Ok can someone try and tell me whats going on here? My car was idling too fast, so I decided to try and tune it today to get the idle down. The best I got it to idle was 850rpm in park. I had to back the idle screws out to 4 1/2 turns for the left, and 5 1/2 turns out for the right screw. This was the only way I could get the vacuum gauge to read 18-19 pounds. When I could get to a certain point of backing the curb idle screw out to lower the idle, I would get a squealing noise coming from the carb throat. It almost sounds like a ringing in your ears. When I would turn the screw in just a little to make it idle faster, the squealing would stop at a certain point to where I could get it to squeal or not by turning the screw in or out just a little bit.

On the test drive the engine was feeling bumpy and sounded kind of like a helicopter where instead of a smooth idle, it was billowing at a constant rate. Does this mean im getting too much air or too little air? The timing is set at about 8 BTDC. I had this car checked out at the shop and they said there are no vacuum leaks. So does anyone know what seems to be my problem or what I can do to get this thing to idle like a normal slant? I know my mixture screws seem to be turned out way too much, but when im in the 2 1/2 - 3 1/2 range, I can't get this thing to idle under 1000rpm in park. Plus it reads at 15 or 16 on the vacuum gauge at that setting.

What carburetor do you have? Is it a Carter BBD with holes drilled in the throttle plates?

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

It's a Carter BBD. I just tried looking in there to see if there were holes in the throttle plates, but I couldn't see well enough in there and I won't be able to take it off and check until at least tomorrow. Is there a difference in tuning the thing if I do or don't have holes in the throttle plates?

In the later years of the slant, they kept idle emmissions down by letting in more air at idle, and retarding the tming to slow it down. That is why the initial timinng was as low as 2 BTDC in some charts for some years. This does indeed work, but produces a fairly sluggish off idle feel. The holes in the throttle blade are to make it impossible to cut out all of the air at idle. If it were me, I would find a BBD with solid throttle blades.,which is a pretty nice carb. Of all the carbs I tried over the years, it was the most flexible, and the most tunable.

Sam

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Actually...

Base ignition timing was, depending on year and emissions package, as late as 5° after TDC on the '66-'68ish CAP-equipped cars...and a setting of TDC was called for in the '73-'75 year range. But starting in about '76, the base timing was actually quite far advanced on many versions of the slant-6; as much as 10 to 12 degrees before TDC. The holes in the throttle plate were to improve fuel atomisation, mixture precision, and engine speed consistency at idle — they came into general use several years after the early "add idle air and retard the base timing" emission control strategy. The holes are calibrated to one particular engine configuration, though, so using a carb or a throttle body w/plates from a 318 (for example), or using a 225 carb but advancing the base timing significantly from the factory spec, will give too fast an idle speed on a 225.

The carbs with holed idle plates can be used successfully, but adapting them from other engine configurations is harder than adapting a carb with unholy throttle plates.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I didn't have time to take the carb off and check the throttle plates. But I took some measurements, so maybe you guys could let me know what you think is up.

Park idle 750rpm
Drive idle 525rpm
Park vacuum 15psi
Drive vacuum 11psi
Mixture screws turned out 3 1/2
15 BTDC

I don't know which mark is my timing mark either. Theres a really clear one at 15 BTDC and then a nasty looking one that ranges from 5 BTDC to TDC.

Doesn't this sound like a vacuum leak to you guys? I can get the vacuum to go up to around 18 in park if I have the screws turned out 5 turns or so. After spraying the manifold to head, carb and egr blockoff plate, is there anywhere else that could possibly be sucking air in? I also have a feeling that I need to pull the distributor and turn it because I couldn't get it to retard past 15. The thing isn't running very smoothly and I have a good feeling it'll die without me being on the gas when I put it into gear when the engine gets a little colder.

Vacuum is measured in inches of mercury (In. HG), not in PSI. Unless you live at very high altitude above sea level, and assuming your vacuum gauge is accurate, your vacuum readings are very low. This could indicate a large vacuum leak, or severely retarded ignition timing, or exhaust restriction, or improper cam timing. How did the vehicle run before the Super Six swap?

3½ turns is quite a lot for the mixture screws to be turned out. Suggests a vacuum leak. 750 in Park vs. 525 in Drive is a larger-than-normal drop. Suggests any of several tuning problems.

15° BTDC is too far advanced on the base timing...if that's where it's really set. You need to get your timing mark sorted out first. See this post for instructions. Also: Are you checking your timing with the vacuum advance connected, or disconnected? Have you checked to make sure your vacuum advance is not ruptured? What gasket did you use for your EGR blockoff plate? Is your intake manifold iron or aluminum? And, what is the origin and history of the carburetor you're using?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

One way of setting the idle bleed screws and timing is to find the combination of settings for both that yields the highest vauum, measured in inches of Mercury,(not psi ), while still maintaining the correct idle RPM. The best tuners are quite proud of the fact that they can get their slants to idle at 650 RPM, but it never bothered me to have mine idle at 850 in park, and then drop to 750 in drive. It is normal for the vacuum to drop when the rpm drops in drive, especially if your mechanical advance springs are weak, and putting advance in around your idle speed in park. When you watch the timing marks with the light at idle in park, is it steady, or does it bounce around some. If it bounces, then there is a good chance your centrifigal springs are not holding tight at idle. It will not idle stably if this is the case.

It sounds to me like you have a workable combination there. I am not sure what is leading you to conclude that you have a vacuum leak. A vacuum leak is often associated with a shakey, uneven idle that looks and sounds like a spark plug is misfiring. If you can get it to idle in park at 18 inches vacuum, and get it to idle at 850 RPM, I would pesonally not be upset about that. Play with it awhile, and see what you can come up with.

Do you have your vacuum advance hooked up to ported vacuum? If you have it hooked up to manifold vacuum, that is another reason the rpm will drop when you put it into drive. At idle, in park, does the rpm drop when you unhook the vacuum advance canister. If it does, you likely are tapping into manifold vacuum and not ported vacuum. A ported vacuum source is located above the throttle plates when they are closed, and will not influence the timing until you open the throttle a bit. Having the vacuum advance tapped into manifold vacuum leads to idle instability as the timing is directly affected by the RPM and vise versa.

If you are hooked into manifold vacuum for your vacuum advance pot, then if the idle drops a little, so will your timing, dropping the idle RPM a little more, and thus dropping your timing a little more. This is a death spiral that will cause the engine to stall. It happens so fast that you are left scratching your head; "What hppened?". Check these things out. I think you might be able to get it to work. Good luck.

Sam

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Yeah I figured there was something wrong with using psi as a measurement, but I was too lazy to figure it out. Anyway, the car ran smoothly before the super six swap. It has a freshly rebuilt engine (by a shop) and ran fine with the exception of gas mileage. So instead of buying a new holley 1920, I decided to go with the super six. So far, even with the crappy tuning, ive gone from 10-12mpg with the 1bbl to 15mpg with the super six.

The carb is remanufactured, rebuilt and rebushed. I had the carb place run tests on it and they said its fine. I had the shop check for vacuum leaks and they said there were none. Take that advice for what its worth. Personally I dont trust any of them.

The vacuum advance was blocked off. Well I plugged up the carb. Was I supposed to plug the carb or the hose off? Sorry im still learning. Anyway the gasket for the EGR block off came out of the kit from Napa that had the manifolds to head and intake to exhaust metal gasket. Im using the australian gaskets for both of those and the one from the Napa kit for the EGR block off. Now the holes on the gasket didnt line up totally right, but I took it off and checked it and it looks like it still seals up both of the holes. Its just off by a 1/4 inch or so. I don't see how air could get through because nothing is open or overlapping. Intake is cast iron and I have the vacuum advance going from the Distributor to the port under the fuel inlet on the Carter BBD. Thats the correct one right? But yeah the engine shakes and rattles more than i'd like. I just can't figure out where the vacuum leak is. Im wondering if I should just take it into a shop and have them put it on a scope?

You are not alone.
I got the same "problem", I am not sure if I have the throttleblades that is bored or not, but as you say, the idle screws out about 4 turns, timing at 15 degr.
With the Holley 1920 it had a much nicer idle, but it launches much better now.
I will "solve" the problem with a "Offy" intake and a Holley 600.

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Lasse

Signet -66 225
Holley 600 Offy-5270 Dutra Dual,regrind. cam 272 0,420 110, head shaved 0.100 slight.ported
MSD6A
A904
7 1/4 3.23

Procedure is simple, get can of carb cleaner/spray from auto parts sore and go back to your place, warm up car and leave in neutral, shake can, pull top off, then spray any 'suspect part' trying not to get any on the exhaust manifold...

My list of priorities would be:

1) all the intake runners
2) carb base
3) carb throttle shaft

If you hit a 'spot' where the leak would be , the rpm will change....

All things considered, if the carb was good and rebuilt by a reputable shop, I would then ask if you had your intake/exhaust manifold 'stack'
milled at a machine shop for flatness as these warp pretty good especially if you got the stack out of a junkyard car (usually abused and never maintained cars).

good luck,

-D.Idiot

Thats the deal. When I first put everything on, I had leaks at the intake runners as well as the throttle shaft. I had the carb rebuilt and rebushed. I also had the manifolds surfaced flat by a good shop. Ive sprayed everything you have mentioned and there is no rpm change.

I sounds to me like the carb needs to be jetted up a few notches.


Jess

Nope. The BBDs (from a 225, a 273, or a 318) with stock jetting generally tend to run well and give good driveability, performance and mileage with stock jetting. There is something the matter with this setup. Might be internal to the carb, might be external, might be both, but something's not right. This is not the time to start drilling or replacing carburetor jets, it's the time to figure out what's causing the poor running and fix it.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Start with some basics. Are the valves adjusted correctly? . Make sure the spark is getting to all 6 plugs. Make sure all plugs are in good shape. Make sure your wires are not shorting out. Make sure the gasket under the carb did not de-laminate up into the base of the carb. I had that happen to me once with a rebuild kit carb gasket. Who made the rebuild kits with the bright orange box years ago? It was their kit. Check your rotor and cap, and see if it is carboned up, or carroded away. Make sure your fuel filter is OK. Check your voltage at the battery and the alternator to see if they are the same. If they are not, you have high resistance in your wiring somewhere. Check the high side of the ballast resister to see if it is the same as the alternator. It all should be close to 13.5 volts.

On some BBD's there is also an adjustable cam set up for the step up power needles (pardon my old guy brain, I cannot remember exactly what they are called) that go down into the jets. This must be adjusted corretly for things to work right.

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