Poor Idle On Rebuilt Carb

[D Denzin]

I can't get my Rebuilt Carb to Idle decently.
It surges.   When it Idles at lower speeds it seems labored. It is inconsistent idling. Speeds up, drops down runs rough, if I blip throttle it'll idle faster, eventually will idle down and resume rough idling and surging. I have to adjust Idle screw out like 3 turns to get it to stay running. Then it runs at about 2000rpms before it seems to smooth out.

Specs.
Pontiac 65 326 HO with 9.2:1 comp.
Cam is basically very close to stock.
carb 17057253
Engine is rebuilt a couple of years ago. Switched carb from original AFB and Intake to Q-Jet and Edelbrock alum intake.
Car ran good with old set-up.

Was rebuilt using kit from Cliff and his recommendations about a year ago. I haven't had the time to mess with it until lately.

Jets .073
Metering Rod .044
Idle Tube .034
Idle Down channel .043
Upper Idle Air Bleed .070
Lower Idle Air Bleed .070
No holes in throttle plates
Idle bypass Air .081
Holes under Idle Mixture screws .090
Air horn Air Bleed .070
Main Body Air Bleed .029

I have checked for Vac leaks with a unlit propane torch. I have disconnected Dist Vac advance. I have disconnected Vac Gage supply. All with no effect. I even supplied propane into the disconnected open vac supply for the gage and it made no diff. Right now I have mixture screws out 2 1/2 turns. Adjusting them in and out has minimal effect. They will cause the engine to get rougher if I screw them way in. I have adjusted the APT in and out thru its full range and it seems to help being set almost all the way out.

Any Ideas on Where to go?

[carmantx]

sounds like you have too much idle air possibly.  With the idle bypass and .034 in the tube.

[D Denzin]

Trying to understand, too much idle air.
I can't get it to idle at all unless I open the throttle blades by turning the idle adjustment screw on the throttle linkage about 3 turns. My eyes are burning at times from the exhaust, thought that was a symptom of too rich.
the best I can get is by turning apt all the way up.
Any easy way to test if I have too much idle air?

[D Denzin]

Big improvement. I disconnected the PCV hose from the carb and capped it. WOW. Now i can get it to idle down to 750-800.
I reconnected the dist advance to the manifold vac. source on carb.
I can adjust the idle speed down using the screw on the throttle linkage to the point that its not even touching.
At that point I tried to turn in the mixture screws to get the idle down. Still can not get the idle down before the engine starts to surge, run rough and die.

When adjusting idle mixture screws and even idle speed it seems odd in that I can make an adjustment, I can see a response in a second or two, then in about 5 to 10 seconds the idle will continue to deteriorate surge and get rougher and rougher. I can blip the throttle 2-3 times and drop it back to idle and it'll be fine for about 10 seconds or so. Then it continues to get worse till it will eventually die.

So now I'm at the point that it will idle decently, a littler faster than I'd like, for about 10 seconds, if I let it sit it eventually will die. I can speed up the idle to about 1000 and it will stay running but then its idling too fast.

Any thoughts?

[429bbf]

id say your float is ether set to high or your needle and seat are leaking ,or your seal on your seat is leaking . what your saying is you are flooding at idle? did you get the gasket under the seat nice and clean before you installed the new one?and did you install a new gasket under the seat. i hate to ask these questions but one little spec of crap will cause the problems you have. do you have a choke is the pull off working?is the choke wide open?fwiw

[von]

Sounds to me like the intake getting air from somewhere with the throttle blades closed. First I'd check to make sure there is play in the linkage rod on the driver side that goes from the primary to secondary throttle shafts. No play could mean it's holding the secondary throttle plates open a hair at idle. If no help there I'd plug the .081 idle air bypass. A 326 with stock cam wouldn't need it IMO. Also I'd check to be sure your distributor vacuum advance can doesn't have a vacuum leak and also that the carb choke pull-off doesn't leak vacuum. Plus, I've seen Q jets where the choke fast idle screw (down low on pass side) is adjusted too far to let the primary throttle blades close enough for warm idle. Try backing it off (counterclockwise) a little.

[D Denzin]

id say your float is ether set to high or your needle and seat are leaking ,or your seal on your seat is leaking . what your saying is you are flooding at idle? did you get the gasket under the seat nice and clean before you installed the new one?and did you install a new gasket under the seat. i hate to ask these questions but one little spec of crap will cause the problems you have. do you have a choke is the pull off working?is the choke wide open?fwiw

I'll check on the needle and seat tomorrow. I have a electric choke, its seems to be working, closed when cold, engine runs on a faster idle, opens up as engine warms. Choke is wide open when warmed up fully. Thanks for the ideas, will check and report back.

[D Denzin]

Sounds to me like the intake getting air from somewhere with the throttle blades closed. First I'd check to make sure there is play in the linkage rod on the driver side that goes from the primary to secondary throttle shafts. No play could mean it's holding the secondary throttle plates open a hair at idle. If no help there I'd plug the .081 idle air bypass. A 326 with stock cam wouldn't need it IMO. Also I'd check to be sure your distributor vacuum advance can doesn't have a vacuum leak and also that the carb choke pull-off doesn't leak vacuum. Plus, I've seen Q jets where the choke fast idle screw (down low on pass side) is adjusted too far to let the primary throttle blades close enough for warm idle. Try backing it off (counterclockwise) a little.

I already checked the linkage rod from the secondaries to the primaries, OK there.
I'll look into all the other suggestions that are "easy" checks and leave the plugging the Idle Bypass till last.
I'll check into the vac advance, I did check operation before putting dist in by applying vac to be sure it worked, to be honest didn't check to see if it would hold and not leak down. Also checked the choke pull off but again for operation not if it held with a leakdown.
I'll check the screw on the choke to be sure its not holding up the primaries.
Thanks for the suggestions will check tomorrow and report back.

[D Denzin]

429bbf,
Checked float level, was 9/32, changed it to 13/32 as spec'd in rebuild sheet.
Disassembled seat and checked gasket and mating surfaces. All looked good reassembled.

I don't have the degrading idle anymore. Your excellent advise on checking float level helped.

Von,
Secondary to primary rod was OK as I mentioned before, so I plugged the .081 Air bleeds.
I also checked the the vac advance and the choke pull-off. Both held vac without leaking down.
I also backed off on the choke fast idle screw, it wasn't touching, made no difference.

So now I can get it to idle down to 680-700 with pvc attached and vac advance attached to manifold vac. Idles very nice and smooth. If I slowly work the Idle mixture screws in it won't reduce idle, just degrades it till the engine runs rough and dies. I can back the idle adjustment screw on the throttle blades all the way out and the slowest idle I get is 680-700.

If I pull the PVC it will idle down to like 550 or maybe even less. If I pull the vac advance and plug or switch it to the ported vac source I can get the idle down. But the idle is rougher.

The timing is set at a total of 34 deg all in. Intial timing is 12 btc. When at Idle with vac advance on man. vac I get about 24 total at the 680-700 idle. 

I'd like to get the idle down to about 600-625. I can do it with no PVC or ported vac for the dist advance but then the idle is rougher. Seems like idle is lower cause the engine is struggling. Maybe I should work on getting a better idle quality using the ported vac. At 680-700 with vac advance on man vac and pvc attached the idle is silky smooth. I'd just like to knock it down about 75 rpm's.

Von, I think your suggestion of plugging the air bleeds helped knock the idle back about 100 rpm's. Thanks!!

Anymore ideas to help me over the hump?

Thanks both Von and 429bbf for your help so far!!

[77cruiser]

Are your throttle plates adjusted evenly?  You might try a different PCV valve some have different ratings for the amount of air they pass.

[D Denzin]

I looked at the throttle plates, the secondaries close nice and tight. Primaries look good too.
I bought a new PCV and tried it. No Difference. I messed around with moving it from the primary side of carb to rear then from there to the intake. None of these location seem to matter. I didn't think they should but I tried. I'm going to move the PVC back to the front of the carb. I guess I'll have to run to town and pick up a couple of different pcv's and try them.

In the big picture it seems as I'm getting too much air somewhere. I'll have to take a second look at everything.
I checked for vac leaks with a unlit propane torch, any better way?

[429bbf]

i use starting fluid to each his own . i agree with jim on the pvc i used to think they were all the same but there not. pull the pvc out of the valve cover or where ever it goes and leave it hooked up. put your thumb over it and slowly start restricting the vacumn and see what happens . hth   you may also try plugging all the other vacumn lines ,brakes ,trans,etc. you never know where the problem is.

[D Denzin]

I pulled the carb off Sunday and took a real good look at the base plate. No, the throttle plates don't close perfectly tight. By that I mean they close tight, can't close them up any more but You can see light around them if you hold them up to the light. Its a issue that the plates are a couple of thousands smaller than the bore side to side. I looked at some older unrebuilt carbs and they appear to be air tight but after cleaning the carbon and soot they have the same gaps. Not much can be done about it.

 I had some other issues (leaking water pump) that I needed to fix. In the mean time I ordered a couple of different PCV's that I'm going to try. Not much in-stock in town.

I am only tapping the Q-jet for Vac for the PCV and Vac Advance for the dist. Car has manual brakes, manual trans.

I also drained the gas tank as the gas for the most part was over a year old. And I am replacing the add on fuel filter that was in between the pump and the carb. The filter looked to have some "milky" looking gas in it. Taking no chances.
So I have to wait for the PCV and fuel filter I ordered.

I also Changed the carb to intake gasket. It was a open one, I swapped it out for a divided one. I don't think it'll matter but it won't hurt.
Am trying everything I can think of.

[77cruiser]

Are the throttle shafts tight?

[D Denzin]

Throttle shafts are good. I rebushed the primaries using Cliffs Kit.
The secondaries aren't tightest fit I've ever seen.
I have checked and watched to be sure that the primaries always settle back to the same spot and I have tried to turn the primaries while running to be sure that they always seat back to the adjusting screw. No issues there.
I could try putting grease around the secondary while its running to see if that helps.
I'll put it on my list of things to try.