340 vibration and hesitation with a short history

[73_Cuda_4_Me]

Depending on specs of the cam, and if you are running stock distributor with vacuum advance still connected...

My 70 340 engine (spare engine in 73 now) with 600 cfm Eddy Performer carb, Comp cam, and headers had way too much advance with vacuum advance connected... the cam liked about 18 degrees advance at idle, and mechanical added 30 that, then vacuum added another 15... had lots of spark knock under load at cruise speeds...

I disconnected vacuum advance, and built a limiter plate to restrict total mechanical advance to 18 (36 degrees all in at 3000 rpm)...

If you don't have adjustable timing light, it's the best investment I would recommend. Then get timing readings from idle up to about 3500 in 500 rpm increments... if you end up with high 40's advance or higher, you'll have to limit it (plate or electronic distributor).

If you are running stock (or upgraded) dual-plane manifold, the carb setting could be rich on side feeding 2-4-6-8... doesn't take much to darken the plugs a few shades!

[734406pk]

Factory exhaust manifolds are gone...Blackjack Headers were installed back in the 80's!

I suspected the cast iron manifolds were replaced with headers, but wasn't sure. So, the heat riser valve is out of the mix. Take a compression test and see what you have, then tune up changing the high voltage side parts (distributor cap, rotor,coil/ plug wires and spark plugs). Fresh fuel in the tank? Replace the fuel filter as needed. Start with the simple things first. Good luck