Rough Idle - Impala Tech
Troubleshooting Diagnosing problems

 
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post #1 of 22 (permalink) Old 10-14-2016, 07:09 PM Thread Starter
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Rough Idle

I have a 67 Impala with a 350 from a 69 Impala that has a rochester quadrajet 4 barrel carb. Up till now everything ran great. It had a smooth idle in drive @ 500 rpm. Now idle is rough goes up and down to where it almost stalls. Tried spraying carb cleaner with no results. The engine runs fine at driving speeds. I believe this is a carb problem. Should I try to rebuild it or get another carb. It has a number on the casting (29224 OB 0879) which I have not been able to find any reference.
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post #2 of 22 (permalink) Old 10-14-2016, 10:11 PM
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Start changing the inlet carb fuel filter. Then continue with getting car up to operating temp. Check that your choke is fully open. If not, fix that before proceeding.

1. Set timing.
2. Set dwell.
3. Set hot idle speed.
4. Set mixture.
5. Repeat 1 and 3.

Follow FSM instructions for steps 1-4, but there are plenty of good how-tos on the net.

Two doors, four doors, wagons, and ragtops.
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post #3 of 22 (permalink) Old 10-15-2016, 07:02 AM Thread Starter
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I have already done all that. This just started happening yesterday. Do they still make rebuild kits for these carbs? The only problem that I have is that the casting around the fuel filter must have had a hair line crack because it had started to leak. I repaired that with JB Weld. So I don't have a filter in there, I have a in line filter right before the fuel pump. I'm afraid that the carb might crack if I try to loosen that fitting. I will have to remove it at the fuel pump and remove it with the carb.
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post #4 of 22 (permalink) Old 10-15-2016, 12:55 PM
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"I've already done all that."

Have you done all that SINCE the symptoms began? Or, was it done before? If the latter... you need to go over it all again. Yes, rebuild kits are readily available for the Quadrajet. Don't rebuild until you cross off all the other possibilities....

Two doors, four doors, wagons, and ragtops.
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post #5 of 22 (permalink) Old 10-16-2016, 02:06 PM Thread Starter
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Yes, I did all of them before this problem started.
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post #6 of 22 (permalink) Old 10-16-2016, 02:15 PM
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That's exactly why you need to recheck everything before wasting your cash on a carb rebuild. One of those items could easily be the cause of your problems. Stop guessing, start checking.

Two doors, four doors, wagons, and ragtops.
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post #7 of 22 (permalink) Old 10-18-2016, 09:02 AM Thread Starter
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Should I feel any vacuum from the carb tube that goes to the vacuum advance on the distributor at idle. I don"t. I only feel vacuum from that tube when I rev the engine. I need to borrow a good timing light. I have a cheap one and I couldn't get a good reading.

Last edited by whons; 10-18-2016 at 09:06 AM. Reason: added information
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post #8 of 22 (permalink) Old 10-18-2016, 12:56 PM
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If you only feel vacuum when you rev it that is ported vacuum only used to pass emission tests. I use full manifold vacuum all the time on my street driven distributors (I use a Mag on my race car).

Buy a good timing light with dial back that will also work with an MSD or similar ignition box. You may not have one now but it beats buying two timing lights in the future.

I have found that the Sears dial back timing light worked with my MSD 7ALs ignition box. Problem with buying anything from Sears is they change vendors every time they let out bids for tools. So my recommendation may not be valid for today (I've had some of my tools for over sixty years now).

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post #9 of 22 (permalink) Old 10-18-2016, 05:47 PM
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Time to check the FSM. You really need to see if that vacuum is supposed to be timed or ported. Then, check all the connections. Then, check all the things I've mentioned.

Two doors, four doors, wagons, and ragtops.
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post #10 of 22 (permalink) Old 10-18-2016, 07:55 PM Thread Starter
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Thanks for all the info guys.
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post #11 of 22 (permalink) Old 10-26-2016, 08:33 PM Thread Starter
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My vacuum advance is attached to a ported vacuum line on the carb. I am installing an Edelbrock carb, and that has both a ported and direct manifold tubes. So what I am understanding is that the vacuum advance should be attached to full manifold vacuum. My distributor is an HEI , not the original one that had points.When I set the timing after the engine is up to operating temp. should I time it with the vacuum advance hooked up to full manifold vacuum, or unhooked? It seems odd that my old carb doesn't have a full manifold tube.
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post #12 of 22 (permalink) Old 10-26-2016, 10:29 PM
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You time it with the vacuum line disconnected and temporally plugged. You don't want the vacuum diaphragm moving the timing around as you are trying to set it. The weights in the mechanical advance should be full in as well. So that all the distributor sees is the initial timing which on a stock car is four to eight degrees advanced. Cams and higher compression can require more ignition lead (to counter the effects of a later closing intake valve or a small aluminum mountain blocking the flame front in the combustion chamber).

The more efficient the engine (better fuel burn and hence better mileage and more power will require less ignition lead). An inefficient engine, such as an open chambered head on a BBC, will want more ignition advance just to stay running.

Ported vacuum was something added in the mid sixties to meet emission requirements for California. They had a problem with power plants polluting the air but felt it was politically expedient to blame cars for their smog issues. Now that cars are polluting less than he cars were back in the forties and fifties they are just realizing that the power companies are still polluting and we; the other 49 states have given up our muscle cars to appease their spineless politicians (not that I'm Bitter: I hate all politicians).

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post #13 of 22 (permalink) Old 10-27-2016, 07:11 AM Thread Starter
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Thanks to everyone. I am learning a lot of info from every one on this site. I to do not like most politicians.
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post #14 of 22 (permalink) Old 10-27-2016, 09:32 AM
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Don't forget that the HEI will enable you to open the plug gap, along with advancing the timing.
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post #15 of 22 (permalink) Old 10-27-2016, 07:08 PM Thread Starter
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Are you talking about spark plug gap?
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post #16 of 22 (permalink) Old 10-27-2016, 08:32 PM
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Yes it jumps from 0.032" to 0.045" with HEI.

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post #17 of 22 (permalink) Old 10-28-2016, 07:44 AM Thread Starter
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That is very good to know. thanks. It might even take a different plug, I'll have to take them out and see.
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post #18 of 22 (permalink) Old 10-28-2016, 08:16 AM
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There are a couple of different plugs made.

Almost all are now resister plugs due to higher voltages of transistorized ignitions. A resister plug has a small air gap similar to the gap in the combustion chamber. It is used to prevent voltage bleed off prior to building up high enough to actually fire the plugs. The air gap counters the transformer effect as the primary voltage builds (in rush current).

Plugs seal to the head either with a crushable flat washer, or a tapered seat. This is determined by the age of the head. Newer heads use a tapered seat that seals better at higher compression levels.

Finally the numbers on the plug list the heat range. The heat range is determined by the distance the heat has to travel to get to the steel shell that is the heat sink. A hotter plug is used to prevent oil fouling, a colder plug to prevent pre or auto ignition. You want the Goldilocks plug that is right in the middle of the heat range for normal operation.

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post #19 of 22 (permalink) Old 10-30-2016, 11:47 AM Thread Starter
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My spark plugs are the tapered style that seat into the heads, which means they are newer heads. The plugs are AC Delco #R44TX and they do have a .045 gap. Is this the best plug for my engine that is a 350 bored .030 over?
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post #20 of 22 (permalink) Old 10-30-2016, 12:06 PM
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They should be fine. 44 is the heat range. The higher the number with AC plugs the hotter the plug. 44 is a good street plug; a 45 would be used on an older engine that has bad valve guides that leak oil to prevent fouling the plug, while a 43 would be used at the strip on time trial day. There are other numbers in the heat range but I chose these to illustrate the point.

Not all manufactures use a bigger number to indicate a hotter plug. Some are the exact opposite so you have to check.


With AC the R stands for a resister plug.
The first numeral 4 indicates that it has a 14 mm plug thread.
The second numeral 4 indicates the heat range.
The letter T indicates a tapered seat (vs. a washer)
The X indicates a wide gap plug (recommended for HEI).

Here is a pdf that explains what the numerals and letters mean:

http://toms427impala.info/CT/ACPlugInfo.pdf

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post #21 of 22 (permalink) Old 10-30-2016, 07:56 PM Thread Starter
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Thanks to all you guys. you have all helped me a great deal. I'll let you know how the new carb with every thing set properly goes.
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post #22 of 22 (permalink) Old 10-31-2016, 02:44 PM Thread Starter
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Got the new carb on. Timed properly, runs better than ever, more power. Also installed the manual choke, starts instantly when cold. Thanks again.
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