Trying to tune my 383, but can't figure this out.

It has always run fairly smoothly, good enough anyway, but I wanted to see if I could get it running smoother.

Carb is a Carter AFB Competition Series 9755 S.

I used a vacuum gauge - on the driver side port - to set the adjustment screws to 1.5 turns. That port by the way, has always been capped off.
You can see the short tube with the screw plugging it in the attached pic, on the left side.

The passenger side port is the one that has always been connected to the vac advance. When I hooked the vac gauge up to that port, I only got about 5 psi. Vac gauge connected to the driver side port gives me 20 psi.

Then I set timing, with vac advance disconnected, at 15 degrees advanced.

If I leave the vacuum advance disconnected, rev the RPMs up to 3500, the engine runs smooth and timing advances only to about 30-35 degrees (hard to estimate - I just have an old-fashioned Craftsman timing light, not a dialback).

But if I connect the vac advance, engine runs very badly as I increase the RPMs, and advances to around 45 BTDC.

Thanks in advance.

With todays fuels leave off the vacuum hose. That is way too much total. You only need about 32-36 for best power.

Thanks for the reply - yeah, I could just leave it disconnected, but it's always been connected before, so I can't figure out what is different now.


What would make the engine run so rough as soon as I start giving it gas - when the vac advance is connected?

What's the condition of the hose itself? I had a slight split in the hose where it connects to the can and cut it back a little. Ran much smoother. Cut back the ends or try a new hose.

What has been done to your distributor , is it the stock unit for your engine or ? If it's the stock unit and you have not done anything to limit mechanical advance you will get too much advance , stock dist. have 30 and/or more degrees built in.

As far as the different readings from one side to the other , it's supposed to be that way . The drivers side port is manifold vacuum and the pass side port is timed vac that will not see full vac till the throttle blades are open a certian amount .

you might want to put a REAL vac cap on it instead of that hose wit ha screw jambed in it .

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What's the condition of the hose itself? I had a slight split in the hose where it connects to the can and cut it back a little. Ran much smoother. Cut back the ends or try a new hose.

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Thanks for the suggestion - I checked it out, and it's in good shape, no leak.

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What has been done to your distributor , is it the stock unit for your engine or ?

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Not sure, probably stock, same one since I bought the car 6 years ago. I went ahead and pulled it (picture) but couldn't find any numbers on it. Must be inside under the points and weights?

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you might want to put a REAL vac cap on it instead of that hose wit ha screw jambed in it .

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I know - one of those little things I never seem to get around to. Kind of embarrassing...

Thanks for the reply.

You should have the vacuum line hooked to the port with the low vacuum. Its ported rather than full vacuum.

take off the vac adv hose and cap it. Make a mark on your harmonic dampener 2&1/4" (which is 35.5 degrees) to the right (clockwise) from the factory slit. Run the RPM up until it wont advance anymore (note the rpm) and lock the dist down w it timed at your new mark. Bring it back to idle and note your initial (post it). you stated that your eng functions well @15 initial (vac dis/plugged) so if this new total mark puts your initial way off from 15 you will need to change the slot lengths so both your initial and your total are pretty much spot on. THEN hookup your vac adv to the pass (ported) nipple and turning it inside w a 3/32" allen wrench CCW will slow the onset of the beginning of the curve by making it take more vacuum to offset the spring tension in the vac can and may need a can w less total (5-9 stamped on the arm works well which is 10-18 added at the crank). put the vac gauge on the dr side and adj for best vacuum on the idle mixture screws then lean them a bit then reset your idle speed then redo the mixture adjustments. Not sure how far to turn each mixture screw in to lean each one after you get the highest vac??. And what springs are in the dist as ideally you would adj: initial then the total then the springs picking a sp0ring combo that will keep you just under the pinging point @WOT then adj the vac can staying just under the pinging point @ PART STEADY THROTTLE at an RPM where the springs are maxed out. Still a prob check rotor phasing by drilling a 1/2" hole in the top flat of the dist cap 2/3 of the way between the center terminal and the #1 terminal closer to the #1 terminal and see how far from the underside of the #1 cap terminal the rotor is at the speed where it runs ragged by freezing the rotor w your light.

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You should have the vacuum line hooked to the port with the low vacuum. Its ported rather than full vacuum.

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Thanks, Stumpy - that's where it's connected to, the one on the passenger side.

I used the full vacuum port (driver's side) for the vacuum gauge, but not for the vacuum advance.

On a side note, does anyone know if in the attached picture, there should be a number stamped on the pad showing on the distributor?

I have seen distributors with a number stamped on them but for your year it should have a tag with the part number .

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When I hooked the vac gauge up to that port, I only got about 5 psi. Vac gauge connected to the driver side port gives me 20 psi.

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Don't mean to nit pick, but when you are using a vacuum gauge to measure carb port or manifold vacuum you are typically measuring in units of Inches of Mercury (In. Hg) not psi (pounds per square inch).

JohnRR - thanks, appreciate the info. I didn't see a tag anywhere, but I'm not going to worry about that right now. I'll try to check some out at Carlisle, for comparison.

rss - (and everyone else) - please, nitpick all you want. I take correcting very well! I saw the "in-Hg" on the gauge, right by the 20, but wasn't thinking when I typed psi in that post.

RapidRobert - thanks for weighing in. I printed out some of your other posts to try to do this stuff. I'm not sure if I'll have time to do your suggestions today, but I'll do them.

I removed the vac advance from the dist, and the only number I saw anywhere on the can is 3656677 and that is right next to one of the two mounting screws - the one closest to the arm. I looked all over the arm but did not see a 5-9 or any other number.

I'm also not sure yet on exactly where to find the 3/32" allen screw.

For now, I capped off all 3 vacuum ports, both on the carb and the 1 on the can, since it seems to want to run best that way - at least until I can get it adjusted properly.

I have been watching your post and you have gotten some really great advise.

If I am not mistaken, the 3/32 allen wrench is inserted down through the nipple hole on the vacuum advance unit and adjusts spring pressure on the internal diaphram of the vacuum advance.

Just thought of something. Is that still with points? If it is put a dwell meter on it and watch for the dwell change when vacuum is applied. If it does you need t look at the point plate where it pivots it my be worn and cause the dwell to change.

az426john – thanks – makes sense, I probably should have guessed that.

JAKE68 – yes, I still have points in there – I’ve got a dwell meter, so I’ll do that. Sounds like you are saying to hook up the meter and then connect/disconnect the vacuum hose to the can, while watching the needle for a change in dwell, which would not be good and might indicate movement of the plate that the points are on.

It will probably be Saturday before I get back to doing that and RapidRobert’s suggestions, but I’ll use this thread to post my progress.

I wish I could have tried those suggestions yesterday and today – but I had to get back to working on my exhaust. I thought I was done with it, but found out on Saturday that the pipe going into the front of the left muffler was barely in (I guess I'm hijacking my own post here...). Had to use the Lisle pipe expander in the pic – bought it 2 or 3 decades ago, used it once, so in a way it was kind of cool to dig it out and use it again. Tell you what though, it flat out ate my lunch, working with it in that position. Lucky I was off work today. The expander worked, I put the exhaust back together carefully and neatly, took a shower, and went for a test drive. No leaks, no rattles, the resonance is much better (not as intense), and with the capped off vac advance (bought some real caps!), the acceleration and cruising is better than ever.

I’ve been driving it to work a lot lately, so probably will for the rest of this week.

Thanks again to everyone.

Well, might have to do something before Saturday.

Had a good drive into work in the RR at 6 am, ran smooth and strong.

But coming home tonight I had some pinging/knocking under load - going slow uphill in traffic in 3rd while giving it some gas.

I'd like to drive it again tomorrow, but sure would like to do something quick to try to eliminate that pinging, just to get me through the week so I can take my time on Saturday with the whole procedure.

I know I could just keep the RPMs up - not go slow in 3rd/4th, but other than that, would the only quick fix be to temporarily advance the initial timing from the current 15 up to around 20?

Any suggestions?

Not hijacking, just a stupid comment. Looks like a good pic to show how to install muffler bearings.