JamesC

iTrader: (5)

The new 4160 Street Warrior with Sofa/Holley tune runs like a bat of hell as I expected it would. No issues there. However, out of the box, the choke cap was set to two degrees rich and the fast idle on start up jumped to nearly 3000. A call to Jeg's and I was assured that the carb wasn't a repackaged return. So I've set the cap to the center detent and turned the fast idle screw down to 1700 (still fiddling). On a cold start, the engine goes to 1700 alright, but as the engine warms the idle slowly advances to 2500 or so. After research, I found a post on another forum that indicates that if the fast idle is set too high, the mechanical advance takes over, which is why RPM's rise. On the money or way off? The stats for my engine below.

Mechanical Advance:

RPM Advance
1100 0.0 degrees
1600 12.0 degrees
2400 16.0 degrees
4600 22.0 degrees

Can I test the above theory, if viable, by simply clamping off the advance-to-carb line. Then if the engine goes to 1700 and stays, I've found the issue?

Thanks,

JamesC

skinny z

I think I know what you're asking but your next to last question (with no question mark) is a little confusing.
It's certainly plausible that in an excessive fast idle, you'll start to get into the timing curve.
By the chart posted, you're well into that curve at 1600 RPM. If the fast idle is at that value the additional 12 degrees will bump the revs. (For the record, I'm a little suspicious of those RPM vs degrees of advance values.)
But you mention simply clamping off the advance-to-carb line. That to me would be referring to the vacuum advance and if connected to ported vacuum, won't enter into the timing curve given the static position of the throttle blades. If you're connected to full manifold vacuum, then things can get very messy when setting out to get it functioning properly.

To truly test that theory, as I've done when working through my mechanical vs vacuum advance, is to tie down the distributor weights to prevent them opening up as the revs go up. I used small cable (zip) ties to secure them. That way the mechanical advance is excluded from whatever experiment you want to attempt.
Not sure if this is helpful but at the very least it'll bump your thread and maybe Mr Kingdom will see it.

JamesC

iTrader: (5)

Thanks, skinny, for wading through my carb ignorance, lol. In that vein could I simply reduce the fast idle by several hundred RPM's and be more in the ballpark? The chart info I provide, btw, is from the 350HO Deluxe manual.

JamesC

skinny z

Backing off of the fast idle screw would be step one.
You have to be at it when it's cold obviously. One little trick you can try is to disconnect the feed wire for your electric choke. This will keep the choke valve and associated bits in it's initial state and you'll have a little time to experiment with the fast idle screw before the engine warms up and comes into it's own.
This sort of tuning always goes back and forth as you wait for the "experimental" settings to reset themselves. I've had my car enthusiast brothers tinker with the choke settings over the course of many days trying to dial it in. Then a cold snap comes along, or a heat wave and the settings go out the window. It's generally a compromise.
Keep us posted.

skinny z

Sure enough, there's a significant jump is spark advance between 1100 and 1600 RPM. I can see that being problematic if the fast idle is greater than 1100. It would be like a feedback system as mentioned.

I'd say that unless you've found a suitable workaround, fast idle shouldn't be any greater than 1100 (or so).

Interestingly, I can't find an aftermarket timing curve that compares with the HO's. Nothing goes from zero at 1100 to 12 at 1600.

JamesC

iTrader: (5)

Thanks for confirming a strong suspicion, one that developed after finding the remarks about fast idle/mechanical advance mentioned above. Cures literally years of head scratching. I have found a workaround: Fast idle set to 1200 which rises to 1700. After a couple of minutes I give it a little pedal, then slowly release it so that RPM's fall to curb idle (750). I don't know if the fast idle cam is in play or not.

Now the "I wonders": I wonder if the car will allow a fast idle of 1100 on a cold day? I wonder how that idle speed would affect the cam step downs? I wonder if the choke would require a longer duration? I wonder if it's worth the trial an error?

JamesC

skinny z

Here's something I can relate. All of those "I wonder" scenarios will change with respect to any of the other "I wonder" scenarios.
Get the fast idle set for a cold day and it'll be off for the next not so cold day. And so on.

You want to dial it back 40+years? My daily driven 400 CID SBC in my 68 Beaumont (aka Chevelle), a college man's creation and hot rodding assisted by great instructors, had a 600 CFM Holley carb of the 4160 variety.
Manual choke.
Never failed to start. Even in the most miserable southern Ontario winter.
The automatic choke mechanism has always bugged me. Electric. Heat stove. It's really not so automatic if you have to kick it down or coddle it in some way is it? Just plain finicky.

But it wouldn't be hot rodding is we didn't have to find a workaround would it?

JamesC

iTrader: (5)

Only 40 years? Pup, lol. About 60 years ago, I was messing with my 54 Chevy two-door post. Straight six with dual carbs, dual exhaust and glass packs. Painted 57 Plymouth Canary Yellow. Renault buckets. Three on the floor. And, yes, dagoed. Oh my, how times passes.

JamesC

skinny z

Two things sir.
One. A Canary yellow 54 Chev 2 door post? Dual carbs?
Man! I thought I was cool being the street racing 68 Beaumont guy. (It may be worth noting one of my favourite car models building them in the 60's as I did was the Revell 54 Chevy Highboy Gasser).



Two. Not many people can call me pup.