Still messing with the distributor curve,and decided to try the medium springs that came with the Summit kit.The softest springs allowed 8 initial,and 34 degrees in by 3500 with the stock weights.The problem was with the idle speed after the engine was hot. Seems the springs were allowing some advance at idle,and with the advance,the idle speed came up.I could switch to a ported vacuum source I guess.

I've also been reading up on recurving and it seems the preferred curve is one with more initial,and less mechanical advance.The total ends up the same ,but I guess the low end torque is improved with more initial.

So at lunch today I swapped the springs and of course without a timing light at work,drove home and put the timing light on it.Full advance now pulled only 25-28 total with the original 8 degrees initial.I advanced the initial to 15 degrees,but by then it was getting too late to be revving the engine in the garage.I assume total will come in at 32-35 degrees at around 3500. I'll check it tomorrow at work.
Is there an easier way to get this curve right?Change the springs,set the initial,road test,next day do the whole thing again,and I haven't even started with the vacuum advance.Also the 406 seems to be sucking the carb dry at high rpms,so testing is frustrating to boot.
On the plus side,the wife has agreed to let me get the new carb.I hope to hold off till I get this recurve worked out.Otherwise I'll be chasing too many problems at once. :banghead:

 

That shouldn't hurt anything as long as it idels good. Mine is set at 14 even after Lars recurved it. It seems to advance about 3-5 degrees as soon as I start it. I've never had a problem starting it (being to far advanced can cause hard starting).

 

I used to run at 21 degrees initial with the stock distribitor and with the 15 degrees machanical in a stock unit it gave me 36 degrees total. I do not use a vaccum advance. With my new MSD I am at 18 degrees advance. Unhook you vaccum advance and rev it to 3000 and adjust the timing to get 34-36 drgrees then whatever your left with at idle is your initial timing. :D

 

An initial advance of 15 should not be a problem as long as you have no problem starting and the total mechanical plus initial advance dosesn't get to high (36 to 39). Changing the springs should not affect the total timing just the rate at which it comes in as RPM increases. The total mechanical advance is controlled by a bushig that rides in a slot. You either have to change the slot length or the bushing diameter to change the total mecanical advance.

 

My engine builder recommended 23 degrees initial timing :eek: :eek: even though - I'm not runnint that much advance...

 

Basically run you can run whatever ( the highest ) advance you want as long as it starts easy and idles OK

 

The more idle advance the better (within reason). Problem is that too much initial advance causes hard starting. If you can set it to 20 and still start it ok, that's what I would do. Then modify the weights so you get max of 36.

BTW, I'm running 24 dgr at idle, but I also have EFI and can set the advance during cranking. I set it to 15. So when I start it's 15 dgr and as soon as it runs it goes to 24.

 

This morning I road tested with the medium springs and initial set at 15 degrees.Started fine,not bad idle,let it heat up while on the twisty back raods and once I hit the highway,I gave it hell.Pulls like King Kong to 5500rpms and feels like there's no end to it.My carb is soon to be history,as my wife gave me the go ahead for the new Holley. :grouphug:

Now for the vacuum advance.Here we go again.Drive a day,twist a screw,drive a day,twist a screw,etc. :banghead:

 

Here's my take on my 406. Both the carb (Q-jet) and distributor have been rebuilt by Lars.

1. My 406 seems happiest with about 32* total mechanical advance. More than this and I get detonation. Compression is about 9.8:1, which I have been told is about the limit with the larer small blocks.

2. My cam/carb match requires 18-20* initial. Cold starting is hard regardless of my initial advance setting due to the cam/carb combination (long duration solid cam with a Q-jet).

3. I re-curved the distributor to have 12* of throw, all in by about 2700 RPM. Car seems to like this. If I don't sell it, I'll probably swtich to a Holley or Demon carb.

 

I braught my timing light with me this morning and pulled off the road and checked it out after my hell raising.Looks like 32 degrees in by 3000.
With the initial at 15,that leaves 17 distributor.With the light springs I had 34 degrees total and 8 initial,which leaves 26 distributor. I thaught the springs only effected the rate.(Unless the middle spring set is still too stiff to let the full advance come in.)
Also I read somewhere that the stock HEI distributor has a 20 degree advance from the factory. Is this true?If so how is it I get 26 degrees with light springs?

 

I think it may reach a temp stop at 3000, and then start advancing more at higher revs. The throw is limited by the slots, not springs. Take the cap & rotor off and play with the weights. See if they catch before full advance. You really need to limit travel by making the slots shorter.

 

I have the Lars recurve kits as well. I timed it to achieve 36 degrees at around 2500 rpm. My initial timing is now about 21 degrees. It runs great. If I wanted to reduce it I would have to lengthen the slot or remove the bushing in the pin that goes in the slot on the distributor advance weight plate. If your idle is good and you have good throttle response off idle then all is most likely well. If you are unsure just reduce you initial timing and road test off idle throttle respones. If it runs better lower or higher you may want to make the appropriate adjustments to the distributor. Lars has a paper on how to go about doing this.

 

As ZL1powr and Zwede posted, the weight of the springs should not affect the amount of mechanical advance (unless they are incorrect or really heavy), only how soon or late it comes in. If light springs allows total advance by 3,000 rpm, then a heavier spring will cause total to come in at a higher rpm. The way to determine when all mechanical is in is to rev the engine until the timing stops advancing and then rev it some more to make sure that it didn't just "pause" before reaching it's actual peak. If you got 26º with light springs but less with heavier springs, you just haven't revved it high enough to see the actual total. Or, the heavier spring is too heavy or too short to allow the distributor to reach it's total.

 

I've been thinkering for a few days now and decided to order the Crane Advace kit.I had a local machinist rebuild the distributor last July,and he installed the Crane adjustable vacuum advance,but didn't use the springs that came with it,or give them to me.(not even the allen wrench you need to tweek the advance.)
Anyway,the thaught popped into my head that he may not have installed the top plate correctly.I read somewhere that the stamped numbers should face down,and he has them facing up.Could this be why I have 26 degrees distributor advance with light springs? He also mentioned it was a Performance GM replacement.He replaced the shaft and bearings with the whole advance mechanism.Maybe it has 26 degrees after all?
I have made some progress,so I don't feel it was a waste,but I want to nail this down quickly,so I can start LIKING MY CAR again. :chevy