Crankshaft pulley size - options?

I've been having alternator charging issues since I switched over from generator a couple of years ago. I installed a 105amp 12SI Delco (aftermarket 3 wire) and wired through "AMP" light to excite. New battery, wiring/electrical connections good, belt tight. After starting engine, I rev it to start charging (best guess 1000~1200 rpm), light goes out, alternator charges, lights bright until engine speed drops back down to idle(600~650 rpm). "ALT" light comes back on, lights dim, etc.
I read on another site (http://www.powermastermotorsports.com/power_pulleys.html) about pulley ratio and alternator rotor speed and have come up with this:
Using 5.125” (crankshaft pulley diameter) / 2.5” (alternator pulley diameter) = 2.05/1 (ratio). By multiplying pulley ratio (2.05) X engine speed at idle (600rpm) = alternator rotor speed (1230). Is this fast enough to operate this alternator and charge the battery at idle? It was suggested that the ratio for street driving be closer to 3/1 or even 3.5/1. This would mean either a 1.71” diameter alternator pulley or a 7.5” diameter crankshaft pulley, minimum. The smallest alternator pulley I've found for a wide belt is 2.5" diameter.
Does anyone know of a larger diameter crankshaft pulley for a stock F head 134? :?

monkeysmonkeysmonkeys

 

Thanks for the info. I checked out the site and it looks like that'll work. I'm going to call to find out the diameter before I order.

 

I haven't tried taking out the light, but you're the second person to suggest it. In the EZ 21 circuit harness there is a white wire labeled "ALT EXCITOR" that starts (or ends) at the alternator post #1 and connects through the fuse block to the ignition switch "ACC" post which is hot when the switch is either "on" or "acc". I didn't use this wire, because I wanted the "AMP" light to function, but I may have to change plans depending on the outcome of testing the alternator as you suggest. I'm willing to try anything and I did order the smaller diameter pulley from the vendor you suggested. I'm still leaning toward the alternator rotor speed as the culprit but I'll let you know how things turn out.

:? You wrote:
"(The exciter terminal is marked #2 on the alternator case and the dash light terminal is marked #1.)"

I may be mistaken but on a three-wire alternator I think post#2 is the voltage sensing post that normally connects to the output post on the alternator.
Isn't the excitor terminal and dash light terminal one-in-the-same?

Thanks,

 

Go for a larger size pully on the alternator, if increasing it's speed is what you are after. Much easier to replace.

 

Also, are you sure the alternator you purchased is not a self-exciter model?

 

You sure about that? I'm thinking the other way around.

Yep! Delco 12SI

 

Larger pulley on the alternator will have the same effect as a larger pulley on the crankshaft. I think. gotta go out and play with drive gears on the motorcycle and see if my perception of that is right. Some one correct me, please.

And from some of my reading on the 10SI and 12SI they could be either self-exciting or not. But, there again I am not 100% on that. Need to go do some research on both of these items.

 

Thanks for the link Steve. The schematic/diagram on the left is how I have mine wired. Just waiting on the pulley now.

monkeys

 

Little late to post on this issue here but to increase the rpms of your alt you wanna either increase the size of your crank pulley or decrease the size of your alt pulley. Since the the crank pulley is so small on most jeeps it would probably be easier to find that. As a added bonus of increasing your crank pulley if you have the original fan and not an electric conversion you will be also adding more rpms to your fan as well. Don't go too big on the pulley though as it may seem insignificant it will rob some hp at low rpms.