318 rough idle (20+ years in storage)


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Aug 21, 2022
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Ontario, Canada
To re-cap:

318 poly in my '67 Monaco was replaced in 1986 by reman LA318 bought from the same chrysler dealership where the car was originally bought from. I had to acquire oil pan, rocker covers, exhaust manifolds and possibly intake manifold, distributor and carb to complete this engine swap.

I put about 10k miles on the car from that time up until maybe 1995, car was in storage from 1999 and only last year did I fire it up for a few miles of driving before it went back into storage until a month ago. I don't recollect if it idled well or not back then, I'm assuming it did.

Current issue - it doesn't idle well. Very rough. Carter 2-bbl carb.

Warm (not hot) compression test came up with 142 - 152 PSI numbers.

I've taken the carb apart, throttle plate shaft has hardly any play, very clean inside. I've set the mixture screws 2 turns out and then have tried to set them to give best vacuum, turned them out an extra 1 or 2 turns, but vac needle bounces around too much to see any difference. I'm doing this with RPM set to 600-650. Vac bounces between 12 - 15.

Engine does not hesitate and responds instantly to opening the throttle either slowly or quickly. Runs better at 1000 and much better at 1500 rpm. Vac reads steady 20 but I should verify that.

I've disconnected the vac line to the brake booster and plugged it. No change. Choke pull-off works very quickly and strongly immediately after engine is started.

Prior to starting it for the first time in 20+ years, I changed the oil, squirted oil into the cylinders, cranked it by hand a few turns, put in new plugs (Champ RN14YC).

Taking the PVC valve out of the rocker cover and blocking the valve with my thumb, I feel vacuum, engine doesn't like it, rpm's drop a little.

A few days ago I put in new points / condensor / rotor / ballast resistor. Did not touch the cap or wires. No difference. Set the gap to 17 thou. Did not remove the distributor when changing parts. I could (or need to) check dwell and timing, need to get timing gun and meter. I bought a cheap little battery-powered digital RPM / hour meter, it has a wire you wrap a few turns around a plug wire to sense RPM. Very slow response time, but it does show RPM, that's what I'm using to set idle rpm. I'm assuming it's accurate.

I will aim at doing a leakdown test next.

What is on my mind is this:

- acquire new timing gear/chain set for future install (will include chain tensioner)
- change to electronic ignition
- how to determine if I have a valve or valve-spring issue?

Has the gasoline changed that much in the past 25 years? I'm using el-cheapo 87 octane gas here. Best we have here in Ontario is 91 octane if that's an issue.

I know it really looks like it's a carb issue, but I want to know if something more serious valve-wise or timing chain is going on.

Does carb float level play a huge role here in idle stability? I have a bit of gas weeping through the airhorn gasket, I should look into that, maybe check the castings for flatness.

Maybe the distributor has too much play? Worn lobes?

No easy way to look at timing chain without draining coolant / removing water pump?

Why not just rebuild the motor, considering all you are thinking about doing?

The vacuum bounce can indicate a valve train issue, as in a burnt valve of similar, yet your compression figures do not support that. IF you've got good compression, a leak down test will confirm the quality of the seal of the rings, valve, and little more to make the engine run smoother. The main thing is that you can hear where the air is escaping from, as to if you hear it in the crankcase (rings) or exhaust system (exhaust valves). Not really needed, to me. Just confirmation of other tests.

A carburetor mixture issue will usually mean a "drifting" vac gauge reading rather than "a quick pulsation/bounce" vac gauge needle action. The "wide bounce" is more indicative of a valve sealing issue, though.

Not an ignition points issue, even with a worn breaker cam in the distributor. So get that tach/dwell meter/timing light BEFORE doing the electronic ignition conversion. A self-powered inductive-pickup timing light will work well. Some claim the dial-back timing lights are not good, but the one I bought from Craftsman back in the earlier 1980s has always given good results for me. But for normal timing checks and such, a more-basic, high-quality timing light is just fine. @halifaxhops might have some in stock?

IF you can find a repair shop with a diagnostic machine with an ignition trace scope on it, then you can watch each plug firing in real time. As to dwell and other factors, plus how many volts are used to do the firing. If one of the traces might be low, it can indicate lower compression on that cyl. Might not really find a "smoking gun", but then you'll know exactly what is going on in the ignition area.

The main thing which has changed with the gas between back then and now is the decrease in the lead content. Plus lots of other small things in the formulation and current blends now have many more cleaning additives in them. Still, it's still what it is and usually will not cause a rough idle situation.

It might not make a lot of difference, but you did not mention new plug wires in your equipment changes. Perhaps some newer/fresher wires might help? Might find a good NOS or used condenser, too?

You might seek out some higher-octane gas from a local vendor (usually, or used to be, some sold at drag strips by "drive-up" vendors) or from some speed shops (5 gallon drums/containers). I doubt this might help, but would rule out the fuel issue, typically.

If the engine runs normal after about 1300rpm level or so, the issue can be in the idle system of the carb. Normal "kit" activities and solvent soaks will not remove hard deposits from the fuels, so might need to mechanically remove them. PM me for details.

Now, as a point of reference, while Chrysler V-8s will idle smooth, they are not as smooth as a Ford FE (390 family) V-8 or modern EFI engine, by observation. Might the motor mounts being aged amplify this?

One other way to approach the fuel issue is to get some fuel system cleaner with "PEA" content in it. PEA is the nitrogen-based cleaner that is in some current Shell gasolines, just in a small bottle for spot use. It's supposed to be the best cleaner, from what I've seen online. Then drive the car, gently at first, then more normally for 1000 miles or more and then re-evaluate.

DO get the timing light and dwell tach and verify the tune-up specs in the process, though. You can use those things in the future. No need to spend massive amounts of time and money chasing a rough idle situation when the car runs normally otherwise. Perhaps tweak it a bit, which costs little to do. Simple, less expensive things FIRST!

Just some thoughts and observations,
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In reading back over your comments, this is not a timing chain issue. Nor is it a carb float level issue. The main jet feeds are from the bottom of the float bowl. If there's enough fuel supply to keep those jets covered nicely, not a critical issue past that. Especially in a low-demand fuel use situation as the engine being at idle.

The only time breaker cam lobe wear can become an issue is when you set the gap to specs and the dwell reading is not in specs. Been there, done that. Using a dial indicator to set the point gap, with the distributor out of the engine, is how I discovered how much wear the lobes can have, with their heights not being universally "the same". Yet we've always used "a lobe" (randomly) to set ignition point gap for centuries, not worrying about or considering that they were not "all the same" after 60K+ miles of use. Perhaps I was being a bit nerdy in using a dial indicator, but I wanted to discover what was going on. Which later led me to an old Mopar Performance (when Chrysler was still using distributors OEM) electronic ignition conversion kit. Made no real difference in idle quality, but I knew it was all new.

Distributor shaft bushing wear comes from the force of the rubbing block on the breaker cam lobes as it turns. There is an acceptable spec range for that wear, but it should not cause the issues you describe. After all, when the engine is running, that pressure from the points' spring is keeping things in place. Most probably not a cause of "rough idle" or the bounding needle on your vac gauge.

Same thing with the timing chain, with the engine running the "drive" side of the chain is tight as the "other side" is where the flop can happen.

Now, if the timing chain is OEM to that reman motor, IF it is the nylon-toothed cam gear, you might put that on your schedule to do in the future. As an "insurance" issue for long engine life before a tooth fractures and falls off due to age, more than anything else. Will not change how the engine acts, though.

Sorry for the lengths,
I don't have an actual 2-guage leakdown tester, but I improvised. I have an air compressor that I use to charge a 5 gallon tank. The tank can't give much air without giving up some PSI.

I charged the tank with a little over 100 PSI and used an inline valve with the compression-tester adapter line to give each cylinder a shot of air. Each cylinder at TDC, crank turned by hand, with a socket extension bar in the spark plug hole so I could see TDC. Marked it on the flywheel, the various positions overlapped nicely (2 cylinders will be at TDC at the same time, one exhaust / one compression).

All plugs out, engine is cold.

With a stop-watch, I gave each cylinder 10 seconds worth of air and recorded the tank pressure drop. Charged the tank back to 100 when it got to 85.

I got a 5.3 average PSI drop (range 4 to 8).

BUT, I swear that when I did 7, it acted funny, like a huge leak. I re-adjusted the crank, tried it again, I think it was still there. At one point I thought I was getting air out of CLY 5 next door. But then the crank moved so I re-adjusted and I thought I'll put all the plugs back in finger tight except for 7 and try it again. This time it held pressure. I removed the plugs one at a time, last one was 5, it still held pressure. I did the last 2 cylinders and then rotated back to 7, it still held pressure just fine.

I think at one point when working with 7, that I applied air and heard something. I thought maybe there's a head gasket leak between 5 and 7.

While doing this, it seemed that for all cylinders the air was escaping through the rings because I was feeling a slight breeze from the oil filler hole, not the carb intake. But when 7 was leaking bad, I wasn't feeling that from air intake nor the oil hole. I was alone so there was nobody around to listen to the tail pipe.

But like I said, I wasn't able to reproduce this leaky 7.

Sticking valve?

I'll take the valve cover off next and see what I can see.

I have not checked for a leaky distributor advance, but it's the only thing left as far as vacuum lines goes. It's a pretty small line though...
Regarding leakdown testing - I came across an ASE instructional video explaining the leakdown test, all the various tricks and things to watch for. The lecture seems to end, then the guy comes back and says well, if you're still here, I might as well tell you that even though the ASE still has this on the exam and wants you to know about the leakdown test, the truth is that in the shop, because time is money, you'll probably use a dual-camera borescope inspection camera to see the condition of the piston and valves. You can take a picture and show the customer what the problem is.

I was thinking a few years ago of buying one of those things...

But would a camera view show you a head gasket problem?

I think the cameras are used mostly to show the customer they need a carbon flush / decarbonize the engine...?
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A leaking head gasket can show up by ballooned radiator hoses and possibly coolant loss too. If the radiator upper hose expands before "the water gets hot", that might be an explanation. You can also rent a tool (with appropriate fluid) that checks for combustion gases in the coolant, too.

A borescope camera (aka "Critter Cam") can also be had from Harbor Freight. You can also order the camera cable and software online to plug into your laptop or phone, too. Even less money!

I'll take the valve cover off next and see what I can see.
and while you're there, pull the rocker shafts. check for bent push rods. with it torn down, put your shop air straight into the cylinders as you were doing. you'll know pretty quickly if you have any compression leaks.
The vacuum bounce can indicate a valve train issue, as in a burnt valve of similar, yet your compression figures do not support that.
that was my first thought but the vacuum gauge doesn't lie. it has to be there somewhere. would a dead skip due to a single cylinder ignition failure produce such a large vacuum variation?
Took driver's side valve cover off, everything looks very clean, inside of cover, etc, like it was rebuilt yesterday. Not sure what I can do here to inspect pushrod / valves / springs without taking the head off (I do not want to do that unless I absolutely know it has to come off). No head leaks anywhere (oil, etc) so I don't want to mess with it.

Heads are 4027593 (1977-1984, 1.78/1.50 valves)

Pass. side exhaust manifold 3830687, driver side looks similar, different outlet position, can't quite make out the number. These might be correct for a 360? No heat riser.

From what I see on the web, the heads had small ports under the exhaust for an air pump? I seem to recall that I needed to get manifolds to cover those holes.

Intake is 2468960 which seems to be 67-69 318? I assume intakes for the polysphere vs LA blocks were not compatible?

Distributor vac advance is fine, does not leak.

Out of curiousity, when did they start making heads compatible with unleaded gas?

Will check plug wires with ohm meter next. Or maybe pick up a cheap set and replace. They look relatively new.

New plugs (Champ RN14YC) were put in a year ago, like I said have only burned about 7 gallons of gas since then, a couple weeks last year and now a few weeks this year, mostly idling, maybe 10 miles of low speed driving. Plugs are black / sooty, not wet. Oil looks great. Coolant (several decades old) is still bright fluorescent green.
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First model year for full-unleaded fuel use was 1973 or 1974. I know that 1972 Chrysler recommended one tank in three of leaded fuel.

Plugs should not be sooty black. That means either an over-rich mixture or poor spark. The oil probably has a fuel smell to it, on the dipstick? Even idling for extended periods, the plugs should not be sooty. Presuming timing is to spec?

Main thing to check plug wires for is continuity. Ohms can vary with the length of the wire, which is normal.

Take care,

Compression test on all cylinders give 150 to 160 psi after 4 cranks. First crank seems to put them all at or above 100, 3 cranks puts them all at least 140. This is even when cold.

My "custom" leakdown test seems to show cyl 7 leaking more air than all others by a factor of 2 to 3.

Spark plugs are Champ RN14YC and were put in new last year, probably have a grand total of 20 miles and 4 hours of run time on them, are sooty. I have old plugs, will verify these were same. Old plugs had definately been worn, gap increased, electrodes rounded.

Vacuum reading was really bouncy, but I connected it to the easiest place - at the remote trunk vacuum can. That's a long, thin line. I repositioned it to the nipple coming off the brake booster, using a very short, larger line.

Started the car today, verified the idle was still rough.

First thing I changed was the coil. Secondary resistance was 24k, this seemed off by a factor of 2. Had an OLD replacement that was in spec. In changing the coil, the ignition wire from coil to distributor was disturbed, the connection came off, there was corrosion. I have new replacement wire set, same resistance as current wires. Put in coil wire. Didn't want to change 2 things at once.

I had previous measured resistance from dist. cap inside aluminum flats to the spark plug ends, I was getting readings from all wires. My wires are 500 ohms per ft, silicone, 20+ years old.

I think engine runs smoother, but I think can run better. Vacuum needle is more stable, 17 PSI is best I can get at 500 - 600 RPM, needle fluctuation is about 1 PSI. At 1400 rpm engine is very stable, vac was at least 20 or maybe 22, but it was also stable at that RPM previously.

I am concerned about gas seepage or wetness seen on the carb air horn gasket. I don't know why the gas level in the bowls should be so high as to reach that gasket and seep through.

I can dial in the idle screws, even turn one of them all the way in, and the engine still runs (but very roughly) at 500 - 600 rpm.

I took the needle valve (at the fuel inlet) out, have to look at these things with a magnifier to see imperfections. This one might have had some, I had 4 spares, one with no visible imperfections so I put it in. Maybe now I'm seeing LESS seepage at the horn gasket?

Idle screws seem to want to be 1 turn each or a little more to give best idle / best vacuum. Seems on the low side?

I have yet to mess with timing, don't have a gun, maybe I will look for cheap gun locally. I'm sure I set the timing years ago at the best position.

I could change rest of spark wires and cap (blue streak, brass contacts) and that would mean everything on the ignition side is new (or different) - points, condensor, rotor, cap, wires, plugs, ballast resistor, coil.

That would lead me back to the carb.

Is the carb running too rich? But I can't change that with the idle mixture screws?

Should I be seeing 20 inches of vac at 600 rpm given the great compression numbers? Change gas from "cheap" 87 to expensive 91?
I wouldn't worry about it. Sounds like you have it all tuned up now. Unless it has a dead miss (which is easy to diagnose) it is just a little rough running. It may come out of it. It could be a valve or spring. It could be a whole list of things. Run it!
Looks like that with the ign coil and connecting wire issues taken care of, you ARE closer to where you desire to end up, by observation. Manifold vac is more normal and smoother. You will NOT probably be able to get a rock-solid steady vac gauge needle, so having one with only about 1" Hg variation is good.

As to the idle needle adjustments, the 1.5 turns out spec is only the staring spec, not the finished spec. If the engine like a bit leaner mixture, closer to 1 turn, so be it. Nothing wrong there. Same if it likes a 1.70 turns out ending setting.

As to the old spark plugs, with a point file, you can put sharp edges where rounded edges now are, with a point file. Then re-gap to .035"-.039". Just takes a little bit of time. Then they can be used later. IF the heat range number might be off a number or two (colder), no issues there, either.

One thing I learned many decades ago, if the engine runs well at whatever the factory specs are (or close to it), don't worry about the manifold vacuum levels. Vacuum levels will increase a bit with 1000 no-load rpms, from idle (which should be closer to 600rpm in "P", not 500rpm), but nothing related to decent piston ring seal or compression pressure.

So . . . verify where the point dwell reading is, set the ignition timing to the general specs for the year of the vehicle the engine is for, set the carb idle mixture (which is ONLY for the idle mixture) for highest rpm per needle. Adjust the hot base idle speed to 575-600rpm in "P". Then take the car for a short drive, noting off-idle throttle response in normal driving, nothing "Performance" as such, just normal traffic. If all does well, take it back and park it, letting it cool down. Then check cold start performance and drive it for about 10-15 miles. Return and park until cold.

As the use cleans/burns-off the spark plugs sootiness, things might improve a bit in how it drives. No "miracles", though. Just incremental improvements, hopefully. Sure, you can pull a plug or two and see what they look like, if desired, but then re-install them and start putting more road miles on the car, not idling for longer periods. The rest of the car will probably quietly be yelling "YEAHHH!"

Also monitor the atf and other fluid levels, including the BRAKES.

I'm going to post an update dealing with ignition, so look there for the next post.
At 17 inches vacuum, what is the timing set at?
I recommend bumping up the timing to 10 degrees BTDC, reset idle at 750 RPM and vacuum should increase to 20 and a smooother idle should be the result.
Yes,avoid 87 octane corn swill.
In Ontario, I burn and store my cars with 91 octane non ethanol; available at Canadian Tire, and Shell and Ultramar stations.
PetroCan and Esso offer 94 octane but DOES CONTAIN EHTANOL.
I burn that too but do not store the cars with it.

I have two 318 cars. 69 Fury III and 73 Satllite wagon. Both have higher initial timing than factory to run smoothly on today's gas. The 69 has 12 degrees with Pertronix ignition inside factory point distributor, and the 73 has a recurved electronic ignition distributor set at 20 degrees, and all in at 2800 rpm with a total of 37 degrees.
Both cars have hotter Accell Super Stock coils, and plug gap is at .37 ( not .35) for a better spark.
You can benefit by going electronic ignition and recurve the distributor.
Hope this helps


At 17 inches vacuum, what is the timing set at?
I recommend bumping up the timing to 10 degrees BTDC, reset idle at 750 RPM and vacuum should increase to 20 and a smooother idle should be the result.
Yes,avoid 87 octane corn swill.
In Ontario, I burn and store my cars with 91 octane non ethanol; available at Canadian Tire, and Shell and Ultramar stations.
PetroCan and Esso offer 94 octane but DOES CONTAIN EHTANOL.
I burn that too but do not store the cars with it.

I have two 318 cars. 69 Fury III and 73 Satllite wagon. Both have higher initial timing than factory to run smoothly on today's gas. The 69 has 12 degrees with Pertronix ignition inside factory point distributor, and the 73 has a recurved electronic ignition distributor set at 20 degrees, and all in at 2800 rpm with a total of 37 degrees.
Both cars have hotter Accell Super Stock coils, and plug gap is at .37 ( not .35) for a better spark.
You can benefit by going electronic ignition and recurve the distributor.
Hope this helps

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Non ethanol is no longar available in Ontario, came into effect Jan 23.
There's this trick of taking today's pump gas and mixing it with water (really shaking / mixing it up) and then let it sit overnight. Preferably use or make a container with a valve / hose arrangement on the bottom. The water will pull the ethanol out and form a layer under the gasoline. You open the valve and drain the water/ethanol layer out, giving you non-ethanol gas. But I guess you'll end up with lower octane gas vs what you started with.

The car is in storage now, but I did buy and install one of those "Dromodary" brand BBD clones off Amazon for under $100:


It came with a couple thin gaskets, and my original wasn't up to snuff so I had to go buy a new one locally (about $15). It's about 3/8" thick. I'll have to do something about the choke pull-off linkage because it's not long enough. This carb has a bunch of ports I had to close off. It's got integral throttle spring which apparently doesn't replace the existing spring, I guess at some point regulations required it.

This type of "tin-hat" BBD is new to me. The power piston is directly driven by the accelerator linkage, vs only vacuum with the older type. I was putzing with this carb for a week around mid-october, it works, maybe it's better than what I had, maybe the idle screws are more responsive and actually do something. But maybe the so-called gasoline that I was using (87 octane, unknown amount of ethanol) is a confounding factor. I drove the car to storage with the carb, so it worked ok for that.

I've got it in my head that what I need is a tail-pipe sniffer that will tell me if I'm too rich or lean. Has anyone every done this? For a while I was also looking at one of those Air/Fuel meter kits you can install, with 02 sensor mounted in the exhaust pipe near the engine. I'd really like to have this engine doing some clean burning and not sooting up the plugs.

But I've got bigger plans for the car next year. Like swapping in a new front stub frame (which I have) and new suspension parts (which I mostly also have)......