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Old 04-26-2012, 02:07 PM
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Timing guru's---ADVISE ME!!

OK
The engine is a 390 Cadillac it always ran good,but had ton's of blowby and would just spit&sputter from stop to WOT .So had a total rebuild done all new internals and added a performance cam. 478-786 lift 268-274 duration 114-106 lobe center. I'm running 2 Stromberg 97's simultaneously,have pertronix igniter 3 ignition and headers. It ran like a 6 cyl!! Same thing from stop to WOT and even going down the highway if you stomp on it would spit & sputter then try to take off,but not too good. So I took it out yesterday and drove it a little,then advanced the timing a little--it helped ,kept advancing and the more I did the better it ran. Had to stop because Vac can was hitting manifold. Today I checked where the timing was. At 900 rpm with vac disconnected it was at 18 btdc --rev it to around 2000 ad goes up to 26 hook the vac up and rev it and it goes to 43. Starts fine even when hot and is not burning lean----should I leave it there or is that too much timing???I think if I would advance it a little more it would be scary fast. Oh, factory timing was 5 btdc .

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Old 04-26-2012, 07:41 PM
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Quote:
Originally Posted by yragat
OK
The engine is a 390 Cadillac it always ran good,but had ton's of blowby and would just spit&sputter from stop to WOT .So had a total rebuild done all new internals and added a performance cam. 478-786 lift 268-274 duration 114-106 lobe center. I'm running 2 Stromberg 97's simultaneously,have pertronix igniter 3 ignition and headers. It ran like a 6 cyl!! Same thing from stop to WOT and even going down the highway if you stomp on it would spit & sputter then try to take off,but not too good. So I took it out yesterday and drove it a little,then advanced the timing a little--it helped ,kept advancing and the more I did the better it ran. Had to stop because Vac can was hitting manifold. Today I checked where the timing was. At 900 rpm with vac disconnected it was at 18 btdc --rev it to around 2000 ad goes up to 26 hook the vac up and rev it and it goes to 43. Starts fine even when hot and is not burning lean----should I leave it there or is that too much timing???I think if I would advance it a little more it would be scary fast. Oh, factory timing was 5 btdc .
Before going further, the vacuum advance is considered separately from the "total timing" when setting up a performance advance curve.

You need to set the timing curve up correctly. To do that you need to confirm TDC is correct on your damper/timing tab. If you don't have a timing light that shows advance, make a timing tape to use for setting up the curve.

Most of the improvement you are feeling is coming from an increase in initial timing. That is not the least bit unusual when a performance cam is used. But if you use a lot of initial timing, that may mean limiting the amount of mechanical advance to keep the total advance (initial plus mechanical) from exceeding 36-38 degrees. There are advance weight and spring kits (don't use the weights, just the springs) to adjust the RPM when the mechanical advance is all in.

Once the timing curve is sorted out you can go about setting up the vacuum advance. You may find using about 10-14 degrees of vacuum advance is about right. There are adjustable vacuum advance cans for the early points-type distributors that can help you set up the vacuum advance. You will often see as much as 50 degrees BTDC at light throttle cruise conditions. Any more is unnecessary, and if the engine surges w/that much vacuum advance it can be lowered to suit the engine and conditions.

On the HEI distributor page in the wiki there's a section on tuning the advance curve that will help.
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Old 04-27-2012, 06:38 AM
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Two 97's work pretty good on a 239 inch flathead, you need more air.
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Old 04-27-2012, 07:36 AM
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Quote:
Originally Posted by hpete
Two 97's work pretty good on a 239 inch flathead, you need more air.
Now that the engine is rebuilt,when I get the $$$ I will have the other 4 rebuilt and run all 6
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Old 04-27-2012, 08:19 AM
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Quote:
Originally Posted by yragat
OK
The engine is a 390 Cadillac it always ran good,but had ton's of blowby and would just spit&sputter from stop to WOT .So had a total rebuild done all new internals and added a performance cam. 478-786 lift 268-274 duration 114-106 lobe center. I'm running 2 Stromberg 97's simultaneously,have pertronix igniter 3 ignition and headers. It ran like a 6 cyl!! Same thing from stop to WOT and even going down the highway if you stomp on it would spit & sputter then try to take off,but not too good. So I took it out yesterday and drove it a little,then advanced the timing a little--it helped ,kept advancing and the more I did the better it ran. Had to stop because Vac can was hitting manifold. Today I checked where the timing was. At 900 rpm with vac disconnected it was at 18 btdc --rev it to around 2000 ad goes up to 26 hook the vac up and rev it and it goes to 43. Starts fine even when hot and is not burning lean----should I leave it there or is that too much timing???I think if I would advance it a little more it would be scary fast. Oh, factory timing was 5 btdc .
Hello Gary,

With the vacuum advance disconnected and plugged and the engine idle at 900 RPM, you are already getting some mechanical (centrifugal) advance. This means that when you see the timing at 18 degrees at 900 RPM the actual timing at idle is lower. This is proven when you say that the timing only goes up to 26 degrees @2000 RPM. This is not too much timing and in fact may not be enough with the cam you are running.

Set the idle to 800 RPM or less. This is so no mechanical timing is being introduced. Be sure the vacuum advance is dis-connected and plugged off. Set the timing to 18 degrees BTDC at idle. Increase the engine RPM to the point where the timing no longer advances when looking at the mark with your timing light. Make a note of the RPM when that happens. That figure should be in the 36 degree range (+ or - 2 degrees) That will be your Total Mechanical Timing. For best street performance it should happen between 2500-3000 RPM. If you were to have the vacuum advance hooked up and performed the same check, the timing numbers will be about 10-15 degrees higher. Do not set the timing with the vacuum advance hooked up. If you do, the overall timing will be retarded and performance will be as you described above. In a word, lousy. This is because the vacuum will drop to 0 when going to WOT under load. Hook the vacuum advance to full manifold source.

You mentioned that you were unable to advance the timing any higher because the vacuum canister would hit the intake manifold. This can be corrected by lifting the distributor up and then rotating the distributor shaft one tooth in the opposite direction and then re-installing the distributor. You will have to rotate the oil pump shaft as well to get it to line up with the distributor drive.
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Old 04-27-2012, 08:20 AM
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Quote:
Originally Posted by yragat
Now that the engine is rebuilt,when I get the $$$ I will have the other 4 rebuilt and run all 6

Run the outer four or if you really want to run all six, then get progressive linkage. ENJOY!!!
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Old 04-27-2012, 08:29 AM
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Thanks,Frisco
Your advice is always right on and appreciated!
According to all the old manuals I have the Caddy distributor only has 8 degrees of centrifugal built into it,so if I set it to have say 34 total then initial would be 26??????
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Old 04-27-2012, 08:43 AM
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Quote:
Originally Posted by yragat
Thanks,Frisco
Your advice is always right on and appreciated!
According to all the old manuals I have the Caddy distributor only has 8 degrees of centrifugal built into it,so if I set it to have say 34 total then initial would be 26??????
You posted that you were running a pertronix igniter 3 ignition. Is that a kit that installs in the original Caddy housing or is it a complete distibutor? If in fact you are running a Caddy distributor and it is limited to only 8 degrees of mechanical advance, I'd be looking to replace that unit to one that has around 20-24 degrees of built in mechanical advance. If it is a complete Pertronix distributor, it should have much greater than 8 degrees of built in mechanical advance.
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Old 04-27-2012, 08:48 AM
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Its the Caddy dist with pertronix inside
Don't know if any aftermarket dist are available for it???
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Old 04-27-2012, 08:54 AM
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Quote:
Originally Posted by yragat
Thanks,Frisco
Your advice is always right on and appreciated!
According to all the old manuals I have the Caddy distributor only has 8 degrees of centrifugal built into it,so if I set it to have say 34 total then initial would be 26??????
You have to verify how much mechanical advance the distributor supplies. Same thing for the vacuum advance.

If the distributor is limited to a small amount of mechanical advance, the vacuum advance most likely will be giving a lot of advance. This is not needed, an adjustable vacuum advance is needed for the best results.

There's no way to know what the best initial timing setting is for an engine (especially one that has had the cam changed to a performance grind) until you have worked your way up from a safe setting. You will know when you're too far advanced on the initial timing when the engine is hard to start hot, or runs on when shut off, or pings when accelerating hard from a low RPM.

That said, starting out at 14-16 degrees BTDC of initial timing is a starting point you can work from. If the engine will start hot, and not ping, and not diesel when shut off w/the initial set at 26 degrees, then you can run it that way- but I doubt it will want that much advance.

So if the distributor doesn't give enough mechanical advance (meaning 26 degrees of initial is too much), the slot of the advance plate can be lengthened a bit at a time to increase the mechanical advance to give you the amount of mechanical you need to go w/the initial advance that you found to work best from your previous testing.

Once that's all worked out, add the vacuum advance back in by reconnecting the hose. You can try both ported and manifold vacuum, there is no way to know which will work best w/the cam you have until you try it both ways. Manifold vacuum is most often recommended, but if the initial is set high enough, adding more timing at idle by using manifold vacuum to the vacuum advance can over advance the engine at idle.
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Old 04-27-2012, 09:00 AM
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Don't have anyplace to connect ported vacuum---always used manifold on everything
It seems to be very responsive where it is now---might try little more advance & see if any better---if not will probably run it where it is now
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Old 04-27-2012, 09:19 AM
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How much total does it have (not counting vac. adv.)? Is it all in by 3000 rpm or less? How much advance is the vacuum advance giving you?

Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. If this is a w/e "toy" then maybe mileage doesn't matter. But it may have a better 'feel' by using enough vacuum advance to get it up to at least 45 degrees BTDC at cruise rpm. The only way this would be a problem is if you did high speed cruising and the vacuum advance was set to come in at a vacuum lower than the cruise vacuum. In that case it could be over advanced, but that's where the adjustable vac. adv. can help.
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Old 04-27-2012, 09:41 AM
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I revved it to about 2000 and it was at 26
my vac can is non adjustable and was new last summer---damn I forget if it was pre-set at 12 or 17---its one of them
this is not a race car---just a hotrod that I drive the crap out of--put on 5500 miles lasy summer. It is a light car--don't know weight,but it is a 30 Ford hiboy roadster---has highway gears (2.79)
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Old 04-27-2012, 09:48 AM
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As cobalt states you should actually take readings to determine the different aspects and limits of the distributor. Know what all the actual timing characteristics and limits are rather than just go by the way it runs, thats my take on it. I would think it would like at least 16-18 base timing.

And going along with what frisco is saying, some of the stock 390 distributors started to add mechanical at 400RPM! and were all in by 2000RPM with a maximum mech advance adding only 8 deg BTDC. See if you can find a part number on it. This site has some specs on the stock distributors

http://www.secondchancegarage.com/pu...utor-specs.cfm

Sounds like a nice ride
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Old 04-27-2012, 10:00 AM
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Quote:
Originally Posted by Custom10
Sounds like a nice ride
Was thinking the same thing, early Cad in it is a classic swap.
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