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Old 08-06-2010, 05:42 PM
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Timing issues with 1974 sbc 350

I have a 1974 sbc 350 with stock pistons and rods, Crane Energizer Cam with 260 duration (204@.050 & .427 lift with 1.5 rockers), 062 vortec heads, aluminum vortec intake (dual plane), HEI distributor w/heavy springs & adjustable vacuum can, Edelbrock 600cfm carb (no spacer), long tube headers (1-1/2" I think) with dual 2-1/2"exhaust.

I have my initial timing around 12 degrees with the vacuum line disconnected and plugged. With the vacuum line plugged into manifold source on carb I get close to 40deg. Total on initial, vacuum and mechanical I am get around 46. How can I be getting 25deg from my vacuum? As soon as I even touch the gas, it jumps to the 46 degrees timing. Doesn't matter which vacuum port I use on the carb. It still has the same reading. I can adjust the vacuum can on the distributor, but not sure whay I am getting 25 from my vacuum sources. The engine sounds really smooth and runs good at idle and when revved up. Any ideas?

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Old 08-06-2010, 05:59 PM
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timeing

put 0 degrees initial timeing in it try that.then youre total should be around 36
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Old 08-06-2010, 06:29 PM
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Ok, was playing with the adjustable vacccum can and I got 15 initial and 25 with the vacuum advance. With that setting, even with mechanical advance, it it stays arou d 40 at 3000 rpm. Seems to start easy with around 15deg with ported vacuum. Should I try the manifold vacuum and lower the inital timing back? Is that what you meant by setting it at zero? Cam was installed straight up with dots aligned on crank and cam.
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Old 08-06-2010, 06:55 PM
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No matter what I do, it will not time right. I think the springs are not the right weight for this setup. I guess I'll try some medium or light springs in it and see what it does. Seems wierd that it goes all in as soon as I touch the gas. From what I read, initial + vacuum + mechanical should = about 50 right?
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Old 08-06-2010, 07:04 PM
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You should never hook up a vacuum advance to manifold vacuum, only ported vacuum. That's why carburetors have a ported vacuum connection.
Hooking up to manifold vacuum renders the vacuum advance useless as it was designed to work. If your getting too much advance from your vacuum can, you need to limit the total amount of advance that the vacuum can will deliver under full throttle.
One option other than limiting the vacuum advance is toss it and get a full mechanical advance distributor and have it set up on a dist. machine where you can tailor the curve controlled by the weights and engine rpm.

Right now, your vacuum advance is giving you more than you need. that's why your having problems with the total amount at 3000 rpm. You need to limit it so you can run more initial without too much total timing. I don't have experience with adjustable vacuum cans but I know from other post's that there is a key that you can install that will limit the amount of advance from the vacuum can. I'm sure that someone will pop up on here and tell you all about it. Hope I explained it good enough for you. Hang in there! Wait for the reply's
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Old 08-06-2010, 08:31 PM
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People use manifold all the time for radical cams so they can have a good idle timing. But, manifold is "on" all the time so you'll get full advance from your vac canister at idle.

Your adjustable vac can needs to be adjusted, its simply too much advance coming from it. You also probably have good MANIFOLD vacuum so its pulling in ALL of your vac advance. If you use ported vac then the vac advance shouldnt have an efffect at idle. With manifold vacuum you're vacuum canister WILL increase your advance at idle and almost until wide open throttle.

If you set your INTITAL timing to 0, then your vac canister will be the only advance you get at idle which, as you said, is almost 25. The problem then is at wide open throttle you have NO vacuum so you lose that 25' timing leaving ONLY your weights. You'll never have correct total timing

I think you should get the initial to what the motor wants on the dizzy alone with NO vacuum conncted. Rev the motor to see that the weights give you TOTAL timing when needed. Then youre set. USe ported vac on the vac canister. Your timing should NOT change at idle but will change under part throttle or slight load. You might get detonation, pinging, which means you have too much vac canister advance so lower it a bit and try again until there is no ping.

On the adjustable vac cans, someone posted recently that adjusting them also affects "how soon" they come in. Meaning, if you adjust for less advance on the "can" it also means that MORE vacuum is needed to make the canister begin its advance. Using manifold you wont see this happen but on ported you'll have different vacuum advance under varying vacuum
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Old 08-06-2010, 11:36 PM
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Quote:
Originally Posted by bubbahotep
People use manifold all the time for radical cams so they can have a good idle timing. But, manifold is "on" all the time so you'll get full advance from your vac canister at idle.
That is so basackwards. If the cam is that radical, why even run one?
It just proves that at that point (stated above) you need to toss the vacuum advance dist. and go all mechanical. That is basically what your doing when you hook the vacuum line to full time manifold. Your taking away the operation of the vacuum advance and putting it all on the initial and centrifugal.
A vacuum advance is not designed to work that way. Anyone that does it like that is only fooling themselves if they think there getting away with something by skirting the proper operation of vacuum advance systems.

Either hook it up right or go mechanical. I'm done with it.

I hope there is someone who knows what their doing out there who can back me up on this one!
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Old 08-07-2010, 12:17 AM
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Sqzbox, you are completely wrong about the vacuum advance. Ported vacuum is an emissions device and is rarely used on a performance engine. Same to Bubbahotep, you guys need to read the "ported vs vacuum" thread here at hotrodders, very informative for those who don't know

To the original poster, you will need to physically limit the travel of the vacuum advance can's pull rod with a purchased(Crane has one) or fabricated travel stop, so that it only adds 15° or so, it has too much travel as is. All the adjusting to the vacuum mechanism won't change this one bit, he screw adjustment in the nipple only controls the rate and Hg" start point.

Get your mechanical straightened out first with the weights and springs and degrees added(travel limit), then work on the vacuum advance
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Old 08-07-2010, 02:23 AM
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VACUUM ADVANCE LIMITER PLATE- Crane #99619-1: 99619-1 INSTRUCTIONS.
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Old 08-07-2010, 05:16 AM
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Quote:
Originally Posted by stfinney
The engine sounds really smooth and runs good at idle and when revved up. Any ideas?
If you're not pinging, and not overheating...I'd say forget the numbers. You're done.
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Old 08-07-2010, 06:46 AM
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Quote:
Originally Posted by sqzbox
Right now, your vacuum advance is giving you more than you need. that's why your having problems with the total amount at 3000 rpm. You need to limit it so you can run more initial without too much total timing. I don't have experience with adjustable vacuum cans but I know from other post's that there is a key that you can install that will limit the amount of advance from the vacuum can. I'm sure that someone will pop up on here and tell you all about it. Hope I explained it good enough for you. Hang in there! Wait for the reply's
Thanks Cobalt. And Ericnova, A vacuum advance really has no bussiness being on a performance engine. You can do so much better with an all mechanical adjustable advance dist. like mallory has or had. It's been a long time for me (70's) since I had one but with a good dist. machine you can tailor your rate of advance and total timing so easily without having to rig a vacuum advance to work. I used one when I was in the army as we had one in the post shop and even though I had to pull the dist 2 times to get it right,
The reward was worth it. My engine pulled through the rpm range a lot better and times droped at the track over whatever I tried with the vacuum advance system. My Crower 3/4 cam 280D/460L didn't make much vacuum and was a dog off the line. The mech. adv. really woke the engine up. Just by installing the mech. adv. dist. bumped my car into the next fastest bracket
by .100 that by itself was a big improvement in the racing world, especially at a desert track , above sea level, with 0 humidity. and hot air temp.

Last edited by sqzbox; 08-07-2010 at 07:14 AM.
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Old 08-07-2010, 07:37 AM
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Quote:
Originally Posted by ericnova72
Sqzbox, you are completely wrong about the vacuum advance. Ported vacuum is an emissions device and is rarely used on a performance engine. Same to Bubbahotep, you guys need to read the "ported vs vacuum" thread here at hotrodders, very informative for those who don't know
Can't find the thread. Can you link me up or something?
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Old 08-07-2010, 08:55 AM
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I took the cap and rotor off and really examined the mechanical parts. The springs look like light springs and the little wings pieces are a bit worn it seems. There was a snap ring missing on one side of the weight also. I guess I will be buying a new kit. Will a local auto parts store carry the travel stop? I know they have the recurve kits. Thanks for all the help!
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Old 08-07-2010, 10:03 AM
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not sure what brand distributer your using but this is what you are after, it is not unusual for those parts to wear. BTW this is for a mechanical advance distributer.

http://www.summitracing.com/parts/MSD-8428/
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Old 08-07-2010, 11:04 AM
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Its a stock HEI unit that came off my old 1975 350 motor. It has been quite a few years since reworked. It was running fine with my old setup and figured it was fine, but I guess not.
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