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| /6 misses at idle when warm...help! https://slantsix.org/forum/viewtopic.php?t=58670 |
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| Author: | cam.man67 [ Sun Nov 08, 2015 5:04 am ] |
| Post subject: | /6 misses at idle when warm...help! |
Hi all...I'm struggling here with my daily driver '68 Dart. It's a 225, 904, holley 390 car and has been pretty well maintained since I've owned it. I just did the valve lash at 12 and 23, points are .022, base timing is at 12 deg. My idle is a little higher than I prefer but I'm not sure what that is since I don't have a tach. I'm guessing 1100-1200 and 8-900 in gear. Just rebuilt the distributor (new body and bushings) because I had erratic timing and replaced the timing chain about 500 miles ago. Timing is now very steady and idle vacuum is 19-20 The car runs great, but it won't idle when warm. It starts easily cold or hot, never have to crank it much. Idle is smooth as the car is warming up, but once it gets good and hot it starts to miss and sputter and usually stalls unless I throw it up into neutral. I was wondering if the ballast resistor was bad, but no, it's at 1.5 ohms. I replaced the cobdenser, still no change. The idle mixture screws function, but they're not affecting the miss at all. I'm fairly sure that this is either fuel or spark related, should I consider replacing the fuel pump? The ignition coil is new, is it possible it's faulty? How would I test either of those? What else am I missing? |
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| Author: | wjajr [ Sun Nov 08, 2015 8:51 am ] |
| Post subject: | |
One cause of high idle rpm with a 390 is having too much of transition slot exposed or in other words throttle plate is cracked open too much and idle circuit is being over whelmed from added fuel from transmission circuit. Check and adjust primary throttle plates so that only a small portion of transsition slot is exposed in the shape of a square. Also if there is too much air being let in by secondary throttle plates, in other words their throttle stop is not allowing the plates to close down enough which acts just like a big vacuum leak. You will need to pull carburetor off, invert it to check and make any need adjustments. Another source of excess air entering manifold can be from sloppy egged out throttle shaft holes. Still another location to check for vacuum leaks is the secondary diaphragm for a rip, and any of the little cork gaskets sealing the electric choke mechanism to body of carb. A dab of Vaseline on diaphragm cover screws will prevent destruction of membrane. Once extra air is eliminated, transmission slot exposure corrected A/F mixture screws will react and be about 1.5 turns out from bottomed out. When idle drops below 850 rpm, excess mechanical advance should drop off to zero lowering idle rpm to drop some more. You will have to fiddle around with idle A/F mix screws, primary throttle stop which controls idle rpm, and base timing until engine idles around 650 rpm, and will settle down after blipping throttle. Using a vacuum gage will help to tune carb. Look for highest idle vacuum, at 650 rpm. |
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