I don't know the specs of the engine, I didn't build it and the last owner didn't know. curious - what carb would you suggest?
You more than likely already diagnosed it in its entirety. The carb base gasket and possibly the carb throttle plate to body. Start at the base gasket, cheap n easy and you know it leaks by the testing you initially did. Do it before anymore books are thrown at it.. lol then look over the timing and its curve...jetting and power valve if so.
Know what you're jetted at too, as was mentioned. Your distributor may have some quicker advance springs in there....find out the curve. Another thing, if this motor burns a bit of oil, that can definitely cause detonation issues.
When you get the vacuum leaks solved it needs at least a 700/750 Holly or a 700 demon. I would start with a 800 thermoquad, i like them and can tune them, that is what originally came on a 340. But first you really need to find why it has a giant air leak. I would also do a compression test to see how hot that motor is. Mine is a 10.5 to 1 and have compression around 200. It just gives an idea of how much squeeze your cylinders are giving. I need 93 and up to run. It is in Charger but B Body sites barley talk about small blocks.
Updates. Found the carb base plate is cracked. So I’m in search for a good replacement. I did do a compression test. Highest cylinder reading 180 lowest 165. The plugs I just put in 30 miles ago look like this. Guess I’ll continue the work after either I find a replacement base plate or a carb. I’ll follow up once I’m back into it
Dead lean, some of which being caused by your busted carb plate. Timing is way shy. This may help FBO Ignition - Home of the HRR688 - Spark Plug Reading
Depends on the altitude and octane but you are right for sea level pump swill. 340 I built in my Challenger many moons ago would run ok on 93 during the winter in Atlanta, but as soon as it got hot during the summer it would run on and knock like hell. Had to run octane boost. I also knew nothing about rounding the sharp edges and cleaning up the chambers back then so it likely would have made enough difference if I had.
I fix this problem in my friends GTO by putting in a cam with more duration on a wider lobe separation angle to close the intake valve later. Is 10.2 to 1 but it has quench, and it runs great on 91 now. Changing a cam can make an engine that would otherwise not run on Pump Fuel, work okay
Be sure to hang on to that paper with the cylinder pressure readings. If all else fails and you cant get a tune without detonation, you may have to pull the heads to check for hot spots or carbon build up. If so, the highest pressure cylinder chamber would be the one I would look at first. Polishing the valve faces, smoothing the chamber, and knocking down sharp edges on the head would likely get you back to good if needed.
Wow that is scary lean. Thankfully only 30 miles. See how it runs with a new carb, try to get something in the 700 cfm range or an 800 thermoquad. I am not sure you can run 91, see if it is OK after new carb. I run 93 non ethanol in mine timing 34° all in at 2800.
People have run much more than that and there are other factors involved than just cylinder pressure. 240 psi on pump is possible when you know what you're doing.
Is that yellow or tan on the strap? Next time you take photos of plugs wrap them in your hand and take a close up so we can see what it looks like into the shell.
It can still detonate in the higher RPM range as cylinder pressures increase. It's harder to hear and diagnose.
What carb is it? If it’s a Holley call BLP or maybe ProForm is still making parts and get a billet plate.
You can do it. You just have to have a purpose going in. And that’s one reason why cranking compression isn’t a very good predictor of detonation.
