Timing settings help

[IT Cowboy]

I set my timing yesterday at 16 degrees btdc for 3500 ft elevation where I live and unplugged the vacuum advance and plugged the port.

It runs great and I am getting a reading of 35 degrees at 3000 rpm with the vacuum advance unplugged.

With the vacuum on the dist. plugged in at 3000 rpm I am getting a reading of 55 degrees advance.

Is this an issue, am I misreading something or doing it wrong. This is with a harbor freight timing light with the advance dial on it.

The truck runs good now like a streak of **** and will turn over the rear tires. It also stays cool on highway or in town or just idling.

[MySons68C20]

Your vacuum advance is adding 20 deg of timing in a no load situation..
With 15 initial connect the vac advance and you should see 35 deg on the timing light.
Sounds like it is working correctly.

[Dead Parrot]

2nd the thought it is working properly. 50~55 is the normal expected total light throttle advance.

The higher initial timing normally does cool the idle temps a bit as it is using less fuel to keep running.

Don't worry about exact numbers. There is some variation between balancer timing marks, the mass produced timing cover, the mass produced timing indicator that is spot welded to the timing cover. Yours may be +/- a few degrees compared to the next one.

[Willshook]

Those numbers are all within workable ranges. The initial is somewhat high, but you're at altitude.

As long as you're not getting any detonation under acceleration, you're good.

[IT Cowboy]

Ok, thank you all and I thought I was ok since it seems happy and runs well. The difference at 3000 rpm when plugged into the vacuum (55) vs when not plugged into it (35) is what made me question it as ChatGPT acted like the 55 advance number was way too high.

It does seem just a little harder to start when it is warm but I guess that is normal and it still snaps right off but maybe just a second or so more to crank.

[Willshook]

Vaccuum advance IS a lot of advance! But it has a huge impact on drivability and fuel economy.

People disconnect them because <internet>, but there's no reason at all to do it, and lots of reasons not to.

[cadillac_al]

Heavy trucks used to have less vacuum advance, closer to 10 degrees, but I don't think anybody cares anymore. I special ordered a couple truck vacuum cans for my trucks a few years ago. I wanted them to run more like stock and didn't want to have 20 degrees of advance like a car. I don't remember what the parts stores were trying to sell me but they were much cheaper online anyway.

[truck-kid]

I'd say all is ok as long as your vacuum advance is hooked to manifold vacuum not ported vacuum verified by MySons68C20 check timing at idle 15* initial hose unplugged plus 20*more with vacuum hose plugged on =35* verifies your hooked to manifold vac.

[IT Cowboy]

Originally Posted by truck-kid

I'd say all is ok as long as your vacuum advance is hooked to manifold vacuum not ported vacuum verified by MySons68C20 check timing at idle 15* initial hose unplugged plus 20*more with vacuum hose plugged on =35* verifies your hooked to manifold vac.

Here is where my vacuum advance is connected to my carb. Sorry the pic came out sideways. One post I found said this

The stock routing for the vacuum advance is to use a "ported vacuum" source for the vacuum advance. A ported vacuum source produces no vacuum signal at idle, but produces normal vacuum signal once the throttle blades are opened up just a tad. The ported vacuum source on the early Q-Jets is the port on the forward, driver's side of the carb just above the idle mixture screw.

[IT Cowboy]

Ok, not that I don't believe you members here but according to the corvette and other chevy forums then 55 vacuum advance at cruise rpm of 3000 is not really all that crazy or out of spec. ChatGPT says it is waaay too high and not right so I think it is full of ****.

[MySons68C20]

You set the initial advance btdc (before top dead center) in your case lets say 15 degrees....
Your mechanical centrifugal advance adds another 20 degrees of timing all in at approx 3000 rpm for a total of 35 degrees total timing.
Your vac advance then adds another 20 degrees of timing depending upon load (throttle position) to give you the 55 degree total.
My 383 likes 20 deg initial 34 total with 10 deg in the vac can...
Give the engine what it wants but I feel you are already there.