694spd
Well-Known Member
Is it possible to make adjustments to the vacuum advance unit on a Prestolite dual point distributor?
What size is that Rusty? 5/32"?
I think You're right. I know it's small. (so is my memory) lol3/32 sticks out in my mind, but it's kinda foggy up there.
Yes sir. 3/32"3/32 sticks out in my mind, but it's kinda foggy up there.
Prestolite. Do not all Prestolites use a stack of washers on the vacuum spring?Is it possible to make adjustments to the vacuum advance unit on a Prestolite dual point distributor?
Yes, but only if the vacuum can has the hex where the vacuum nipple is. Also, the adjustment is not how much advance, but when it comes in. The amount is not adjustable.
I think I have a photo of mine.It also looks like it has a nut on the end where the vacuum nipple is, so if it does, it adjusts with the washers as described above.
Prestolite. Do not all Prestolites use a stack of washers on the vacuum spring?
If so, inside it will look like the one in the picture here:
1951 Imperial and Chrysler Service Repair Book - Distributor Repair
The washers are how those are adjusted.
That's not out of line. It will vary depending on application. What year, engine, transmission? Your testing is good, but did you take into account of the rpm when it advanced with the vac pump? If not, go back and subtract the mechanical timing at that rpm. Hope that makes sense... I ended up getting an additional 25 degs advance from the unit at about 12" hg vac. I was a little surprised that it advanced that much. I was expecting more like 18-20 degs.
Correct. It should be zero. Its gets vacuum from a hole just above the throttle blade when the blade is at idle.At idle, I assuming that there should be very little "port" vacuum at the carb, correct?
That's probably not full advance at 2000 rpm. Very few factory advance curves look like the hot-rodded curves. Most are two stage. I may have '68 340 curve plotted as an example. If not I'll do it later. Or, if Halifaxhops has the '69, even better. (also, remember, the advance specs in the back of the electrical section are distributor degrees and distributor rpm)Mattax, The car is a bone stock 69 dart 340 4 speed. You are right, I forgot about the added mechanical advance when when the engine speed pick up after I had applied vacuum on the unit.
Earlier, when I tested the mechanical advance alone with the vacuum advance disconnected and plugged. I was getting about 25 degs mechanical advance that was fully in at about 2000 rpm.
For 1/4 mile, yes, you may want more or a little quicker than factory at the top end (3200 - 6000 rpm). Come up with that number by on-track testing for your car, engine, and fuel.The factory spec list the initial timing at TDC at 750 rpm. That setting would only give you a total advance of 25 degs. If I go with a 10 degs initial setting, I will have a more desirable total advance setting of 35 degs.