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Back at this Finicky 340.

Tim Chavez

Tim Chavez

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well Gents
with the cab swap completed on the Cummins, I'm back to trying to figure out this 340 in the charger. originally i wasn't getting cylinder 8 to fire and thought that was the major issue but turns out the new plug i had installed was also bad and timing was a bit off. both issues have been resolved, so all 8 appear to be firing now but its not really wanting to run at a normal idle. it runs strong at around 12-1500 rpm and seems to rev fully without sputtering. it will run at its high idle for 10-15 minutes then will suddenly act like its starving itself of fuel and shut off. if i can get to the throttle before it dies it will pop like its backfiring then straighten itself out. ( but will die shortly after) if it does die it needs 2 or 3 squirts from the accelerator pump to fire back up. my guess is vacuum but i cannot find the leak. its manual brakes so no booster to be the culprit. the booster hole on the carb is plugged with a bolt that seems to be threadlocked in. and all other ports on the carb are plugged or being used. settings on the carb seem to be doing nothing ( air fuel mixture screws all the way in or all the way out and everything in between give no changes to the engine running.
spec wise its a J head 340 from 1970 with an edelbrock 4 barrel 650 cfm carb manual choke.
thanks for the help guys
 
Does this happen cold & hot or just cold?
 
How old is the carb, might be sucking air by the throttle shafts. Also check your vacuum advance to make sure it's working.
 
69 GTX said:
How old is the carb, might be sucking air by the throttle shafts. Also check your vacuum advance to make sure it's working.
Click to expand...
Not exactly sure. I rebuilt it last year, but it was my first carb rebuild so I wouldn’t put it past having done something wrong. I do not have a great history on the car other than I bought in 04 and it sat in my parents garage until about 2016 when I bought my house and finally had a place to work on it. I do know the previous owner wa pulled over in it back in 97 for 110 in a 65 so I’m guessing it was running well in 97 haha
 
Last edited:
69 GTX said:
How old is the carb, might be sucking air by the throttle shafts. Also check your vacuum advance to make sure it's working.
Click to expand...
The distributior is new, I switched He car to an all in 1, 2 wire distributior. It’s still at its “out the box” settings. But I’d assume the vac advance is working, how do I check that? I’ve also upgraded to a high output ignition coil.
 
If idle doesn't change with adjusting the screws all the way in, something is big time wrong. My bet is on the carb but you said timing was set, what is your initial and your total? Timing should be set first before going anywhere else. You need a piston stop to located true TDC, mark the damper at TDC and then set your timing with a light and distributor advance unplugged and capped. You should be somewhere between 12 & 18 BTC initial and that depends on the engine/cam and what it likes. Engine should be up to operating temperature before checking. Then rev her up (if it will run) to around 3000 rpm and check again, now you should be around 32-34 BTC. As for checking the vacuum advance, hook the distributor up to manifold vacuum and check again at idle, you should now have more advance than your initial without it. Make sure for the test that it is manifold not ported vacuum or it will do nothing at idle. One your timing is sure, then you can move on. Take a propane torch (unlit obviously) and go around everything checking for leaks. If you can't locate a leak, the problem is likely the carb itself. Shout back with what you find out
 
Bill Monk said:
If idle doesn't change with adjusting the screws all the way in, something is big time wrong. My bet is on the carb but you said timing was set, what is your initial and your total? Timing should be set first before going anywhere else. You need a piston stop to located true TDC, mark the damper at TDC and then set your timing with a light and distributor advance unplugged and capped. You should be somewhere between 12 & 18 BTC initial and that depends on the engine/cam and what it likes. Engine should be up to operating temperature before checking. Then rev her up (if it will run) to around 3000 rpm and check again, now you should be around 32-34 BTC. As for checking the vacuum advance, hook the distributor up to manifold vacuum and check again at idle, you should now have more advance than your initial without it. Make sure for the test that it is manifold not ported vacuum or it will do nothing at idle. One your timing is sure, then you can move on. Take a propane torch (unlit obviously) and go around everything checking for leaks. If you can't locate a leak, the problem is likely the carb itself. Shout back with what you find out
Click to expand...
If I’m using/reading this timing light correctly initial timing is set around 13 and at rpm it’s around 33. But I’m unfortunately teaching myself to do everything as I go.. tdc is still legible on the damper and seemed spot on when I checked with the cyl 1 spark plug out. I’ll try the propane torch. When I had it running I sprayed the engine down with starting fluid but never heard a change in the motor.
 
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All the above is “spot-on”. I wouldn’t use starting fluid(choke clnr maybe or propane). Too much chance of a fire for me.
 
well i ran through the Gambit again, propane seemed to yield no changes to idle same as the starting fluid. initial timing reads between 13 and 14 and at mid to high rpm (no tach) its reading between 33 and 34. same issues, needs pedal pumps while cranking to start cold (and strangely doesn't want to fire at all under choke) once she warms up it dies on its own( like its running out of fuel) a few pumps from the pedal (before cranking not during) and she fires back up idle for a few minutes and die again. the vac advance is plugged into the manifold vac port on the carb. Should I get another kit and try to rebuild the carb again? Or just start lookin for a new carb?
Thanks gents
 
Sure sounds like you need to recheck all the settings in carb(too low float level?).
 
Reuse shouldn’t be a problem. If your explaining correctly, it almost has to be carb. If settings are still right, I’d blow out every passage/hole you see with choke clnr & compressed air if you can.
 
When I’m wrong I’m wrong. Turns out I read the diagram wrong and the vac advance was actually on ported vac. Switched it to manifold vac and it wouldn’t stay running long enough to even check the timing. Gonna try to find the sweet spot in the distributor again and give it another go... This is why you send your kids to tech school instead of getting a worthless Psych degree
 
Based on what you said earlier, the timing is not the issue. It should idle OK with or without the vacuum advance.
Sounds like you have a mixture problem. Either try another carb or find someone knowledgeable to look at yours.
The timing is set with the vacuum advance unplugged and port capped off. After it is set you can plug in and test to see if you get more advance.
 
Tim Chavez said:
An old edelbrock 650 cfm manual choke. (1406) I think
Click to expand...
ok, a Holley is easy to check and adjust the floats, but I'm not as familiar with the edlebrock on how you adjust them, pretty sure you have to take it back apart on that one
 
Bill Monk said:
ok, a Holley is easy to check and adjust the floats, but I'm not as familiar with the edlebrock on how you adjust them, pretty sure you have to take it back apart on that one
Click to expand...
yes the needles have to be pulled and the top unbolted. The rest can stay installed setting the floats is tricky because it’s set by bending the little metal tab on the back of the float
 
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