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'68 Dart off idle stutter
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Author:  Dave145 [ Tue Sep 16, 2025 7:29 am ]
Post subject:  '68 Dart off idle stutter
Hi everyone, been a long while since I've been on here. I've got a '68 Dart 270 with a 225 in it that's giving me fits. The engine is completely stock. I had it professionally rebuilt January of this year. The problem I'm having is an annoying off idle stumble, and rough idle. The car has almost always had somewhat of a tremor at idle even before the engine was rebuilt. The car runs great once you clear the stumble and has no problem on the highway doing 75+. I routinely get around 19mpg. At one point I got upwards of 22, but that was two years ago and before the engine was rebuilt.

The carb is a Holley 1920. I think it's original to the car. I rebuilt the carb this weekend using the manuals on this site. The float is set properly wet and dry, my main jet is a #57, and my accelerator pump produces a very strong stream/spray of gas when the throttle is opened. I have a new base gasket that's about 3/8" thick installed. My idle speeds are 700-725 in park and about 650 in gear. My mixture screw is turned out 2.5 turns and I have a steady 20" of vacuum. My choke works properly and comes fully off when the car is about 160 degrees.

I seem to have good fuel pressure at roughly 6psi. When the engine was rebuilt I replaced the original leaking fuel pump with a new Carter one. My gas tank is new, and I have two fuel filters installed that are about a year old. I have tried getting gas at multiple different gas stations to rule out bad gas, and that has not helped.

My timing is set to 6* BTDC. I tried the factory spec of 2* BTDC and that did not help. My vacuum advance can is new, comes on at 12" of vacuum and provides 10* of advance. I forget what my centrifugal advance is set to, but I know I haven't changed it from whatever it was factory set at. The weights in my distributor move freely. I replaced the points and condenser and gapped my points at .019"-.020". The distributor is original to the car to my knowledge.

I adjusted the valves with the engine hot and idling in gear. I have run them at .10/.20, .12/.22 and .14/.24. The car seemed to accelerate the best and idle the best at .12/.22 so that's where they are currently set.

My coil, plugs and wires are new. The coil is a generic car quest coil, the wires are Delphi I believe and the plugs are NGK plugs with the crush washer removed. I will get the number off the plugs on lunch. Last time I pulled them, they were all evenly a nice grey color.

My intake gasket does not leak, my compression on each cylinder is about 160lbs and my exhaust is not restricted. My heat riser opens and closes smoothly. I have a 180* thermostat in the car because I drive it a lot in the summer. I tried a 195* thermostat and that did not help. It has this problem when it is both warm 80*+ or cold 40-50* outside.

I'm fairly stumped here. I don't want to fling parts at it, but I'm not sure what else to check. Does anyone have any ideas?
Author:  Dart270 [ Tue Sep 16, 2025 8:42 am ]
Post subject:  Re: '68 Dart off idle stutter
Hi Dave,

Thanks for your very exhaustive list of tests and results. It appears you are doing everything right and trying all the right things to track down this problem.

After this first reading, I have two thoughts:
1) You may have a damper that has slipped and means the ignition timing mark is not correct. I have not had this happen to me personally, but others have. You could take a degree wheel on the end of the crank, find TDC with a stop tool (make sure not to let the stop tool get close to a valve, which can happen if you make your own stop tool that is not straight, ask me how I know...), and then check that the damper mark is hitting zero on the timing tab when you are at TDC. This takes some time, disassembly, and fiddling, but might be worth checking.
2) Did you degree your camshaft? If you replaced it and/or the timing chain set, then your cam timing could be on the retarded side, which can cause or exacerbate a stumble off idle. Please tell us what are your cam specs too, if it's not a factory cam. This is more involved to fix, but if you are running out of options....

This still could be a carb issue, so I wonder if you could borrow someone else's known-good carb to swap on?

I'll contemplate further. BTW, I LOVE 68 Dart 270s! I still have mine, which was my first car I bought in 1988.

Best,
Lou
Author:  Dave145 [ Tue Sep 16, 2025 9:36 am ]
Post subject:  Re: '68 Dart off idle stutter
I can make a piston stop tool and check the damper on my lunch. I've never seen that before either, but anything could be possible!

As for the timing chain and cam being degree'd I would hope the shop that built the motor did that but ya never know.

I do actually have another carb that I've tried. It's a Holley 1945 that's pretty much new in the box from '76. I put a couple hundred miles on with that carb and the issues persisted. If anything it idled a tiny bit better, but the rest stayed the same. I'm not against trying it again though just for giggles at this point.
Author:  Dart270 [ Tue Sep 16, 2025 10:27 am ]
Post subject:  Re: '68 Dart off idle stutter
My guess is that 99% of shops will not degree a cam, and if they do they will charge you for it with a line item on your bill. They will just line up the dots. Most cams/chains I do are within a couple of degrees of right, but a few times they have been way off. If you have the degree wheel on the crank and have a dial caliper and mount, you can measure off your #1 intake valve pushrod (or even the valve retainer - reduce lash to zero) without pulling the head or anything else.

Lou
Author:  Dave145 [ Tue Sep 16, 2025 11:07 am ]
Post subject:  Re: '68 Dart off idle stutter
I'll have to check into the degreeing then. I kinda wondered about it, but I know I never asked about it.

I made up a piston stop at lunch and was able to verify the balancer is correct at TDC.

I did some more checking around since I had a minute and this problem has only managed to get worse.

Plugs 1 and 3 had a drop of oil at the tips of their electrodes. Not too worried about that. All 6 plugs are the same color.

The spark out of the coil while cranking the engine leaves something to be desired in my opinion. With a little over 12V in the battery at the time, I had a thin, orange spark.

Popped the cap off the distributor. Two terminals have been being ground down by the rotor hopping around. The rotor itself doesn't sit tight to the shaft and bounces/spins around with ease. The points don't look aligned either, despite the fact they were when we installed them a couple months ago. The distributor shaft moves up and down a lot. I'm not sure how much is too much, but I do know it moves at least 1/16" of an inch. It has *some* side to side movement, but I don't think it's enough to cause this big of an issue.
Author:  Reed [ Tue Sep 16, 2025 4:20 pm ]
Post subject:  Re: '68 Dart off idle stutter
Back when I was still running Holley 1920s, I pretty much had to increase the main jet size on all of them to 63-64 to cure low-speed lean issues. Try swapping your main jet to a 64 and see if it helps anything.
Author:  Dave145 [ Tue Sep 16, 2025 4:55 pm ]
Post subject:  Re: '68 Dart off idle stutter
Quote:
Back when I was still running Holley 1920s, I pretty much had to increase the main jet size on all of them to 63-64 to cure low-speed lean issues. Try swapping your main jet to a 64 and see if it helps anything.
I think I have a 64 laying around. I'll give it a try and see!
Author:  Dave145 [ Tue Sep 16, 2025 8:32 pm ]
Post subject:  Re: '68 Dart off idle stutter
This evening's update. I pulled the distributor out and regapped, realigned, and cleaned the points. .019 gap on every lobe. Put distributor back in and set the timing to 6* BTDC. No change. For giggles, I swapped a known good coil into the car and that did nothing as well.

I had a #64 jet laying around, so I installed that in my 1920 and went for a drive. It made things better...somewhat. At cruise it was extremely rich and the sputter on take off remained.

I opted to reinstall my NOS Holley 1945 carb in the name of science. Seems to run a lot better currently. I put a fresh tank of gas in this evening and plan to drive it to work tomorrow to see if the problem persists. So far it seems promising.
Author:  Reed [ Tue Sep 16, 2025 11:21 pm ]
Post subject:  Re: '68 Dart off idle stutter
Hmm. Then I second the recommendations to (a) verify the damper outer ring has not slipped and (b) bite the bullet and pull the timing cover to verify the cam was degreed properly. I have had brand new timing sets be as much as 8 degrees out.

You might also want to install a wide band O2 sensor to help diagnose rich/lean mixture issues.
Author:  Dart270 [ Wed Sep 17, 2025 1:46 am ]
Post subject:  Re: '68 Dart off idle stutter
I would not worry about the up and down movement of the dist shaft. Many of them are like this and I've seen no issues with running them. If your rotor is contacting the cap, that sounds bad. Try replacing the rotor and probably the cap. It should not wobble at all on the dist shaft. Maybe the top of the dist shaft is damaged, if it is not the rotor. Measure the voltage between the coil terminals with the ign switch to ON. Depending on whether the points are closed, it should be around 11-12 V (open) or around 6 V (closed), IIRC. It's been a while... Best to do everything else before you pull the timing cover.

Oh, I just discovered I have a bunch more spare distributors than I knew. If you need a good electronic dist to switch to HEI, I can ship it soon. I might have a good used points unit too. Where are you located?

Lou
Author:  SlantSixDan [ Wed Sep 17, 2025 9:29 am ]
Post subject:  Re: '68 Dart off idle stutter
Quote:
I had a #64 jet laying around, so I installed that in my 1920 and went for a drive. It made things better...somewhat. At cruise it was extremely rich and the sputter on take off remained.
Also, your gas mileage would have dropped off a cliff.
Quote:
I opted to reinstall my NOS Holley 1945 carb in the name of science. Seems to run a lot better currently. I put a fresh tank of gas in this evening and plan to drive it to work tomorrow to see if the problem persists. So far it seems promising.
The 1920s are known to just end in ways that cannot be addressed by a rebuild. Corrosion/etc deep inside the metering block, and there's just nothing for it any more, since new metering blocks have been NA for decades.

Distributor end play can affect how the engine runs because of the helical distributor drive; if the shaft moves up/down, that's going to mess with the spark timing. More end play = greater extent. I can't get onside with don't-worry-about-it/a-lot-of-them-are-like-that; take a look at how specific the FSM is on the minimal allowable end play in the distributor. Even more end play = broken parts at the top and bottom of the distributor.

The HEI upgrade is a very good one.

Cap/rotor: Read and follow the suggestions in this long (but worthwhile) thread.
Author:  Greg Ondayko [ Wed Sep 17, 2025 10:05 am ]
Post subject:  Re: '68 Dart off idle stutter
A tip that I found that is even easier to confirm TDC compared to a piston stop is the digital borescope camera from HF. It worked well when last I needed it for such things.
Author:  Dave145 [ Thu Sep 18, 2025 5:59 am ]
Post subject:  Re: '68 Dart off idle stutter
I'm back! So far with the 1945 the car seems to be running better. I still get a slightly lumpy idle from time to time, but it's a lot better than it was. The off idle sag is gone, so I think as Dan suggested the 1920 may have just gotten too corroded inside to function anymore. I have ordered up an electric choke kit to get the choke to function properly with the new carb. Right now I just have it wired open to allow me to drive it.

Between the coil terminals I get approximately 8-9 volts with the points closed, and about 6 volts when they are open. I went ahead and ordered another cap and rotor, because I don't like how the rotor fits on the distributor shaft.

Currently it's too early to tell what kind of mileage I am getting, but I should have some sort of clue by the end of the week.
Author:  Dart270 [ Thu Sep 18, 2025 7:32 am ]
Post subject:  Re: '68 Dart off idle stutter
Sounds like progress! I had 1920 carbs that would never come off idle right, and maybe one 1945, but the 1945 seems like a better choice if you have a good one.

Lou
Author:  Dave145 [ Fri Sep 26, 2025 8:14 am ]
Post subject:  Re: '68 Dart off idle stutter
Well it's been a little over a week since I did the 1945 swap so I figured I'd give an update.

I added an electric superchoke kit to aid in starting on the cooler mornings. So far it works fantastic.

I got 352 miles to a full tank of gas, which for my car is roughly 20-21mpg. This was mixed driving, everything from stop and go traffic to 70mph highway runs.

The idle has mostly improved. There are times it will idle rougher, but that's mostly after a harder stop. I'm guessing it's excess fuel in the intake making it's way to the end cylinders. 90% of the time I'd say the car idles fairly smooth.

The stutter off idle is gone!

I'm considering playing with my timing curve to give the car some more prep while accelerating, as it is pretty slow. For comparison, I have an '87 Duster with the N/A 2.2 in it and it "feels" quicker, even just putzing around town.

For the time being however, it seems to live!
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