Hey man, my 49 Ford had a motor meter installed when I got it. The dang thing tends to die at stoplights and other inopportune times. I’m sure the carb needs to be gone through, but I might have a vacuum leak. What do you make of these readings? Will idle fine at around 20, start dipping down and then when it dips down to around 17, it’s about to die
I would start by looking for a vacuum leak. Could be as little as a carb base bolt being a 1/4 turn lose. What is in common when it dies? I was going round and round with the car dying at stop lights. Turns out it would even do it if it was in park and I put my foot on the brake! Couldn’t find a leak and tried to over come it by bumping idle speed up. I knew it was a bandaid but thought worth trying! Didn’t fix it and the vacuum readings were all kinds of wonky. I ended up changing the idle mixture of the carb and got a ton of vacuum back, dropped idle speed back down and the brake issue was gone. It was just off enough that when I hit the brake the booster took the vacuum available and killed it. You’ve probably just got something minor to hunt down. Vacuum leak is the most likely. Set it in park idling and spritz some wd40 around and when the idle speeds up you’ve found your leak!
That is about as good of a vacuum reading as you can get if it is bumping 20 inches of steady vacuum . This is one vacuum chart that a lot of us refer to that shows how to figure out issues that you probably don't have. They all say pretty much the same thing but some are easier to decode than others. Still I would be looking at Idle speed especially if the car has an automatic. Quite often someone will set the idle speed at a low level where it idles great sitting in neutral in the shop but the car dies when it is in gear at a stop light. If you have access to a Dwell meter/tachometer I'd hook it up and see what the idle rpm is and how that jives whith the book for your engine.
Thanks, it’s a 350 Buick, with a two barrel Rochester. It makes sense on that vacuum leak. It pulls a good 20 inch, then suddenly jerks down to less vacuum thanks. I was going to do some engine bay cleaning, so while I’m there I’ll probably find it. Tim, I’ve ran into a loose carb. Took it to a mechanic, said it was fixed. Next time I checked the oil, I bumped into the air cleaner and it rocked back and forth nicely.
3Fiddy Buick, I'd start by looking at vacuum hoses one at a time. Just spend some time checking each hose on the car including the one to the gauge it's self. Vacuum hose is pretty inexpensive by the foot at the parts house and if any looks suspect I'd change it with the correct size hose. That includes the short piece of hose on the modulator that often doesn't hold a good seal if it is comprimised by oil and grease and what not.
If your engine is dying from lack of fuel for example the vac is going to drop off so what you’re seeing isn’t necessarily a vac leak. You can have a spray around the intake with some easy start or butane to check for that. But I’d give it a basic tune first, check points and plugs, set the timing, set the idle mixture
If it will idle at 20" it isn't a vacuum leak unless it is some kind of on/off type leak. It is more likely it is running out of fuel even possibly an ignition problem. What happens if you raise the rpm to 1500 or so? Does it stay steady or do the same thing?
I tend to think the idle mixture is too lean. I have developed a braking technique where I keep my toe near the accelerator just in case I need to idle it up a little bit. It has a hand choke on it and most of the time it doesn’t die if that’s pulled out just a little bit. It did die once like that so it’s not 100%. It’s hard to say for sure. I have really just been like driving to the store and back; little errands here and there. I really just need to sit and let it warm up properly before I start jacking with a whole lot.
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