rpm. Failure to do any of those steps will result in low readings.
If the engine isn't operational, that makes reaching full operating temperature impractical. The test will result in low compression readings, but you'll just have to live with it for now. Cylinder to cylinder variations are more important for the moment than absolute pressures.
If the starter and/or battery are tired, they may not be capable of spinning the engine at 200 rpm. As above, carry on with the test, but the readings will be low.
Wide open throttle is not negotiable. A closed throttle test tells you very little.
I find it curious that 1 & 2 are low while 3 & 4 are still low, but at least have a reading. I'm wondering if you did the compression test with closed throttles (no air gets in, so very low pressures across the board), and if the carbs are not balanced? (the front carb is more closed than the rear carb? ).
Strombergs or Dellorts? Imbalanced Dellortos will result in a distinct difference between the 1-2 pressures and the 3-4 pressures. With Strombergs the balance tube in the intake manifold will mask carb imbalance at low rpm.
If carb balance and test methods are not issues, then...
Check the cam pulleys to be sure they are installed properly. .. the appropriate side of the pulley facing forward for each cam, and the correct timing marks aligned (some pulleys have multiple sets of marks).
With the crank set to TDC, the timing dot next to the IN mark on the front of the INtake cam pulley should align with the dot next to the EX mark on the front of the EXhaust cam pulley. The teeth the dots are on should align face to face, or be off by less than a tooth width (the belt can only be moved on the pulley in whole tooth increments).
If the proper dots are on the centerline between drawn through pulleys, but on far opposite sides of the pulleys, then turn the engine through one more revolution and back to TDC. Now the proper timing dots should align. If they are off by more than a whole tooth increment, then re-time the cams.
Check the valve clearances. Did you re-shim the valves? It's almost a lead pipe certainty that they would have to be re-shimmed after a valve job. If you have negative clearances on any cylinders, those valves will not seal and pressures will be low to non-existant.
Shim all valves to the high end of their ranges. .005" - .007" for intakes and .010" - .012" for exhausts. The clearances tend to close up with wear, so starting at the top will give the most miles before the job valves must be re-shimmed. High or low within range won't significantly affect the compression pressures.
Good luck, Tim Engel