Carb question

Maintenance/Repairs
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44

The screw in carb 5 looks like the idle speed adjustment screw. This is only adjusted after the idle mix screws are adjusted properly.

To adjust the idle mix screws, turn them in until they stop. Be very careful here not to tighten them too much because you can deform the tips. Just lightly grip the screwdriver with the tips of your fingers and you will know when you hit bottom. Then back out 1.5 turns on each.

Then start up and warm up the engine. When up to temp, turn the screws one at a time, in or out but not more than a half turn in either direction and set for the best idle. Go back and forth between the screws for overall best idle as each screw has an effect on the other.

Now once that is done, use the idle speed screw to adjust the idle to between 550 and 650 rpm IN GEAR, should be between 700 and 800 in park/neutral. BUT

Carb 3 shows a big dashpot with a bolt head touching the throttle linkage. Must be a California car. This is not the choke pull off that I mentioned before. It’s purpose to keep the engine speed from dropping off to fast as you take your foot off the gas. Instead of the rpms dropping right to idle speed, it kinda keeps the revs up a little. Yhis is to keep the sudden change in engine speed from going too low and the engine dying.

This little device could also cause the high idle speed if it sticks out too long and doesn’t retract as it should, and I see a little rust on yours. You need to make sure that plunger works freely. IIRC they were known to be problematic but usually because they didn’t work and the engine would die every time you let off the gas.

Now about the motor mounts. The chevy motor mounts did not use a captive design. By captive design, the rubber was sandwiched between the two metal plates that could only separate so far. In other words, the mounts only had about a 1/2 of play before the hooks on one plate engaged the hooks on the other. The later model mounts, still used today, the rubber could break all the way through and cause the engine to be really rough, but it could only rock so far.

The worse case in the old design was if both motor mounts broke and the engine torqued over enough, one of the mounting plates could slip so far out of position that the motor would not drop back in place when you let off the pedal. Now the engine was stuck in the torque over position and you were off to the races.

It has been a minute since I had my 66 Catalina with the 389 4V engine and I only had the car for 7 months due to a crazy lady who got mad at the guy behind me and rammed him while were were stopped for a light. She took out 6 vehicle in all. I had the Pontiac 7 months, the guy she rammed had his Mustang 2 months and the Ford Ranger (at the time the Ranger was the top of the line Ford Pick Up) Had just pulled out of the dealership where that guy had just bought it. He hadn’t driven it 100 yards yet.

Point is that I didn’t get to know the car all that well and I didn’t do much work on it as it only had 21k miles on it. IIRC, two oil changes and a tune up (plugs, points and a condensor) was about it.

45

Good catch!
I saw that and wondered why it existed on a 1966 carb, because I didn’t begin seeing those until a few years later. The “California car” is likely the reason.

Those dashpots were used for emissions. They held the throttle open on hard deceleration to prevent an overrich mixture.

46

I am guessing you are trying to describe “loping” at idle. A stock Bonneville, no matter which engine size, should not lope at idle but should be smooth as butter. Loping is generally from “performance” cams.

PB blaster might be okay, but I would use a carb cleaner spray.

47

Are the plugs/wires/points/condenser/cap/rotor all pretty new? Timing and points adjusted correctly? Vacuum advance working correctly?

48

Sticking throttle shafts is a somewhat common problem, sometimes the secondary throttle on my carburetor doesn’t fully close. If the engine does not return to idle, close the throttle manually. Also check if the dashpot is retracting properly.

1 Like
49

yes. all those things happen.

50

gotcha. ok maybe glug glug isnt quite right. it just sounded lower and meaner. thanks for that info i didnt realize that. i wanted it to sound like my old road runner. didnt occur to me the was a performance engine only thing.

51

yes. it was all done recently

52

Yeah, good catch on you too. It was initially for the emissions but also it did keep the engines from dying on sudden throttle off under certain conditions. Not sure if the 65 had the air pumps or not, but the 66 did and California required that the carburetor idle mixture screws be adjusted to run too rich just to prove the air pump was working. That is why the plastic caps were put over the idle mix screws.