BBCorv70

I recently set my idle timing, vacuum advance disconnected, to 18 degrees per suggestion from another member.
70 454, 9.5 compression, Competition cams 280h, part number 11-208-3.
Runs much better, cooler, idles well, etc.
I haven't tried WOT under load yet, mainly light cruising.

My concern is whether I'll be getting into trouble with overall ignition timing.
I'm running a stock distributor.

If the factory mechanical advance provides 26 degrees (taken from a Chevrolet service manual), my idle timing set to 18 degrees, it would seem the total advance should be 44 degrees.
Way too much?
Assuming 26 degrees mechanical advance, desired 36 degrees all in, this would limit my idle timing to 10 degrees.

10 degrees initial timing seems too low for this cam, may be why it was running like a dog.

When running a performance cam, do people routinely modify the factory distributor to shorten the mechanical advance curve?

Unless I have the total mechanical advance wrong for a factory distributor, it would seem I may need to either modify the distributor or consider a smaller cam?

It's possible the distributor had been modified, total mechanical advance shortened, though I don't recall asking for this modification when I had the distributor rebuilt many years ago.
I will be retesting the all in RPM and total mechanical advance, something doesn't make sense.

stingr69

Seems that you do understand the issue!

You should reduce the available mechanical advance in the distributor mechanism if you want to change the initial advance setting.

Not that hard but that is how it is done. Distributor needs to be modified.

Initial advance plus all the centrifugal advance needs to sum up to 36 degrees. Therefore to increase initial timing, you need to reduce the available centrifugal advance.

derekderek

regardless of how bad at idle you DO NOT want more than 36 deg total. do you have a set-back timing light? no? ok, go get some white-out from staples. paint tdc mark. now move tdc mark to 15 degrees. paint another mark at 0. move that mark to 15. paint another at 0. now you have a 30 degree mark on your balancer. you do not want that mark past 6 on timing pointer at full advance. you can also see how much vac advance you are getting too. also just cuz you are at 3k doesn't mean distributor has stopped advancing. you assume it has and it does 4 or 6 more, you are roasting pistons.

BBCorv70

I have a set-back timing light. Will be retesting the all in degrees and RPM in the next few days.

lars

I have never recommended setting initial to 18 without checking total timing and verifying that it is at 36-38 (I checked my e-mail to you: I stated to verify that "Total mechanical timing (centrifugal fully pegged out) should be about 38, occurring between 2800 - 3000 rpm."). You need to set and verify total timing to 38 (big blocks with stock heads like about 38 total - small blocks will run 36), and then check initial timing to see where you end up. Ideally, with that cam, you want initial to fall in the 18-range, as I stated. You have to modify your centrifugal curve to achieve that spec. But never set initial to 18 without checking to assure that total is at 36-38.

Lars

BBCorv70

I did check the total timing, appeared to be around 36 degrees, though I'm now not sure it was entirely all in. The springs are a bit stiff, could have been more to go. I'm questioning whether my measurement of all in timing was accurate assuming the distributor wasn't modified. If the distributor is indeed stock, it shouldn't be stopping at 36 degrees with initial timing set to 18 degrees. I'll check again, this time removing one spring from the mechanical advance to make it easier to reach all in.

The answer to my question seems to be I may need to modify the distributor, limit the mechanical advance to work well with the cam installed. Not what I was after many years ago when I had the cam installed by the last owner swapped out.

Does anybody know of a reputable place I could send the distributor to be recurved, advance limited?

Other option I'd consider is another cam change, something closer to the original.

Thanks for the help.

Mooser

MelWff

You need to buy the spring kit that Lars recommends, Mr Gasker 928G. It comes with springs and a brass stop bushing. You will have to remove and disassemble the distributor to install the stop bushing. The original bushing was nylon and is either gone or on the verge of falling off. Using the original weights install the silver and black springs and then you can set your total timing to 38 degrees using a timing tape or dial back light.

resdoggie

You didn't mention what heads you have but If you have aluminium heads you can retard the total timing 2-4*. If it's within your budget, I would forgo sending out the dist'r and use that money saved for a programmable ignition and dial in any timing curve you want. Money better spent imo.

BBCorv70

Original iron heads, closed chamber.
I'm trying to stay as close to bone stock as I can, the cam wasn't what I intended 25+ years ago when I had the engine refreshed.
My fault for not being more specific when I requested a cam close to the original.
Out of curiosity, what programmable ignition did you have in mind?

It appears the Mr Gasket spring set comes with a bushing to limit the advance, something I wasn't aware of.
I was under the impression the only way to limit the advance was to shorten the mechanical advance slot, brazing or welding...

resdoggie

I have the Mallory Maxfire but it isn't available anymore. MSD makes them. No screwing around with springs. Just dial in your curve electronically.

BBCorv70

Does anybody know what mechanical advance I can expect to see after installing the Mr Gasket bushing?
How much does it reduce the advance?

derekderek

with vac adv unhooked, rev motor until you see it stop advancing. if you need a quick blip to 4500 or 5k, it better to do that than have it under load at that rpm with 42 degrees and roasting pistons. plus, you don't know what rpm it quits advancing at. before you change settings on anything, you need to know what you are changing, how much and is it necessary. for all you know, you have correct mech advance as-is...

BBCorv70

I'll be testing again either with one spring installed or lighter springs.
The factory service manual states the distributor has 26 degrees of mechanical advance.
If this is the case, too much to allow keeping the initial timing at 18 degrees.

derekderek

OTOH, some centrif comes in as soon as distributor starts spinning. so your 18 may be with3 or 6 deg of advance already there. plus vac raises idle timing a lot. so you may not need 18 initial.

BBCorv70

It's possible some of the 26 degrees has been taken at idle.
Bottom line is the all in cannot exceed 38 degrees.
I will be retesting this coming weekend.

MelWff

you cannt set it correctly without changing the springs to the lighter ones in the kit. It is not only the 38 total, it is the 38 total occurring at about 2,800 to 3,000 rpm. Ask Lars for his set up paper, you accomplish nothing with the stiff factory springs.

BBCorv70

Yes, thanks, I realize this, ordered a Mr Gasket kit last night.
My first concern is limiting the "all in" to 38 degrees, next step will be "all in" between 2800 and 3000 rpm.
Thanks everybody for your input.

MelWff

Im trying to tell you they are not steps you cannt do one without the other.

BBCorv70

Sent a PM.