I have a rebuilt 1966 427 L72 and a rebuilt Holley 3247. New engine has just 200 miles on it. Car fast idles at about 1500 RPM and then settles at about 900-800 RMP once warm. Sometimes, at a long stop light, the idle will dip below 700 and then threaten to stall and sometimes drop below 500 and cut out. Is it necessary/normal to have to rev the engine at stops to keep it happy and spry coming out of stops? Second question - how many pumps are other L72 owners giving the car before starting cold? (My car has K66 - transistor ignition system). Thanks. (running 91 octane in California).
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Re: 1966 L72 Idle and Stalling / Cold Starting
William,
Have you warmed the engine and adjusted the timing and then carburetor curb idle speed and emulsion screws.
Does the engine restart with accelerator pump shot after stalling? The problem could be proper adjustment of the Holley. -
Re: 1966 L72 Idle and Stalling / Cold Starting
I have not yet adjusted the timing and idle speed after engine warmed only because I don't know what to adjust it too (probably because it seems fine at about 9000 RPM - it's after running it about 20 minutes in that I seem to have the issue. Does that idle seem proper or should I adjust for more RPM)? Yes, the engine does re-start immediately after stalling no problem with an accelerator pump (thankfully). Appreciate your reply.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
William,
It should not be necessary to keep your foot on the gas at stop lights. While you're in there, I would suggest converting from ported vacuum to full time. One advantage is the engine will run cooler at idle. Maybe Duke can remind me of the other advantages. Here's a thread on how to do it on the L71:
https://www.forums.ncrs.org/showthre...anifold-vacuum
Also, what valve springs are you using? The originals are prone to breakage and can drop a valve. I'm thinking you probably don't want to try that!
JoeComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
All things working properly and as designed by the oe, the L72 should behave just like any grocery getter of the era. The one I know intimately runs smoothly enough and makes enough idle (factory spec) and off idle torque to pull it away from a stop light in 3rd gear with it's 4:10 gears better than my 340 hp in first with it's 3.08's did. The L72 is a pleasure to drive.
It is definitely not normal with the stock cam to have to get it up to 900 rpm to keep it running/get it to pull away properly. Also, the word "sometimes" in your description tells you something is wrong. The attributes, like lots of camshaft overlap, that define the low end characteristics of the engine don't change periodicly.
I would first check thoroughly for vacuum leaks and pcv operation. Confirm smooth dist advance, vac and mechanical, operation. Check fuel pump pressure and flow. Eliminate everything until it leads you into the carb.
As to the cold starting, start by ensuring that at cold start the choke flap closes fully with the initial pump of the throttle. It also puts a squirt of raw fuel into the throat. Some engines start fine with that 1 shot of fuel, some like 2. It really isn't a critical thing. If you have to pump it repeatedly to get it to fire when cold, you have a problem. That problem is usually the choke flap not fully closing.
SteveComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Steve and Joe, I appreciate the responses. I have some work and investigation to do. This car is new for me (acquired in December) so I am learning it all (I had a 1971 454 (LS5) - which was great too). Thanks for the ideas.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
May not be a problem for your cold starts if it is driven every day, however when it sits gas is evaporating out of the Holley. Since my Holley equipped Corvettes may sit not being used, I always remove the air cleaner and install new fuel through the vent of the front fuel bowl before I attempt to start the engines. Doing this they always start up immediately.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Here is an update. Drove the car last night 25 minutes in all gears up to 65 mph. all worked fine. Stopped, had dinner and 50 minutes later the car started but then quit on me. Unless I floored it to get it started, it would not start. Then, on the way home, it stalled on me at every light or stop. And I had to really press on the gas to get it started. Something is wrong with the car once it is warmed up. Warming up it is fine. Warmed up it has problems keeping idle without gas pedal. If I try to anticipate this as I slow down and throw it into neutral and try to rev it to keep it alive, it cuts as soon as the RPMs drop and I touch the gas. Then this AM, 11 hours later, the car started fine, idled fine and drove just around the block fine for 15 minutes.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
William,
Shot in the dark, but I had that happen to my 427 with a Holley in my Chevelle. I found that the float level was to high and at long idle, was dribbling fuel and causing my engine to load up at idle. I set the float level and cured the problem.
DomComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
I had a similar issue with my 1965. It developed some time after the restoration and not initially. Problem was traced to a kinked rubber fuel line between chassis and fuel pump. The hose was too long but kept a nice bend while cold. After being warmed by engine heat, it kinked. Problem resolved after shortening hose.Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Thanks. Were you having any issue when it was cold warming up ... ?Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
The fact that you have to hold it to the floor to start it tells you that you have a flooding condition. That can be too much fuel or not enough air. The first thing to check is to see if the choke is coming off fully. When it has warmed up fully, shut it off and lift the air cleaner lid off and hold the throttle off the idle stop screw and look to see that the choke plate is standing wide open. If it isn't that's where you start.
Given your comment earlier about poor cold start/pumping the pedal more than normal, it wouldn't surprise me to find that either the exhaust crossover in the manifold has been blocked off and/or the heat riser has been removed or disabled. Typically when these "mods" have been done the perp will notice that hard hot start thing. There isn't enough heat to take the choke off. So they try to fix that by reducing the choke thermostatic spring tension. That results in poor cold start because there is insufficient tension to close the choke when it's cold. So when I see these two problems combined, that's the first place I look.
SteveComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Steve, thanks for your reply and advice. I will try that this weekend. Any idea why the car then is shutting off on me once warm ... is it flooding itself? It seems to be not getting enough fuel at the point of RPM drop and shut off. I'm very perplexed as to why it is only happening once warmed up ...Comment
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Re: 1966 L72 Idle and Stalling / Cold Starting
There's no short answer for this.
The engine requires a rich mixture to start when cold. The air is denser and the fuel won't vaporize properly. To overcome this the choke, on a cold engine, must fully close. This lowers the pressure above the throttle plates and causes more fuel to flow. This along with a couple squirts of raw fuel via the accelerator pump gets it started.
If your choke isn't closely fully/properly you need to compensate by pumping more raw fuel into the throat to get it started.
Once the engine starts the choke is pulled open a preset amount by a vacuum diaphram or piston. This allows it enough air to run, but is still richening the mixture.
As the engine, and along with it the choke heat source, start to warm up the engine requires less additional richening. As this is happening the choke stat is warming up and the coil looses tension, which allows the choke to open up. This continues until the engine is completely warmed up at which time the choke must be fully off.
The warm engine does not want the richened mixture, so if the choke is not coming off in sync with the engine warming up, it will start to run excessively rich. This can cause the idle speed to drop and can lead to stalling. When the engine stalls, or is shut off warm, the choke thinks it is starting a cold engine and closes the flap, perhaps not all the way, but more than it needs to.
This makes the engine hard to start. Holding the throttle wide open activates what is known as a choke unloader which overcomes the stat tension and opens the plate. Additionally the wot allows excess air through and dries the cyls.
But this is just one scenario. Your choke may be functioning perfectly fine and what you have may be a lean condition. If you have a vacuum leak even with the choke fully closed it may be too lean for a proper start. During warm up the choke could be richening enough to offset the lean condition. When the choke comes off the engine is lean and prone to stalling out. Although your hot start procedure suggests it is rich.
So both rich or lean can cause the rough idle/stalling. There are distinct differences in the symptoms and standing beside your car or driving it I could tell you in a minute which it is. But I've been a journeyman for almost 40 years and did this stuff on a daily basis in the 70's and 80's. So you are best to go with a logical sequence of testing and checking.
So that sequence goes something like this.
We assume you've made all the tune-up ign checks. Initial timing, mechanical advance, vac advance all working properly and set correctly. K66 is a pretty reliable and consistent system and doesn't sound like a problem. Plugs and wires are all checked and good.
Cold engine, check to see that the heat riser is there. Has it been wired open? Does the counter weight move easily and return to it's stop with a reasonable spring tension?
Lift the air cleaner and pump the throttle once. Did the choke plate close fully? If not, examine why not.
Start the engine. Watching the heat riser, jazz the throttle a couple times. Does the heat riser blow open? If not, the flap may be missing or broken. If the heat riser is working okay gingerly feel the exhaust cross over around the choke stat. It should start to heat up very quickly, well above and much quicker than the heater hoses.
If all this has checked out so far, watch the engine run. The choke should gradually open as it warms. By the time you can feel heat in the heater hoses the choke should be wide open.
There's more, but that gets you started.
SteveComment
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Re: 1966 L72 Idle and Stalling / Cold Starting
Thank you so much for taking the time to write that all out. Helps a lot. I'll get to work. Today's other theory (offline) is that the problem is the Holley 3247 and the lower octane gas (lower than what was run in 1966). A friend had a very similar problem with his Holley in his '72 LT-1; when it got warm the car would shut down at stops. I drove this car again tonight for 20 minutes and it was perfect. Shut it off, had dinner and 50 minutes later started it up and the stalling problems started to happen. Something is happening when the car is warmed up, sitting, and then having to go again. I'll start trouble-shooting via your steps above. Again, truly appreciate the response.Comment
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