The mechanical advance in any distributor is a set amount of degrees. You CANNOT adjust it without taking the distributor apart and doing some welding and filing. There is a pin that moves in a slot and that is what determines how much mechanical advance there is in a particular distributor. You can put a bushing on the pin which limits the pin travel and therefore, the amount of mechanical advance. That is all you can do without disassembling the distributor. When you change the springs, you change the rate at which the mechanical advance comes in, NOT THE AMOUNT, THAT IS SET FOR YOUR PARTICULAR DISTRIBUTOR. If you have the stock distributor for the 68 GS400 (1111285), it has between 30 and 34* of mechanical advance. That is why the initial timing is specified as TDC (0*). The stock springs allow 30-34* at 4600 RPM. You can boost performance by putting in lighter springs. Instead of getting 30-34* at 4600 RPM, the lighter springs allow the 30-34* in at more like 2500 RPM. You still run the initial at TDC, but your mechanical is all in at 2500 or less RPM. Problem is that most cammed and modified BBB's like at least 12* of initial advance. You CANNOT run that because you have too much mechanical advance(30-34*) in that distributor, and you would end up with 42-46*. The lighter springs can help alot by themselves, but when you use them you have to shorten the amount of vacuum advance to 8-10* That you do with an adjustable vacuum advance or you fashion a block off plate. Hope that makes it clearer.
Larry you just cleared so much up for me. So you're saying a modified BBB will need the initial readjusted, whereas with my stocker I can just use the lighter springs and change the vacuum advance to an adjustable one and keep the initial at 0*? Edit: Is the vacuum advance the Crane Cams kit comes with adjustable already with that little black plate?
Yes, while most engines will like more initial timing, it is more important with a modified engine (bigger cam, more jetting, etc.) If the engine is stock, just changing to lighter springs will boost performance and low end. Leave the initial at 0* This all assuming you have that 1111285 distributor. Are you sure it is the original distributor? The Crane Cams adjustable vacuum advance comes with some really nice springs. The unit itself has that little black cam that limits how much advance you get from the cannister. However, the Crane instructions tell you to mount it onto one of the cannister mounting screws. It works alot better if you drill and tap a hole in the arm and mount it to the arm. Look at the pictures in my first post. With Cranes instructions, moving the black cam pushes the link further into the cannister, and that moves your initial timing up. You don't want that. If you mount the cam on the arm, it stops the link from moving it's entire travel and limits the vacuum advance that way. With a stock engine, you could also just use the stock cannister limited with a home made block off plate. Again look at the pictures in my first post.
Where would I find the numbers on the distributer? I'm 90% sure its original since all the rest of the engine is, aside from the water pump. I'll check just to be sure. I don't think I've got anything sitting around at home to make a block off plate out of. Could I use a piece of scrap aluminum that I could get from work? It's really thin aluminum coil for capping windows and doors and we always have alot of small pieces sitting around. Edit: Never mind, I went and looked at it and found the numbers. It does have have 1111285 distributer.
OK, You have the original distributor so you should stay with the 0* initial timing. You can use any metal as long as it is thin enough. The screws you need have to be short enough not to interfere with the link. You don't need to modify the vacuum advance until you install the lighter springs. Take a look at the underside of the weight plate and see if there is a bushing on the pin. There is a picture of the pin in slot and bushing in post #3 of this thread.
I'm pretty sure I can see the pin with a bushing on it. I'm having a tough time keeping the cap off, holding the flashlight and using a mirror to look at it at the same time (I'm not in a good position to get the car out and start taking vital parts off the engine right now). I doubt it could be anything but the pin after comparing with the picture. I just ordered the Crane Cams advance kit.
I just thought of two questions Larry. Do I have to pull the distributor for this, and how will this effect my gas mileage?
It can be a little tricky putting a bushing on the pin but it can be done in the car. For the vacuum advance mods, you just need to remove the vacuum advance (2 screws and the link). It helps to use a vacuum pump to pull the link all the way into the cannister. It gives you more clearance to get the screws out. How will it affect your gas mileage? It shouldn't. When you have the stock springs in the distributor, you don't reach full mechanical advance until 4600 RPM. When you are tooling down the interstate at 60 MPH, the engine is probably only turning 2200-2400 RPM. At that engine speed, only part of your mechanical advance is in. In your case, you have about 23* or so from the mechanical advance at that engine speed. The throttle is open only a small amount and engine vacuum is high. The vacuum advance unit gives you about 18* more advance, so your timing is at about 40-42*, which gives good economy. When you floor the gas, the engine vacuum goes to 0, the spring in the vacuum advance cannister pulls out all the vacuum advance, and as the engine reaches 4600 RPM, all your mechanical advance is in ( 30-34*) for best power. When you put lighter springs in the distributor, all your mechanical advance is in at highway speeds. If you add in the stock vacuum advance, your at over 50*, and that is too much. That is why you must restrict the vacuum advance to 8-10* max. When you're at 60 MPH on the highway, you'll have all your mechanical advance (30-34*). Add in 8-10* from the vacuum advance, and you're at 40-42*, same as before. Does that make sense? Speeding the mechanical advance will give you a real seat of the pants increase in performance at low speeds. Good Luck.
Thanks for answer my questions Larry. So do I have to change the stock bushing that's on there for a new one?
Thanks Larry. I gotta say, you're the best, you really simplified this whole thing for me. I can't wait to get this done now! I shoulda had the parts overnighted ou:
I'm glad you understand. That is really why I wrote the opening post to this thread. If you have an advance timing light, you can check your total advance when you get the kit. Plug the vacuum advance, install the 2 yellow springs. Set the advance dial to 32*. Point the light at the timing tab. You should see the timing mark below the 0 on the tab. As you rev the engine to about 2500 RPM, the mark should move up towards the 0. If it stops at 0, your total timing is 32*. If it stops at 2*, 34*. The specs for your distributor call for 30-34* of mechanical advance. Manufacturing tolerances back then were not as good as today, so it could be 30*, 34*, or something in between. That's why it is a good idea to check the total timing. If you only have say 30* in your distributor, you could run 2* initial which would bring you to 32*, or 4* initial which would give you 34* in that case. Then again, your distributor might have 34*, in which case you stay at 0* initial. after you've verified the total, install the 2 silver springs. They are probably the best, but you can experiment. Then restrict the vacuum advance(8-10* like in the pictures, and plug it into a manifold vacuum source. Running it off manifold vacuum will give you the vacuum advance at idle as well as highway speeds.
Mallory magnetic breakerless disty (my first buick, 68 lesabre conv 350 engine). I am replacing the distributor and thought about using the mallory unilite magnetic breakerless disty, with vacuum advance. SUMMIT: MAA-5764301 And how will I know how much initial advance this distributor has? BTW setting mechanical advance in this disty is quite a bit differrent, no springs in there. Has anyone had any experiences with this, I dont want to use the old points style or hei distys, so I thought this might be a good one? this does come with vacuum canister but do I need to swap the crane adjustable vacuum canister in it to ba able to limit the vacuum advance? The engine will be stock except TA intake, so would the initial 10-12 and 30-34 degrees be good? THANK YOU
I figured I might add a bit to this awesome thread. I recently acquired a '73 Estate Wagon with a 455, and have been going through everything to make it roadworthy. I got the distributor all torn down, and it was in sad 152K mile shape. I ran into a small problem with the plastic seal that is at the top of the distributor body, it looks like it was supposed to be plastic, with a felt seal on top. Is this necessary or can it be omitted? What is it called and where can one be found if it is needed?
Some distributors came with a wick that was supposed to lubricate the distributor cam. I'm not sure if they are available. You can do without it.
Should be fine as long as you have a start retard. Got to watch your plug heat range also. Plug tips will run hotter with full advance all the time.
Larry, My engine fires up cold or hot with a stock starter every time and with no retard. The dist is locked @34*. Obviously it depends on the actual cylinder pressure whether you'll need a start retard. Edit, maybe I do have a start retard after all. I keep seeing this guy looking at me when I turn the key and look in the rear view mirror at the same time.
You are lucky Andy, Most guys have a problem especially when Hot. The starter can kick back and damage things. I have a 20* selectable start retard on my MSD Digital6. I don't use it because the engine doesn't like starting at TDC. It would work great in your case.
