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One for you OLD School Mechanics

Having an issue (among many others) with my 1950 cadillac. (Stock 6 volt 331 V8). On very low throttle it doesnt burn clean - it hiccups. Driving around with normal throttle its fine. Ive got recent/new plugs, wires, cap, coil, rotor, points, condenser. Essentially replaced everything in the ignition system - although I have not rebuilt the distributor itself. If I remove the vacuum line from it I DO have vacuum on the line. Ive put the timing light on it and tried various settings, and adjusted the carb idle screw. Fuel pump is rebuilt and works fine.

The only ideas I can think of is replacing the vacuum advance unit on the distributor and perhaps the timing chain. Thoughts?

You can test the vacuum advance by putting the timing light on it and revving the engine. No sense changing it until you find out if it’s broke.

If the valves were off their timing due to chain wear or stretch, I would think you’d feel that under acceleration rather than at idle. Have you put a vacuum gage on the intake?

Have you actually checked for erratic spark? While a scope is the way to go if you know how to use one, you should be able to get an idea if the stumbling is ignition related by checking for regular timing light flashes. If you have hat, then you may be looking a valvetrain problems.

Which brings me to the valvetrain… have you removed the valvecovers, run the engine, and checked for sticky valves? A vacuum gage on the intake will also disclose sticky valves by wobbling the needle. Have you checked the lash adjustments?

Post back with the answers.

By the way, you may want to get a book on rebuilding this carb, if you can find one. Adjusting the idle screw without adjusting the mixture screw doesn’t compensate for an incorrect mixture at idle. First the mixture needs to be adjusted, then the idle. I’m willing to bet that the carb manufacturer provides a manual online. I’ve discovered that many old manuals are now online for things like this.

Nice ride, by the way.

I am not a mechanic but seen a fair share of carbs and I think TSM might be right about the carb. Might need a rebuilt or cleaning.

have you had the carb rebuilt? does the accelerator pump diaphragm have a hole or crack or bad seal?

ethanol is not your friend and all rubber fuel lines will eventually succumb to it s evil

a re built carb should have parts that will handle the ethanol, unless you found some new, old stock. in which case it will eventually deteriorate too

EXCELLENT comments about the ethanol and old carbs. I’m ashamed I didn’t think of it.
A tip of the hat to you, wes.

Count me as a second one thinking it’s a carb issue. Sounds like the idle circuit is a bit wonky (technical term) on this one.

I can’t talk “nuts and bolts” credibly here: my carb experience is all small-engine and motorcycle, and I’d sooner “Keep my mouth shut and be thought a fool…than open it and remove all doubt!” Notwithstanding that, the carb employs SOME mechanism that controls mixture–predominately at low CFM, and with a moderately large manifold vacuum. That’s what I’d look at first.

I dont have a vacuum gauge, I wonder if I can rent one or buy one cheap? Same for the scope. Learning as I go here. I bought one of the “nicer” timing lights from harbor freight. I planned on rare usage and this particular one got good reviews. That being said, a few times tonight I had trouble getting the timing light to light up. Not sure if this was from the gun or not. Once I did get it lit, the pulse was consistent, so Im assuming that doesn’t indicate erratic spark. I have not removed the valve covers, although I plan to when I repaint them and replace the gaskets (passenger side oozy). Thanks


Buy a vacuum gauge on the cheap

Either buy a good used one cheap on ebaby

Or buy a cheap new one at sears or some such store

Sorry - should have mentioned carb has been rebuilt, and I was told the rebuild kit was ethanol compatible. I have messed with the mixture screws as well by moving it until it runs rough and then dialing back until smooth. It should be noted, that although I cleaned the gas tank, there is a fair amount of rust, which I have a clear glass cleanable fuel filter to help prevent. Its possible that some crud got up into the carb, but the car starts fine, and once warm, seems to idle fine.

A vacuum gage is very cheap. And easy to use. You can buy a kit at any parts store with the common fittings included as well as simple operating instructions.

A scope not so much… and you gotta understand how to use it. It isn;t something you can just figure out when you get it unless you already understand how CRTs basically work (although the signals are now converted into digital simulations for display on flat screens), sweep rates, and a number of other basic settings related to the size of the pulse etc. In short, you’d need some basic electronics training first.

I’d explain why I call the digital displays “simulations”, but that would get a bit deeper.
Oh… what the heck. With a Cathode Ray Tube, the electrons stream from the gun is bent directly by putting the signal between deflection plates, totally analog. With digital displays, it’s all turned into I’s and O’s, totally binary, and then converted to a representation of the original signal. So I call it a “simulation”. Technically I shouldn’t, but I’m biased against digital. I’m old. I’m analog. I like to fill in the whole signal.

Digital displays are really fine. It’s just an old bias of mine.

Oh, regarding that timing light, does it have a 6VDC setting? The light needs its correct operating voltage to enable the induced pulse to flash the light. Understand too that the light only flashes when it senses the pulse from the ignition wire.

A scope may be beyond the budget for the OP.

You can check for erratic spark by hooking up the timing light on the #1 plug wire and watch the light to see if the flashes are consistent. Then just go to the next cylinder and repeat.
Try to be sure to clamp onto the plug wire closest to the plug.


Are you sure the distributor vacuum is hooked up to the correct port? It should be above the throttle plate. Here’s a description:
Is the centrifugal advance working correctly? Is the vacuum advance?

Have you adjusted the points (how), then the timing at idle (vacuum disconnected and plugged), then checked for centrifugal advance (still with the vacuum disconnected) by revving the engine?

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Straining my old brain on this one. As I recall, those distributors used venturi vacuum rather than direct manifold vacuum to advance the timing. They also used centrifugal advance to advance the timing as the rpm increased.

Also, as I recall, they used either a Stromburg or a Carter carburetor both of which used an accelerator pump piston rather than an accelerator pump diaphragm. Malfunction here would cause hesitation when accelerating.

To the OP: the carb manufacturer and ID should be either cast into the carb’s body or on a label on the carb body.

The more I think about it, the more I wonder about the ethanol effect… and the more I think a rebuild is in order. Age of elastomers + ethanol = a really suspect carb.

The light is designed as a 12v timing light as I read they still work fine, but are just dimmer in flash. Does it make a difference where the clamp is placed on the wire? I clamped it on the #1 wire roughly 6 inches off the distributor.

At idle there should be no discernable vacuum at the distributor advance dash pot. Disconnect the vacuum advance hose and check for the “hiccups.”

Spark advance vacuum is ported vacuum from above the throttle plate. And while there might be enough vacuum to pull a vacuum gauge up from zero at idle it should not be enough to be felt on your finger.

It is a carter carb. I could feel the vacuum on my finger when revved. Points were adjusted with a feeler gauge.

If Im at a stop sign and am leaving at a normal or fast pace there is no hiccup. Its only if I apply minimal throttle to creep the car forward, or reverse.

Question: if I remove the vacuum line and the problem disappears, what problem would that indicate?

You can use a new battery to power the timing light. If you have a local car club in your area see if there are any members that are willing to lend you a hand. A few tools that will help are a timing light, dwell meter, and vacuum gauge. I think you have a fuel issue as said above, but it would not hurt to find an “old timer” with the right tools and knowledge to double check things.


A 12VDCV battery to power the light… Steve, that’s a capital idea! Why didn’t I think of that?