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Old 05-14-2009, 06:51 PM
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sbc timing question

Hi, 350 chev, performer RPM manifold, Demon jnr carb, not sure of cam. I just removed twin point distributor, and replaced with single point vac advance dizzy. Immediately it drives better, more throttle response, seems to have more power, haven't heard any pinging, yet.

However I would like to taylor the timing a little better, to the 36* total timing as recommended in the posts.

Most articles (including my rebuild your small block chev book) say to set initial timing between 8-12 BTC. I did this to get it going and as i say it runs good.

When I plug in vac advance to manifold (again, I have taken the general consenus from the posts herein to be manifold rather than ported), this bumps up idle timing to about 16*. Again, as mentioned in posts, this does seem to bring the engine temp at idle down.

However, my two questions.
1. The idle is now fairly high, over 1000. I imagine this has something to do with the advanced timing, but I can't turn the idle screw any lower. Does this mean there is too much timing at idle, and I should revert to ported vac to lower idle timing?

2. When I checked total timing to see if it was in the ballpark of 36*, with the initial at 8, I don't seem to be anywhere near 36*, only to about 20* (with vac advance disconected). I am concerned if I advance the timing any further, ie another 10 or 12* that my idle will go thru the roof, that the initial timing at idle would be about 18*, and worse, if I stay connected to manifold vacuum advance (to keep idle temp down) this would increase the idle timing to about 28 or 30*!

So, does this seem normal? As far as I can tell the damper mark is on TDC, so I don't think it has slipped. It looks like if I want anywhere near the 36* total as recommended, I am going to have to run ported vacuum, but I still believe my inital timing wouldbe up around 18*. And I am still confused as to why some people say 8-12 is good initial timing, and others say 18-25 is good initial timing. Is this because you set with vac advance hose discon at say 8*, and then once plugged in to manifold it adds another 10* or so?

Please help??!!!
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Old 05-14-2009, 07:56 PM
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Any points distributor belongs in one place, the trash. A HEI out performs points in every way possible. I would also check your harmonic balancer, it`s not uncommon for the outer ring to have slipped giving you incorrect timing readings. most SBC`s live off vacuum advance, the vac advance is supposed to be connected to a manifold or full vacuum source. Lean mixtures burn slower, this is why the high advance at idle (with vac advance) was called for. When I ran my last engine which was a low compression 350, 12 degrees before top dead center with vac advance disconneted and plugged. When plugged in it bumped it up to 24 degrees. With this setting it had excellant throttle responce and power, this was with a road demon jr. 625.
If it pings in your case then the thing to do is retard the spark timing 2 degrees at a time until pinging is no more.
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Old 05-14-2009, 08:05 PM
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As noted, even the cheap $45 chinese HEI is ahead of a points dizzy. Yeas, it can idle with vacuum advance coming on, yes your balancer may have slipped, or it may not have. What is your cranking compresion? Low CC can require a little more initial advance.

The first step you need to take is to go to Summitracing.com and get you the in house brand dizzy, they work well and are priced very reasonabley.
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Old 05-14-2009, 08:21 PM
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thanks guys, at least i know i'm not alone with the high initial timing setting!

I'm also thinking that maybe the mech advance may be slightly seized, or gummed up it does not seem to advance much, maybe if i give it a clean and lube it might let it swing up to the 36* mark without too much more initial timing.

And yes, I have aware of the benefits of a HEI, but I have firewall clearance issues with the Chev HEI, a MSD one is over a thousand bucks in NZ with coil, and and Ignitor module to fit in my current diz is about $260, which is a little out of the budget at the moment so a must persist with stone age technology for the present. Once I have replaced my diff and done the vavle guide seals, then maybe I will start saving for electronic.
cheers
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Old 05-14-2009, 08:50 PM
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What rpm are you checking your total advance at? With stiff factory spec springs, it may not hit full advance until 4500 rpm or higher.
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Old 05-14-2009, 09:01 PM
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Do you have a electronic choke thermostat? If so, does it have battery voltage with the key on? Your high idle gives me this thought.

What is your point dwell? This will effect your ignition timing. It should be between 27 and 32 degrees. Your vacuum advance will effect this, and a adjustable vacuum advance will help in fine tuning.

A manifold vacuum source is the key, and it should be unplugged while setting the initial base timing.

Another thought is the point breaker plate, it could be real loose, and giving you a false timing setting. RE: The point dwell effects the timing.

Check your choke setting, and throttle cable (unhooked) for binding, and a weak throttle return spring. If you have an automatic trans, check your kick down cable adjustment. (this may be holding it open)
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Old 05-14-2009, 10:39 PM
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you can replace the points with a Pertronix replacement. That will do away with the points without the major expense. A Ign box (MSD or the like) can be added later to get the hot spark. Have you had the dist recurved on a distributor machine yet? Looking for 36 total with 12 initial means you need about 14 in the mech advance and 10 in the vac can. In the past I did a lot of them. Start the mech advance about 250 rpm above idle and finish it all at cruise rpm. In my experience you can then use the manifold vac to give you the total you desire while allowing the advance to drop back a little under load. You can play with the split between the mech and vac advance to get the best performance.
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Old 05-14-2009, 10:51 PM
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Hey guys, note the OP is in New Zealand. With currency exchange rates and import duties/taxes, spark plugs probably cost him $25 each. LOL
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Old 05-15-2009, 10:30 AM
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Quote:
Originally Posted by 350pano
I just removed twin point distributor, and replaced with single point vac advance dizzy. Immediately it drives better, more throttle response, seems to have more power, haven't heard any pinging, yet.
This points to a prob. w/the dual-point dist. But properly tuned, a DP set-up is better than single points. If, however, there's excess wear or some other mechanical failure of the DP plate, you're prob. better off w/o it.

Quote:
I would like to taylor the timing... to the 36* total timing as recommended in the posts. * SNIP* The idle is now fairly high, over 1000. I imagine this has something to do with the advanced timing, but I can't turn the idle screw any lower. Does this mean there is too much timing at idle, and I should revert to ported vac to lower idle timing?
That the idle screw is no longer in contact w/the linkage (so the idle speed can't be lowered) causes me to think that the fast idle cam (inboard of the thermostat- if electric- on passenger side of carb) is on the lowest setting. The 2'ndary throttle blades may be open too far. Possibly the idle mixture screws may be too lean as well.

The carb should have ample adjustability to set the idle lower, even w/the ignition advanced so check to see what's up with that.

To tune the advance curve, you need a set of springs like sold by Mr. Gasket, etc. I don't use the weights (keep the stock ones, at least for now).

Replace the OEM springs w/a pair of light ones- just heavy enough to bring the weights back closed when you shut it off.

With these light springs, recheck the total advance (w/o vacuum advance for now) by bringing the engine speed up to where the timing quits advancing and see what you get.

Initial plus mechanical is what should equal 34-36 degrees. What the vacuum advance adds to that will put you over 34-36 but that's OK- this will only happen for any length of time at cruise (low load) and idle- which is OK, too.

This is over-simplified, but will get you headed in the right direction, I think. Main thing it'll do is let you know what your mechanical advance is able to provide. And from that, you can see what (if any) modification to the distributor is needed.
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