I was wondering if anyone knows how to get rid of a hesitation on a 71 Buick Skylark 350 I bought a remain carb from a place called Carbs direct on the net and it works great at full throttle idles well and all that I have the mixture screws adjusted pretty good timing is good engine does not shake too much. The problem I have is when I give it a little bit of gas when I am cruising around 2500RPM and have a slight hesitation. I was told maybe adjust the accelerater pump rod I guess you have to bend it not too sure how this will help me or not? Anybody have any thoughts? thanks Bill
If the carb was just remanufactured, then assume (for the moment) that everything is correct on it. Check a few other items first, such as... Heated Air Intake - make sure the plumbing for the heated air intake on the air-cleaner snorkel is working. The carb is jetted pretty lean because it's assuming the intake air is around 120F. If the heated-air intake plumbing is missing, the vacuum hoses disconnected or the vacuum motor broken, then cold air will be coming in and the carb will run too lean. Manifold Heat-Valve - the is located on the right (passenger) side exhaust manifold. It forces some exhaust gases to flow through the cross-over passage under the carb in the intake manifold. If this valve is broken, it usually stays in the open position and the intake manifold doesn't receive all the heat it's designed to. This is more noticed as a cold-engine drivability problem. PCV Valve - Make sure this valve isn't stuck (shake it - if it rattles, it's good). All other vacuum hoses / leaks - make sure no other vacuum hoses are loose, cracked, plugged, disconnected or leaking. While you're at it, check the ignition timing and dwell. All of these checks shouldn't take more than half an hour. Once done, you can be fairly sure that you aren't trying to find two problems at the same time (a daunting task...). If all of the above checks out, then get your service manual and verify all of the adjustments on the carburetor. Except for the float level, all of the settings are external. There is one adjustment for the accelerator pump rod length.
Thanks I will take a look. I know that I don't have the heat riser hose on which I will put on that is probably why it runs twice as bad when cold. Also do you have any vacuum diagrams to show me where all the vacuum hoses go to the air cleaner assembly? It is for a 71 350. thanks Bill
Bill, Very simple. There is a sensor inside the air cleaner. one nipple goes to a manifold vacuum source, the other nipple goes to the air door vacuum motor on the snorkel. Then run a vacuum line directly from the carb to the distributor vacuum advance. That's all you need on a 71 350. BTW, you may want to make sure that the vacuum cannister (vacuum advance) is actually working. Sometimes the diaphram in the cannister goes bad. It's easy to verify with a timing light.
Thanks Larry I know the advance works I used a vacum pump on it and it holds vacum so I should good there. There are two nipples under the air cleaner where does the other one go. I have one nipple that comes out of the block right in front of the aircleaner then I have the thermo vacuum switch with three nipples on it do I need to plug something seems like too many fittings and not enough places to plug them in. Any help would be great. I also noticed my charcoal canister has one side plugged there should be a hose from there that goes somewhere any ideas? thanks Bill
Forget the thermovacuum switch, it's absolutely useless. The nipple in front of the carb is manifold vacuum. Run a hose from there to one side of the air cleaner sensor. The other side of the sensor goes to the snorkel door. Then run a hose from the ported vacuum nipple on the carb to the vacuum advance. That's it, nothing else needed.
The thermo-vacuum switch is used to keep the engine from overheating when idling on a really hot day. With the 'ported' vacuum hooked up to the distributor, there is no vacuum advance at idle. This reduces emissions but also increases the heat load on the coolant. On a hot day, the slow fan speed at idle may not draw enough air across the radiator to keep things from overheating. Enter the 'thermo-vacuum switch'. It has three ports - two inputs, one from the 'ported' vacuum (labeled "C" for carb), one from the manifold vacuum (labeled "MT"), and one output to the distributor (labeled "D"). Under normal conditions the switch sends ported vacuum to the distributor. When the engine starts overheating, however, it switches to manifold vacuum. This advances the ignition timing about 14 to 18, causing the idle speed to increase. This speeds up the water pump action and cooling fan airflow across the radiator. The advanced timing also reduces the heat rejection to coolant. When the temp drops to normal, the switch goes back to ported vacuum. For the most part, you can leave the switch out of the equation and simply connect the carburetor port directly to the distributor advance. As for the air-cleaner temp sensor - it has two ports on the bottom of the air cleaner. They're both identical. Simply connect one port directly to manifold vacuum (either to the manifold itself or a T-fitting supplying some other manifold vacuum device, like the choke pull-off). The other port is connected to the vacuum motor on the air-cleaner snorkel. Full manifold vacuum will cause the motor to close the snorkel and suck in hot air through the hot-air pipe from the exhaust manifold. As the air in the air cleaner heats up, the temp sensor opens a bleed valve. This bleeds off the vacuum from the filtered air inside the air cleaner, causing the motor to open up and admit cool air through the snorkel. The sensor balances the bleed air to maintain a temp of about 115 inside the air cleaner. This hot air serves several purposes. One, it prevents carburetor icing in humid weather. Two, it promotes better vaporization of gasoline, especially in cold weather. Three, by maintaining a constant temp the carburetor can be calibrated more closely, rather than trying to be rich enough for -20 temps and too rich everywhere else. This tighter calibration results in lower emissions and better fuel economy. The one disadvantage is that hot air is less dense so the engine produces less power. However, it's been calibrated for this higher temp so simply disabling the hot-air system without any other modification will not help. In fact, it will make the mixture too lean and hurt things. Plus, the only time you need full power is at full throttle - at that point, manifold vacuum is so low the snorkel motor relaxes and cold air is drawn in anyway. The engineers at GM knew this and sized the main jets to provide a proper mixture at WOT with cold air. So, you get full power at full throttle. At part-throttle the hot air returns; the rest of the carb circuits are calibrated lean for this. Piston airplane engines have manual carb heat and mixture controls so that they can develop full power throughout their entire operating range. However, it requires that the pilot change the mixture control for every change in RPM or altitude. And the carbs do ice up on occasion as well - quite attention getting when the engine begins to stumble and cough, or even stop! The heated-air intake of a modern car makes it very reliable and enjoyable. It's well worth the effort to make sure it's working correctly.
