I recently swapped a older manifold onto a 73 engine. I have the distributor connected to a vaccuum port on the carb but there is no vaccum thre until the throttle starts opening. Should the distributor be connected to a port which has vaccum at idle or a port that doesnt have vaccum until the throttle blades start opening more?
This is "ported" vacuum, meaning the source is above the throttle blades. Try both and see what you like. I used to run manifold vacuum (sourced below the throttle blades) until last year, with the new heads & cam the engine likes ported vacuum better. Devon
Depends on the year model,,,, The ported vac advance was one of the first changes that the factories made on account of emmissions... The old cars had vacume ports either on the carb , below the throttle plates or from a fitting tapped into the intake manifold... a few ,,,like Fords had a spacer plate between the carb and intake that had vac fittings for power brake and vac advance in them.... Personally , I like the ''live all the time'' approach , for the better mileage...and throttle response...
Good advice, Try both to see the difference. I used to run manifold vacuum before but had problem with hesitation when launching from zero. Worked on the timing advance as described in Larrys power timing thread and changed to ported vacuum. Now the trottle response is crisp as a cold winterday, both when launching from zero and when passing someone on the freeway. Christian Link to larrys power timing: http://www.v8buick.com/showthread.php?t=63475
What is the difference between "ported" vaccum and "manifold" vaccum? I never understood this. My vac advance goes to the bottom of the carb. When I nail it, it takes like 5 seconds until terminal velocity kicks in. Like it's just waiting for the vaccum to build up or somehting. The sudden increase in acceleration is drastic, all the while the pedal is buried.
Ken, ''straight ''vacume is hooked to a direct vac source all the time,,,''ported'' vacume is hooked to a port on the carb that has its source above the throttle plates so that the vacume is created by the air coming thru the carb as the engine revs up... First ck. out the vac advance system.... There are passage ways under the carb that can stop up with carbon, lines can leak, the diaphragm can get a leak in it...[the most common thing]... ect....ck to see if the system holds vacume, and if it physically moves when the vac is applied,,,, also the mechanical advance....needs to be clean and lubed... look at the advance limiting bushing, these are bad to dry rot and crumble all to pieces ....also I would look at the accelerator pump shot and adjustment... Set the basic timing at a hot idle, with the vac line disconnected,,,,when the line is reconnected , the engine should speed up....if not , there is something wrong,,, also the timing light should show the advance at the harmonic balancer... With ported vacume the advance will not come in untill the engine comes off idle...
Ken, I'll try and explain this. When the engine is running, the pistons moving up and down in the cylinders create a vacuum pull against the closed or nearly closed throttle blades. All vacuum is, is a difference in pressure between outside the engine (atmospheric pressure) and inside the engine. As you open the throttle blades, pressure equalizes between the outside and inside the induction side of the engine. Therefore, below the throttle blades, vacuum is constant, and any source taken from below the throttle blades will have vacuum anytime the engine is running at idle or small throttle angles. These sources include those on the intake manifold runners, as well as some on the carburetor. It's easy to tell the difference. If you unhook a manifold vacuum source while the engine is running at idle, it will hiss, and the engine may run rougher. If you unhook a ported source, nothing will happen. The ported source is taken from above the throttle blades. It isn't exposed to vacuum while the throttle is closed. As you open the throttle, it is exposed, and a ported source will develop vacuum both from remaining vacuum inside the induction system and air rushing past the opening throttle blades. As the throttle blades go to wide open, pressure outside and inside the engine equalize, and vacuum is no longer present. Air rushes into the engine at atmospheric pressure. That is why engines make more power when the barometer is high. Air is pushed into the engine by higher atmospheric pressure. Both ported and manifold sources will go to 0 at wide open throttle. Manifold sources will be at their highest vacuum level during idle, and small throttle openings. Ported sources will be at 0 at idle, but rapidly increase as the throttle is opened, then follow manifold vacuum, and also go to 0 at wide open throttle. As to the second part of your post, I hope you are exaggerating when you say it takes 5 seconds to get to full power and pull on your engine. That sounds to me like either your timing isn't coming in quickly enough, or your secondaries are too tight and aren't opening early enough. Vacuum doesn't build with your foot on the floor, it is at 0 or very close to 0 at wide open throttle. Vacuum advance doesn't work at all under those conditions. Vacuum must overcome a spring in the vacuum advance canister to advance the timing. At wide open throttle, as stated before, vacuum is at 0. The spring in the canister removes whatever advance there was at higher vacuum levels. Your timing at that point is the sum of your initial timing and the mechanical(centrifugal) advance. Do you own a timing light?
I don't own a light but I have friends with them. Just to give you some basics, I think total timing is 34 degrees. I have no idea of the other breakdowns though. i.e. initital, etc The carb is a QJ from a 72 455, it was rebuilt by a QJ old timer who does carb work for the shop that did my engine. Everything on my engine is new. Using pertronix electronic ignition. I wonder about the vaccum advance on the carb though, that's what it really feels like. And I literaaly mean like 3 seconds till she comes alive. Maybe 5 is exagerating. I have a half inch spacer under the air cleaner to clear the carb casting and I wonder if one of the flaps or linkages is getting hung on it. Seems almost too consistent to be a problem like that though. I'm tempted to throw an 850 DP on just to see if it's carb related.
Ken, Vacuum advance has absolutely NOTHING to do with it. When your foot is on the floor, THERE IS NO VACUUM ADVANCE. Your timing is initial + mechanical. It should all be in by 2500 RPM or sooner. Initial is unimportant also, you need the total advance by 2500 RPM. You shouldn't think it's 34*, you have to know. Borrow a light, and see when your advance is all in. Confirm that your secondary throttle valves are going wide open when your foot is on the floor. With the engine off, sight down the carburetor while someone floors the gas. Push the air valves open and make sure the secondary throttle blades are vertical. Check the secondary air valves to make sure that the primary vacuum break is holding them closed at idle. It will bog if they flap open too soon also. It should scream when you nail it. When a Q-jet is adjusted right, it's seamless. What you describe is not normal.
Ok, when I pull it out of the cacoon in the Spring I will have to do this. I only say I think because nothing except death and taxes is absolute. I picked that engine up and watched them dial it in at 34 degrees total at the time I watched them dyno it. Retarded to 32 degrees the engine made less horsepower. It liked 34 degrees. Sorry I'm thick about the vaccum stuff. :error: I do know the last time you told me to check the carb plates that the secondary pulloff worked. The secondaries closed and I couldn't push them open. When I started the car, within a few seconds the diaphragm opened all the way (or closed whatever it is) and as soon as I shut the engine off it did the opposite all the way.
