Posted by Chris W. (216.172.142.169) on April 12, 2001 at 02:36:39:
In Reply to: Problems... posted by Brian on April 11, 2001 at 13:09:03:
First, there is a chance that the 780 might work. Except for your cyl. heads, you have basically the Edel. Performer RPM Package. If your buy the complete package, you get their 750 carb with it. It is just that your heads don't flow as well as the Edelbrock's.
The first thing I would go back and check is your valve adjustment. The problem is probly with your carb, but incorrectly adjusted valves can cause similar problems.
When are you checking your plugs? After the engine idles for awhile? The backfiring sounds like there is a "hole" or a lean spot in your fuel delivery. But if the engine is idling rich, then it will blacken your plugs and it will kind of throw you off when tuning it.
Check to make sure your secondaries are adjusted correctly. There is an adjustment on the bottom of the throttle body near the secondary vacuum diaphram. You might have to take the carb off to do this. Turn the screw in until it just makes contact with the throttle shaft, then turn it 1/2 turn more. One of my carbs had it's secondaries closed up and it was very hard to get it started agin. By doing this you let a little more air into the motor. If the secondaries were already adjusted like this (they should be), then turn the screw in 1/2 more turn.
Now to the Power Valve. You need to make sure you have the correct size. If you have between 13-16" of vacuum, then you should have either a 6.5, 7.5, or an 8.5. Either one should work, but you may have to try each of them.
Fire the engine up. Warm it up and readjust the mixture screws. Use a vacuum guage and set the screws to the highest vacuum reading you can get.
Reset your idle to about 1000 RPM's in park (auto trans). Also recheck your timing. Let the engine run awhile and then recheck the plugs. If they are still black, then try adjusting the secondary screw agin. Go about a 1/4 of a turn more each time. But don't go over 1 1/2 turns. If you do, you might not be able to use the idle speed adjusting screw to control your idle. Then readjust your mixture and idle speed. This should clear up your plugs or atleast get the engine idling "cleanly".
Now to the backfiring and power problem. This part is pretty much just trail and error until you get it right. If the above helped, increase your jetting a few sizes. Then see if it improves any. If your power increases but it still backfires, increase your squirter size. I'd try a "31" sqirter. You shouldn't have to use to big of a sqirter, so concentrate on the jetting until your happy with the way it runs.
I had the same problem with one of my engines and this is exactly what I did to fix it.