L-82 Tuning Question

Discussion in 'Troubleshooting & Diagnosis' started by Greg73Z28, Apr 17, 2009.

  1. Greg73Z28

    Greg73Z28 Veteran Member Lifetime Gold Member

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    Hello All. I have just completed a rebuild on the stock L-82 in my 73 Z28. I am having trouble getting the engine to idle smooth and consistent. Does anyone know how much manifold vacuum I should be getting at idle in gear? Also, the GM book calls for valve adjustment to be 1 full turn past zero lash, which sounds awful tight to me. I have them set a zero lash right now which could be partially responsible for low vacuum readings I am getting (5-6" Hg in drive @750-800 rpm). Motor is numbers matching and bone stock, GM factory spec cam, valves, ect. Only variation is .060 over on the cylinders. I have extensively searched for and eliminated any vacuum leaks that I can find. I have found that the vac advance on the distributor is wrong (requires too much vacuum to operate) which I am replacing this weekend. Just trying to eliminate all possibilities before looking to the carb.

    Thanks in advance for any help. Even though I am newly registered to this site, I have been using and reading it for years. Hands down, the best site out there (not just Camaro sites, ANY site). Your tecnical info has been invaluable during this frame up. Keep up the good work.
     
  2. Greg73Z28

    Greg73Z28 Veteran Member Lifetime Gold Member

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    I'm not much on the book either. Timing is stable at idle and I have installed a Pertonix unit in the distributor to eliminate the points and the dwell. When I install the new vac advance unit, I am going to check the springs. I agree that 6" seems low, however the correct vac advance unit opens at 7" which is a lot lower than a standard 350 motor. As for the Advance vacuum source, I originally had it hooked up to a ported hole on the carb, like the factory did. I tried it on manifold vacuum and it actually helped considerably with throttle response.
     
  3. danbrennan

    danbrennan Veteran Member Lifetime Gold Member

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    When I put an L82 cam in my '74 back in the '70s, I got about 18 inches of mercury at about a 750 RPM idle speed.
     
  4. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

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    You definetly have a problem. The L82 is a very mild engine, and the cam is not particularly aggressive. A 355 at 750 RPM should have at least 18" vac. I've gotten 17" out of the 350/327 cam and that's a bit lumpier. If using stock llifters, go 1/2 to 3/4 preload when hot. Do it while the engine is running. Loosen untill it starts to clatter, adjust 'till it stops...then down 1/4, wait untill idle stabilizes, then down another 1/4 etc...If you zero load the lifter, you have no ability to take up any clearance due to wear or low oil pressure. Preload has NO effect on llift in a hydraulic lifter. First, find that leak! You have a big one someplace. Either that or your timing is waaaaay out.
     
  5. danbrennan

    danbrennan Veteran Member Lifetime Gold Member

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    So the '60s 151 cam is lumpier than the '70s L82 cam? I had been wondering about that. I put the 151 cam in my '78, and have been having trouble getting more than 15 inches vacuum Hg at idle with it, plus the idle does seem to lope, whereas the L82 in my '74 never really did.

    I had a broken rocker stud on my '78 last fall, intake if I remember right, and that was making a mess of the vacuum reading - bouncing all over the place, and couldn't get it above 10 inches.
     
  6. Greg73Z28

    Greg73Z28 Veteran Member Lifetime Gold Member

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    I was thinking that these readings were low, even for the hi performance motor. Any thoughts on where to look for vaccume leaks? What has been done so far is I have tested the tranny modulator, brake booster (new) vac. advance and charcoal cannister. All hold vacuum except the cannister cap where the vac lines plug in was cracked and is for now capped off. Heads were pressure tested & magnafluxed and carb professionally done (stock quad). Intake manifold?? Maybe a wrong base gasket inside the carb? I don't think the timing is off, at least not by a lot. Set at 8* & runs out at full advance at 42-43*. No cruise or AC circuits to worry about. Running out of ideas on where to look.
     
  7. danbrennan

    danbrennan Veteran Member Lifetime Gold Member

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    Some people have had luck spraying Gumout around the intake manifold and carb flange, to check for leaks - the engine speed will flare if there's a leak. I think a Propane gas cylinder works for that, too.

    A plugged exhaust system will also give a low vacuum reading.
     
  8. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

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    Yes, the old 151 cam is lumpier than the L-82 cam. 15" with that cam is ok depending on your manifold choice, compresson etc...to look for leaks, use a propane torch (unlit please), turn it on and then point it at potential leaks. The gas gets drawn in and the idle flares up. When that happens, you found the leak. I would suspect intake manifold runner leak into the lifter valley otherwise you'd hear the hissing off a smaller leak. Carb cleaner also works as a good source to find leaks.
     
  9. danbrennan

    danbrennan Veteran Member Lifetime Gold Member

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    Also, how's the cranking compression? No chance of the cam/crank timing wheels being a tooth off?
     
  10. Greg73Z28

    Greg73Z28 Veteran Member Lifetime Gold Member

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    I don't think so but I have thought of that. What is concerning me is that this is the same problem I had before the rebuild and I'm concerned that they missed a crack or something like that. I sure hope not though considering everything is matching numbers.

    Thanks for all the ideas. Keep em comming. I start searching on Monday.
     

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