396 big block. New distributor new carb.

I thought I had everything good to go. But I guess not. Vacuum advance disconnected with a bolt in the hose to stop the vac leak -- I set my timing gun to 36* and revved the engine to about 2600 and got my marks to line up. Great. Engine sounded ok, I guess. When I tightened everything back up I double checked it. Seemed ok still.

When I put the vac advance back in the rpms went up so I figured I could just adjust the idle screw down if anything. For the heck of it I checked the timing at idle with the vac advance hooked up. The marks didn't align until the gun was at 46*.... Meaning my timing at idle was 46*?? I thought it should be between 12-16, no?

What the heck is going on? What am I missing?

 

That means your advance is giving you 10 more degrees,doesn’t sound bad to me,but
let the experts help you they will chime in. Smart people here.

 

What was the timing at idle with the vac can disconnected? 30 degrees vacuum advance does seem a bit much, but it's not unheard of. Check the specs on the vaccine you have, it should show the total vac advance in distributor degrees. Double that for crank degrees.

Your total advance without vacuum is good. If the car runs well at part throttle cruise and doesn't buck or kick in high gear at modest speed, you're probably ok.

 

I think the timing at idle with the vac disconnected was maybe 14 or 16, somewhere around there.

Here and there, I forgot what setting I had on at the time, I'd get some popping out the exhaust. That's what made me recheck everything. I'm just trying to not blow anything up haha. I mean is there a setting I should aim for for caution? Like maybe not 36* maybe 32? Not sure. I've built maybe 20 cars before, but this is my 1st carb car. Stressing me out a bit!

 

you are guessing at base time from your description.
idea is to check mech timing.
check timing, vac unplugged at idle.
watch timing as you increase rpm looking for when timing starts to advance
and what rpm it quits and the total amount of adv increase.
this is total mech.
with the info you can see what changes to springs and weight movement you need to
correct.

once mech timing is setup then you deal with vac adv amount

 

Yepp, that's how I had it set up. I set my gun to 36 and revved it till my tdc mark on the crank lined with the 0 mark on my block. Got to about 2500-3k and it was just about at the line. Kept going to about 3600 and it didn't keep advancing so I figured it was good.

I think the thing that freaked me out was the 46* idle with the vac. How would you adjust that? Or do you just live with it?

 

you can limit the vac amount by putting a "stop" to limit travel.
ask lars for detailed tune papers, he is a member
email him
I don't remember the addy
someone will be along

 

https://www.corvetteforum.com/forums...long-post.html

 

Nope, but I did now!

I'll re-re-re-time the car again tomorrow. I feel like I'm doing all of the above, but I'll give it another go just to verify! Appreciate the link !!

 

if you are wanting to maximized performance then you need some specific goals.
a BB has ideal specs on idle mech advance, what rpm the advance starts, what rpm
it is max at a rpm.
then vac advance has what amount of inches and max advance it adds.
specific info and how to are found in @lars papers.
have a goal.

 

thats normal...read Lars paper...small blocks can see as much as 50 plus degrees of timing...16 initial , plus 20 of mechanical plus 10 or 12 of vacuum advance is an easy 46 to 48. Dont worry about it. If you were over 50 you could be nervous. My 327 was over 50 at times and my big block is and easy 46 to 48 total. Im at 38 all in before 3000

 

There are three things that are interacting with timing: base timing without any mechanical or vacuum advance; mechanical advance that should just begin around 1000-1200 rpm and top out somewhere in the 2600-3600 range; vacuum advance that is active at idle and diminishes as manifold vacuum decreases with higher rpm. There is a lot of overlap of all three that will affect what the total timing reads at any given rpm.

1) Base timing is only of note to those who just want a quick and dirty setting; most of us use the mechanical and vacuum components to arrive at optimum timing and just let base timing fall wherever it falls. Base timing is only important to determine when the mechanical advance begins.
2) Mechanical advance is adjusted through the spring tension on the centrifugal weights. We don't want it active at idle but we want its full timing advance effect somewhere between 2600 and 3600 rpm. There is a balance between too soft where the advance may come in and top out too early and where it is delayed in coming in and tops out at an rpm higher than desired.
3) Vacuum advance is full in at idle, will affect idle rpm, and decreases as manifold vacuum drops with hard acceleration. The variables are the sensitivity of the diaphragm to vacuum and the amount of movement of the arm that moves the base plate. The "2-inch rule" determines which vacuum unit to get according to its vacuum sensitivity and mechanical limiters affect arm movement.

When each is set according to your desires, your actual timing will be a combined timing measure of all working together. The final determinant will be the degree to which your engine and fuel can optimally run without pinging.

 

Rookie question...Did I miss whether he is using ported or manifold vacuum?

 

Pretty sure I'm using manifold -- the lower plug on my Holley, passenger side, tucked in almost underneath the bowl

 

yep thats manifold, ported is higher up on a holley, generally sticking out of the metering block on an older Holley

 

Ok, I think I finally got it (again)

Vacuum not hooked up:

12* at idle w/ 700 rpm idle
34-36* at 2600 rpm (hard to tell the exact. Might be a bit under the line, but it's essentially lined up)

Vacuum hooked up:

31* idle w/ 950 rpm idle
I forgot to check what the * was with the vac connected at 2600, but it looks like it would have been into the high 40s-50 maybe

Problems I ran into:

At one point the car was fine until I plugged the vac back in. At that point it would start to putt/pop out the pass side exhaust. What could that have been?

It sounds a little chattery out the pass side when I drive it. Could I maybe have a valve that's out of adjustment? I checked everything like 4x when I put it back together but I supposed something could have loosened. I guess I need to watch some vids on what that sounds like, or maybe do a compression test or something. But so far so good!



​​​​

 

Try decreasing your timing 2 dedrees at a time to see if the.popping goes away. Its either detonation or a lean backfire or just running lean

 

Your engine is quite beautiful!!!

 

What is the compression ratio?

 

My final timing isnt doing it, so I'm assuming maybe a prior setting I had it at was too advanced? I wasn't sure if it was an advance or retard issue.

I'm trying haha!!

To be honest I have no clue haha. I could probably do research on the numbers on the block and figure it out. It came with the car... I just spruced everything up on it