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  #16 (permalink)  
Old 02-02-2013, 12:05 PM
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I agree with many posts here. I would check the total timing and go from there. Either with a dial back timing light or revving the engine to over 3k rpm when the mechanical advance should be full in.
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  #17 (permalink)  
Old 02-02-2013, 12:06 PM
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Quote:
Originally Posted by yragat View Post
I haven't been able to find out what total should be on the Caddy engine ----it is a 1960---390ci new rebuild with mild cam my vac advance (connected to manifold ) is 12* and I believe my centrifical is 12* also---If I remember ---have it written down some where--will have to check.
And I probably should have 4 of the carbs running instead of 2 ,but can't afford to have them rebuilt & get linkage right now plus the gas stations would eat me alive. Like I said the darn thing runs like a rabbit,except from idle to wot. And who doesn't like to blow off a tuner at the stop sign once in a while ha ha
You are prollably seeing a lean condition at the hit of the throttle from not having enough accelerator pump shot. I would suggest you readjust the idle mixture screws and then add pump shot if the problem remains after dealing w/the timing. Using a progressive linkage could help, too. Are you using a pump on both carbs? If you're getting a puff of black smoke when you floor it, you could have too much pump shot. Someone following you can see this- don't try to see for yourself!

TIMING:
You need to verify what the mechanical advance is supplying- if you have 12 degrees initial and 12 degrees mechanical, that's on the low side- 24 degrees total timing is not enough, I'd be shooting for 32 degrees, all in by no more than 3000 rpm.

You can make a timing tape to see what the timing is even w/o a dial back timing light.

You shouldn't need more than around 8-10 degrees initial timing for your Cad 390, but that's not written in stone. Again, give the engine what it wants, then tailor the mechanical advance to keep the total timing from being too high.

The vacuum advance on these engines often gave more advance than the engine needs when recurving the distributor with a performance curve. Use a limiter to keep the vacuum advance from supplying more than about 10-12 degrees, or replace the vacuum advance can w/another that has less advance. Connect it to a manifold vacuum source.

Last edited by cobalt327; 02-02-2013 at 12:11 PM.
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Old 02-02-2013, 12:13 PM
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Don't you figure the vac advance into the total?
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Old 02-02-2013, 12:18 PM
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Quote:
Originally Posted by yragat View Post
Don't you figure the vac advance into the total?
No, not unless it's clearly stated. Normally vacuum advance is separate from "total timing". Total timing = initial plus mechanical.

More here.
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Old 02-02-2013, 07:34 PM
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I have a 11:1sbc the works great with 25 degrees at idle. Less than 20 and the headers glow red. Long duration cams is another thing that effect timing requirements.
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Old 02-02-2013, 08:02 PM
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This is the cam I have
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  #22 (permalink)  
Old 02-03-2013, 04:52 AM
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A engine will not start if the initial timing advance is 25 degrees BTDC.

You are including 13 degrees mechanical advance with 12 degrees initial advance.
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Old 02-03-2013, 06:45 AM
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That isn't true, an engine can start with over 25 degrees of timing. The compression ratio and cam timing has a huge effect on this. I also have a 280 turbo (retrofit) and I lock the mechanical timing at 30 degrees. No mechanical advance curve and 18 more with the vacuum advance on ported vacuum. Engine spins fast on the starter. Tiny stock cam and 7.4:1 cr.

I don't use vacuum advance on my 11:1 sbc due too not much vacuum (242/242 cam at 0.050). The starter spins the engine very quickly with 25 degrees of timing. I have see other people use as much as 35 degrees at idle (locked timing).

But yes, my 454 doesn't like to start with over 20 degrees of mechanical timing at idle. 9:1 cr with a little cam 208/214 at 0.050".

Engines are different so blanket statements are almost never true.

In general, I would run as much initial timing as possible, before the starter begins to complain or if there is detonation, while limiting the total mechanical to mid to high 30's or maybe 40 if you have an especially inefficient engine. (like a 454).
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Old 02-03-2013, 07:12 AM
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You cannot check the initial timing advance above 700 RPM. Centrifugal (mechanical) timing advance mechanisms start advancing the timing above 700 RPM so if you are checking the timing above 700 - 1000 RPM, you are seeing centrifugal plus initial timing advance.

Factory high performance engines with more radical camshafts are equipped with a mechanical advance mechanisms that are designed to delay centrifugal advance until about 1100 RPM so the initial timing advance can be set. I have no idea how the various aftermarket double-throw down, million volt ignition systems work. Comparing those are like comparing apples and oranges.

Many high performance engines with radical long duration camshafts will not idle below 1200 RPM and the initial advance cannot be checked because the centrifugal advance is in operation. Checking the timing in those engines must be done based on mechanical plus initial timing advance.
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Old 02-03-2013, 10:58 AM
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The idea that "you cannot set initial timing if the idle is above 700 rpm", and "engine won't start if the initial timing is over 25 degrees" is wrong. How in hell does bs like this get started? Oh wait- I see.

If the mechanical is coming in at idle, heavier springs will delay the mechanical while initial is set. In fact, the mechanical coming in at idle is not wanted anyway, and heavier springs should be used. Having the mechanical trying to come in at idle will cause an erratic, hunting idle speed that will never settle in to a steady idle.
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Old 02-03-2013, 11:01 AM
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Timing

Which is precisely why they make recurve kits with heavier springs to begin with eh Cobalt 327?
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  #27 (permalink)  
Old 02-03-2013, 11:20 AM
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That would be my guess...

This can get tricky having to delay the onset of mechanical advance, yet have it all in by a low enough rpm- but in the case of an engine built w/a cam that won't idle below 1200 rpm, some compromises are bound to come up if using an analog timing advance set up on the street.
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Old 02-03-2013, 11:32 AM
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It is quite obvious the some people don't know the difference between initial advance and centrifugal advance or they have never worked on anything but a racing engine.

You cannot lock down the initial advance if the engine is idling above 700 - 1100 RPM because the centrifugal advance has started and the timing has started advancing. That is why the initial advance of a high performance engine cannot be set or even checked unless you modify the centrifugal (mechanical) advance mechanism with heavier springs. Heavy springs will make the advance begin too late except for WOT operation or street engines that leave a traffic light at 6000 RPM.

How can you check the initial advance of an engine if cannot idle below 1200 RPM?
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  #29 (permalink)  
Old 02-03-2013, 11:41 AM
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"Some things" ARE quite obvious, I agree.

You are NOW saying the same thing we just did- use a heavier spring. Just a little different than "it can't be done"...

While you're putting a spin on your "advice", care to explain what you really meant when you said "A engine will not start if the initial timing advance is 25 degrees BTDC"??
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Old 02-03-2013, 12:12 PM
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No engine will start with 25 degrees INITIAL advance. The 25 degrees advance referred to in this thread includes the centrifugal advance.

The pre-1970 high compression engines will not start with more than 14 degrees initial advance. l

Examples: A 1970 427 CI L88 engine will kick back against the ignited mixture if the initial timing advance is more than 13 degrees BTDC and a 1970 Pontiac Ram Air IV engine must have no more than 13 degrees BTDC initial advance. The centrifugal advance mechanism in a RA IV distributor starts at 1100 RPM and advances timing to 24 degrees. The centrifugal advance in a 1970 Pontiac regular production distributor starts at 800 RPM. If heavy springs are used to delay the centrifugal advance the timing may be too late.

Some people don't even know what initial advance is because they never have worked on a stock engine.
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