318 hunts and surges at idle or when cruising at low rpm

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Str8up

Str8up

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please read full post before replying so you don't suggest something already done.
since problem started I have replaced.
distributor. wires, plugs, cap rotor, coil, ballast. all done twice to ensure not problem. even put old ones back on once. firing order checked many times by me and friends.
new intake and carb. pump, filter and all lines.
timing and carb settings done over and over.
NO VACUUM LEAKS, as I know that is the first thing everyone is going to say. 18invac!
so at idle it hunts. as you increase rpm all of a sudden it is like a cylinder kicks in and smooths right out. also if driving with your foot into it and rpm is up. it's fine. try cruising at a nice even speed (low rpm) at a steady 50mph it starts feeling like it's missing. give it a little gas clears up. tried everything. today as it was running I pulled one spark plug wire at a time. when I removed cyl 8 it started backfiring through the exhaust . seemed like on that cylinder???
with that wire of it just pops, pops pops.
what causes this???
thanks in advance
 
Magnum cam in LA motor or visa versa. Timing set one tooth off. Carb low circuit plugged. This for the DR House of mopar to conduct a differential diagnosis. You know who you are!
It's not me!!!
 
Maybe it's a vacuum leak!....Just kidding!! I would do a compression test, and also an engine running compression test. Maybe a valve hanging up or broken valve spring.
 
I think a look at a few plugs including #8 would give a lot of info.
That's where I'd start, and then I would probably pull the valve cover and see how much those #8 valves move because this sounds like a valve issue to me also.

From doing this you might see something really obvious like a broken spring or a rocker only moving 1/4 inch.
 
My 273 with Eddy 1405 doing similar stuff. I took Eddy off and apart. Cleaned up reassembled with orig factory stuff. Just put fresh orange springs in it. Set choke (somewhat lol)
And out for a rip. So far so good. I hope to know more tomorrow.
Thanks for the link @krazykuda
 
thanks everyone. I should have mentioned I did compression test. all cylinders within 2 psi of each other. I am going to work on it today. I am also starting to think valves as I have pretty much replaced all external parts. I also was thinking carb issue but it seems funny that considering I installed new carb it would be doing the exact same thing as the old one.
 
It's lean, a slight increase in part throttle enrichment is called for. Your statement of giving it more gas and it seems to clear up tells the story......accelerator pump is supplying the gas to richen it up and smooth out. Go one step at a time.

You didn't say what carb you have.
 
JBurch said:
It's lean, a slight increase in part throttle enrichment is called for. Your statement of giving it more gas and it seems to clear up tells the story......accelerator pump is supplying the gas to richen it up and smooth out. Go one step at a time.

You didn't say what carb you have.
Click to expand...
 
it is lean as my plugs are gold to white. no one plug looks any different . they all appear to be pretty much the same color. what cause lean burn. I should have mentioned . putting my hand over carb to block air flow increases the rpm. smoothes out but not totally. it is like the first 1/4 inch of throttle is the issue. when ur at idle and begin increasing throttle it is boggy , lazy and then you suddenly get to that spot it just kicks in and revs right up. like a cyl. kicked in.
 
If you put your hand over the carb blocking the air and it diesnt die and speeds up...its a vacuum leak at the carburetor.
 
How did you chk for vacuum leaks? I use a small propane torch with a rubber hose a foot long. Rpm should rise as it sucks in the propane.
 
Been a long time since I've been inside a Carter AFB/AVS..........foggy memory says needles, springs and jets required for tuning.

You're not that far off from what you need, needles are numbered, they may be step tapered, as are jets, springs MAY be color coded, otherwise id them by wire size, heavy/lite.

MAYBE, you MIGHT be able to use Edelbrock pieces for tuning.

I would start with a needle 1 step smaller than what you have in the carb now, next I would do a spring change of 1 step, vacuum pulls the needles out of the main jet, change spring to change rate that happens.
 
ok, so today I looked inside carb while increasing throttle, and JBurch is correct. no fuel coming out on side of airhorns/main jet. when I increased the throttle I noticed the left side was not dripping fuel until I got the throttle to a certain spot. then the engine would rev and serge to the rate the gas was dripping on that side. took apart, cleaned and blow all out. reassembled same thing. no flow on that side. going to take carb of and go over it again. funny thing is car started doing this before I installed new carb. is there something that controls vacuum to that side of carb causing needle not to lift
thanks
 
problem solved.
bought edelbrock 1406 and installed it. car idles and runs great.
my carter is any identical carb to this edelbrock. will have to rebuild carter
thanks to all for your help.
 
You found your own problem no one said the main well or booster was clogged, so pat yourself on the back instead.
 
You still may have a problem. The pop-pop-pop is a tell tale.Disabling a plug, introduces an unburned A/F charge into the header. When it catches fire from another cylinder, AND air from somewhere, it explodes and the expanding gasses rush down the pipe and it makes a pop,usually heard out the tailpipe.. So Ima guessing the timing may be a little late and the header has an air leak into #8 pipe. Normally an air leak into a manifold is no big deal. But an air leak in a primary pipe destroys the strength of the scavenge pulse, and that means that cylinder will be down on power when the others are pulling hardest. Just something to think about.The air leak could be at the end of the header where it bolts to the exhaust pipe,in which case it would not effect power. But if the primary pipe is pulling in air up near the head, I would be scheduling it in for a gasket,no hurry.But if you are running a manifold, I would be in a bigger hurry to fix it, as this situation could damage the flange. Could damage.Not; will damage.
There is another possibility. If an exhaust valve is hanging up, when the descending piston is trying to pull in a fresh charge, then the piston may try to pull atmospheric air in through the tailpipe.This will/may also cause popping, as the fresh air meets the burning charge. This can be felt by putting your palm over the tailpipe on an idling engine. When the valve sticks, it will seem to suck your hand onto the pipe. If you have a crossover, you will have to cup both pipes, or plug one. This is more noticeable on vehicles with very short systems like lawn and garden equipment.Or small motorcycles, but I have witnessed it on cars too.
 
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