[Mwieczorek]

When I first got my 66 Bonneville a few months ago, it had idle problems. I pulled the carburetor and did a rebuild (it was indeed varnished up and had a lot of green sludge in the bowls), and it still won't idle. I have worked through all the vacuum issues, and I'm certain they are all sealed up. I even bought a new anti-stall dashpot, and as neat as it is to watch it actuate and bump the throttle linkage, it still hasn't solved my issue.

I am getting a solid 16" of vacuum at roughly 700 RPM of idle, but I can turn that idle bleed screw back and forth all I want and unless I completely close it it basically does nothing. I need to get the idle speed to increase a bit to keep the car from stalling when I put it in gear, and it just seems like all the fiddling in the world isn't helping.

I am using a tach and vacuum gauge to tune, the way I was taught. It's always worked fine on other carbs with an actual idle speed screw and idle air mixture screws, but this air bleed setup is new to me.

Just in case it helps, IIRC I have about 12 degrees of timing advance at idle without the vacuum advance hooked up, and it runs up to around 36 degrees at 3000 RPM.

I think my throttle shafts are still reasonably tight.

I'm about ready to pull the distributor to check to see if something is loose and screwing with timing, but it seems pretty consistent.

any other ideas? I'm about ready to chuck this carburetor and install fuel injection...

Thanks!

[padgett]

I always used the linkage idle screw on the side to adjust the idle RPM, not the mixture screws. It opens the primary butterfly and I would not call it a bleed screw. Do like dual quads.

[gary bennett]

I agree with padgett

[Mwieczorek]

On the OEM Carter AFB, as far as I can tell there is no idle adjustment screw on the linkage. There is an adjustment for fast idle, but once the engine heats up and the choke rotates the fast idle cam disengages and you are down to hot idle adjustment via that big center air bleed screw. There is no hot idle adjustment on the linkage.

Even the service manual says to use the air bleed and idle fuel mixture screws to adjust idle speed.

LOL, looks like I just needed to surf the web a bit more... This guy suggests adding a conventional idle adjustment screw to the carburetor due to modern fuel (its the last section on the page)...

https://www.thecarburetorshop.com/AFBtuningtips.htm

[alleyCat]

I was just going to ask if its a Carter with the large center air bleed ! Make sure your primary throttle blades are closed at idle . When you rebuilt it did you spray Gumout or some other solvent thru all the transfer slots and hole when you had all the blled and mixture screws out ?

[Sirrotica]

Originally Posted by Mwieczorek

On the OEM Carter AFB, as far as I can tell there is no idle adjustment screw on the linkage. There is an adjustment for fast idle, but once the engine heats up and the choke rotates the fast idle cam disengages and you are down to hot idle adjustment via that big center air bleed screw. There is no hot idle adjustment on the linkage.

Even the service manual says to use the air bleed and idle fuel mixture screws to adjust idle speed.

LOL, looks like I just needed to surf the web a bit more... This guy suggests adding a conventional idle adjustment screw to the carburetor due to modern fuel (its the last section on the page)...

https://www.thecarburetorshop.com/AFBtuningtips.htm

Not all AFBs are not the same, some had the brass idle air bleed, while others didn't. Some had a curb idle adjustment screw, and no idle bleed.

My way of setting the ones with the air bleed would to be to set the idle speed where it is spec'd, then adjust the mixture screws for the smoothest idle in gear, if it's an automatic, or just for the smoothest idle with a manual.
If the idle speed is within spec, then you're finished. If it changes the idle RPM you'll have to reset the idle speed, then re-adjust the mixture screws again. You may have to do this more than once to get it right. This is how you'll be able to get a smooth idle, at the correct RPM.

The final adjustment is the mixture screws, I always adjust them last after setting the idle RPM.

The setting of telling you to bottom them out, then backing them out so many turns is just to get you within an adjustment that will run after a rebuild. The final mixture adjustment is made for smooth idle warmed up, and running of course. Some people like to watch a tach to get the adjustment at the maximum RPM, I just lay my hand on the engine to feel where the engine is the smoothest, whichever you prefer.

[Mwieczorek]

Originally Posted by alleyCat

I was just going to ask if its a Carter with the large center air bleed ! Make sure your primary throttle blades are closed at idle . When you rebuilt it did you spray Gumout or some other solvent thru all the transfer slots and hole when you had all the blled and mixture screws out ?

Everything got hosed out with carb cleaner followed up with a pick or brush. I have a set of really thin brushes for cleaning carbs and they are perfect for that.

With that said, I can't get enough idle speed out of that air bleed to keep the engine running. It seems like after about 2 turns out it just doesn't do anything anymore. In park, I can shut it off by closing the screw, but then when I open it up it seems to max out around 670-700 RPM in park and that just isn't fast enough to keep it running in gear. As soon as I put it in gear it stalls. I need a little more idle speed.

I'll figure it out yet!

[Goatracer1]

Is your engine stock? My 389 with an AFB has between 19 and 20 inches of vacuum at 600 rpm and 6 degrees of advance. With 12 degrees of advance I would think you should have more vacuum at idle. Have you checked for timing chain stretch or compression?

[padgett]

BTW something else that can screw up a Pontiac idle: sticking distributor weights. Also the kind of idle/vacuum you have is determined by the cam , I like a 744 but have a Muncie.
Is it possible you have a vacuum leak elsewhere ?

[8LUG]

Bent shaft on the bottom by the secondary throttle blades.

[padgett]

MBATR the original issue waaay down at the bottom, was " need to get the idle speed to increase a bit "

Bent shaft or vacuum leak will keep the idle speed from decreasing and it need to increase.

Guess I need a front/driver's side picture of the carb, does not sound like any AFB I have.

[gary bennett]

Loosen the 4 nuts holding carb down. Slide the carb toward firewall just that little bit. I had your problem when I restored my 63 Bonne. Then do your adjustments. Worth a try.

[Mwieczorek]

I did a compression check today. All cylinders had between 150 and 170 psi of pressure, 2 were down at 150, most were 155-60, and one was higher at 170. I think my rings and valves are ok. I ended up following the advice from that Carbshop.com article and drilled and tapped the boss on the driver's side of the carb and installed a 10-32 screw and spring. i just closed the air bleed all the way and set the idle with the screw like a more normal carburetor. Now I have it idling at around 900, and it drops to 550-600 when I put it in gear. Before it was dropping to under 400 in gear and it just wouldn't stay running.

Now, if Summit would just find my 4th wheel and tire I could finally go for a spin! I ordered 4, only 3 showed up. Sigh...