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Registered
Join Date: Jun 2019
Posts: 75
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CO spec for ‘77 US car
Hi all, I’ve been driving my ‘77 911S for a few months now after a top end rebuild and it has been great overall. However I have a cold start issue that has been annoying. It will start but seemingly on a few cylinders only for 30 seconds or so and sometimes backfire a bit. Then it fires all 6 and runs great!
I wanted to ensure my air/fuel ratio was good on this car and also my 79 928, so I got the GasTester a couple weeks ago. I finally busted it out today to check my well warmed up 911 and noticed that the ideal spec in the books for a US car is very different from a ROW car. The 1.5-3.0 for a US car and 1.0 to 1.5 for the ROW car. Is this due to the emissions equipment on my US spec car? Since I’ve removed all of that, would I use the ROW spec? As it is, I set it to 1.6 at 960rpm and it seems to be behaving quite well though I can’t test the cold start until much later Photo taken during testing - using my timing light to set the RPM 
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09-02-2024, 08:12 AM
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PCA Member since 1988
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Yes, the differences in CO% are for different emissions requirements. If you deleted the US stuff, use the Euro spec, but feel free to adjust it a little richer or leaner to get it to run best.
However, that's probably not why you are have a rough cold start. Next step is to check CIS pressures, especially cold control pressure. Does it start smoothly when warm?
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners. Cars are for driving. If you want art, get something you can hang on the wall! |
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09-02-2024, 12:17 PM
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Registered
Join Date: Jun 2019
Posts: 75
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Hi, and thanks for the info. That’s what I figured!
It does indeed start well when warm. I’ve had the fuel distributor rebuilt by CIS Flowtech and have tested all the fuel pressures a few times to make sure they’re in spec. I have an 033 WUR System Pressure is 5 bar CCP is 2.1 bar at 21C I’m guessing I might have an issue with the cold start injector not working for long enough. It always fires right up - but then stumbles for 30 sec or so until it starts to run well |
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09-02-2024, 01:02 PM
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