fluctuating tach

Old 01-17-2008, 01:20 PM
  #1  
bigstu
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Default fluctuating tach

hello fellows, I've a question/ concern. I only have 4 - 1/4 mile passes in my camaro and I'm still working the bugs out. I've got 1 problem that I'm concerned about. Above 7000 rpm my tach flutters a lot, causing it to bounce off the rev limiter much sooner than I expect it to(7600rpm). (555ci dart cnc 355's, hp unknown est. 900, t-400, 9" 5000 stall conv - 33.5 inch tire/ 4.88 gear - 3300 lb w/ me in it). once I turned the dial up to 8000 on the my last pass, it still fluttered, but not high enough to hit the limit .It pulled strong thru the traps and recorded my quickest pass to date which is 9.41 @ 142.??. Now, the 60ft was a 1.35 spinning with a soft 3000rpm launch off the t-brake with old slicks. I have yet to really turn it loose, due to other concerns.

Now, the tach is an autogage that was in the car when I got it and my 1st thought was to replace that and go from there. but a friend mentioned it could be bad valve springs. Now that made me think - the tach is only showing the signal that it receives from the msd, is that right?

the springs are comp cams 947's that were not new when I got the heads which came off of a 572 that had been out 9 times, maybe 50 passes. So far, it's a tough read for me.

What are your thoughts?
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Old 01-17-2008, 02:32 PM
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v8Fiero
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whats your ignition setup? locked timing?

When I had an HEI crap cap dissy in my fiero my tach would jump quite a bit when vibrations got bad, idle and high rpm. after the 3rd pass it got worse so I looked and the tach output wire was loose on the dist, once I got back home I re-soldered it and it hasnt had a problem since. you might want to check the connections between your rev limiter (6al?) and your dist.

if you ran your best time to date with that problem I dont think it would be a mechanical problem
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Old 01-18-2008, 10:50 AM
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bigstu
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It's a msd distributor with a 7al2 and 3 step with the selector knobs, not rpm chips. I'll double check the connections. Unfortunately, here up north the tracks don't open 'til april so I won't get the opportunity to try it 'til then. thanks
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Old 01-18-2008, 11:36 AM
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Your tach shouldn't have anything to do with the rev limiter. Even bouncing it should only be showing rpm. You might check the resistance of your high side limiter and see if it is wrong. I have a chart someone sent me showing the resistance values compared to rpm I can email you if you need it.

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Old 01-18-2008, 01:13 PM
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bjuice
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if you had a bad valve spring ( broke) i DO NOT think you would just see it in the tach ..but also hear and feel it in the way your motor runs,sounds etc...

i will say the tach has a bad ground,or some sort of electrical problems with the tach..even the tach could be bad....they do not live forever...before i would go tearing stuff apart or assuming the worse..get ( borrow ) another tach and plug it up..if you got the same problem then go from there...try to keep it as simple as possible from the start....from all my years experience most major problems turn out to be minor ones...
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Old 01-18-2008, 01:35 PM
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hammertime
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Originally Posted by bjuice
if you had a bad valae spring ( broke) i DO NOT think you would just see it in the tach ..but also hear and feel it in the way your motor runs,sounds etc...

i will say the tach has a bad ground,or some sort of electrical problems with the tach..even the tach could be bad....they do not live forever...before i would go tearing stuff apart or assuming the worse..get ( borrow ) another tach and plug it up..if you got the same problem then go from there...try to keep it as simple as possible from the start....from all my years experience most major problems turn out to be minor ones...
I am with Brian, sounds like its electrical to me. I had this happen last year and ended up being a loose connection. It also could be ignition.
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Old 01-18-2008, 02:13 PM
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lookingaround
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i bought a autogage tach back last year and it says your not suppose to use them with msd boxes. ill go to my bosses performance shop tomorrow and double check but i dont know if it would make your car miss though.or Ron Owens could probably check if he carries them.
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Old 01-18-2008, 03:14 PM
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bigs1975
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We had one do the same thing with a autogauge tach on a sbc and it turned out to be a ground.The body had no ground from the motor or chassis and once we added the ground problem went away.I run an MSD with a Autogauge tach and have not had any problems.Double check grounds and the resistance values for you high speed limiter.It could be like a bad pill but in the knob function.
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Old 01-19-2008, 04:44 AM
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caprice327
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Could be the canootin valve overhead pressure relay switch.
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Old 01-19-2008, 06:37 AM
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autogage is made by autometer isn't it?
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