L-79 timing / idle problem - NCRS Discussion Boards

L-79 timing / idle problem

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  • Rick K.
    Expired
    • December 1, 1996
    • 15

    L-79 timing / idle problem

    I have a 1966 327/350 Vette with 4spd & currently running a Edelbrock 1405 without a choke. The engine was rebuilt (26 yrs. & 28k miles ago) and was bored .030 over with an OE replacement cam installed. The distributor is single point type with <.010 end play and a new VC-1810 (B-28) vac. can.The car has developed a banging noise & virbration at start-up. As the car warms and/or the rpm exceeds 950-1,000 the noise subsides.

    Timing is set at 12* BTDC / 800 rpm / vac plugged (timing will fluctuate 2* while running). Advance is all in at 2200 rpm & 34*. Vacuum at idle is 18-19" (full-manifold) and steady except for a very faint flutter (about 1/2") that seems to coincide with the timing change. On quick acceleration the vac. drops briefly to about 3" & then back to 23". Settling back to 18-19" on de-acceleration. I have used carb. cleaner to look for vacuum leaks around the intake but have not found any.

    Valve lash has been set to 3/4 turn past zero. Engine cold.

    Is there something obvious here that I have missed ?
    Last edited by Rick K.; July 15, 2008, 06:11 AM.
  • Timothy B.
    Extremely Frequent Poster
    • April 30, 1983
    • 5186

    #2
    Re: L-79 timing / idle problem

    Rick,

    Are you saying centrifugal timing at 2200 is 34* or is this taking into account initial as well as centrifugal? Could you be hearing exhaust leak or loose rocker arm, may be time to remove valve cover for a look.

    If this engine has original L79 cam, no way idle vacuum is 18-19. Sounds like 300 HP cam. My experence with L79 is 750 RPM idle at 14" vacuum.

    Comment

    • Duke W.
      Beyond Control Poster
      • January 1, 1993
      • 15667

      #3
      Re: L-79 timing / idle problem

      I agree with Tim. It's not an OE L-79 cam. In addition the carb is non-OE and the spark advance map has been considerably altered from OE.

      These non-OE issues may or may not be a source of the problem, and beyond this your description of the problem is too vague to be of much diagnostic use.

      The best thing you can do is get your head into the engine compartment at start-up and try to determine the source of the noise.

      Duke

      Comment

      • Tim S.
        Very Frequent User
        • May 31, 1990
        • 704

        #4
        Re: L-79 timing / idle problem

        Is the noise at crankshaft speed or at camshaft speed? If you take a timing light to it and the sound is consistent with the crank you have a bottom end problem. If you think it sounds half as often as the timing light flashes, you have a valvetrain problem.

        Comment

        • Joe C.
          Expired
          • August 31, 1999
          • 4598

          #5
          Re: L-79 timing / idle problem

          Originally posted by Rick Knight (28484)
          I have a 1966 327/350 Vette with 4spd & currently running a Edelbrock 1405 without a choke. The engine was rebuilt (26 yrs. & 28k miles ago) and was bored .030 over with an OE replacement cam installed. The distributor is single point type with <.010 end play and a new VC-1810 (B-28) vac. can.The car has developed a banging noise & virbration at start-up. As the car warms and/or the rpm exceeds 950-1,000 the noise subsides.

          Timing is set at 12* BTDC / 800 rpm / vac plugged (timing will fluctuate 2* while running). Advance is all in at 2200 rpm & 34*. Vacuum at idle is 18-19" (full-manifold) and steady except for a very faint flutter (about 1/2") that seems to coincide with the timing change. On quick acceleration the vac. drops briefly to about 3" & then back to 23". Settling back to 18-19" on de-acceleration. I have used carb. cleaner to look for vacuum leaks around the intake but have not found any.

          Valve lash has been set to 3/4 turn past zero. Engine cold.

          Is there something obvious here that I have missed ?
          Rick,

          Welcome aboard!
          Please describe the "banging noise" and "vibration" in as much detail as you can.
          Goes away when warmed up could be piston slap.
          Unfortunately, you don't know enough about the internal components of your engine to know for sure. Agree that your idle vac is 4-5 points too high for an original or reproduction (cloned) 151 cam. Do you know whether you have forged or cast pistons?

          Joe

          Comment

          • Joe C.
            Expired
            • August 31, 1999
            • 4598

            #6
            Re: L-79 timing / idle problem

            Originally posted by Tim Schuetz (17356)
            Is the noise at crankshaft speed or at camshaft speed? If you take a timing light to it and the sound is consistent with the crank you have a bottom end problem. If you think it sounds half as often as the timing light flashes, you have a valvetrain problem.
            Tim:
            Timing light flashes every 2 crank revs.......same as bumpstick.
            Joe

            Comment

            • Rick K.
              Expired
              • December 1, 1996
              • 15

              #7
              Re: L-79 timing / idle problem

              Thanks for the input everyone. I willl pull the records on the car and reply when I have the info. at hand.

              Comment

              • Rick K.
                Expired
                • December 1, 1996
                • 15

                #8
                Re: L-79 timing / idle problem

                The only source for info. on my cam & pistons is a 26yr.old garage invoice that describes the cam as P.N.122052 & the pistons as P.N. LZ165F030.
                The noise & vibration at start up sounds like a "dull metallic knocking" sort of like an exhaust pipe rattle. The noise seems to only be on the right side (I can actually hold the right sidepipe and the noise subsides). Both the noise & vibration will subside at above 900rpm and/or the engine has heated up. If you listen at the right sidepipe the noise sounds sort of like a burnt valve type of noise.

                I thought the vacuum was high also.

                I have reworked the right side pipe , changed heat riser, tried B-1/B-28/ B-20 vacuum cans , checked compression, the plugs look good, checked valve springs & checked rocker arms. The car run strong as ever & starts very easy.

                I hope this info. helps as I have checked every thing I that I think could be causing this. This past week the car had as many as 4 test instruments on it at the same time & I have checked every service manual I own.

                Comment

                • Joe C.
                  Expired
                  • August 31, 1999
                  • 4598

                  #9
                  Re: L-79 timing / idle problem

                  Very possible that it's the sidepipe inner tube rattling. True especially if they are more than a few years old. Some sidepipes will resonate, and almost sound like detonation during cruising........especially when sound is reflected off of a neighboring car, guard rail, viaduct, etc.
                  First, try wiring the heat riser wide open, and be sure that it is tight. Next, temporarily disconnect the sidepipe from the manifold.

                  Joe

                  Comment

                  • Timothy B.
                    Extremely Frequent Poster
                    • April 30, 1983
                    • 5186

                    #10
                    Re: L-79 timing / idle problem

                    Rick, L2165 is replacement piston for 300 HP car, it is flat top with four valve reliefs. Look at you heat riser for the anti rattle spring for the noise.

                    Consider changing the vacuum advance to B22 which matches you engine idle vacuum better. Check archives for more info.

                    Comment

                    • Joe C.
                      Expired
                      • August 31, 1999
                      • 4598

                      #11
                      Re: L-79 timing / idle problem

                      Originally posted by Timothy Barbieri (6542)
                      Rick, L2165 is replacement piston for 300 HP car, it is flat top with four valve reliefs. Look at you heat riser for the anti rattle spring for the noise.

                      Consider changing the vacuum advance to B22 which matches you engine idle vacuum better. Check archives for more info.
                      I see................... "LZ165F030" is translated as "L2165F, .030" over". If so, then it is a FORGED clone of the cast flattop piston piston originally fitted to 250/300 HP engines. Piston slap is a therefore a possibility.

                      Joe
                      Last edited by Joe C.; July 16, 2008, 09:26 AM.

                      Comment

                      • Duke W.
                        Beyond Control Poster
                        • January 1, 1993
                        • 15667

                        #12
                        Re: L-79 timing / idle problem

                        Even though the 2165 is forged, the recommended installation clearance is only .0015", so there should not be any noticeable slap if properly installed.

                        I don't recommend this piston for any application. The compression height is a few thou less than OE and it will not provide sufficient compression in most applications.

                        For a 300 HP rebuild I recommend Federal Mogul or Keith Black flattop hypereutectics that should yield 9.5-9.75 true CR with nominal block deck height, chamber volume, and suitable head gasket selection.

                        For L-79s I recommend the L2166F domed OE replacement piston, with a target CR range of 10-10.25:1, which should be achieveable as above with suitable head gasket selection.

                        Duke

                        Comment

                        • Rick K.
                          Expired
                          • December 1, 1996
                          • 15

                          #13
                          Re: L-79 timing / idle problem

                          Thanks again for everyone's input & information. Tomorrow I will change the vac. can to VC1802 & continue chasing this problem down.
                          I have tried 2 different heat risers & even a spacer that eliminates the heat riser.
                          I played with valve adjustments today. I think part of this problem could be valve related on the right side. I also think the problem is multi-faceted.
                          The side pipes are 5 yr old Allen's. Very LOUD. I may cobble up a temporary exhaust to help eliminate this part of the problem.
                          Thanks again.

                          Comment

                          • Timothy B.
                            Extremely Frequent Poster
                            • April 30, 1983
                            • 5186

                            #14
                            Re: L-79 timing / idle problem

                            Duke, I thought 2165 compression height is the same as OE for 300HP (1.671). The compression height of the 2166 piston is more (1.675), maybe I got it wrong.

                            Comment

                            • Duke W.
                              Beyond Control Poster
                              • January 1, 1993
                              • 15667

                              #15
                              Re: L-79 timing / idle problem

                              All the OE 327 pistons have a nominal compression height of 1.675", which yields nominal deck clearance of .025" with the nominal 9.025" block deck height. The 2165s are 1.671" as you stated and the 2166s are 1.675" since they are essentially exact replacements for the OE domed pistons. Keith Blacks are typically a little higher - like 1.678".

                              The mfgs. compression height specification should always be verfiied because even a few thou can have a meaningful effect on compression ratio when you are targeting a specific value or narrow target range.

                              Duke

                              Comment

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