Distributor Springs - When do they need to be changed or flawed logic.

[74 challenge]

Hi Guys,

It's been a long time!

Got the 360 in the car and it runs great except for 2 exact circumstances. on light throttle and cruising.

I have jetted the carb down to a 65 and it picked up a bunch of response and it doesn't smell like a gas truck dumped it's load in my garage. However I get this stumble right off idle. The engine can almost stall but as soon as I let off the throttle or giving it more throttle it corrects itself and runs like a bat out of hell. In the second scenario while cruising on the highway it has a stumble. The car likes to nose down and it happens rhythmically but in the same stumble I get if I play with the timing.

When I put the engine in the car I put in a lunati cam - Part Number escapes me now but it's quite mild.
I am running this distributor, with a stock ignition box: https://www.summitracing.com/int/parts/sum-850003-1/overview/

I was playing around with the timing without a gun the other day in the garage and I advanced the car too much but the stumble was identical to the one I got from my light throttle response. This made me think could the issue be the springs giving me too much advance too quickly? or to slowly?

I know someone is going to ask my timing and I don't know the numbers I need to get a dial back light - so that information is coming once I can get a hold of a buddy.

I jumped on the internet (I know I know but bear with me) and read that springs control when you get your timing advance. So if I were running a disto that was stock and the initial timing from the factory was at TDC it would need 35 degrees of mechanical advance so the springs would be lighter, where a car with a mild cam that runs at 14 or so degrees initial advance and somewhere around 35 degrees of total advance would need stiffer springs?

am I onto something or out to lunch?

Someone is going to suggest a fuel issue. The car is finally running well fuel wise. My secondary's are a tad rich but this all takes places when I am not into my secondary's. I have about 7 pounds of fuel pressure, float levels are good, choke operates as it should.

[Chryco Psycho]

first problem I would look at the accelerater pump with the leaner jets you need more duel as the throttle is opened .
 swcond , stumble with no throttle ,movement ?

[1 Wild R/T]

stumble with no throttle ,movement ?

Sounds like a lean surge or to much timing to me...

[73_Cuda_4_Me]

Stiffer springs won't change the 'amount' of total mechanical advance, only 'when' it advances compared to engine RPM... depending on dizzy, you would have to limit amount of advance, or set total amount of advance with switch (newer electronic dizzy) or limiter plate (stock type dizzy)...

[74 challenge]

first problem I would look at the accelerater pump with the leaner jets you need more duel as the throttle is opened .
 swcond , stumble with no throttle ,movement ?

No stumble without throttle movement.

Adjusted the accelerator pump to give more fuel and less fuel obviously it changed but made no difference to the stumble.

[74 challenge]

Stiffer springs won't change the 'amount' of total mechanical advance, only 'when' it advances compared to engine RPM... depending on dizzy, you would have to limit amount of advance, or set total amount of advance with switch (newer electronic dizzy) or limiter plate (stock type dizzy)...

I'm thinking this is my problem. As soon as I touch the throttle I get too much timing that it can almost cut out the engine.

I'm putting this theory out to the people of this forum. However it does not seem common to me that one would change a distributor after putting in a mild cam.

[73_Cuda_4_Me]

Are you running only mechanical, or mechanical and vacuum advance? If only running mechanical, the timing won't be changing much in just adding some throttle...

If running vacuum too, then as soon as you add throttle, vacuum drops off, so actually retards timing a bit until vacuum comes back up when RPM = throttle position...

[74 challenge]

Only mechanical.

[73_Cuda_4_Me]

Then the next step is setting timing and idle mixture screws to best idle (highest vacuum and idle speed), keeping idle down around 800-1000 RPM with idle speed screw...

Without an advance limiter in dizzy, do timing for highest vacuum/RPM at idle speed, then SLOWLY adjusting each of the idle mix screws till get fastest RPM, then lean out each screw 1/8th turn (lean burn idle).

Knowing exact timing with light is critical... can get adjustable timing light, or mark 36 degrees advance on balancer so can use standard light... Chryco gave a dimension in one of his posts for how far to the right of TDC (when looking at front of engine) to make the mark... just a tad over 2 inches, I believe, on standard balancer.

Chryco would recommend setting timing at 3,500 - mechanical is all in by then, so will set your maximum advance... but then you are stuck with whatever the idle timing is... in my case, it put it at 6 degrees at idle, and my cam likes 18 for starting and idling best. Since I do mostly low/part throttle cruising, with occasional 'stomping', I set idle timing to cam's optimal, then made limiter plate to only allow another 16 degrees advance, for total of 34 all in...

[73_Cuda_4_Me]

... a good reference from a previous thread here...

http://automotivemileposts.com/garage/v2n8.html

[brads70]

Neil installed this in mine at Carlisle one year... http://www.4secondsflat.com/Mopar%20Mechanical%20timing%20limiter%20plate.htm

[74 challenge]

Neil installed this in mine at Carlisle one year... http://www.4secondsflat.com/Mopar%20Mechanical%20timing%20limiter%20plate.htm

Brad what situation caused you to get one of these?

I think I have played with my Timing a billion times checking and re-checking. When it's good at idle and full advance I get this insane stumble  That's why I'm questioning the "when" my timing comes into play. I have a factory distributor that I removed for this one. I might swap it back in and see if that does anything. It worked fine it was just ugly.

Also for the lean surge when I put the car up to the point where it starts to stumble and I force the choke closed it doesn't change. Wouldn't it resolve itself if I limited the air into the engine?

[brads70]

Brad what situation caused you to get one of these?

I think I have played with my Timing a billion times checking and re-checking. When it's good at idle and full advance I get this insane stumble  That's why I'm questioning the "when" my timing comes into play. I have a factory distributor that I removed for this one. I might swap it back in and see if that does anything. It worked fine it was just ugly.

Also for the lean surge when I put the car up to the point where it starts to stumble and I force the choke closed it doesn't change. Wouldn't it resolve itself if I limited the air into the engine?

Its useful for getting more initial timing at idle and low rpm's without having too much at the other end old school method was to weld up the slots making them smaller/shorter. I'm still playing/tuning my carb as I haven't spent a lot of time doing so, to busy driving it! 

[Chryco Psycho]

.

Also for the lean surge when I put the car up to the point where it starts to stumble and I force the choke closed it doesn't change. Wouldn't it resolve itself if I limited the air into the engine?

 it would so maybe you have a different issue

[HP_Cuda]

Starting to think this is more carb then timing.