vacuum lines, tuning carb.

[willstruck]

So, I decide to tune my truck up. Get a hold of a timing light, timing was like 13 btc. Set it to 4 btc. Can't find the vacuum gauge, so I still need to tune the carb. I was looking at a video on tuning the Edelbrock carb I have, and they say that the vacuum line to the distributor is generally better hooked to the right port on the carb which always has vacuum, and usually only hooked on the left one for emissions vehicles. Mine was hooked to the left one, so I looked at what was the right one, it looks like it runs to the transmission. Didn't hop under the truck to double check though.

Basically, are the vacuum lines hooked up correct, with the drivers side on the carb going to the tranny, and pass. side going to dist? Or does it even matter?

[cableguy0]

Why did you lower the timing to 4? If it was starting and running ok at 13 you shouldnt back it down. The more timing the better performance for the most part.

[willstruck]

Originally Posted by cableguy0

Why did you lower the timing to 4? If it was starting and running ok at 13 you shouldnt back it down. The more timing the better performance for the most part.

Really? This is my first carb'ed engine. Everything I read said to just put the timing at 4. So, the farther I can go with the timing, the better the performance?

[cableguy0]

4btdc is the factory setting based on emissions and smog controls of the time. Ignore the factory settings completely. I would put it back to at least 12btdc. Sbc's like a good amount of timing. As long as the engine isnt pinging and it starts up and shuts down nicely theres no reason to back the timing off.

[ChevyC102007]

The 350 in my '80 is set at 10 degrees, starts easily and shuts down well.

[willstruck]

Alright, I'll fix the timing tomorrow after I fix the leaking water pump. I'll also pick up a vacuum gauge and tune the carb. Any hints? It's the basic Edelbrock 650, I think 1408 was the model. I know the basics about tuning it, but any hints are welcome.

Also, anyone know anything about the vacuum line question I posted?

[James McClure]

Will, there are 2 different vacuums. One is called manifold vacuum and the other, ported vacuum. Basicly one is above the throttle plates (ported) and the other below (manifold). Ported vacuum is part time, manifold is full time. The distributor, the trans and heater AC controls get manifold vacuum. There are others but that all depends on emissions level. Most of the emissions stuff is ported which gets redirected by thermal switching devices in the water jacket. Any fitting on the carb that has vacuum at an idle is manifold vacuum. Other fittings will only have a vacuum signal when you crack the throttle a bit. jim

[babyJay]

You don't want the vacuum advance on the dist connected to full manifold vacuum. It should be connected to the ported vacuum. If you wanted it advanced all the time, (manifold vacuum) you should just remove the vac advance from the dist and mechanically advance it, which would be the same either way?

[willstruck]

Thanks guys, I tried googling it a different way last night, and learned a lot. There are actually 2 diff. types of vacuum advances for dist. One uses manifold, and uses how the vacuum decreases as throttle increases to advance timing. Ported is way more common, and works just like you guys said.

[James McClure]

Will, I worked as a GM mechanic for 33 years and a few years before that in a service station. Without reguard for what other manufacturers do GM has had full manifold vacuum to the dist vac advance for as far back as I can remember. There is a good reason to do it this way. Ported vacuum is to difficult to regulate and is adversly affected by to many things to easily. A small change in idle speed will make a huge difference in vac levels where man vac isn't affected be such things. Ported vac will also drop to "0" very quickly and will be unable to operate the vac advance. Manifold vac will hold above "0" for quite a bit longer makeing it more effective. Both suffer when there is a mechanical issue inside the eng though. I don't know if you are aware that total advance will excede 20 degrees at idle on a normally functioning system. As you accelerate the vac drops and timing will decrease (never below base timing) to prevent pinging. As eng speed picks up the load drops, vac increases until it reaches almost to idle levels. Above that, the advance weights (under the rotor in HEI and below the advance plate in point ign) bring the timing up even more. The higher you can run the timing without excessive pinging or haveing a hard start condition on a hot eng the better off you will be in both performance and fuel economy. I know I'm going to get a lot of crap on this one. jim

[willstruck]

I'll try hooking the line to the other port, that's what the other sites recommended, was trying it out. Now that I have it tuned, I notice flat spots in the acceleration and power, like right at mid throttle. cruising along is fine, push the pedal a bit, it seems to bog down, put my foot into it a little bit, and it picks up...