AMC 304 Tuning

TKFireman

TKFireman

Active Member
Joined
Feb 4, 2009
Location
Apex, NC
I have a '80 CJ7 with a 304, edelbrock performer intake and truck avenger carb, OEM ignition, replaced by previous owner. I can not get this thing dialed in.

I set the timing at 5 to 7 BTDC and the RPMs go up over 1000, idle screw all the way out. Can not get idle down to 700 rpm range.

Mixture screws by the book says set by factory, shouldn't have to touch, but if you do they should be the same, one was about 1.5 turns out and one was about 3 turns out. I tried 1 turn 1.5 turns and 2 turns, resetting timing each time. Engine wants to backfire when under a load, like when your going slow say 40 mph in 4th gear (t176) and gas it, it backfires and chokes out. Seems to do better when 2 turns out, but idle rpms were 11 to 12 hundred, and again, idle screw all the way out.

What am i doing wrong? Or better yet if there is anyone in the Apex area that can help me dial this thing in, i would appreciate it.
 
I found that i had a vacuum port not capped, i fixed it and still have same problem.....found 4 ports, 3 now capped and 1 going to distributor
 
Even though you capped the vacuum ports, you could still have a vacuum leak. Also, if your carb gasket isn't sealed good, it will do what you describe. Also, check that the plastic spacer under the carb isn't cracked or broken.
Now, a good way to check for a vacuum leak is, while the Jeep is idling, take a propane torch, open the gas knob(but don't light it), and point the nozzle at the base of the carb, along the intake gasket area, and anywhere there might be vacuum hoses and fittings. If the idle raises, that's where the leak is.
 
I tried the propane trick, didn't have any luck, engine speed never changed. I did try putting a larger throttle return spring on, and that got the rpms down to about 900, even though the throttle cables are not tight, but pulling it back by had, it still wouldn't go below 900, again with idle screw backed way out. I could see two shot of gas in the front two ports, nothing in the back two, is this right?
 
TKFireman said:
I tried the propane trick, didn't have any luck, engine speed never changed. I did try putting a larger throttle return spring on, and that got the rpms down to about 900, even though the throttle cables are not tight, but pulling it back by had, it still wouldn't go below 900, again with idle screw backed way out. I could see two shot of gas in the front two ports, nothing in the back two, is this right?
Click to expand...
What size Truck Avenger? Might be too much carb for a 304. Jets too big, maybe? Floats need adjusting?
 
its a 670 cfm......may be to big, i know nothing about them and am trying to learn. Supposed to be sight glasses to see fuel level in bowls, but their foged up and can't see through them
 
carb might be to big. I run a performer intake and cam in my 80 cj and i only run a 600 carb
 
i hope thats not the problem.....I have been reading, and it appears that i have no PCV valve installed, carb was but on by previous owner. Vacuums capped on carb, but line coming from back of intake where i think the PCV originally was is open. Could this be my problem? There is a small amount of oil in that open line, is that normal?
 
ok, after talking to a friend/fellow cj owner, i played with the timing some more and found that it actually runs best with timing set just after tdc, don't know why, but it runs better, however still backfires when lugging gears. Starts off in first uphill no problem, going slow in 2nd, backfires unless you just ease into the throttle slowly, same for 3rd and 4th, any ideas?
 
TKFireman said:
its a 670 cfm......may be to big, i know nothing about them and am trying to learn. Supposed to be sight glasses to see fuel level in bowls, but their foged up and can't see through them
Click to expand...
670 is too big for a 304 w/stock cam. Timing should be 8 degrees BTDC.
 
TKFireman said:
Mixture screws by the book says set by factory, shouldn't have to touch, but if you do they should be the same, one was about 1.5 turns out and one was about 3 turns out.
Click to expand...

What book?
Factory manual?
Haynes/Chilton?
If any of the above, it's referring to a MC2100 and NOT to oversize holley...
 
Caver Dave said:
What book?
Factory manual?
Haynes/Chilton?
If any of the above, it's referring to a MC2100 and NOT to oversize holley...
Click to expand...


Thats from carb's manual, from holley's website
 
so if the carb is just too big, is there anyway for me touse this carb at all?
 
Is it a true backfire or a loading up heavy fire?
 
i don't know the difference, but blowing out of carb and not exhaust if that matters. I dialed down the mixture to one turn out and reset timing to about 8 btdc and its running decent, still backfires when lugging a higher gear, but starts off fine and runs strong, and idles at around 900 rpm, thats gonna have to be good enough for now. My next jeep money will go to upgrading ignition and we'll see about the carb......thanks for all your help!
 
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