MelWff

Have you checked if the outer ring on the harmonic balancer has slipped causing false timing readings.

Evette76

If I knew how to check that I would. I'm imagining it has to be something like that with how advanced it has to be.

Jebbysan

Yes, in an above post I mention the true TDC check......you can do this with a 5/8 socket on a bar and a thin screwdriver.....start to blow thumb off #1....now put the bar on and turn crank bolt until you start to feel the top of the piston through the sparkplug hole......turn crank back and forth slightly while you fee the piston "high spot" with the screwdriver....this will get you within a degree or two if you are good.....this is close enough to tell you whether the outer ring is spun on the balancer.
Lean has nothing to do with it......verify timing first before you do anything else........you are shooting for 15 initial and 36 total....or close to that......lock it down and diagnose other issues. The timing numbers are a standard baseline for ALL small and big block chevrolets.....and should not deviate far from that......big bores want more total, big cams want more initial......
If you are experiencing issues with running after the timing is spot on then you need to troubleshoot further...such as vacuum leaks or carb jetting......I see you are running a Holley.....

Jebby

Evette76

Ok that all makes sense. I'll check that tonight when I get home. Yes it's a Holley 650 ultra double pumper.

Evette76

It has a big lopey cam in it, at the most, how much advance should it have both initial and total? Here is a link to what it is.
https://www.summitracing.com/parts/lun-60105

MelWff

Total mechanical is 36 BTDC at 2.8k to 3k rpm. Initial will be whatever number of degrees the distributor can generate. So if you set it to 36 and the distributor generates 24, Initial will be 12.

Evette76

So I'm to just set the total timing around 36 and then move on?

MelWff

Yes but at the stated engine speed which requires spring experimentation.

wwiiavfan

Don’t be afraid of a lot of timing. Low compression motors with a lot of cam overlap like a LOT of timing. Really rough Rule of thumb is if the motor likes it, keep adding until you get pinging under light load at cruise. Then back off a little.
I have a crate motor with 8:1 compression and an L82 spec cam. I can run high 20’s initial with vac advance and into the 50’s all in with vac advance, and a decently smooth idle. With the vac advance disconnected and 14 initial and 36 all in it idles like a pro stocker but is really lazy on the low end.

stingrayiii67

Yes, this is true.
I have mine set at 22 initial with a big cam. After that I had to readjust and tune the carb a little to get it just right.

Jebbysan

About 20 degrees.......but set for 36 total and note where the initial falls......you do not set timing for initial.....initial timing specs were for manufacturers to curb emissions......

Jebby

Evette76

Alright got the total set to 36 degrees around 3000 rpm. At idle (about 1000 rpm) it's still at 22 degrees. Since it has so much timing at idle I just left the vacuum advance on the ported side of the carb. TDC was also verified correct. Couldn't find bushings to fit the stock advance weights so just installed the advance kit weights and middle piece with the medium springs. It now is back to just having the slight stumble if while you're driving along steady and then give it more throttle. I'm assuming it needs more accelerator pump shot.

Jebbysan

Rather than have it on the ported...adjust your can for 6-8 degrees and put it back on full manifold vacuum.....you want about 28 degrees at idle ideally with the vac plugged in.....
Once you do that, run a slack adjustment on your accelerator pump arm. The Ultra DP 650 has 63/73 jets 28/31 squirt.....if you are still stumbling.....add 2 numbers to the primary and a 31 primary squirter with the tubes......

Jebby

Evette76

Ok that all sounds good. Just out of curiosity, why the higher advance like that at idle? Does it make the idle even stronger? I am running vintage air and when the compressor kicks on it will not hold an idle. Maybe this will help? Otherwise I need to try the idle step up.
Right now the primary jets are 64 and the pump shot is a 31. Maybe try 66 jets and 34 shot?

MelWff

Size of jets doesnt effect idle circuit. Did you balance opening of primary and secondary throttles before installation.

Jebbysan

Yes...it makes it "stronger"......more efficient really. Get the timing sorted first, check the accel pump gap...should be .015.....

Jebby

REELAV8R

Your fuel burns slower under less pressure and turbulence. When you have a cam with a lot of overlap (the period in which both intake and exhaust valve area open) You have a loss of compression at less than peak torque RPM. Every cam has a loss of pressure due to overlap, just more so with a large cam. So the fuel burns slower due to compression loss, swirl loss, lack of turbulence etc.
In order for an engine to produce it's peak power at a given RPM it needs to reach peak pressure approximately 17* after top dead center (ATDC)

If the fuel is burning slower then it needs to be ignited earlier in order to reach peak pressure at this time. Any timing that achieves that peak pressure before or after that 17* is a loss of power.

So when you are idling the fuel is burning slower taking longer to reach is max pressure. If you want it to reach max pressure at that 17* ATDC then the fuel needs to be ignited earlier.
That is why more ignition timing is needed at idle. Lacking that and you get hesitation, low power, low idle vacuum, greater throttle displacement needed to idle, perhaps to the point of engaging the main circuit which in turn makes it idle rich, on and on and on.

For a street driven engine you need to get the IDLE timing right in order to get your idle and off idle performance correct. It has to be right to get the idle jetting right.

I do not believe that idle timing is incidental to the total of 36* total timing. It is important and critical to a large portion of the driving you are going to be experiencing in a street driven car.

Jebbysan

I know it doesn't but he was complaining of an off idle stumble.....squirt and jet size does effect that.....squirt is important but jet has a role as the fuel needs to get started through the booster.....a larger jet helps that IF the jet is too small to begin with....

Jebby

MelWff

i picked up on the 1k idle speed and turning on AC causes a big drop.

Evette76

The jet change alone has gotten rid of the stumble. Engine seems to be running really nice right now. Definitely want to thank everyone for the help! I am trying to get the car done before Good Guys in Des Moines this weekend. I did a ton of work to the car to get it ready. I think I'm actually going to make it. Thanks!!