Some background: I recently swapped out a particularly pathetic reman carb that was on my '78 when I got it for a fresh Motorcraft 2150 (the old one was even missing a couple of jets! No wonder I couldn't get it running right). After about two days of being driven to school and back, it (re)developed a bad hesitation out of the blue. The fuel-air is currently set at 3 turns out, as the stock setting of 2 turns caused it to idle so roughly that it nearly rattled my teeth out (It's still idling a bit roughly. idle improves dramatically if I turn the fuel-air screws out to 3.5 turns, but I'm pretty sure that this is far too rich). The issue is definitely worse when the engine is cold, often stalling smack in the middle of intersections. If you slowly ease the throttle on, it will either not do it, or barely do it. Pumping the throttle when it cuts out will usually cause it to fire again before the engine stops turning and get it past the hesitation point. When warm, it will still do it, but only under specific conditions. The engine has to be under load of some form, and the throttle has to be pressed at a medium-fast rate to about half throttle. It will not do it if you stomp the pedal to the floor when it's hot. I'm relatively certain that it isn't the accelerator pump. The only change that has been made other than swapping the carb are is follows: I switched the vacuum advance from ported to manifold vacuum. Initially, I thought that the EGR had blown the diaphragm out, but swapping it for a new one made zero difference. However, I wouldn't be surprised if the parts store sent me a broken/used/incorrect EGR for a FIFTH time in a row...

 

Set your idle jets with a vacuum gauge. Start with 2 turns out then adjust to highest vacuum on both sides. A stock setting of 2 turns out is always just a starting point. If it takes 3.5 turns out to get a proper fuel ratio, so be it.

Also is your electric choke working?

 

Sort of. The choke itself works, but the fast idle does not. I have to manually hold the pedal down for about a minute before it will idle. Not sure if it matters or not, but it will occasionally either lose power rapidly at high rpm, or die completely, in which case I have to let off the throttle and/or pump the gas a few times before it will cut back on. At this point, I believe that this is being caused by the original 1978 fuel pump.

 

Hm.... Start from the beginning - either check the fuel pump output or better, replace it. They aren't usually real expensive? Or is this one in the tank.

Anyway, THEN set your float level. Make sure the linkage is free and the butterflies open up all the way when floored (engine OFF!) Very common mistakes with new carb, ask me how I know), check that the accelerator pump shot is adequate.

IF everything else is setup right (timing, ignition, etc) at this point only you now want to use a vacuum gauge for idle mixture setting. Everything in the way the engine is tuned up to this point will change the optimum setting of the idle circuit to some degree, so it is done last.

The rough settings will not be close enough. When adjusting the idle mixture screw(s) the RPM will change, reset RPM to factory spec, and then go back to the idle mixture settings. The idea here is to lean everything out as far as possible tho no more.

This will trim your idle circuit, speed and quality out with juuuust a slight turn, you can see it on the vacuum gauge as well as hear it. Sometimes our ears are better than meters. Done!

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Sounds like a plan. The only thing that makes me nervous is the fact that the condition seems to have spontaneously generated during the day while the truck was parked. I drove it for two days, and it ran perfectly (The old carb was so bad that the new one probably added 50 horse. Now it can cruise at 80!). Drove it the morning of the third day, still ran perfectly. Drove it the afternoon of the third day, and now the idle sucks and it's got a bad hesitation. That SHOULD narrow it down to the EGR, as I can't think of what else could cause something like that off the top of my head, but, as stated, replacing the EGR did nothing. I'll use a vacuum tester to check the new valve tonight and post the results. Any suggestions on what to look for as far as pressure? Applying pressure to the port should actuate the valve, so it'll be pretty straightforward... I hope.

 

What do you mean when you say "fresh"? New or new to you? Could have a small piece of trash in it causing issues. How does your fuel filter look?

 

This is why ya have to start at the beginning - esp. if giving advice sight unseen. Too many assumptions to make accurate guess. You mention that the fuel pump is original to 1978. It might be fine, but that's not the way to bet. If the pressure was too high, or two low, the old carburetor would have the float level set wrong or compensated somewhat, maybe. - or not.

Once a new carb is put on, it would have to be set again. Ideally don't pair an old fuel pump with a new carb. See where I'm going with that?

For all that, how many miles does the distributor have? The bearings get all wallered out and wobbly. A new dizzy will often improve idle at least as much as a new carb or electronic ignition. Start with known good baseline for fuel input, ignition timing, valves adjusted, etc. and then start adjusting the carb as final measure. Sent from my iPhone using IB AutoGroup

 

By "fresh", I mean a reman ordered from LMC Truck. The vehicle has 54,000 miles on it, so I sincerely doubt that the distributor is shot. That said, the voltage regulator WAS shot, so you never know. Note: In the near past, the roll pin that holds the stator in place fell out and got ground to dust inside the distributor. I did take the entire unit apart and wiped/blew it out with a compressor. Could that be causing any issues?

 

Just keep doing what yer doing. Does the carb have a spacer? Make sure the carb flange bolts are evenly torqued, the metering block/bowl too. Engine should be at normal (hot) operating temp.

If engine is healthy and tuned It should idle smooth down to 500 or so and "hold" without stumbling or wandering and good high steady vacuum. Even if your final idle RPM is set or spec'd higher, (Holley's might like 600+) an engine that is tuned well will idle smooth down low. Then dial in the idle and idle mixture screws for the highest vacuum achievable with the smoothest idle; at the specified RPM in the manual. Use a tach unless your ears are calibrated.

You can tell when you are closer because the plugs won't look like they've been sprayed with high-temp stove black. What do your spark plugs look like right now?

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Edit: in your first post you mention you are using manifold instead of spark port timing. Be sure this is correct for your application.

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The plugs looked good last time I checked. Not glazed and not coated in fuel or anything. I have been checking at full operating temperature, and the carb does not have a spacer. For the old smog era 302's, they were run with the vac advance on ported vacuum for emissions control reasons. It helps add power/smooth out the powerband to a surprising degree if you switch it over to manifold vacuum.
Note on the dying at high RPM: It just did it while I had the window down, and I noticed that when it does die, there is absolutely no ignition whatsoever, so I'm either losing spark/fuel for some mysterious reason. If it's spark, that likely leads me to something interfering with the pickup unit in the distributor. I had something behave similarly when It got water and metal dust inside the cap. If it's fuel, the first suspect is the pump, which IS an external unit to my knowledge, so that's the first thing I'll replace if this weekend's tuneup doesn't fix the issue. Also, while it's dead and I'm pumping the throttle, it is making a rather loud "pissssh, pissssh, pissssh" sound akin to the pressure release valve on an air compressor.

 

Fuel pump? They will often (fail) work at idle maybe, but unable to provide sufficient pressure at highway speed. WAG. Lots of possibilities. Plugged filter, clogged lines, air leaks at loose clamp, all of the above. Important that fuel pressure is steady at all times whether before the pump or at the carb, full tank, half tank, etc.

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If you do not have a carb spacer, then how are you running an EGR valve??? The EGR valve is bolted to a carb spacer.

 

Hm, guess I was wrong. I'm a bit more used to 1970's motorcycles that didn't have all that nonsense.

 

I addition to suggestions above, a basic ignition tune-up may help. On mine, replacing the 35 YO factory carb with a reman fixed about 80% of the idle and off idle roughness. But it still wasn't 100%. New cap, rotor, wires and plugs got her to 100%. The old wires did not have a lot of miles but lots of years. It seems wires may deteriorate with time almost as much as mileage.

Also agree the vacuum gauge idle tuning approach works well. I'm pretty sure mine is more than 2 or 3 turns out and it idles great.