Demon carb on 289???
 

Hi. This is my first post.

My neighbor asked me to help him with his '65 Mustang coupe today. He has a 650 vac. sec. Street Demon carb on a .040 over 289 and it is obviosly running rich (black smoke at idle). I told him that I thought that carb is too big for a 289.

It has a 284 adv. int. dur. hyd. cam (sounds healthy), high rise dual plane, aftermarket c.i. heads with 1.94 intakes, headers, crane ignition, and 10.5 to 1 slugs. It is pulling 11" vac. @ 800 rpm idle so I left the 6" power valve in. It seems to be jetted close as it does o.k at half to WOT.

The carb has four corner idle set up. I have each screw turned out only half turn and it still smokes and smells rich but not as bad.

Is the 650 too big? Should he go to a 600? Also, I turned his total advance up to about 34 degrees and it seemed to like that under load. Is that about right for a mild 289?

Sorry for all the ?'s but my hobby lies with Mother Mopar.

What does it do under load at full throttle? Does it still put out black smoke as it accellerates down the quarter mile wide open? Might just need another 1/4 turn in on the idle mixture screws. Vacuum secondaries are usually pretty forgiving as to carb size.

Rev

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'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.

What are the plugs looking like? and how about jetting down a couple of sizes? 2 on the primary and 2 on the secondaries.

Ron

How about the power valve?? Or the float level??

Both of these will cause overly rich conditions. I don't think the size of the carb is the problem.

Thanks for all the replies. The plugs are black. At WOT it seems to be fine. It is just the idle circuit. It has sight glasses on the bowls and the levels are good. The main jets shouldn't affect idle,right? I am going to replace the powervalve today. It might be letting fuel pass at idle.

Just FYI...The main jets do effect the way the adjustments react. The fuel for the idle circuit comes through the main jets. But, they are adjusting some you said and I don't think that is your problem. It won't cause overly rich at idle and such. I think you'll find the power valve blown. If you remove carb and don't tilt it or turn it upside down. Then remove the jet plate that holds the powervalve and if you see fuel in the chamber on the vacuum side of the power valve, well it surly is blown. Common problem with Holley style carbs.

For your info. The Mustang is running great now. After I performed a vacuum reading on it, it was pulling about 6 to 7 inches @ 800 rpm. The power valve that was in there was a 6.5. I put in a 3.5 " p.v. and runs like a champ.
Thanks.

Why is the vacuum so low?? Do you have enough intial timing in there?? Just sounds very low to me.

I have intitial timing at about 14 btdc and total advance at about 38. But I have no idea if he actually checked his timing marks when he put this thing together. As for the vacuum being low, must be the cam. No vac. leaks and signal is steady. He can't tell me much about the cam other than it is a 284 adv. dur. I don't think he is even sure. Someone built this car for him.

It sure is lumpy for a 284. It idles rougher than a 292/548 that I had in a sb Chev years ago. The car seems really flat until about 3000 and it pulls hard all the way to 7000. I didn't take it any higher than that. Anyway, it seems to be fine now.

I did jet it down some using the plugged power valve up a hill method.