Engine Help with stubborn misfire.

Nov 26, 2020
54
11
18
41
Southern Virginia
Hello, I am running out of ideas trying to solve a stubborn misfire. I have a rebuilt carburated 302 running a light cam, but is otherwise stock. Problem is I am getting a fairly severe misfire at 0-25% throttle. Only misses while under load, no miss out of gear. This problem started a few days ago when I discovered a substantial vacuum leak and fixed it. Now that the leak is gone the thing just wont run right. I readjusted the carb so it idles correct, but now under light throttle it misses hard. I doubt its a spark issue because it idles and works under higher rpms just fine. All cylinders have around 180 psi compression and the timing is set a 9 degrees btdc running manifold vac. advance. I made sure my choke was opening all the way as well. I don't know what else to try. What else could the be?

Thanks.
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
19,646
6,567
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polk county florida
Put a vacuum gauge on it and tell us what it does at the rpm where the miss is.
Also check plugs wires, plugs and cap just to be sure.
 
Nov 26, 2020
54
11
18
41
Southern Virginia
Put a vacuum gauge on it and tell us what it does at the rpm where the miss is.
Also check plugs wires, plugs and cap just to be sure.
Wont be able to see my vacuum gauge while its missing because it only happens when its under load (whoops, forgot to mention that. Will edit original post.) of normal driving conditions. It seems to miss up to 2000 rpm. Checked ignition earlier today everything looks good. No corrosion inside cap, wires look good, plugs look good ensured gap of .035.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
19,646
6,567
193
polk county florida
Looking at and testing/ diagnosis are two ways of doing it. One will allow you to find the problem, one will not.
You can run a vacuum line into the interior or zip tie it to the wiper arm and drive it around. If you have a vacuum advance on the distributor that could be not working correctly or out of adjustment.
Keep in mind the higher the rpm the less noticeable a miss is.
What ignition system are you running?
At what rpm does the vacuum advance start working?
Is the timing set with the vacuum to the advance can disconnected?
9* at idle seems a little low to me :shrug:
 
Nov 26, 2020
54
11
18
41
Southern Virginia
Looking at and testing/ diagnosis are two ways of doing it. One will allow you to find the problem, one will not.
You can run a vacuum line into the interior or zip tie it to the wiper arm and drive it around. If you have a vacuum advance on the distributor that could be not working correctly or out of adjustment.
Keep in mind the higher the rpm the less noticeable a miss is.
What ignition system are you running?
At what rpm does the vacuum advance start working?
Is the timing set with the vacuum to the advance can disconnected?
9* at idle seems a little low to me :shrug:
Ok so I've fixed the problem. I switched to ported advance, set my initial timing from 9 to 12, and readjusted idle/mix. Now it runs like I would expect it to. No miss.
 
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