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930 runs uneven on steady rpm

Hi.
I have a friend that has a 930 with 26000 miles that run uneven when steady rpm on any gear. As soon as you give it throttle it runs normal. After ignition adjust it improved, but not disappeared. The rpm is as well a bit low on idle, at about 700 when warm and cold...

Any hints ideas?

Regards,
Roar
Old 10-14-2011, 11:53 AM
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Uneven...as in missing? Or like surging? Can you be more descriptive?

May be just mixture related. Try leaning it out just a little bit by turning the mixture screw counterclockwise a degree or two. If you're running too rich, that should cause your idle to rise. If it causes the opposite, then it was running too lean to start with and you'll want to richen it up accordingly. Remember, just very small adjustments. You'll need a 3mm allen wrench to do it. Once you found the best idle, then finish up with the idle adjustment screw to get the idle up to 900'ish.

These cars typically run better on the rich side, BTW.
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 10-14-2011, 01:11 PM
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Sounds similar to a problem I chased for a while that I would describe as a part throttle miss. It only happened while cruising at steady rpm with very light throttle. In my case, timing light revealed ignition was too advanced.
Old 10-14-2011, 01:37 PM
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when you adjusted the timing, did you notice if the timing was solid or was it wandering around a bit. I had similar issues on my SC and it was the distributor that was loose so if I would hold the timing light on the mark at steady RPM (even idle), the timing would hold for a while, jump to another value for a while and do that back and forth. A new distributor fixed it.
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David
1985 Black 930, K27 7006, Garretson Intercooler, Adj boost, TB: 23 - 31mm, bielstein sport shocks, ER polybronze bushings front and rear, ER monoball joints front and rear with offset camber plates, 935 X triangulated strut brace, raised spindles, racing bump steer kit
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Old 10-14-2011, 02:02 PM
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Hei Roar,
Have you verified the AFR?
First verify correct mixture, check timing with timing light.
Could also be a air leak in the inlet.
I had similar problem on my US imported 930. When I removed all the air pump stuff, it was stable
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Trond R.
1979 930: Garret GT35r turbo, EFI, carerra intake, Link EMS, custom GT2 cams, 98mm JE P/C, 964 crank (stroker), custom valves & ported (XtremeCylinderHeads) etc..etc..
1972 914-6 GT replica project
1986 944 Turbo
Old 10-14-2011, 02:43 PM
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Hei Roar,
Have you verified the AFR?
First verify correct mixture, check timing with timing light.
Could also be a air leak in the inlet.
I had similar problem on my US imported 930. When I removed all the air pump stuff, it was stable

BTW, how is your 959 project going?
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Trond R.
1979 930: Garret GT35r turbo, EFI, carerra intake, Link EMS, custom GT2 cams, 98mm JE P/C, 964 crank (stroker), custom valves & ported (XtremeCylinderHeads) etc..etc..
1972 914-6 GT replica project
1986 944 Turbo
Old 10-14-2011, 02:48 PM
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What year? My '87 did this and I'm convinced it is the lambda system causing the mixture to cycle between rich and lean. The key test for me was to use a dwell meter and adjust the idle mixture so that the frequency valve duty cycle was averaging 50% at cruise RPM. With the O2 sensor connected the frequency valve duty cycle bounces around between ~45 and 55% with about a 1-2 second period that matches my light surge or chug period. Replacing the O2 sensor with a brand new one didn't affect anything

Unplugging the O2 sensor causes the frequency valve to run at constant 50%, giving the same averge fuel mixture, just no lean/rich cycling coming from the lambda system. This pretty much eliminated the surging.

On the advice of Mark and others I set my idle mixture at 2.5% CO, after removing my cat... for testing purposes. It runs infinitely smoother in this test condition.

Incidentally, recent emissions testing without the cat but with the O2 sensor hooked up measured 0.6% CO and 34 ppm HC. Definitely on the lean side.
Old 10-14-2011, 03:48 PM
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My car was doing this also. Turned out to be a leaking cold start valve.

You can check by blanking off the line at the fuel distributor by substituting a solid bolt (m 8 x 1, 15 mm long) for the banjo bolt. This will stop the flow of fuel to your cold start valve. If, after doing this, your idle stabilizes, change the cold start valve.
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Last edited by 911nut; 10-15-2011 at 05:40 AM..
Old 10-15-2011, 04:07 AM
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thanks a lot for all the replies.
I will forward them to my friend with the problem

The 959 is still in pieces to say it that way, neither was Rome built in one day
Hopefully the chassis can be sent away before xmas, though no stress....

Regards,
Roar
Old 10-16-2011, 01:06 PM
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