350 engine unknown cam not running well

just bought an Old camaro with what looks like a pieced together 350. the cam sounds pretty radical. it has vortech heads and edlbrock RPM intake with edlebrock 1411 (750 cfm) carb and factory HEI ignition. I changed the plugs because it was just not running great and figured that I would start tuning. I am having a heck of a time. Its backfiring through the carb when I rev it. It sounds like one cylinder is missing, but not all of the time. Compression is 190 on every cylinder. I have adjusted the timing every which way possible but it runs the same. Plug wires are new. I have checked firing order over and over. when I looked up the edlbrock web site it says that the 750 is not recommended for small blocks. it also said that the 750 carb is not recommended for the RPM manifold. I haven't had a muscle car is 25 years and I was a holley guy back then. any advise or suggestions would be much appreciated. I do not know where to start...

Welcome to CHR!

Sounds like you’ve already started to eliminate some of the obvious “easy” things, i.e. new plugs gapped correctly (.035 for standard ignition, .045 for HEI), new wires, set the timing 8-10 degrees with vacuum advance plugged and look for total timing of 32-35 degrees.

I think you also have touched on what the root problem(s) might be. The 750 is in all likelihood too big for a 350. If the cam “sounds radical, it’s probably too big as well and perhaps has a trashed lobe (or several bad lobes?). You’re probably going to have to open this one up to really fix. Good news is the 350 is a great platform for a rebuild. If it hasn’t been bored, a .030 over with some good pistons will work well with your Vortecs however; with the compression being 190 all the way around, I’d be tempted to select a more appropriate cam, new lifters and timing set and put things back together. The Performer RPM is a good manifold and I’d go for a 600 Edelbrock 1406. Select a cam that performs well with your overall combination considering your rearend ratio, transmission type, and c/r your piston/head combination provides. Remember too that cam break-in for non-roller cams is critical and the cam manufacturer’s instructions must be followed exactly or your new cam may be trashed in the first few minutes of run time.

Give us a bit more info and we’ll chime in with more suggestions. Techinspector1 is a whiz and if you ask nicely, he will probably help you with a dyno simulation as well as point you toward multiple helps/articles to determine the compression ratio numbers and how to accurately calculate what you’ve really got going on inside the engine.

Again – Welcome and stay active – we’ll sure try and help you out!
Glenn

The main problem here is that the cylinder pressure is too high for pump gas. I don't expect you to believe me, take a look at what the CraneCams engineers have to say here.....
Crane Cams |

Trust me, fuel octane boosters are not the answer. Read the bottle/can that junk comes in. When it says it raises the octane rating of your fuel by 2 points, it actually means 2/10ths of a point, like from 91.0 to 91.2. Utter foolishness. Makes you want to grab the makers by the throat.

There is nothing you can do for this motor except disassemble it and re-engineer it for use with pump gas (9.5:1 max static compression ratio with iron heads, 10.5:1 max with aluminum heads). I suspect that whomever built it used pop-up pistons and that the static compression ratio is 11.0:1 or higher. I further suspect that you have one or more cam lobes goin' south due to improper installation and break-in by the builder. Here's a tutorial on flat tappet cams that you may find interesting......
http://www.crankshaftcoalition.com/w...ips_and_tricks

Experience will show that a 350 will make max power with a 750 carb. If max power is not what you are looking for, then use a smaller carb for better driveability (smaller primary throttle bores contribute to faster air which contributes to a better signal at the venturi which contributes to better mixing which contributes to a more complete burn). My personal favorite is the 750/800 Rochester Quadrajet on a spread-bore RPM intake manifold. Tiny little primary bores for best driveability and mileage, huge secondary bores for best power.

Let us know when you have the motor down to a bare block and we can instruct you how to proceed.
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