Vacuum advance location

[razor66]

I had an engine builder tell me one time that running 20+ degrees of initial timing would likely result in having engine run-on "dieseling" issues on ignition shut-off. I didn't buy it, but perhaps he had an engine that had dieseling issues running higher initial timing that was helped by retarding it more but the root cause was likely something else. That's what I thought anyway, so I didn't argue the point. What are some of your thoughts on this?

[swatson454]

I had an engine builder tell me one time that running 20+ degrees of initial timing would likely result in having engine run-on "dieseling" issues on ignition shut-off. I didn't buy it, but perhaps he had an engine that had dieseling issues running higher initial timing that was helped by retarding it more but the root cause was likely something else. That's what I thought anyway, so I didn't argue the point. What are some of your thoughts on this?

My humble thoughts on this are that trying to run 20* on a stock-cammed engine may not be the best idea but on a performance build that has over 240* @ .050 on a 106 LCA, it's a must.

[Dodge Freak]

Dieseling happens when the throttle is open too far. Lots of initial timing would help to close the throttle more, therefor minimize the chances of dieseling--not increase it.

When the ignition is shut off the sparks stop, timing does not matter unless is so far off the throttle needs to be open too far to allow the engine to idle.

You can make any carb engine diesel, just open the throttle up as you shut off the ignition.

[paul s.]

with manifold vacuum... you can get that big cammed street engine to purr like a kitten. (as long as the combo isn't out of whack)

That has been my experience.

Excess advance is often used to band-aid poor carb calibration--in particular, not enough fuel at idle. Be careful...

nope. poor ignition tune can cause you to band-aid the idle with too rich of an idle mixture. LOL, funny how that hole "perspective" thing works .

I tend to use ported most of the time. I think half of that "GM Engineer" article from the Corvette site is BS, as I have seen many Chevrolet engines built in the 1960s that used ported from the factory.

I tend to limit my mechanical with a bushing, run a lot of initial (16-20 deg), and adjust the vacuum deg of advance to 10-15 deg or whatever works best. Running high initial with manifold can give too much timing at idle. The guys spreading the "manifold source" gospel fail to mention that most cars with OEM manifold vacuum advance also had about 2-6 deg initial timing with the vacuum unplugged, and a lot of centrifugal to allow for ~32-40 total timing. I'll bet the initial was dialed back because they also had a bunch of vacuum advance at idle with the manifold source.

Manifold vacuum seems to give a bigger rpm drop from neutral to drive in tight converter automatic street cars. I don't like that, so I use ported when it seems to work.

I am far from an expert... just posting what works for me. One thing that is evident from this thread is that vacuum advance is STILL one of those things that is misunderstood, and seldom agreed upon, even after all these years!

why would i or anyone else mention what the oem's did back in the 60's? it's TOTALLY IRRELEVENT. you had high octane leaded gasoline at every gas station; high factory compression ratios with aluminum cylinder heads a rarity; and mild flat tappet cams, carburetors that flowed on average... what; 600-700cfm. the oem's wouldn't need to dial in excessive initial advance, they used high compression and mild cam timing (for the most part). JUST THE OPPOSITE of what most guys are running today.

one thing i think alot of guys fail to mention is.... they don't TUNE the vacuum advance. manifold vacuum can be tuned to give minimal rpm drop in gear.

A wise man once said, "a sh!tload of anything can be detrimental".

[razor66]

This would have been a 240 something cam @ .050" on a 108 or 110 LSA, so not a stock type cam engine. I had actually mentioned the possibility of the throttle blades being opened too far, but he said that he had the transfer slots set where there were virtually none of the secondary transfer slots showing and less than 0.020" of the primaries showing. He also said he tried another carburetor with the same results. I thought perhaps he had an imperfection in a combustion chamber (aluminum heads) or something causing a hot spot. He said he didn't worry about it anymore as dropping the initial timing below 20 degrees resolved the issue as far as he was concerned. I agree with your thoughts on it Dodge, but this one seemed like a strange case to me as it didn't leave me with a satisfactory answer.

[MadBill]

Re dieseling, retarded timing increases combustion chamber temps and makes an engine more susceptible. High coolant temps and a fast idle speed are the final ingredients.

[Curveit]

Ah, late '70s GM motors, with Thermal Vacume Switches, and Idle Stop Solenoids.

[falcongeorge]

Hope I don't get my ass kicked for dredging up this old thread, but it seems to fit best here.
I recently bought an s10 with a 10.5/1 355 in it, runs good, 244 @ 050 solid on 110 lsa, 4 corner 80779 750 dp, msd distributor and 6al. When I bought the truck the vacuum advance was unhooked, which I planned to rectify once I started tuning, but it ran quite well this way, crisp throttle response, and pulls hard at wot.The idle is pretty fat, it's been bugging me, so today I thought to hell with it's I'm going to start tuning this thing, so I figured before I start playing with the carb, I'd better start by hooking up the vacuum advance . So I start by plugging the vacuum advance to the spark ported source. Whacked the throttle from idle, BIG lean pop on tip-in. Before you jump to conclusions, remember, this wasn't there with no vacuum advance, and initial is 18 degrees. Ok, so that didn't work so good, so I try manifold vacuum, whack the throttle, no pop.
This strikes me as kind of odd. Remember, the flat spot isn't there with NO vacuum advance, so it's not insufficient advance at idle that's causing it, why the BIG flat spot with the vacuum advance hooked to the spark ported source, any ideas?
I know, I know' who cares' just hook to manifold vacuum and forget about it, but I hate not knowing.

[falcongeorge]

Sorry, I just realized there's a very misleading typo there. Should say "plugging the vacuum advance into the spark ported vacuum source"...

[F-BIRD'88]

Your engine with that cam can use a lot more initial base timing than 18deg.

I suggest 26deg initial base. ( shorten-limit the mech adv curve to 10deg to allow this.)

Be sure that the carbs throttles are correctly oriented (T slot exposure at curb idle )
Getting this right is critical to correct idle set up and good throttle response.

[falcongeorge]

Once it's warm, it idles like a stocker with 18 initial, and no vacuum advance. I don't see any reason to add more initial, that's why I tried ported first. I know from your other posts you like to lock out the curve. I have done that on occasion with truly BIG cams on the street (like 275@050+) but I don't see any reason to on a mild streeter. It's a 4 corner carb, the throttle plates are well down in the slots. I am currently running the vacuum advance to manifold and it seems pretty happy, but I'm still perplexed by the big tip-in flat spot on ported.

[F-BIRD'88]

Its simple. The engine wants more initial base timing at idle.
Get your head around that. 26deg minimum.
Yup I run mine locked out ( cam 240-246@.050")
I also use vacuum advance ( ported, limited to 12deg max)

The carb idle off idle transition circuit probabily also needs more work.
Tuning the idle air bleeds and IFR so the AFR is correct as you open the throttle
from idle. But to start, this engine just needs more idle timing.
The exhaust gas recycle reversion at idle delutes the combustion charge
( exhaust does not burn). This SLOWS the combustion speed. (same simular to dumping water on a fire)
This requires more spark advance. 18 is not enough.
The engine does not care what you think. It wants more idle timing
to burn the fuel on time.
it pops when you open the throttle because the fuel is still burning
when the intake valve opens. Lite the fire sooner. ( more idle timing.)

A afr gauge will help you see what the afr is at idle and off idle
so you can dial in the carb's idle circuit.

Trouble with using full manifold vacuum advance to give the engine the idle timing it wants is that as soon as you rug it, that required spark advance disappears right when you need it most. Especially with a automatic transmission.
The biggest barrier to getting this engine dialed in is your incorrect assumption (belief)as to what this street motor needs for timing.

[F-BIRD'88]

It probabily wants locked out mechanical advance + vacuum advance, just like mine does.
Wether you then use ported or full manifold vacuum to control the vacuum advance system, on top of that is
a minor detail. Thats up to you.

[bmcdaniel]

Hmm, I guess the old rule of thumb "When running vacuum advance, initial advance should never be less than 10* or more than 20*" is out the window.

[falcongeorge]

When I bought the truck the vacuum advance was unhooked, which I planned to rectify once I started tuning, but it ran quite well this way, crisp throttle response, and pulls hard at wot....
So I start by plugging the vacuum advance to the spark ported source. Whacked the throttle from idle, BIG lean pop on tip-in. Before you jump to conclusions, remember, this wasn't there with no vacuum advance, and initial is 18 degrees. Ok, so that didn't work so good, so I try manifold vacuum, whack the throttle, no pop.
why the BIG flat spot with the vacuum advance hooked to the spark ported source, any ideas?
.

Clearly reading comprehension is not one of f-bird88's strong points. Neither are good manners, or for that matter, analytical thinking. I'll break it down, so no-one else gets confused, at this point, as far as I'm concerned, he has had his say, and is done.

1) 18 degrees initial, NOvacuum advance, no flat spot runs good.
2) 18 degrees initial, spark ported vacuum advance, big tip in flat spot.
3) 18 degrees initial, manifold sourced vacuum advance, no flat spot.
Since combo 1 has no flat spot, and 18 degrees initial, that rules out the need for more initial as the cause of the flat spot.