Idle fluctuation

88_AHB

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Hey everyone,
Been trying to get my rebuilt 318 dialed in. Im having a issue with the car while idling. The rpm’s fluctuate about 300-400rpm. Ive checked for vacuum leaks nothing obvious, retimed and set idle air mixture. Im gonna check the float levels and see if they need adjusting. Is there something im missing to check/correct? Thanks
 

volare 77

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since you checked the obvious things then I would take a look at the distributor and check the movement of the weights, vacuum advance and the condition of the springs Have you put a timing light on it to see if the timing is bouncing around or staying steady?
 

Aspen500

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since you checked the obvious things then I would take a look at the distributor and check the movement of the weights, vacuum advance and the condition of the springs Have you put a timing light on it to see if the timing is bouncing around or staying steady?
I was thinking of the same thing but you beat me to it.:cool:

Usually a vacuum leak will cause a rough erratic idle, not a 400 rpm fluctuation.
 

88_AHB

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I haven’t put a timing light on it since this idle issue started. I’ll try that this weekend, the distributor is brand new MSD billet with matching coil.When I Initially fired it up and a few times after it never had this idle issue until a few days ago. Still haven’t driven it yet just put it into gear and fix fluid leaks etc. Would a blown power valve on the Holley might Cause this idle issue by chance?
 

Aspen500

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A blown power valve would most likely cause it to run VERY rich, similar to if the choke was stuck closed.
 

BudW

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I don’t know much about MSD distributors, but I know the (newer) factory appearing Chinese made distributors that fit our car has flimsy lightweight counterweights that will do just what you are describing. Even the later model Mopar Performance distributors use the small weights and are also “troublesome”.

Dist guts EI.png

2642261 Dist Shaft.png

The thick and heavy Chrysler style weights.

Cheby Dist Weights.png

The thin and troublesome weights used today.

Another thing I run across is the grease used in the distributors get hard after a few decades and I’ve seen that cause the ignition advance to do funny things.

The first thing I would do is either purchase, rent or borrow a timing light. If purchasing one, getting one with a dial-in advance is nice (but not a requirement).

Then watch the timing at idle. If this is a fresh rebuild and you have a new timing chain, then the timing light should not vary much. Loose timing chains can cause the timing jump around a few degrees, at idle – which can affect timing a bit.

On most cars (not just Chrysler), the (centripetal) weights in distributor shouldn’t start to move until over 1,000 RPMs (most cases at 1,500 RPMs).

If you have a Mopar with the skinny weights in them or an older one with rock-hard grease in it, then that could be the problem. If ignition timing is varying at idle (because of mechanical or vacuum advance) then that would be a valid reason for your timing to vary that much.

I don’t have an as extreme issue as you have, but the distributor weights on my ’77 wagon needs to be cleaned and re-greased – but I’m going to wait until something else in the area needs repaired, first.


If you have the “skinny/wimpy weight style distributor (aka: Chinese built) – then the only recommendation I have is to find an original Chrysler distributor and install and that most likely will fix your problem (but your MSD might be a factor). As to which distributor (advance curve) to get depends on your current application. If you have a highly modified small block, I would look for a 340 or 360 HP version. The rest of us, I would look for a 318 (2-bbl) version. Either version need to have a vacuum advance on it. If not, stay away (for Lean Burn or computer-controlled versions, which also has no mechanical weights inside).

If you purchase an older distributor, but before you put it in use, I recommend taking the top half apart (see top picture), cleaning well, add grease to areas the weights contact (pivot points, etc.) and a drop of engine oil where the two shafts touch (under the small felt under the rotor), test the vacuum advance (for leaks), install and set ignition timing, and call it a day (if test drive is good).

I have two almost new small block distributers (and a 3rd brand new one) in my garage with the skinny/wimpy weights (of Chinese descent) that I took in trade (for the older ones) - that “I” refuse to use. The older ones fixed their problem(s). I suspect the same will work for you (if you have a newer distributor). I don’t have any more, older distributors, either (for small blocks).

If the vacuum advance is disconnected when idle fluctuates, then there is not much else it can be to cause that drastic of a change.
BudW
 

88_AHB

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So,I hooked up the timing light to the car after it has warmed up. The timing was NOT jumping around while the idle was still fluctuating.

For the hell of it I replaced the power valve anyways since it’s very quick and easy to do.When I first fired up the engine after the rebuild the distributor was 180 out and popped a few times it idled normally. Then pulled it and reset it to TDC, thereafter the distributor was far out on timing and backfired through the carb.Got the timing where it needed to be, set the idle/air screws and tried to set the idle. After this time I first noticed the fluctuation in idle.

I will pull off the distributor cap and checkout the weights in there tomorrow.I’m not running any of the lean burn system. The 318 has a mild Howard’s roller cam in it stock heads for a 88, eddy rpm intake,600 Holley vacuum secondary single feed, headers.
 

88_AHB

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I had it set around 850rpm in park and it falls and rises around 300-400rpm difference from that.
 

mgoblue9798

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Hey everyone,
Been trying to get my rebuilt 318 dialed in. Im having a issue with the car while idling. The rpm’s fluctuate about 300-400rpm. Ive checked for vacuum leaks nothing obvious, retimed and set idle air mixture. Im gonna check the float levels and see if they need adjusting. Is there something im missing to check/correct? Thanks


Just a shot in the dark, but I had a power brake booster diaphragm fail and cause similar issues to yours. Unplug the vacuum line from the booster and put a bolt in the end of it and try it.
 

BudW

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I was thinking about the possibility of a bad brake booster. I don’t see a lot of bad Dodge brake boosters out there – but it does happen.
An easier method than to
Unplug the vacuum line from the booster and put a bolt in the end of it and try it.
is to grab ahold of the hose and plastic valve (where it goes into the booster). In most cases a good twist/pull and it pops out. Most cases, the valve has clamps on it - which makes disconnecting the hose, um, a pain. With engine running, you can place your thumb on the inside of the plastic valve and listen to engine run. If no change, then booster is not the problem.
20161019_181846.jpg

This is from my '86 Fifth Ave.

Note: It helps to use a fingertip of soap on rubber gromment to reinsert the valve.

Note: by listening to the air entering the booster (when valve is removed), after booster has sat for a while (10 min or longer) is one way to tell if booster is good (or not). It should hold vacuum indefinitely, or until you press on brake petal. There should be enough vacuum in booster to give you one complete power-assisted stop and maybe even a second power-assisted stop if car is in motion with engine off.
BudW
 

88_AHB

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I'll try that this weekend.I did install a brand new booster for a K car in it to clear the valve covers.I could of gotten a bad one from the parts store?
 

AJ/FormS

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300/400 is a lot; why is the idle so high? maybe you're getting into the transfers too far.
If it's 850 to 1200 then yeah I could see the timing curve bopping around.
But if it's 850 to 500 then no, not flyweights. But if Vcan is hooked directly to manifold vacuum, at the wrong carb port, then in combination with a vacuum leak, then Yeah 300/400 might be possible.
But if it does it in park, with the Vcan defeated and the unused ports all plugged,and the choke valve hanging verticaly, then IDK, cuz 300/400 is too much hunting for a carb with a PV, (Holley-type).
Then my guess would be a rapidly changing fuel level, or contaminated fuel.

BTW, as to signalling the V-can there are two port feeds; one picks up off the front and is a full-time manifold vacuum port; don't use this port. The other exits higher up on the metering block at the passenger side. This is called the spark-port and there should be no vacuum there at idle, zero. This spark-port is the one to signal the Vcan.This port might not start to pull vacuum until perhaps as late as 1700rpm in park. And that would be fine.
If the spark-port pulls vacuum at 850rpm, then the throttle valves are waaaay to far open, and with normal ignition timing between TDC and 20*advance this should not happen; So then I would suspect late valve-timing and/or too tight valve lash. But as to how that would cause a fluctuating idle all bt itself, I again don't know, unless the Vcan gets involved.
 
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