L6 timing question

[jfnar]

I decided to bump up the timing on my 292 a little so I unhooked and plugged the vacuum line as the manual directs. After a slight adjustment I reconnected the vacuum line and double checked. The timing is exactly the same at idle no matter if the vacuum line is connected or plugged. Is it supposed to be like that? If so, why disconnect the vac line to time it? By the way I have a HEI distributor from a '77 250. Thanks for any insight.

[geezer#99]

If you’re plugged into a manifold vac port and the timing doesn’t change then your vac pot on the dizzy is kaput.
If you’re plugged into a ported vac port (zero vacuum)then your timing won’t change.
The reason to always plug it off is then you know for absolute sure you’re not adding any timing from the vac pot. Sometimes if your curb idle is set too far open you’ll get some vac from the ported port.

What did you set your initial timing at?

[jfnar]

I haven't hooked a vac gauge to the line but it's hooked to the port on the Holley 390 carb where the carb instructions said to hook it. I'll check with a gauge tonight. I've currently got it at 12. Thanks.

['68OrangeSunshine]

Nominal Vacuum reading on my 292 is 17.5''Hg. With 290 * H2O temp, idle 750 RPM, 40 PSI Oil Pressure.
I plug the vacuum line with a golf tee, and also put a small vinyl cap on the carb nipple.
The L6 HEI vacuum canister is the same as the V8. I have the lowboy HEI with the remote position coil ['75-'77 model year stuff.]

[geezer#99]

Originally Posted by jfnar

I haven't hooked a vac gauge to the line but it's hooked to the port on the Holley 390 carb where the carb instructions said to hook it. I'll check with a gauge tonight. I've currently got it at 12. Thanks.

The port above your idle mix screw is ported vacuum.
The port below is manifold vacuum.

[jfnar]

I checked a couple things last night. I put a vacuum gauge on the distributor vacuum line and it gets no vacuum until the engine speed is increased. I don't know what speed it kicks in because I couldn't see the tach. I put a vacuum pump on the distributor and it does rotate when the vacuum reaches about 10 to 12 in hg. Not exact measurements but I'm working with cheap HF tools. I advanced the timing until it seemed to be running best but it was way off the scale. I backed it down to approximately 17 to 18 degrees. Again, this is an estimate because my scale only goes to 12. I drove it and I really couldn't tell any difference. I'll probably back it back down to around 15 or so just to ensure I don't have any detonation.

[chevy_mike]

If you distrib port is going to that one, it is the wrong one. You need the one under the fuel bowl.

You can see the Tee where I have the trans and HEI port hooked to and it goes to the port under the fuel bowl. You want manifold vacuum, not ported vacuum.

[jfnar]

I was going by the holley instruction sheet.

[geezer#99]

Try both.
Use whichever works best.

['68OrangeSunshine]

When I had the Holley 390, I used 8.5" Power valves. The PV cuts in at one-half optimum manifold pressure or 17 Inches of Mercury [17''Hg].
At idle your suction or Manifold Pressure is high. [Even higher when decellerating]. When you accelerate, the MP goes down. At full throttle, the Vac Gauge reads 0. As you gain momentum, then the pressure gains. The higher the reading on your Vac gauge at speed, the better your fuel economy.
When tuning the carb, you want the highest MP you can get at idle, at optimum temp, on each air/fuel needle. For my 292 that's 17 - 17.5"Hg. My engine is bored .030 over, and has a Crane 260H cam, however.
My timing is about 12 degrees advanced.
I use an Edelbrock 1404 500 CFM now, but I ran the 8007 Holley 4Bbl for 25 years.

[jfnar]

I started messing with the timing because the idle seemed a little rough, especially if I set it below 900 rpm. Last night I was looking for the lower vacuum port so I could move the distributor vacuum line to it. It's hard to find because it's on the back side of the carb. I got a small mirror and a light and started looking for the blue cap that I put on that port when I put the carburetor on. No blue to be seen. I finally found the port and the cap is gone. Now I know the problem; the Mother of all Vacuum Leaks. I put the distributor line on the lower port and plugged the upper port. It idled smooth as silk. I adjusted it down to about 600 and still smooth. I have no idea what happened to the cap on that port. My only hypothesis is that I had a couple backfires when I first set the timing and maybe one of them blew it off. I don't even know if that's possible. Anyway it's idling good now. By the time I got this far the mosquitoes were dragging me towards the woods so I retreated to the house and watched the all star game. Tonight I'll take another look and use the vacuum gauge to fine tune the timing and idle mixture. I have no idea what size power valves my carb has and I'm not well versed at tuning carbs. I bought this one from a guy who was abandoning a Ford 300 project. It was new in the box and I just used it as it was. Got a great deal on the carb though. $100 plus he threw in an offy manifold that was not correct for his 300; just happened to be correct for chevy 6. I don't get lucky like that very often.

[chevy_mike]

Glad to hear you got it dialed in.