427 stalls when you stop

steve meltzer · #1 (**permalink**) 06-15-2005, 09:38 PM

I've got a 427 SO, toploader, new factory Holley 4160 (vac secondaries), MSD box and typical Ford distributor (from Southern). This Kirkham has only about 300 miles on it, and, based on the intake manifold vacuum, I replaced the "65" power valve for and "85". (Don't really think that's an issue here, but thought I'd mention it.) The problem I'm having is that the car stalls and dies when I come to a stop...not necessarily a hard or "panic" stop, but even a gentle stop. The RPMs dip into the 600 range, and the car dies. I know that Holleys can do that under a hard stop if the secondary floods the intake, buth this happens under all stops.

Both floats are just at the lower lip at idle...this is a little tough to discern, as the idle is somewhat rough, at about 1000 RPMs. I called Holley and they said that I need to add a "dashpot" to sustain the normal idle, but Southern (Lyle) thought the distributor springs might be weak, allowing the advance to fall too low. The timing is at 36° BTDC, all in at 3200 RPMs. I don't know what the base idle is at this moment (never paid any attention to that).

Any thoughts as to what the problem might be and how to solve it?
Thanx steve meltzer "unencumbered by the thought process"

Excaliber · #2 (**permalink**) 06-15-2005, 10:31 PM

I'd check the base timing first thing. At least 12 if not 15 BTC or even higher. I had the symptoms until I bumped up my base.

steve meltzer · #3 (**permalink**) 06-16-2005, 05:24 AM

thanx for the info. When I spoke with Lyle at Southern, he actually thought that the base timing should be near 20! Sounds like checking the timing would be a good place to start. You must have had the same symptoms. thanx again. steve

chopper · #4 (**permalink**) 06-16-2005, 11:10 AM

FWIW, I had the timing set on my engine (Southern Automotive 427 SO/484 stroker, aluminum heads and dual Holleys) set to 10 deg at idle (900 rpm), increasing to 38 deg at 3200 rpm, and it returned to idle without any problem. I might suggest that you check the carb float settings again. My personal experience with Holley floats was that the most reliable setting for the float level was made with the float bowls off and inverted. I adjusted the needle valve so that the floats were horizontal with the bowl inverted, and then made only minor adjustments once the engine was running. I found that if I had to make a significant adjustment (more than one full turn of the float adjuster nut), there was something else wrong. Keep in mind that the final float level should be adjusted with the engine running at a sufficient speed to allow the fuel level in the bowl to begin to drop, which will cause the needle valve to unseat and allow fuel into the bowl. The fuel flow at idle may not be high enough to allow an accurate adjustment. Every time I've tried to adjust the floats on a Holley at idle, I ended up with the floats being adjusted too high, which in turn may directly cause the problem you describe.

I assume you've already checked this, but you should also make sure that the needle valve is clean and not sticking, nor leaking at the seat. In my experience, the needle valve design of the Holley is one of the weak points of the carb, and I used to carry spare needle valves in my tool kit to change if they stuck or got dirt in them. Any minute piece of trash on the face of the valve will prevent the valve from seating and will cause the carb to flood.

One other item to check is the power valve. I exchanged the power valves on my carbs from the stock 6.5's for 8.5's like you did, and when I went to the speed shop to get the valves, I took a power valve checker and a hand vacuum pump with me. The speed shop manager and I went through eight power valves before we found two that actually opened at 8.5 inches. The low from this sample was 4.0 and the high was 12.5. It may be that what you (understandably) assume to be an 8.5 valve, isn't. If the actual valve opening value is very close to the engine idle manifold vacuum, it may cause the valve to open momentarily during pressure fluctuations following throttle closure. Holley says that opening the power valve is equivalent to increasing main jets by 8 to 10 sizes. This may cause the carb to run temporarily rich upon throttle closure. Just a thought.

Frank Messina · #5 (**permalink**) 06-16-2005, 12:28 PM

You're on a slippery slope there. As the idle speed drops, vacuum drops which may be allowing the 8.5 power valve to open, sending in more fuel and killing the idle further until it finally quits. The only way to be sure is to go back to the 6.5, hook up a vacuum gauge and measure your vacuum at idle. There are different rules of thumb for selecting the correct power valve, but on a street car I would suggest 1/2 idle vacuum. Set your timing, then go back and fine tune the idle mixture using the vacuum gauge and set your idle speed. Float level should be so that fuel just reaches the dribbles out of the hole on the front, and barely reaches the bottom of the hole on the rear. You should be goood to go at that point.
Frank

mj_duell · #6 (**permalink**) 06-16-2005, 12:35 PM

Quote:

Originally posted by chopper

I assume you've already checked this, but you should also make sure that the needle valve is clean and not sticking, nor leaking at the seat. In my experience, the needle valve design of the Holley is one of the weak points of the carb, and I used to carry spare needle valves in my tool kit to change if they stuck or got dirt in them. Any minute piece of trash on the face of the valve will prevent the valve from seating and will cause the carb to flood.

This is the exact problem I had. After changing out the needle valves all was well.

--Mike

Frank Messina · #7 (**permalink**) 06-16-2005, 01:13 PM

I was going to PS about the needle and seat but Mike beat me to it. They stick on brand new carbs! Whack the bowl with a big screwdriver handle. If that clears it up, buy a new one and put it in.
Frank

chopper · #8 (**permalink**) 06-16-2005, 06:32 PM

With all due respect to Mr. Messina, the "rule of thumb" which advises to use a power valve rated at 1/2 of the idle manifold vacuum may or may not be adequate. A more accurate method is to mount a vacuum gauge to the dashboard and run a vacuum line from the intake manifold to the gauge. Then, drive the car on level ground and have a passenger record stabilized manifold vacuum at 10 mph intervals in each gear up to about 60 mph or so. From this data, you can determine exactly the lowest stabilized manifold vacuum under normal ("unloaded") operation. In all probability, the lowest vacuum will occur at idle, but this way you will KNOW what it is rather than guessing. Based on the lowest stabilized manifold vacuum, select a power valve which is 1.5 to 2.0 inches lower than your lowest recorded stable vacuum. See this link:

http://www.holley.com/HiOctn/TechSer...o/TI-222A.html

Not to beat a dead horse, but power valves can cause a multitude of problems if not tuned to the application. A power valve tester is almost a necessity for really knowing what's going on inside the carb:

http://www.jegs.com/cgi-bin/ncommerc...61&prmenbr=361

FWIW, my money is still on a faulty needle valve.

steve meltzer · #9 (**permalink**) 06-16-2005, 07:42 PM

wow! i've learned a bunch about this and will try to get to the carf in a few days. (damn work always gets in the way!) I'll post the list when I learn more. thanx again...very interesting. steve

mr0077 · #10 (**permalink**) 06-16-2005, 08:34 PM

Hey, Doc, you ought to hook up with the Houston Area Cobra Club boys...some pretty sharp guys there that would like to see your car AND would be happy to help with the problem. Check them out at http://www.rokfordesign.com/hcc/ or join the Yahoo group, email at houstoncobraclub@yahoogroups.com

Excaliber · #11 (**permalink**) 06-16-2005, 08:58 PM

I'm not familiar with the power valve circuit in an Edelbrock carb. Piece of cake to change out on a Holley. How so on an Edel?

I used a vac guage to determine my PV setting. Idles at 11" and pretty much cruises at 11. I went with a 9.5, works well. Vacuum gauge tapped to the windshield wiper arm.

flipper35 · #12 (**permalink**) 06-17-2005, 09:43 AM

Edelbrocks don't have a power valve. They use springs on the metering rods. The rods have two diameters on the end and when the vacuum drops to where the springs are rated, the rods get pushed up to where the smaller diameter is in the jet allowing more fuel flow.

Excaliber · #13 (**permalink**) 06-17-2005, 09:59 AM

Thanks Brent, I see the principle is the same. Extra fuel based on engine load\vacuum. Sounds like they might be easier to change out than the Holley power valve setup.

A ruptured power valve will cause major "flooding" and a run rich condition. I wonder if those Edelbrock springs\metering rods could "hang up" also causing an extreme rich condition.