Poor Idle On Rebuilt Carb

[von]

The primary throttle blades must be installed one way only. Did you mark them before removal? Also the secondary blades have beveled edges and must be installed in the proper orientation too. If you installed everything exactly as it came out, it's always possible someone incorrectly installed them in the past.

[Cliff Ruggles]

The secondary throttle plates need to be correctly indexed, centered and closing fully so you don't have any air leaks around them.  The factory used a pretty "loose" tolerance on the secondary throttle shafts so they can return to fully seated each time they close.  It's a pretty important part of the rebuilding process, so spend some time with the secondaries to insure that they are fully seated each time they close.

It's not uncommon to find a shaft that is twisted slightly, linkage holding them open slightly, or the plates not correctly centered up and indexed so they fully close.  If not done correctly, they are a vacuum leak and will not allow the engine to idle down low enough, and leans the idle mixture out at the same time.....Cliff

[D Denzin]

How tight are the secondary blades supposed to be. They look good. But if you hold them up to the light there is a gap around them. There are touching on the higher side (higher up in carb). If I had to guess I'd say there is less than .003 gap. Any person with decent eyes can easily see a .001 gap with light behind it. You can see that they are narrower than the bore. So no way will they ever seal closed completely. Maybe I should take a feeler gage and check the clearance. I'm sure its very little.

[D Denzin]

Ok, here is where I'm at.
I have fresh gas in the tank, I put in a new in line fuel filter.
I pulled off the carb and got out a feeler gage and checked the secondary bores. There is a small amount of movement left to right (side to side) of the butterfly shaft and butterflie assembly in the bores, its no more than .0015. No more than .0005 (that's right 1/2 of .001) of gap between the bores and the butterflies front to back when they are closed. I am sure that the fit of the secondaries to the bore is good.
I have to run the car with the PVC plugged.

At this point my idle is still poor. It isn't smooth, its a little rough, not real bad, but it has been better.
I can get it to idle at about 650. But what it does is it'll idle there for about 10 seconds then it drops down to about 500-550 for a second then it speeds back up to 650 and it'll idle there again for 10 seconds and it then repeats.

If I adjust the mixture screws out a half turn it'll loose the speed changing but then its idling at 700. I then back off the idle screw at the throttle linkage about 1/8 of a turn and then the idle drops but I'm back to the idle changing speeds. I started out with the mixture screws out 3 turns. I kept repeating the adjustments now I'm out 5 turns and nothing seems to be improving. Eventually I'm at the point again that the idle adjustment screw is not touching the throttle linkage. If I start cranking in the mixture screws then the idle gets real rough.

I don't see any dribbling of gas going on in the primary bores by the booster rings while its idling.


Any thoughts?
Thanks for all the help so far.

[blazer74]

Must keep the PVC plugged.   Just a thought but are you using power brakes?

If so could you be loosing vac thru the brakes?

[D Denzin]

No power brakes, only VAC. hookups on carb are the PVC and the Vac advance for the dist. Manual trans. Very simple setup.

[Cliff Ruggles]

Part of the trouble here may be too lean at idle.  Going back and looking at the idle tube size and DCR's that's a pretty lean set-up.

How far out are the idle mixture screws set at?  Do you have full control of idle quality from rich to lean using the mixture screws?

Check the timing as well, and make sure it's rock solid steady at idle speed.

How much vacuum does this engine produce at idle speed?....Cliff

[D Denzin]

I read Cliffs last post and decided to go out and see if I can see a difference if I adjust the idle mixture screws out. I know if they are too far in they will kill the engine so thats too lean right?
So After the engine warms up I adjust the mixture screws out 1 more turn. Now at 6 turns out, was at 5 turns when I started the car.  Car is still idling. I crank out the screws another turn. 7 turns. I'm thinking 7 turns out thats pretty far. I let it sit and idle. Seems if anything its idling rougher, slightly. So I adjust it in to 6 again. Idle does speed up a hair and does smooth out a hair. Is that now too far out? Is that what I would see if its Too rich? I guess I'm asking the question how do I know if I'm too rich?
So in answer to Cliffs question "do I have full control of idle quality from rich to lean using the mixture screws?" I don't know. I'm not a wizard on these things, I'm very mechanically inclined but my knowledge is lacking. I'm asking the question back "How do I know if I'm too rich? and "how far out can I go with the mixture screws?"

"check the timing as well, and make sure its rock solid steady at idle speed"
My answer to this. It is if the idle is steady. I have the dist advance attached to the man vac source. If my idle is floating around, man vac is floating so is advance. Should I maybe working on idle without using any vac advance? Plug both manifold and ported sources and then adjust idle? Again initial timing is 12 and total is 34, at idle its about 24 with manifold vac attached.

"How much Vac does engine produce at idle speed?"
At 700 rpms I get a good 16-17 inches.
When the engine idles at 450-500 its 12 inches, at 600-625 its 15-16.

Here is the kicker. From my last post on March 25 where I describe where I'm at things have changed.
Now I can get the idle down. What I did to get this change? I don't know, I walked away for a day, went fishing and gave it another try. When I came back it changed. What I do is adjust idle to 600-625, I get a 15-16 inches of vac. But the idle sort of floats up and down that 25 rpms. If its idling at 700 its rock steady. At lower speeds it sort of floats very little. Not to worry?

Now when I shut off the engine and restart it the idle is at 450-500. I don't have touch anything for it to start. I'm standing out side the car, reach in the window, turn the key off, then engine kills, no dieseling or anything. I can count to 10, touch the key to start the engine, fires right up but its idling real slow. If I blip the throttle up and hold it up so its running at 1000-1500 rpms for about 1/2 a minute and drop it back to see what the idle is. Then the idle might be about 550-600. So if I adjust the throttle screw just a hair. And I mean its just a very fine adjustment, less than 1/8 of a turn on the screw, just barely turning it and the idle is again fine. 600-650.
I turn off the engine again and restart its idling 450-500 again. I can get the idle back to the 600-650 by running the engine for along period of time at an increased speed.

What is going on here?

If anything this car is consistent in its inconsistency. If that makes any sense. I mess with it. I think I know where I'm at. I go away, come back and then its doing something else. At first I couldn't get it to idle down. Now it idles down, I can get a decent idle, at least I think so. But if I try to turn the car off and restart the idle is bad and needs adjusting.
Is 10 seconds of sitting enough to cause some sort of heat soak? or is the vac being shut off completely then coming back causing something to change. Is something maybe sticking somewhat in the carb and when I shut off the engine and vac it moves causing the really slow idle?.  Maybe I should fix up the other couple of minor issues I have with the car and take it out and drive it. Right now its not drivable, seat is out. Everything I've done is just been done in the garage. I probably have a couple of hours of run time in the garage.

I'm still running with the PCV plugged.

Please don't give up on me here. And don't feel like your going to insult me by asking what to you are insignificant questions. Like I said I'm no wizard on this stuff. But I am perfectly capable to fix this if I can get some direction/knowledge from some more informed sources. I'm just at a point I don't know what to try next. Maybe I should go fishing again and it'll be better. Maybe there are some sort of garage gremlins messing with me.

[77cruiser]

At what rpm are you checking the timing? If at 600 does it change when you bring it up to 700 rpm? If it does your spring are to light for the mechanical advance.

[Cliff Ruggles]

+2 on checking the timing.  Inconsistencies in idle speed and  idling too slow after a restart are often distributor related, or inconsistent timing.

Hook the PCV system back up.  Millions of engines used that system, it needs to be there to keep the engine cleaned out.  Figure out the issues with it in the system working like it's supposed to.

Your problems seem to me like to much timing at idle combined with carb a tad lean at idle speed.  You should have full control of the mixture screws, are better sensitivity with them without backing them out so far.......Cliff

[68 Firebird]

Most carb issues are actually ignition issues!!!  That is a true statement I learned a few years ago.
When I had my DUI HEI curved I had them setup curve to start at 900-950rpms.  This worked wonders
for my idle in and out of gear.  And running ported vacuum advance takes another variable away from
idle equation.

[D Denzin]

Well I pulled the dist apart and found it had apparently the wrong weights in it. I checked them against an old dist I had laying around. So I swapped them out. Car ran better. I ended up reattaching the PCV and swapped vac advance over to the ported source. Car ran OK. I turned car on and off. Everything seemed better. No changes.

I figured I check total timing, adjust mixture screws using the vac gage and do any final adjustments.
I thought I had it licked.
While I was getting my stuff ready to restart and do the adjustments I notice there is smoke coming off engine on the pass. back corner. Where the PCV goes into the Valley cover. I figured something was screwy with the PCV. Well long story short it seems the coil was leaking oil. I could see oil bubbling out of the rolled seam on the top of the coil. This was causing the smoke. I pulled the coil and appparently the black plastic top section is bulged and cracked. Its also low on oil. You can hear the oil sloshing around when you tilt the coil over.

So now I need to get a new coil. I was using a Pertronix coil to go with the Pertronix ignition set. I'm going to get my hands on a old GM coil and try that. I might be able to prevent a new pertronix coil from going bad if I remount it up and away from the engine but it seems that the original GM stuff lasted in the original location why shouldn't the aftermarket one? Off to find a coil and try again.

Thanks again all for helps and tips. I'm optimistic that I'll have this licked soon.

[Cliff Ruggles]

Progress is a good thing.  The distributor is a big player with these things, and you've got to get it working correctly before carb tuning will be effective.

You didn't mention if the timing was actually moving around at idle speed or not?

I would also check the mixture screws now that you have went to a ported source, to make sure they have enough fuel for the engine without backing them out too far?.....Cliff

[D Denzin]

Timing is solid. doesn't move around at idle when using ported or plugged. Moves slightly when idle is fluctuating when I'm using manifold vac source. Centrifugal advance does not kick in till around 1200 rpms.

Mixture screws were turned in about 2 turns on ported vac. Seemed to help smooth out engine. I was going to do some final adjustments as mentioned in last post when I noted bad coil. So thats where I'm at now.
Thanks for all the help. I'll advise what happens when I get a new/different coil.

Always seems to be 2 steps forward one back.

[D Denzin]

So I got a new coil.
I can get it to idle at a decent rpm. 600-650. That's good.
It doesn't idle smoothly, kinda labored. I get about 13-14 in. of vac at idle.
I've got timing at 13 deg at idle. Ends up giving me about 35 total with no vac advance. Am using ported vac source for dist.
I've got the PCV hooked up.

 I've got the mixture screws out 7 turns. Seems best about there. Not a lot of difference in the adjustments of the mixture screws. If I get them in too far, maybe 2 turns or less it really gets rough. I can crank it out and all it does is increase the idle speed alittle. Does not really make the idle smoother. So then I adjust the throttle linkage to lower the rpms. Shouldn't I see more of a dramatic difference in adjustments on the mixture screws? How far out is too much? how do I tell if its too rich on the mixture screws? I'm hesitant to go beyond 7 turns out. There is no resistance from the springs at this point and the mixture screws seem very loose.


So all in all its idling at a decent speed but its idle is not smooth, minor surges and it floats up and down between 600-650. The Vac is lower than I think it should be and it too floats between 13 and 14 in. I checked the timing with the vac advance disconnected and plugged. Timing is solid. Removing the vac advance does not change the timing as I would expect it shouldn't when using the ported vac source. When adjusting timing I notice that the engine speeds up and smooths out when advancing the timing to about 20-25. Why not go back to man. vac for the dist advance? If there is no downside that anyone can clue me into I'm going to try that.

I got a couple of other issues fixed while waiting for the coil so I could take it for a spin down the road. Seems to run pretty good.  No bucking, secondaries open nicely. I would say that other than the idle this carb seems like an improvement from the stock AFB that was on the engine.

On last issue with the idle. When I turn the car off and restart it after any period of time the idle decreases about 100 rpms. So after restart its idling around 550. running rougher too. A couple of blips on the throttle doesn't cure it. But it seems after about 3-4 minutes of running it goes away. The car is also running hotter than "normal" when idling for about 15 minutes or so. So when I took it out it was on the hot side. Only took about of minute of driving and the temp was down to more of a normal range. Maybe some sort of heat soak thing going on with my idle issues?