Technical Low vacuum reading 283

Engine idles and revs great with no load. Runs smooth down the road. Under hard acceleration I have a bad misfire. Standard Chevy 283 with an upgraded set of springs in the cylinder heads. Comp cams 12-344-5 lift int/510 exh/525 and a 650 Holley. The points gap was set by a friend with a dwell meter and float levels were both set. Carb screws set at One and a half turns out each. When I connect a vacuum gauge to full manifold it hovers around 5hg and carb screws do little to change it? Hosed the bay with either and the idle never budged. Advancing the timing leads to hard starting. Seems a bit of blue smoke out of both pipes. Pulled plug wires one by one and every time the engine dropped rpm’s significantly. Any ideas?

 

Ok, it's a 283, with near 300 degrees of advertised duration and more than .500 lift. The cam operating range is 3700-6500 RPM. Based on the 301s I ran years ago, I don't think you should be expecting a lot of idle vacuum with that cam. Backfire could be lean carb under acceleration or ignition.

 

Yep I was thinking too much ignition timing, how much @ 2500 and how much total?

 

I agree, cam is huge for the little cube 283....it's never going to make a loft of vacuum....if you can get 8" I'd be amazed. 246°/253° @.050" duration is a whopper in that small an engine.
That 3700-6500 rpm range given is based on use in a 350 cubic inch.

It's going to like a lot of initial timing, like 18-24°, if not just locked like the racers do at 34-36°. Distributor is definately going to need a recurve if you want to keep it with an advance curve of some sort, because you've got to keep total mechanical advance under 38-40° max in most cases

On the non-responsive idle mixture screws, I'd bet this is the common deal of "primary throttle idle speed screw cranked up to get enough idle airflow and good idle speed, but now your running at idle on the transfer slots, thus no mixture control from the screws". Very common with a big cam and guys that don't know what you have to do to the carb to get everything right.

If you take the carb off the manifold, you can examine for too much transfer slot is showing below the closed throttle plates....if showing more than about .050"(basically more than just a square opening) it's open too far.


Fix is done a couple of ways. You can try cracking the rear throttle open a hair, to bleed some air in, and close the front throttle screw back down and see if that gets you there. Can't go alot this way, you don't want to expose any rear transfer slot.
If it doesn't, now you are looking at drilling air bleed holes in the primary throttle plates like most of Holley higher performance carbs have.
Start with a 1/16" hole on the front half of each primary blade, about 1/8" from the edge.

 

Wanted to bring this topic back up. I have a 283 in a 64C10 and these past couple of weeks I have been conducting maintenance such as an oil change, plugs and going over my brakes in anticipation of driving to the LSR like I do every year.

After everything was completed I've been driving a few miles each day to let the brake shoes set it and more or less be sure all is on point before I leave this Friday (4/30/25) to Austin. My motor has been running strong and fires up on a dime but I have noticed now that sometimes at a stop my RPM's are dropping below 500 and the motor will now sometimes shut off. Usually at idle in drive it's set at 700 RPM's.

In park it idles fine but has always been a few hundred higher which puts it at around 1,000 RPM. Now here lies my question, during this maintenance I forgot to put my plug wire back on my number 3 cylinder and I turned the motor on. It ran for about 10 - 20 seconds when I noticed right away it was not firing on all cylinders.

I quickly cut it off took another look under the hood to double check everything and that's when I noticed the number 3 spark plug wire was off and not connected to the plug. I snapped it back on and it fired right up.

So it seems to be running fine with the exception of the drop of RPM's while in drive and at a stop. I really don't feel like the timing was affected, I'm leaning more towards a vacuum leak but if I cannot find one is there any other suggestions out there? I did not touch the carb in any way, have not had to adjust any idle mixture screws or anything else on the Edlebrock CFM 500.

As I was saying it's less than 48 hours til I leave for the Roundup and I need to get this figured out before I head out on Friday morning. I appreciate any input to narrow this down and get it fixed.
Thanks,
Mark