The Quadrajet is the proper carburetor for a '69 427. When I bought the car, it had a 454 (not correct of course). The carb had been rebuilt by Lars to stock specs a few years ago (I have the receipt) and it ran perfectly. The newly rebuilt engine (427 - L36) now uses the carb. It has the original and proper jets and metering rods so it's essentially stock. I installed a Howards roller cam: lobe lift = 0.327" (intake and exhaust), duration = 226.6 deg @ 0.050 (intake and exhaust) so it's not an extreme cam, but definitely bumps nicely and has much lower vacuum at idle. It was hunting pretty badly (power piston spring too stiff) so I removed the power piston spring (for testing) so that the primary metering rods would certainly drop into the jets. The engine runs quite well above about 1100 RPM on up but I just can't get it to idle down. It simply dies. I've fiddled with the idle jets, plugged all vacuum lines (including spark advance) to eliminate the possibility of leaks (barring any potential intake manifold leak), check the timing, valve clearance, etc. Spark plugs, wires, coil, points, and cap are new.
Any ideas on what I should try next? Thanks in advance.

 

I wonder if you have an advance issue in the distributor.

I have run into this on an engine where I could not idle it down and even though I could get the ALL IN advance set..when I tried to idle it down it would just die about 1300 RPM's. The advance weight springs were incorrect in the distributor that was rebuilt by someone else.

DUB

 

The distributor is the same one from the 454 and worked very well throughout the RPM range. I started out with 4 deg advanced (per the GM manual) initial timing, and have increased advance to see if that would improve things but it's really hard to get a static initial timing reading since it won't idle so don't really know where it's at. I have the dwell set at 30 deg. Manual says 28-32 deg. Does the new high-duration/lift cam need more advance?

 

I am no expert - but will offer my perspective. My thought is that cams with more duration tend to need more advance - For instance, I run a cam that is 230* at .050 - I run about 18* initial advance and have it all in about 34*. Granted, this is a large cubic-inch SBC/stroker - but I don't think the concept is really any different relative to camshaft duration.

This is with AFR heads, so the chamber is pretty efficient. No hard starts, no kickback at all. The engine seems to like it.

I've experimented with initial advance over 20* with no issues on this motor. 18 seems to be adequate. I didn't see any improvement going higher, so stayed there.

Of course, a lot of other factors will influence ignition timing requirements - but my experience suggests cam duration is one of the bigger factors.

I would try something like 10* initial and experiment up and down from there. Just make sure you can keep total centrifugal from going too high (over 36).

Just my .02 cents - hope that is useful to you.

 

The carb was rebuilt by Lars and it ran well. You should just send him an e-mail [ V8FastCars@msn.com ] to explain your present situation and ask for his advice. Why ask us about your carb when you can ask the guy who really has the answers?

 

Good point, and I will. I was seeking some general theories, but you're right. I was also wondering about timing, and I will try Coopers suggestion and bump the initial timing.

 

I also have a 69 L36 and think your problem is in the initial timing. The chassis service manual says 4 degrees initial advance which really chokes it down even when stock. With that cam it would be hard to get it to idle at all with only 4 degrees initial and is also the reason you have low vacuum at idle. Check how many degrees mechanical advance in your distributor and shoot for about 34 degrees total to start with (vacuum advance disconnected and the line plugged). The original 69 L36 distributor produced 26 degrees of mechanical advance which is why only 4 degrees initial was used. I have mine reduced down to 17 degrees mechanical and use 17 degrees initial with a Comp XR270 hydraulic roller. By doing this it idles smooth as silk (like stock) and produces 18.5 inches of vacuum at the stock 800 rpm idle. If I drop this to the stock 4 degrees initial it will barely run at idle.

 

Thanks for all of the suggestions. I took it all in an bumped the initial timing to 18 deg, and she's running pretty well. I can now idle down to about 800! Much better. I didn't think to go that far. Thanks guys!

 

To OP, did you discover anything that points to the problem? never mind, missed it.

 

Spooky, the engine runs great now. One thing I didn't add to the thread was that the new coil (PerTronix Flame-Thrower) had actually gone bad. PerTronix was great and sent me a new coil, and she runs like a champ.

 

WHAT?

 

That coil should not go bad. It will go bad though if you do not have the alternator grounded well enough and it is spiking the coil and overheating it. Just speaking from experience...

 

"90% of all carb problems are ignition problems."
(From my "Tuning for Beer" Seminar & Lecture series)


Lars

 

A NAPA coil will out last most ANY aftermarket coil these days. MSD red is the worst.

They leak, overheat, burn open, get intermittent, the works,

 

Just get a standard, stock replacement NAPA/Echlin coil. They're black, look stock, and run great.
Lars