67 Monza Hard to Start Cold

[JCrane1]

Hello,

I bought my 67 Monza about 11 months ago. I've never experienced any significant trouble starting the car until just a few weeks ago. It's always been a little cold blood on the first start. A couple of pumps on the accelerator was normally enough to get it to light off and feathering the throttle for 30-45 seconds was usually enough to get it to fast idle on the choke. A couple weeks ago, something changed. The engine acts likes it going to light off and as soon as I let go of the key, it quits running. I found if I hold my foot to the floor, it will run/chug (sometimes for up to 2 minutes) before it clears out and starts running. Once it get it started and warmed up, it will start with a quick snap of the key and no accelerator pedal input. I initially thought I was having a fuel delivery problem, but I confirmed both carbs are squirting a blast of fuel each time I pump the throttle wide open. Please take a look at the youtube video I posted. I'm wondering if anyone can point me in the right direction to fix the problem.
https://www.youtube.com/watch?v=lFrJ_pZurRg
[share_youtube]https://www.youtube.com/watch?v=lFrJ_pZurRg[/share_youtube]
This is what it sounds like after it finally clears out and lights off.
https://www.youtube.com/watch?v=78PY9BATNpE

Thanks,
Jeff

[bbodie52]

I would start by checking the operation of the choke mechanism — particularly the VACUUM BREAK diaphragms, vacuum lines, and linkages on each carburetor. The function of the Vacuum Break automatic choke subsystem is described in the DELCO ROCHESTER - Models H, HV Carburetor Service Manual (attached), and is quoted below...

Model HV Choke System

The 1962 and later model HV carburetors have an individual choke mechanism in each carburetor. The thermostatic coil for each carburetor is located beneath the engine cylinders in the hot airstream and is connected to the choke shaft by an adjustable rod. The thermostatic coil operates in conjunction with a choke vacuum break diaphragm assembly mounted on each carburetor airhorn. The fast idle cam assembly is mounted on the side of the carburetor opposite from the vacuum break unit.

The choke operates as follows:

With the engine cold, prior to starting, opening of the carburetor throttle allows the thermostatic coil to close the choke valve and rotate the fast idle cam so that the high step is in line with the fast idle tang on the throttle lever. As the throttle is released, the fast idle tang comes to rest on the high step of the cam. This gives the engine enough fast idle to keep the engine running cold.

When the engine is started, manifold vacuum is applied to the spring-loaded vacuum break diaphragm. This moves the diaphragm plunger into the assembly until strikes the cover, which in turn, opens the choke valve to a point where the engine runs without loading or stalling. The length of the connecting link determines the amount of choke valve opening.

As the engine warms up the choke thermostatic coil is heated. It relaxes its tension, thereby, allowing air velocity to open the choke valve. This continues until the engine is warm. At this point the choke coil tension is completely relaxed and the choke valve is wide open.

The fast idle cam has graduated steps so that fast idle speed can be lowered gradually during the engine warm-up period. The fast idle cam follows rotation of the choke valve. When the choke valve is completely open and the engine is warm the fast idle tang on the throttle lever will be off the steps of the fast idle cam. At this point the idle stop screw controls normal idle speed.

DELCO ROCHESTER - Models H, HV Carburetor Service Manual.pdf

DELCO ROCHESTER - Models H, HV Carburetor Service Manual

(1.79 MiB) Downloaded 51 times

If a vacuum hose is loose or split, or if the internal diaphragm in one of the vacuum break units has ruptured (or the vacuum break plastic case has cracked or developed a vacuum leak), the choke valve in one of the carburetors may be remaining fully closed, even after the cold engine has started. The vacuum break mechanism is supposed to sense intake manifold vacuum and then open the choke valve partially to permit some air to enter the carburetor throat. Without this function the engine will be starved for air and will load up or stall. Your problem may simply be a loose or split vacuum line on one side, and this may be killing the engine until you have forced the engine to warm sufficiently to open the choke coils and the choke butterfly valves. The vacuum break mechanisms have no function once the engine is warm and the automatic chokes have disengaged.

[64powerglide]

When you get it warmed up & running smooth put a timing light on it & see what it is. Powerglide, close to 500 rpm's in gear.

[JCrane1]

Thanks guys. The information is very much appreciated. I'll focus my attention in that direction and feed back the results.

[UNSAFE]

If holding your foot to the floor is the only way to keep it running it is likely that the chokes or choke pull offs aren't working .

Try tying the choke plates open to see if it makes a difference .

Does it blow smoke when it's doing that ?