SBC timing at idle "walking" .. actually steadily climbing

[bessie66]

Hey all,

truck is: '71 c10 350/th350, eddy performer intake, headman headers, ngk plugs, holley truck avenger electric choke carb, HEI dizzy, etc

Issue is - i noticed shortly after installing the holley carb, that the timing at idle steadily increases as it warms up. The rpms climb to the point where it'd be violent if thrown in Drive. The timing will increase until I kill it. Initial is set at 10(with vac adv disconnected), yet i've watched the mark increase to 16-18 before i decided to shut it down, otherwise it will continue to climb as it runs.

Anyone else been blessed with this occurance?

Thanks y'all.

[RichardJ]

The idle mixture screws and idle speed screw should be adjusted after the engine has warmed up and the choke is off (OPEN).

It sounds like you are adjusting the idle speed with the engine cold and also the choke isn't working properly.

Timing advances, as rpm increases. That is how mechanical advance works. Mechanical timing advance begins to work at around 700-800 rpm depending on the distributor. For that reason, you re-adjust the speed screw to bring the rpm below that threshold in order to adjust the base timing.
As rpm changes when you adjust the mixture screws, you must again re-adjust the speed screw to bring rpm back down.

Carburetors are not Plug n Play.

[no1udknow]

I agree, warm the engine up to the point that the therostat is open before making any adjustments, including the timing. Also, is the timing advancing while the vac advance is hooked up? If so you may have the vac advance hooked up below the throttle plate. This would mean that the engine will constantly pull vacuum at idle which will continuously activate the vac advance as opposed to only doing it when the throttle is activated. If it is fkr sure above the throttle plate, then it is a matter of carburetor adjustment. The throttle plate is open too much at idle, but the fuel/air mixture may be too lean. As it warms up, the mixture becomes more acceptable, the engine ru s better, increasing the vacuum on the advance.

[bessie66]

The engine was/is timed correctly, has been for a while now; this is the fourth sbc I've torn-down and retimed. i just swapped out an eddy 600 manual choke for the holley. I had previously adjusted the carb while the engine was warm - had been idling in the garage for 5-10 minutes on a 71 degree day - choke open. The vac adv is hooked up correctly when it's hooked up. Throughout the years I've rebuilt, installed and adjusted a Qjet w/ auto choke to manifold, eddy elec 650, eddy manual 600 etc without issues ... this is first time i've encountered this issue.

I've also had a feeling it may a choke issue. The speed screw is all the way out.

[bessie66]

Also, I don't believe the choke disengages when the accelerator is "blipped". It doesn't drop. Perhaps I'm looking for a choke solution?

[davepl]

1) Fix choke to factory spec (or in your case to whatever the electric choke setup called for)
2) Fix choke pull-off to factory spec (bending rod to adjust door opening from vacuum can)

If you get (1) wrong it'll be useless until it warms up (unless you like two-foot driving and all of that).
If you get (2) wrong it'll fire up nicely when cold and then die.

The only reason your timing would advance as the engine warms up is if you have the vacuum advance connected improperly or the idle screw is opened too far to bandaid something else. Since you say the speed screw is all the way out, it's not the latter. There should be NO vacuum at wherever you connect it, at least once the engine is at normal idle speed.

I'm curious where the engine is getting air from if your primaries are completely closed though.

If you have vacuum, usually it's either (a) the wrong port, (b) you have a vacuum leak.

Static timing should be adjusted per the emissions decal. With today's fuel you'll likely need a couple of extra degrees initial. And again, vacuum advance is required for proper part throttle drivability but does nothing at idle.

[luvbowties]

Fast idle cam linkage, for on-choke operation, adjusted improperly, or hanging when you blip the accel. [and it refuses to slow down after it runs enough to allow slower idle at its semi-cold operating condition].

Sounds as this choke-cam linkage takes over and keeps it idling too fast--even with idle-speed screw all the way out.
Sam

[bessie66]

Great answers, thank you very much

[no1udknow]

Originally Posted by davepl

1) Fix choke to factory spec (or in your case to whatever the electric choke setup called for)
2) Fix choke pull-off to factory spec (bending rod to adjust door opening from vacuum can)

If you get (1) wrong it'll be useless until it warms up (unless you like two-foot driving and all of that).
If you get (2) wrong it'll fire up nicely when cold and then die.

The only reason your timing would advance as the engine warms up is if you have the vacuum advance connected improperly or the idle screw is opened too far to bandaid something else. Since you say the speed screw is all the way out, it's not the latter. There should be NO vacuum at wherever you connect it, at least once the engine is at normal idle speed.

I'm curious where the engine is getting air from if your primaries are completely closed though.

If you have vacuum, usually it's either (a) the wrong port, (b) you have a vacuum leak.

Static timing should be adjusted per the emissions decal. With today's fuel you'll likely need a couple of extra degrees initial. And again, vacuum advance is required for proper part throttle drivability but does nothing at idle.

Pretty much what I was saying. Timing is advanced as rpms increace to compensate for combustion lag. Although the engine is turning faster, the flame front of the combustion can only consume fuel at a certain rate ( which actually slows down with a higher octane rating). So in order to ensure the fuel is burned before the exhaust valve begins to open, the fuel is ignited as the piston is in the compression stroke, the faster the piston is traveling, the sooner the ignition needs to fire. It's like leading a target with a gun, the faster the target is moving, the further ahead of it you must fire.

[mechanicalman]

Originally Posted by bessie66

Also, I don't believe the choke disengages when the accelerator is "blipped". It doesn't drop. Perhaps I'm looking for a choke solution?

Perhaps, and likely. I'd also check the dist mech advance springs/weights to see if it might be advancing early.

Sounds like you have created a loop where the advance starts to creep, then the RPMs come up, then the advance moves more, then the RPMs come up, then the advance moves more.....til you get where you have been getting.

With the choke not opening, and with the idle speed screw all the way in, you are providing it with the fuel/air it needs to perform this feat.

Clear as mud? You might have a dist prob and a carb/choke prob. You said it started after the carb was replaced, and that the old carb had a manual choke so that may have been covering up a problem.

Did you replace the carb to try to fix a problem or did you just want an electric choke? I'm sure you hooked up the power and ground and it has not blown a fuse? No matter what, having the idle speed screw all the way in is not something that is helping the situation and is part of the problem.

[bessie66]

Hey mechanicalman, nope, i had stated that the speed screw has been all the way out - it was one of the first things i had done to try to tackle the issue.

[mechanicalman]

Originally Posted by bessie66

Hey mechanicalman, nope, i had stated that the speed screw has been all the way out - it was one of the first things i had done to try to tackle the issue.

Oh, I got it backwards, duh. Maybe there is a vacuum leak, it seems to be getting too much air?

Lost now. Any headway?

I'd consider trying the old carb if I still had it. Good luck!

[davepl]

If it weren't for the idle screw I'd assume this is the old "vacuum leak compensated for by opening blades too much thereby exposing the transfer slots and producing ported vacuum at idle" routine, but he's got it backed all the way out.

I'm still guessing the vacuum advance is connected to manifold vacuum and pulling the RPMs up way too much. But he'd need to connect a gauge to whatever port he's got it hooked up to and let us know.

[mechanicalman]

Originally Posted by davepl

If it weren't for the idle screw I'd assume this is the old "vacuum leak compensated for by opening blades too much thereby exposing the transfer slots and producing ported vacuum at idle" routine, but he's got it backed all the way out.

I'm still guessing the vacuum advance is connected to manifold vacuum and pulling the RPMs up way too much. But he'd need to connect a gauge to whatever port he's got it hooked up to and let us know.

It's a little unclear if this happens when he has the dist vac dis-connected, but I'm sure after he reads this he will let us know. I assume he tried it dis-connected, but every time I assume....

[davepl]

Are you thinking that the mechanical advance is already kicking in? Or what do you attribute the high idle speed to? My thought was the vacuum advance over-advancing the timing.

Normally with a vacuum leak you'd run so lean it wouldn't pick up the idle speed, but what if you had a significant vacuum leak and then accidentally compensated by having the choke closed, thereby fattening back up what air makes it through the carb? I wonder if you could get an engine to run that way, and if it did, if it'd idle fast with a large vacuum leak.

[mechanicalman]

Originally Posted by davepl

Are you thinking that the mechanical advance is already kicking in? Or what do you attribute the high idle speed to? My thought was the vacuum advance over-advancing the timing.

Normally with a vacuum leak you'd run so lean it wouldn't pick up the idle speed, but what if you had a significant vacuum leak and then accidentally compensated by having the choke closed, thereby fattening back up what air makes it through the carb? I wonder if you could get an engine to run that way, and if it did, if it'd idle fast with a large vacuum leak.

Well I think you are on to something with the vacuum leak providing the air by-pass and the choke not opening providing the fuel, causing RPM go up to where the mech adv kicks in, then RPM goes up more, then mech adv kicks in more and the cycle continues until it goes full advance and reaches what he refers to as way too fast to put into gear.

But we are still not sure, though I assume, it will do this with the vac adv dis-connected. Until then, I'm just speculating. I'd like to have him try putting the old carb on.

It's an interesting problem, and he says it started after changing carbs.

[68C15]

Do we know if the secondaries are closed and not stuck against gasket or manifold?

[mechanicalman]

Originally Posted by 68C15

Do we know if the secondaries are closed and not stuck against gasket or manifold?

Good question!

[no1udknow]

Originally Posted by 68C15

Do we know if the secondaries are closed and not stuck against gasket or manifold?

That popped into my head last night too. If he has the idle screw backed all the way out, unless he has a vacuum leak or the plates are stuck open, that motor shouldnt run. Honestly even the vacuum leak would still make it hard to keep running. Btw, its a holley??? I don't remember but do those carbs have vacuum secondaries or mechanical?

[71 super]

Check the distributer, I had one in my 71 bb that broke the tack weld at the advance plate and the timing was all over the place. The shaft was actually free wheeling in the housing. I was dumb founded as to how it cranked, but it is a possibility.

[davepl]

I considered the secondaries but ruled them out on account of the air doors - then remembered it's not a Q-Jet. It should be a Q-Jet :-)

[72 K20]

I got a holley carb from a buddy of mine many years ago and when I bolted it on and fired it up I couldn't get it to idle under 12-1400 rpm on a stock small block. Checked for vacuum leaks,,, all was good. Idle screw backed all the way out. It turned out that the throttle plates were not centered on the secondarys there fore not letting them close all the way. Once centered it would idle down .

Something to check.

[bessie66]

Hey guys thanks for getting into it haha. I have NOT had the vac advance connected at any point, yet.
Ive been wondering if i may have some linkage getting tied up...
Deffinately questioning the distributor too. Also, i will re-check.clearances, however i dont see a way that the blades could be getting caught -up; i have been wrong before!
If memory serves me it has vacuum secondaries.

Sorry for delay - my 3 month old son has been keeping me out of the garage lately... someday he'll learn.

[mechanicalman]

Originally Posted by bessie66

Hey guys thanks for getting into it haha. I have NOT had the vac advance connected at any point, yet.
Ive been wondering if i may have some linkage getting tied up...
Deffinately questioning the distributor too. Also, i will re-check.clearances, however i dont see a way that the blades could be getting caught -up; i have been wrong before!
If memory serves me it has vacuum secondaries.

Sorry for delay - my 3 month old son has been keeping me out of the garage lately... someday he'll learn.

Kids gotta come first, I respect your priorities. Trucks are cool, kids are your legacy.

If you don't find anything, I'd like to see you try putting the old carburetor back on for diagnostic reasons.