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Pass Emissions
Livng in Arizona, I have to have my Cobra pass emissions. This year they really tighten up the emissions standards, which I had passed the previous 6 years. I replaced the Barry Grant 750 carb with a new Holley 650 double pumper, hoping that it would run cleaner. I can't get the car to pass the loaded (35 MPH) test for Carbon Monoxide, but does pass it at idle. On the Hydrocarbons test it fails at idle but passes on the loaded test. I'm running a 351 Cleveland on a Weiand xcelorator maniford. I've bumped the timing up 26 degrees BTDC (suggested by tech support at Barry Grant). Replaced the plugs and plug wires. I'm sort of at a loss where to go next? Any suggestions?
Bill |
You are probably running a tad lean at idle and rich at part throttle (35mph). The trouble is that at 35mph, you're probably in the transition between the idle circuit and the main jet. Adjustments will probably take some fiddling with the main jet while monitoring the results on a HC/CO meter.
I disagree with Grant on the idle timing. I'd go toward retard rather than advanced. Advancing the timing tends to increase HC. Are you using a vacuum advance system? If so, consider disabling it for the test. |
Hi Bill,remember HC is unburned fuel - raw gas, CO is partialy burned fuel - rich mixture. I agree with strickly P, back the timing off to pass the test then adjust it to perform. Use the mixture screws on the carb to get the highest idle, then lean it to get a 50 rpm drop (the lean drop method). But you may not be able to over come a radical cam. To help set the idle 50 rpm under the max alowed. In Colorado its 1200 rpm. For the CO under load install a smaler set of jets. You may have to purchace an adjustable metering plate for your holley. I think the factory metering plate is equall to a 70 or 72 jet size. Try dropping it to a 66 jet. Hope this helps :)
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run a 30% xylene mix with your gasoline. Xylene is an octane booster with a much lower vapor pressure than gasoline. You should get a net octane of just under 100 with 93 preimum. You will also see significantly fewer HCs.
In either case I'd lose the double pumper for testing purposes. Good Luck, |
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