440 still idles with screws turned in

[Wietse]

Instead of drilling holes in your throttle blades, try the following first to see if you can get a good tune with more air.
I did this on advise of someone who knows a lot more about this then me.

Hook up a piece of hose on the big vacuum port on the back (pcv connection) with a 1/4" or 3/8" ball valve installed.
With engine running, slowly crack the valve open and feed it with more air until it idles better.
Now play with timing and idle screws all you need.
Later on you can make an educated guess of how much sq.inch of surface the valve shows open and re-calculate that to 2 holes size to drill your throttle plates.
Start drilling small

 

[Geoff 2]

DS,
Post #59. Didn't miss any point. So I will try to address your 'points'.
[1] We know everything we need to know about the patient: it is wheezing from covid [ rough idle, low vacuum ].
[2] The fix has been prescribed by me & by others much smarter than me in post #50. I can find plenty more examples if nine is not enough.
[3] You ask why does vac increases when you advance timing. I presume you mean idle timing & that is what I will answer. Vac at idle is a measure of how well the inhaled mixture burns. A stock engine has a smooth idle because the combustion is good & as complete as it can be. A rough idle due to a big cam that has poor combustion at low rpms [ improves at higher rpms ] & that reduces idle vacuum.
[4] Yes, you might say a mismatch between cam/carb. But the carb company doesn't know what cam you will be using with it's cam so it sells the carb in basic form WITHOUT drilling holes in blades & rich idle settings, & leaves that to the tuner. I doubt the owner is going to remove the big cam he just installed to go faster because it is a 'mismatch''; he will tune around it. And here we are...

I have lost count over 30 yrs of how many off idle flat spots I have fixed by adding MVA. There are two reasons & two things at work here. Once idle timing is optimised, whether it is using MVA, locked dist, ign box, etc, the t/blade position can THEN be adjusted for correct t/slot position. No, you do not want the t/blades completely closed [ these incorrect positions & problems that occur are discussed in the Holley book & many others ]. Ideal is just a smidgin of t/slot showing. This leaves more of the slot for the crucial off idle transition. So number one: t/slot position is correct. Number two: the engine is making more HP at idle with the correct amount of timing. When the throttle is opened slightly to take up the load, the engine already has more hp 'there' to take the 'hit'. There is not a time delay like there would be if some/all of the PVA has to activate first to carry the load.

Also, the IFR will likely need to be enlarged for big cams.....but not for the idle mixture; might come a a surprise to some. There is ALWAYS enough range/adjustment on the mixture screws for proper idle PROVIDED the t/blades are correctly located. IFR also supplies the t/slot for off idle fuel. When more air has been added via drilled t/blades or other means, more fuel is needed to correct the A/F ratio. Sometimes on some carbs the idle down channel [ also called the Economizer ] needs enlarging as well.

Don't know how many times I have to say it: the internal curve in the dist comes LAST....not first. Because obviously what ever initial [ before MVA is added ] is chosen is going to affect the centri curve. MVA or PVA goes to zero at WOT & at WOT is out of the picture.
A few weeks back I was watching a YouTube video of an engine builder who said EVERY engine should have a minimum of 35* init timing. He gets the concept, but 35* might be a bit much for smaller cammed engines. Tried to find it again but have been unable to. My computer skills are pretty poor [ too much time doing engine tuning! ] so wouldn't know how to link it. If somebody can find it, please post it.
Why don't you [ or anybody else reading this ] try the simple test in post 57 & give yourselves a pleasant surprise.

 

[turbine68rt]

Here is an example & close to what I would use for your car: 16 init, 30 centri, 18 MVA. Car idles with 34*. Cruises with 34 + part/all of 30* centri. At WOT, MVA goes to zero so engine sees 36*.

If you have 16° initial and 30° centrifugal, at wot the VA drops out which leaves 46° all-in. How are you coming up with only 36° at wot?

 

[Dan64]

Just throwing this out there, not saying it would be a good/ bad thing and I'm sure many will ask "Why" but have any of you run a centrifugal lock out distributor WITH vacuum advance to add another 15 or so degrees for part throttle cruise?

 

[Wietse]

A rough idle due to a big cam that has poor combustion at low rpms [ improves at higher rpms ] & that reduces idle vacuum.

I think you forgot to mention that it is the late ICA that squeezes a lot of air back to the intake as a source of low vacuum?

My cam gives about 6" vacuum, advancing the timing increases efficiency, very true, but it also increases rpm...more rpm is more vacuum.
The ICA is not going to change, so it keeps blowing back to the intake.
I had the same issue, due to the throttle is too far open and there is no reaction on the idle screws, but running on a fat mixture of the transition slot.
The blades were that far open that it was even putting vacuum on my Ported vac signal line at idle. (i have a TC that is not helping either as it puts too much load on the engine at idle)
Maintaining the same rpm I have barely seen any improvement in vacuum while adjusting initial timing, adding a Vcan just makes my engine ping at light throttle, ported/manifold, it doesn't matter.
A carb is set to meter x amount of air with x amount of fuel, if you engine requires more air to idle well you will have to get it from a bypass and not from the carb throttle blades. (square looking transition slot below the blade is a guide line)
So, a PCV (stock or adjustable) is an alternative source of air. Some carbs have adjustable by-pass screw below the filter stud in the center to add air.
Added air means you need to add more fuel, and that is were you use the idle screws for.
And if you set the throttle blades as they should and you added additional air through a bypass already, it will react directly on idle screw changes as it will need it.
Set your initial advance is to just get the engine to idle normal, Initial + centrifugal is WOT and a Vcan is for anything in between idle & WOT.
Getting the engine to the point where it is getting the air it needs with the fuel to match is first thing, not adding 30 deg of advance to let it idle to compensate for the lack of air and fuel it needs.

 

[Geoff 2]

Turbine,
Post #63 you are correct, I made a mistake. Centri curve should have been 20*.

 

[Geoff 2]

Dan,
Post #64. Very good suggestion. That is actually what I have, but detonation can be a problem with today's crap gas. I have an IR system on my car, four downdraft Weber 48 IDFcarbs, only street driven one in the world that I am aware of.
My dist is locked @ 35*, & MVA adds 13*. So idles with 48*.

 

[RemCharger]

Dan,
Post #64. Very good suggestion. That is actually what I have, but detonation can be a problem with today's crap gas. I have an IR system on my car, four downdraft Weber 48 IDFcarbs, only street driven one in the world that I am aware of.
My dist is locked @ 35*, & MVA adds 13*. So idles with 48*.

4 cyl?

 

[Geoff 2]

Wietse,
First sentence is correct, last sentence is wrong. Back to the first sentence. ICA, I presume you mean Exh closing. I just gave the description for low vacuum in general terms, rather than getting overly technical.
P. 113 of the original Holley book by Holley engineer & later VP Mike Urich covers it well:
' The use of retarded spark requires richer jetting in the idle & main systems to get off idle performance & drivability'.

And:
' Because retarding the spark hurts efficiency, the t/plate must be opened further at idle to get enough mixture in the engine to continue running'.

So, sounding like a broken record: you give the engine the idle timing it wants FIRST & tune ign/carb from there.

GM used MVA. Pontiac & Chev used it, probably the others. My 66 GTO was driven off the showroom floor, idling at 26*. Chevs had about 24*. When my son in law comes over, I will post the relevant section of the FSM so the non-believers can see it. My GTO had 6 init + 20* MVA. With a mild cam that was 197 @ 050. And that was with 10.75:1 CR too.
So suddenly idle timing of 30+* for a hot cammed, low vacuum engine, doesn't sound so high does it???

As far as pinging with VA. Once the reqd amount of MVA has been determined, you need to make a stop to limit VA to that amount. The VA unit should be installed with Allen Key fully CW [ softest spring setting ]. With the engine idling, in gear if auto, check the timing & note. Turn AK 2 turns CCW & recheck timing. Keep doing this until timing wavers or drops. Then go back 3 turns CW on the AK, & the idle timing is set. MVA provides cooler engine running, so the risk of pinging is reduced using MVA because engine heat exacerbates pinging. Other fixes in stubborn cases is to use a stronger spring in the centri curve to delay it. Or reduce static initial timing slightly & make up the difference with more MVA.

 

[Geoff 2]

No, V8, 455 ci. PM an email address I will send pics.

 

[70bigblockdodge]

Double check float level because it's a new Eddy carb.
Put more initial in it till it idles better and then figure out what to do to limit total timing.
After a couple of seasons you can make plans to change cam to something less terrible.
Yes, I have this cam in a 400, makes it even worse. If it was my only car I would have it out already. Sorry to break it to you, but that cam needs a ton of compression. If your building that much compression why use such a small cam. It is a never happy cam, except 4000 plus rpm.

 

[Wietse]

First sentence is correct, last sentence is wrong. Back to the first sentence. ICA, I presume you mean Exh closing. I just gave the description for low vacuum in general terms, rather than getting overly technical.

I mean Intake Closing, the bigger the cam the later it closes. (Guessing from his cam specs given i guess a closing angle of 70-75deg ABDC)
The later it closes, the more volume of air is being pushed back into the intake...cause loss of vacuum.
The later it closes, is less dynamic compression ratio which results in less power.
Why you would want to tune an engine with additional ign. advance if it is obvious it screaming for more air and fuel.
You need to feed it with what it needs first, then find a good time to light it up.
That fact that it needs more fuel and air is already proven since OP has his throttle blades that far open that he is idling on a lot of transition slot.
Too much fuel flowing in and lack of air, he will have a stumble at some point as he will run out of transition slot before he is on the main jets.
Most 440's will run no issue with anything between 16-20deg initial timing.

 

[Dragon Slayer]

Weiste nailed it, and he explained vacuum. The engine is the pump, the efficiency of the combustion changes the rpm. As it rises given everything else stays the same (carburetor blade restriction), vacuum rises. That could be from timing, or it can be from mixture. I am sorry Geof, but you sound like you have a solution and every problem is the problem your solution answers.
I believe you need the right carburetor to work with your combo. So you get the fuel and air mixture right. Carter gave tune up data on their technical forms, because things like engine vacuum, timing, and knowing the engine specification from the manufacture was necessary to calibrate the carb to meet their needs. That universal Edelbrock can't physically be calibrated out of the box to cover an engine with 6" of vacuum and one with 18" at the same time. Idle mixture screw can't do that. So now your talking about drilling restrictions that are hidden, and require removing plugs and such. Most of the books you quote tell you not to do that. So unless you do have a dyno and lots of experience your just going to mess up a carb.
Can you tune a car using MVA, YES. Can you tune one using Ported, YES. But if you need 40 degree or more of initial timing for idle, why is that car on the street in that combination?
Weiste is also correct about the compression. Higher compression motor can create more vacuum because the chamber has less air to start.

At this point to make it work, you want to modify the distributor, modify the vacuum can, and heavily modify the carburetor. OR we can just swap the cam maybe?

 

[4406bbl]

A cam swap to something ground on 112 center would be best, but he can make this livable. 16-18 on the crank, 20 in the dizzy, and do whatever to get the idle screws 1-2 turns out, 1 being ideal and the throttle blades closed. The transition slots will still be exposed slightly, and you want that. I would never try to get this combo to idle on the vacuum can, others would, it will lead to a miserable and disappointing experience with a 6-8" of vacuum engine. As long as you start small on your drilling and increase .005 at a time on blades, and .001 on idle restrictions IF needed, you will not ruin your carb. I also do not like the vacuum can fix because it will fail long before a dizzy goes bad, or a good carb has an issue, it is a piece of rubber in a shitty hot enviroment. I agree with others at the same rpm this likely low compression, long duration engine will have the same vacuum at idle, no matter how much advance it has. The one thing the vacuum can will do for this combo is give it big fuel mileage gains at steady rpm on the highway. I do not agree with the 60 degrees some say, never more than 50 on a big block mopar with normal heads, small blocks less, swirl heads less, gen3 hemis less. The only big block heads that might tolerate more would be the motor home peanut plug, extra cooling for the spark plugs heads and even then nowhere need 60 degrees. As a cheap start maybe pull the timing cover and degree the cam, advance it 4 degrees? It should be ground on a 108 intake center.

 

[4406bbl]

Now to elaborate a little on steady rpm timing. Example 1976 new yorker stock 400 leanburn, it would get 19-20 mpg, 65-70 mph 21-2200 rpm on flat ground in 1980. I removed the lean burn, switched to an electronic dizzy, 38 total and a vacuum can that could provide an additional 20 degrees on the crank. It would high speed mis with too much timing, this engine had better heads, smaller cam and a better cooling system than any muscle car of the day, and it could not accept much more than 50 degrees of total timing at 190 water temp, without detonation at all times over a long term, more than 100,000 miles, adding fuel never made that go away, only backing down the timing. I had a vacuum heater valve control on the dash from an older Chrysler so I could control the timing and the best mileage without detonation with a leaned out thermoquad was 27, 25 was pretty normal, you could not pull grades or pull big headwinds with the timing that high. This car was also quiet so you could hear detonation, our muscle cars are not. I can't tell you how many times I had the carb apart, swapped and limited vac cans at rest stops and campgrounds to get it to that point. Stopping the misfire always required the timing being backed off.
Chrysler printed a fuel mileage pamphlet in the late 70s, engineers I talked to at shows even said no more than 50, some things in print said 56 degrees on a big block, 50 on a small block, I say bullsheet unless it is below zero outside. I even tried a modded aircleaner with 2-4" tubes mounted to scoops under the bumper...a big block open chamber head likes about 48 degrees, 50 max in real word conditions. My point to all this is when people make blanket statements about Chrysler missing the boat on manifold vs ported vacuum advance I call bullshit, they had to produce a product that was safe to operate under all conditions and not blow the pistons or rod bearings out of it. I also call bullshit on guys that mod the dizzy and vacuum advance then drive across town to a car show and think they are heros, stick a trailer behind it and drive 2000 miles thru the desert and over the rockies only stopping for gas and then maybe I will believe the manifold vacuum 60 degrees total timing myths.

 

[Wietse]

To get back to OP his issue:
He obviously has the throttle too far open, the only reason it will continue to run with idle mixture screws closed is the fact it is running on transition slot fuel.
Engine needs the air to stay running so you have to give it but by other means then through the carb bores, like I suggested earlier to install a piece of hose with a small ball valve so you are able to control the additional air you are feeding it.
Remove the carb and re-set your throttle to show only a little rectangle or square transition slot below the blade. (make sure it is on curb idle and not on choke cam)
Set the idle mixture screws 2 turns from their seat.
Reinstall and run the engine, let it warm up a bit. you will have to keep the throttle open by hand likely as it will stall if you don't.
Now start cracking that valve bit by bit, you should reach a point where you no longer need to assist to prevent engine stall.
At this point it is a matter of playing with the idle mixture screws and the valve, keep in mind if the rpms go up it means you are going in the right direction.
You could check the vacuum to see for any improvement at the same rpm, look for 0.5 or 1" because it will not be more than that if at all.
If there is a point where you can't get the idle to come down to your liking, but the engine runs strong, retard the distributor a little and rpm will drop.
What ever you need, rpm up or down, you can play with the ignition advance and the idle mixture screws but do not change the carb idle screw.
Another thing is to ensure your PCV valve is in good condition, make sure the check valve (ball) is free, the PCV is also a source for adding bypass air but it must give a constant restricted flow.
Whatever the initial advance is (12,14,18), don't care about that for now but stay below 20-22*, once the idle is set you can recurve the distributor to set your mech adv. for WOT. Probably around 36* total.

 

[Geoff 2]

[1] If you guys care to have a look at the post above in this section about dual Quad 383 Offy you might learn something. Post #10 has the Edel tuning manual. At the top of page 7, in the procedure for idle with long duration cams it says to USE PLENTY OF IDLE TIMING &/or CONNECT THE VAC ADV UNIT TO MANIFOLD VACUUM.
[2] Quite agree that more compression increases idle vacuum. That means an engine rebuild & a lot of expense. The OP wouldn't be the first one to be in this low vacuum predicament, but he will probably decide to soldier on with what he has.
[3] 6bbl, sorry but a lot of nonsense in post #74 & 75. Where to start. We are talking about mostly hobby cars, not daily drivers, so the VA unit will probably outlast the driver. If it has an elec dist, the p/up wires will probably fail first from the continuous flexing before the VA unit. And your reading skills need a brush up. Back in post #50 I gave examples of idle vacuum increasing with more idle timing.
By increasing idle timing by whatever means, this engine will be able to idle at a lower rpm. Typically with a generic 15-20* at idle, the engine might need 1100 rpm for comfortable idle. Comfortable idle: the lowest rpm it will idle at & be happy. Add another 20* via MVA, & it will idle at a LOWER rpm & smoother. Hard to put an exact number on it but without MVA 1100, maybe 1000 rpm with MVA. Just because your emissions era car pinged & misfired, doesn't mean the OPs car will. Further adjustment may well have fixed your problems. You mention bullshit, well it is bullshit to claim that MVA causes rod brgs & pistons to 'blow out'. Once past idle where the engine really is loaded, MVA & PVA act the same. And more bullshit: Chrys did use MVA. From the early to late 70s, Chrys & other manufacturers used MVA to reduce engine temps....by increasing idle speed. Increased idle speed sound familiar? When the engine reached a certain temp, a temp switched from PVA to MVA.
[4] The LS engines. Very well designed, efficient combustion chamber. Not sure about later LS engines, but the earlier ones were set at 27* WOT. So, 5* at idle? nope, 22*. MVA alive & well in electronic form.
[5] Stay tuned. Will be providing more MVA success stories.

 

[Wietse]

Quite agree that more compression increases idle vacuum.

There are 3 things that affect manifold vacuum, you even might disagree but these are still facts....
1) Throttle plate position
2) Engine rpm
3) Compression

Nobody mentioned a rebuild here to increase compression, dynamic compression comes from the cam and it is installed and he has to deal with that now.

Typically with a generic 15-20* at idle, the engine might need 1100 rpm for comfortable idle. Comfortable idle: the lowest rpm it will idle at & be happy. Add another 20* via MVA, & it will idle at a LOWER rpm & smoother

How are you going to increase this idle timing on said engine?
You have almost no vacuum, you physically cannot pull in 15-20* of advance with the Vac adv....
Don't need to tell you what is going to happen when you set your initial to 30* or more I hope? Yes, it will idle very strong.

maybe 1000 rpm with MVA

And then you throw the selector in Drive......

 

[Geoff 2]

Wietse
[1] Post #71, not me, although I agree with the sentiment, said the engine needs more compression. 'That cam needs a ton of compression'. The only way to do that is to change heads....or pistons. Most would consider changing pistons...a rebuild. I also said the same as you, owner will probably decide to live with the cam & tune around it.
[2] Vacuum. T/plate position is the culmination of a number of other factors that I covered earlier in this thread.

Engine rpm also dependent on other factors. Would be nice if this engine would idle at 400 rpm, but it is too low & engine would stall.

I said at the very start of this thread that I thought the vac seemed too low for this cam. That brought two responses [a] there were some faulty cams that this was normal vac for this cam. Take your pick. I also suggested checking to make sure the cam was not installed retarded &/or to advance it 4*. This would be done BEFORE anything else.
Glad you agree it will idle 'very strong' with 30 or more degrees. That is what I have been trying to convey.
The Accel HEI adj VA unit can add 21* @ 8"of vacuum so I see no reason why the above would not work.

 

[Geoff 2]

As promised, GM info on MVA as used in production. 1967 Pontiac FSM. Note 20*[ 10* dist ] of MVA added.