340 misfire

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4speed73340

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I have a '73 dart sport 340 4 speed that has a misfire that I cannot get rid of. I have been through 2 sets of plugs, new wires, new cap and rotor, 2 different carbs, different timing, and nothing has helped. It does it between idle and around 2000-2500 rpms. After that, I don't know if it goes away, or I just can't hear it anymore. It has a stock electronic distributor with a msd 6al box and msd coil. It started after I cut off the exhaust to where it dumps at the axles. A week or so earlier I put a new cam and intake on it, but I took the valve covers off and watched the rockers, and everything seems to go up and down like it should. I have checked the plugs many times, and at first, it was the number 2 cylinder, and now it seems to be the number 8. I can tell it comes from the right bank because of the exhaust. I hooked a vacuum gauge to it and it pulls 6 inches of vacuum, and doesn't move much at all. I can tell it is down on power. It has drag radials, 4.10 gear, j heads, rpm air gap intake and the fastest it's gone in the 1/8 is 9.41. I keep thinking it could be a burnt valve, maybe a vacuum leak, or some kind of ground. I really want to get this thing running right so I can take it to the Dragstrip, and drive it to school this year. If anybody has had the same issue or has any idea how I can fix it, I would appreciate it. Thanks, Logan
 
Find someone with a leak down tester and start there. Or perhaps start the car up and start unhooking plug wires one at a time to see if the sound of the motor changes.
 
a low non responsive vacuum signal would indicate more than one burnt valve.
valves not closing after cam change due to too much lifter preload, improper cam timing?
 
Broken spring or bent valve maybe? It might be worth it to cut open the filter and see how much metal came out of the cam break in. Valves may be moving but not enough...
 
get a compression tester, test all cylinders.
 
As far as improper cam timing, it ran great at first, then it started. So, a low vacuum reading where the needle doesn't bounce around could indicate more than one burnt valve?
 
What kind of rockers are you using? adjustable or stock? Also did you verify TDC on #1 with your harmonic balancer to make sure it did not slip? What is your timing set up?
 
does the vacuum increase when you increase throttle and hold at steady rpm.
 
steady low at idle, I would be looking at a intake manifold leak.
you need to do a compression test.
 
Are you using vacuum advance? might plug it if you are, may have a leak there if it ran good before, other wise check for vacuum leaks at carb and intake.
 
but then again, that cam might only produce 6" vac at idle.
 
Ok, that will be the next thing I do. When I put my new manifold on, it didn't want to bolt on very easy, and when I got it on, the front didn't make contact with the cork gasket on the front of the block. I could fit my fingernail in the space between the intake and gasket, so I put some rtv on the top of the cork to seal it. Could the heads be milled causing the intake not to fit right? I bought the car almost like it is, I just swapped heads and intake so I don't know. I can't imagine since the stock rocker arms are being used. If so, could that cause it?
 
How much initial timing do you run? I do not use the cork gaskets on the intake, just black silicone and take the roll pins out front and back off the china wall, are the roll pins still in? intake could be hitting those making it not seal very good.
 
Do a compression test before you do anything else. it will eliminate or identify potential problem.
 
you could try advancing the ignition timing.
 
I run my 340 at 18 initial 34 total, the more initial should give you a little more vacuum. I have a little smaller MP solid cam that is 235 @ 50 and has around 9 inches of vacuum.
 
I put plenty of assembly lube on cam and bottom of lifters, and oil on the outside of them. I made sure they spun. I used Lucas oil break in oil and even put in a little extra zinc. I ran it between 2000-2500 rpm for 20-25 minutes. Next morning, I changed oil and filter, and put some additive in it.
 
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