924Board.org

Sharkman · Joined: 18 Oct 2012 Posts: 7 Location: Central IL

Any help would be much appreciated. Back in august I bought my first Porsche a running 1980 931 for $900, which was in need of a little tlc, primary a giant dent repair, new tires, new shift linkage bushings, and electrical connected and ground cleaned. Well the dent is still there, and the gear changes are grindy if not shigted right but the tires are new( the car's suspension is in great condition so it has i nice ride ) and almost all of the electrical issues have been resolved; Except one. At about 3000-3100 rpm under partial throttle the tack will jump any were for 250-1000rpm and then bounce around at the incorrect reading. There is no jump in engine speed, but the is a very slight hiccup/miss (only at the initial jump, and the overtones the engine is producing change. The same jump will occur if I down shift and the rpms are around 3100, but once the actually the engine speed drops below 3100 the tach returns to normal function. It only slightly flicks at 3100 rpm when running wide open throttle

Also the previous owner had the timing belt, cap, rotor, plugs, and wires changed. I changed the plugs this summer as the plugs that were in it were improperly gaped. all the contacts on the ignition module have been cleaned along all of the ground i could find. A second ground for the ground at the batter to the engine block have been made.
It is also running 9-10 psi of boost(I made no modes, nor did the previous owner) , Ignition timing is 25 btdc @ 2000 rpm, and there is not cat, only a glasspak!

Thanks for the help, and sorry for any typos

Chris S.
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1980 931

1972 MGB (father son project)

1982 Datsun 280zx turbo(sold)

1978 Ford 250 W/ 460

Paul · Posted: Sun Oct 28, 2012 1:02 pm Post subject:

I'd try another tach.
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

kcoyle · Joined: 15 Jan 2011 Posts: 712 Location: Long Island, NY

Dude, you avatar is gi-normous.
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1982 931- Stock with MBC at 8psi

Relax, all right? My old man is a television repairman, he's got this ultimate set of tools. I can fix it.

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Joes924Racer · Posted: Mon Oct 29, 2012 1:52 pm Post subject:

Car looks great though.. hmm i was under the impression the board wouldnt let that big of a avitar...hmmm....oh and try another tach
and my feeling is it could be a ground maybe.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

peterld · Posted: Mon Oct 29, 2012 9:17 pm Post subject:

Bouncing Tach Saga.............Try the electrical connections (wiring).....follow through from the back of the tach to the motor....check all grounds.
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80/81 932/8 ROW

Cedric · Joined: 27 Aug 2004 Posts: 2806 Location: Sweden

Hi! Nice car!

If you can get hold of an ignition module it can be worth a try. Sounds very much like a problem i had, although with s2 engine and thus ditc ignition. It started with a flimsy tach and ended with starting problems. The module on the ditc costed peanuts/free, guess the s1 version is more rare..
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1980 924 Turbo
www.instagram.com/garagecedric/

Sharkman · Joined: 18 Oct 2012 Posts: 7 Location: Central IL

Thank you guys for the responses, and thank you Rasta for the non-ginormous avatar. This week/weekend I will try out an aftermarket tach I have. I have cleaned, replaced and tightened as many of the grounds as possible, but It seems ever time I fix one I find that another on is causing another problem.
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1980 931

1972 MGB (father son project)

1982 Datsun 280zx turbo(sold)

1978 Ford 250 W/ 460

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

check the ground wire coming from the alternator and going to the intake manifold.
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Supercharger and EFI kits
https://www.the924.com

Sharkman · Joined: 18 Oct 2012 Posts: 7 Location: Central IL

So after many months I finally got to hook up a aftermarket tach and run it side by side with my stock tach. The aftermarket was hooked up directly to the coil. Both bounced and had the same issues. I have checked and repaired all the grounds I could find (did that when/before I made the first post). Any other ideas of what could cause the issue. Could a bad diaphragm or part in the vacuum advance/ retard cause this? I know it works some what, the other day I removed the ignition dizzy cap and applied vacuum to both barbs on the dizzy and watched the hall sensor move each time. I also have not had the chance to acquire a Ignition module yet.

Once again thanks for the help.
_________________
1980 931

1972 MGB (father son project)

1982 Datsun 280zx turbo(sold)

1978 Ford 250 W/ 460

Sharkman · Joined: 18 Oct 2012 Posts: 7 Location: Central IL

Any one there, has my thread been lost?
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1980 931

1972 MGB (father son project)

1982 Datsun 280zx turbo(sold)

1978 Ford 250 W/ 460

Buwani 931 · Joined: 31 May 2004 Posts: 308 Location: Napa California

I would double check the connectors on the ignition unit. Not just for clean but for tight. Perhaps even the coil itself.
Shawn
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1980 931
That's thirty minutes away. I'll be there in ten.

Sharkman · Joined: 18 Oct 2012 Posts: 7 Location: Central IL

Thank you Shawn. I will try to tighten the connectors( I have done this on other parts of the car but I don't think I did it on the Ignition Unit.) then I will try at new coil, or may be even a msd or crane cams ignition unit.
_________________
1980 931

1972 MGB (father son project)

1982 Datsun 280zx turbo(sold)

1978 Ford 250 W/ 460