Oxnard - the current distributor timing setting was determined without use of a timing light, but by slowly advancing the distributor untilthe engine had reached the point of;no pinging, smoothestand fastest idle, and highest vacuum.This combination is the recommended timing method recommended by Pxxx Carburetors - and seems to have worked well - in terms of driveability and mid-range throttle response. The off-idle flat spot is still there, however.
Advancing the timing more from this optimum point does not allow the engine to turn-over when shut off, and retarding any further back towards the 6BTDC mark only reduces the intake manifold vacuum below the highest 20-21 range.
I can experiment more with the timing set points, but the off-idle flat spot never really goes away.

 

What about the transmission? When was it serviced last? That's about the only item not brought up.
Jim

 

It has to be one of three things. Compression, ignition and fuel. You say that compression is good on all 8 cylinders, the ignition timed and is hot at the plugs and the fuel is fresh. It should be running like a top. If it is still stumbling, I would replace the carb with a fresh one. Somehow my gut feeling is that the carb you have on there is not dialed in and you may not be able to dial it in.

 

You could have just poured in a pint of ahlocol. (I spelled that wrong, didn't I)

 

Have you checked to see how far from dead closed the idle speed screw is turned? I don't know exactly how many turns from totally closed it should be but if it is too far it will cause that stumble. And what about your power valve? If it is too low it won't open soon enough. I've always taken a vacum reading at idle and subtracted 5. I seem to remember my Autolite 4100 having a 10.5 valve. I would put a timing light on it to see just where it is though. Always nice to know that little number.

 

Hi,

I know your feeling. I had the exact same thing. My autolite 2100 Carb rebuilt at P Carb, and same results as yourself on my 1968 289 2V Auto trans.

I tried all the things like you, per their instructions; Timing by ear with the RPM and Vacuum gauge, checked alsofor vac leaks with carb cleaner.

On top of it,I replaced the point, rotor, cap, plugs and plug wires, Cleaned up also the PCV valve. Didn't do much.

Finally I adjustedwhat I didn't want to adjust until I had fix this hesitation stumbling problem, which is fuel mixtures screws.

That was the problem, I had to uncsrew about 1.5 turn the left screw (pass side) and 1 turn the right screw (driver side) What a difference, so my mixture was way too lean for my engine.

So by your post it is notclear if you ckecked that, if not give it a try. Just keep note of the numbers of turns you give each screws, worst you just put it back where it was.

 

To answer some of the suggestions; transmission - shifts OK, serviced (fluid & filter) maybe a year ago. If the transmission were at fault, wouldn't this be indicated at speeds or conditions other than only at idle; i.e. erratic upshifts, or downshifts? So far, the rebuilt C4 tranny shifts great and as far as I know is in good shape.

The 2100 carb mixture screws were adjusted at the time the rebuilt carb was re-installed. I followed the '66 shop manual and Pxxx Carb recommendations - individually screw in to richen, and out to lean the mixture - until the highest RPM, highest vacuum, and smoothest idle is realized. This is where the carb mixture screws have remained - as nothing else has been changed on the engine; same timing setting, same idle speed, etc.
Unless there are more, or different suggestions to fix from this latest post, as a last resort I'll contact Pxxx Caruretor to see; 1) if they can suggest something else, or 2) see if they can repair / diagnose the same 2100 two-barrel...for a third time!
I found it interested during my conversations with the folks at Pxxx Carbs - they know for a fact this same carb did not exhibit any hesitation, or flat-spotting- pulled smooth all the way through the RPM range when hot-tested on their 302 engine. Maybe if they could share their exact installation sequence and set-up prior to the engine testing, then maybe I could duplicate...
I've already thoroughly followed the Pxxx Carburetor instructions provided with the completed carb as-received. The off-idle flat spot still remains, however.

 

how about your valves, have you ever adjusted them? give that a shot, the worst ting that could happen is youll know there set correctly.

 

Thanks for the suggestion. If the valves were incorrectly adjusted; i.e. off at the rocker arm adjusting - too tight or too loose - then this could be seen using a vacuum gage ported at manifold vacuum during idle speed. If the valve(s) are mis-adjusted - this would show up as an erratic needle or weak vacuum at the gage reading. Per my previous post(s) the manifold vacuum at idle remains at a steady, strong 20-21 inches Hg. Also, any mis-adjusted valves could be determined by the individual cylinder compression check. With valves opening too far, or too little - the compression reading would be abnormal for that cylinder. Again, the current cylinder compression readings are OK - taken wet or dry - for all eight cylinders.
Keep the suggestions coming, please! At this point, I'm going to re-check all of the basic stuff before I condemn the carburetor - including the valves, compression, dynamic timing, etc.

 

Jim's got the reason down. The solution is to understand what the problem really is and make the necessary changes. The physics of it is as you open the throttle, the off -idle circuit in the carb is demanded, typically using some of the idle jets and some of the main jets for a fuel source along with the accel pump. Check your fuel pump output pressure and volume and your filter for restrictions. If these are good, it's in the carb and the shop needs to figure out why. Some things to look at in the carb are:
accell pump, linkage, adjustment, check valve. (can you see a blast of black smoke from your tail pipes when you jump on it? if yes then that's too much gas) other things to look at are worn throttle shafts( not every one bushes them on a rebuild), a small intake gasket leak under the carb, and finally mis- adjusted idle screws.

The correct way to set idle screws (and the easiest) is to shut off the engine and run the screws in until they lightly seat. then turn outward 1 1/2 -2turns. Start the engine and separartely turn them inward until a noticable RPM drop (50 -100)rpm. ( u needto use atach) and then run outward until the engine hits a maximum speed and then starts dropping off a little. Stop. You can turn them back in a little but don't go crazy. Don't forget to readjust the curb idle after you get the mixture set.