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Discussion Starter #1
I’m a novice when it comes to timing but I’ve been doing a lot of reading and research on the subject. I bought a timing light because I’ve been curious and also have had idle issues and stumbling/bogging trying to drive at an idle. Today I hooked up my timing light and fired up the engine. I initially left the vacuum advance hooked up and it showed 12 degree advance at idle and 36 degree under load. I was under the impression that this should roughly be what the advance is with the vacuum disconnected. So I disconnected the vacuum and my advance dropped to 2 degrees and full advance was around 24. Ran like crap. Can my timing really be off this much and is this why it’s running poorly at times? This engine had a complete overhaul last fall so I didn’t expect to have any issues. Also, the cam is 218/226 @.05 with .48/.48 lift, 108 LSA. Not wild but kind of choppy.
 

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66 Chevy II, Pontiac powered; 68 &75 Firebirds
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Initial and total advance should be adjusted with the distributor vacuum advance disconnected. Some engines work well with vacuum advance connected to a full manifold source, others from a ported source. The only difference is that a ported source provides no advance at idle, but the advance starts as soon as the throttle plates open. Manifold vacuum provides advance even at idle. That is your case.
Your relatively mild cam is a bit lumpy because of the overlap from the 108° LSA.
 

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Can my timing really be off this much?
Sounds like it is if you checked it and it's at 2 degrees adv. Most of these rigs want more like 16 deg initial advanced with a total at 36ish without vac advance, total of 50ish with the vac. back off the adjustable vacuum if it's lurchy or pinging. That's a pretty standard tuning for SBC with a bigger cam.
 

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You timing is definitely off, and more than likely your problem. 2 and 24 is way too little timing. You may have a decent reading at idle with the vacuum hooked up, but as soon as you open the throttle vacuum drops, reducing your timing to an unacceptable range. You want your total advance timing with vacuum advance disconnected and port plugged, to be around 34-36° on average. But that's a safe range to be in. Since you already know you have 22° of mechanical advance in your distributor, and you need 10 more in advance, just turn your initial timing up to 12, hook the vacuum advance back up, and go for a drive. I bet you it feels like a whole different engine. Engine Masters just did a show on timing and air fuel ratios. They used a 460hp engine and with total timing at 11 degrees they lost over 140hp. And when they bumped it up to 21 degrees they only gained half of it back. Timing is extremely important in how an engine runs.
 

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Leave the vacuum advance off for now and the carb port plugged. Raise the initial timing up to whatever gets you 36 total and take the car out and see if it’s better. Once you see how it runs then hook up then vacuum and go drive.
 

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Discussion Starter #6
Update. Moved total timing from 24 to 34. This moved initial timing to 12. Runs much better and idled better. Had to change the idle screw quite a bit after plugging the advance back in. Much improved driving at an idle. I will tweak it some more tomorrow and experiment with 36-38 total timing. Can’t believe I’ve been driving it this spring with the timing off so much. Thanks for the reply’s.
 

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Awesome. It's kinda hard to tune it to the exact number it likes without being on a dyno, but you're in the ballpark now. Try 36, and if you don't see any noticable difference stop right there. It's better to be a few degree off to the retarded side than to be too far advanced.
 

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Mike - 74 Nova Baltimore, MD
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Glad to see that you are making some progress!
If you are planning to connect your distributor's vacuum advance canister to a full time manifold vacuum source, it's a good idea to know the total amount of advance degrees that the vacuum canister is able to provide.... and at what vacuum level (Hg) the vacuum advance canister provides full advance.

If you just connect your vacuum advance canister to a full time manifold source without knowing these numbers, you could be over advancing your total timing during idle and light throttle cruise.

*What type of distributor are you currently using?
*Does your distributor have an adjustable vacuum advance canister?
*If your initial timing is 12° w/o the vacuum advance canister connected, what is your initial timing once you connect the vacuum advance (at the same rpm as before)?
 

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Discussion Starter #9
Yeah next issue is the vacuum advance. Initially before I changed the timing it was advancing 12 degrees. After changing the timing and I assume gaining some vacuum, it now advances 20 degrees. So it’s running about 32 degrees advanced at idle, too much. I think I need to get a limit plate and limit the advance to 10-12 degrees and then try bumping timing to 36 with no advance. I have an msd hei distributor and would like to get 18 initial, 18 mech, and 10 vacuum.
 

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Someone set your timing with the vacuum advance connected.

Install the black centrifugal advance limited bushing, along with 2 blue springs.

Install the MSD vacum advance limiter plate https://documents.holley.com/frm34674_83623-84281_china.pdf

and set it so you have 13-16 vacuum advance if you have the pro billet hei, 8-11 if you have the street fire. If you have the pro billet and want less advance than that, you need to install a limiter plate on the vacuum cannister body like this crane one. Use it to limit the travel of the advance rod.
https://www.summitracing.com/int/parts/crn-99619-1/overview/

The curve you are thinking of will work well.

Also, with a cam which has a choppy idle, like your 108 LSA cam will, if you have a pcv valve you must run one designed for low vacuum at idle engine. A standard V173 works well.
 

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Mike - 74 Nova Baltimore, MD
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Initially before I changed the timing it was advancing 12 degrees. After changing the timing and I assume gaining some vacuum, it now advances 20 degrees.
That's kinda what I though was going to happen... and you may also experience some engine surging at light throttle cruising due to an over-advanced timing if you leave your vacuum advance connected to a full time vacuum source. But there are very easy ways to fix this.

Pockets, That's exactly what I was going to recommend :thumbsup:
https://www.summitracing.com/parts/...MI4rjPwd3-4gIVtP_jBx2YnwzVEAQYASABEgIMG_D_BwE
It's very easy to install and works well. I have one of these on my Pertronix HEI and set at "D" to limit the vacuum advance canister to 11°.


Since your currently getting about 20° of vacuum advance at idle, I would think you are very close to... or at full pull of the advance pin. After you install a VA limiter plate, your VA canister should definitely be at full pull @ idle. This is where you want to be for the best/most consistent idle.

I have an msd hei distributor and would like to get 18 initial, 18 mech, and 10 vacuum.
What MSD Hei are you using?.... Street Fire?... Pro-Billet?
Note: If you have an MSD Street Fire, there are no bushings that can be swapped to change the amount of mechanical advance. The starting and stopping points of the mechanical advance curve are controlled by the distributor's center plate and advance weights. IIRC, I believe the MSD Street Fire provides about 21-22 crankshaft degrees of mechanical advance.

P.S. You may also want to consider 15°-16° initial, 20°-21° mechanical, and 11°-12° of vacuum. You will still be very close to your desired advance curve, but your engine might be easier to start (especially when hot). This is how I have my Pertronix HEI set.
 

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Discussion Starter #12
Thank you for the replies. I have a street fire distributor so yes unfortunately can’t change the total mechanical advance. I did get some new springs so I can get my timing all in by 3k instead of 4K like it was stock. Also got a vacuum limiter plate to limit the advance. I’ve got the initial at 13 and total at 34 right now but with the VA hooked up it’s like 28-30 at idle and 48-52 at cruise. I’ve heard a little pinging so unhooked the VA for now until I get the limit plate and can keep it at 10 degrees advance. Goal is for 15-17 initial, 21 mech with quicker all in, and 10 vacuum advance. Should be much better than what I originally was running.
 

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Mike - 74 Nova Baltimore, MD
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I think your new timing goals are going to work out fine on your Street Fire HEI... but there are ways to change the total mechanical advance degrees on a "GM Style" HEI (Street Fire, Flame Thrower, etc). It's just a little more complicated.

Below is a post were I provided detailed instructions (with photos) when I was dialing in my Pertronix HEI advance curve.
https://www.stevesnovasite.com/forums/showthread.php?t=661331

NOTE: It's also a good idea to check that your weights and centerplate are installed correctly on your HEI (as my centerplate was installed incorrectly from the factory). If you go to post #6 in the above link, you will see how your weights and centerplate should look when installed correctly in a clockwise rotating engine (Chevy). There is also a YouTube video that shows an example on an MSD Street Fire HEI that has the centerplate installed backwards.

I think you are good with your HEI's weights and centerplate since you have a steady timing reading at idle. If your centerplate was installed backwards, you timing at idle would be very erratic.
 
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