383 problems- rough running
Thread Starter
Melting Slicks
Joined: Dec 2000
Posts: 2,635
Likes: 0
From: Union Ontario
383 problems- rough running
Here's the details guys,
383, Sportsman 2 heads, comp XE274 cam, Performer RPM intake, holley 750 vac secondary carb, MSD 6a. The engine is brand new except the carb( reman unit), and MSD was used. Its running around 10.5:1 compression
When first started up, the engine ran great, idled as good as it can with that cam. I tuned it up(leaned out mixture turned down idle speed screw) and adjusted timing(16 initial, ~36 in by 2500) and am running manifold vacuum. I ran a steady 12 inches of vacuum.
Now I have a backfire out the exhaust when I crack the throttle at 2700 RPMwhen its under load (no backfire when revving it in the driveway). I checked the vacuum its jumping from 5-15 inches with a very lopey idle. I checked fuel pressure and its 6 PSI steady as a rock The float level looks to be perfect. When running in the garage it smells extremely rich, the plugs are a dark brown.
I am thinking it may be the power valve is not the right one for the engine. What do you guys think?
383, Sportsman 2 heads, comp XE274 cam, Performer RPM intake, holley 750 vac secondary carb, MSD 6a. The engine is brand new except the carb( reman unit), and MSD was used. Its running around 10.5:1 compression
When first started up, the engine ran great, idled as good as it can with that cam. I tuned it up(leaned out mixture turned down idle speed screw) and adjusted timing(16 initial, ~36 in by 2500) and am running manifold vacuum. I ran a steady 12 inches of vacuum.
Now I have a backfire out the exhaust when I crack the throttle at 2700 RPMwhen its under load (no backfire when revving it in the driveway). I checked the vacuum its jumping from 5-15 inches with a very lopey idle. I checked fuel pressure and its 6 PSI steady as a rock The float level looks to be perfect. When running in the garage it smells extremely rich, the plugs are a dark brown.
I am thinking it may be the power valve is not the right one for the engine. What do you guys think?
Team Owner
Joined: Feb 2003
Posts: 30,183
Likes: 313
From: Lake Norman NC
i had the same problem...and still have some of the same problems. I had a backfire thru the carb though, and it was the PV..i changed it..it solved my backfiring, but still the engine runs rough with no steady vacuum. Sorry i'm in your boat too.
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Sounds like a lean misfire. Try bvacking out the idle mix screws 1/8-1/4 turn.
Also, if the carb is rich at idle, you can't get rid of that by using the idle mix screws. They only control the volume of the mixture, not the characteristics of the emulsion, ie the actual mix of gas and air. To change the emulsion you need to alter either the amount of air or gas introduced. With a plain ol Holley 750 VS, you will need to put some thin wires in the idle circuit. I've got a pic at work of the metering block and where to put the wires. I can send it to you if you need it.
Brett
Also, if the carb is rich at idle, you can't get rid of that by using the idle mix screws. They only control the volume of the mixture, not the characteristics of the emulsion, ie the actual mix of gas and air. To change the emulsion you need to alter either the amount of air or gas introduced. With a plain ol Holley 750 VS, you will need to put some thin wires in the idle circuit. I've got a pic at work of the metering block and where to put the wires. I can send it to you if you need it.
Brett
Pro
Joined: Sep 2004
Posts: 500
Likes: 1
From: Philadelphia PA
To jump in on the timing, mine is set at locked out 36 and runs great.
Originally Posted by vette64772
Try backing off the initial timing. My 383 is set to 10 deg initial and 34 total and runs very strong..........
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Originally Posted by Highhat
To jump in on the timing, mine is set at locked out 36 and runs great.
Pro
Joined: Sep 2004
Posts: 500
Likes: 1
From: Philadelphia PA
Sorry. I replaced the iginition with MSD ignition, and you can set and lock out the advance in the distributor.
Originally Posted by glen242
What does this mean? How do you lock out at 36*? Not familiar with the "lock out" term. Do you mean the most centrifugal + initial you get is 36* at some rpm, hopefully under 3000? Are you using a vacuum advance can?
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