Thermoquad tuning on 318

[remytherat]

Right now there are two issues I am facing: stumble right off idle and major bog when secondaries open. Idle mixture screws set for highest vacuum (21" Hg). Accelerator pump in either position did not affect situation. Metering rods adjusted 2 rotations from bottomed out. Air valve wound up about two turns. When given gas in neutral, throttle response is very good and secondaries open perfectly. In drive there are the above problems. Off idle stumble can be driven around by very slowly easing onto the throttle, after which the car acts normally. When the peddle is punched to the floor, the car dies and bump starts itself when foot is taken off the gas. When the secondaries are eased into over the course of 10 seconds or so it runs very well.

Engine info

Original unmodified 1969 318
Stock cam
1972 340 intake
1972 small primary thermoquad
Stock advance curve, ~12˚BTDC initial timing
2 1/4" h pipe exhaust

 

[AJ/FormS]

sounds like the accelerator pump is AWOL and or the fuel level is too low, and/or the low speed circuit has a problem. Check for a missing checkball in the pump circuit.
if it was me I would disable the Secondaries first then retest, and leave them wired shut until you resolve this tip-in issue.
BTW on a 318/TQ, if the secondaries open in neutral, the Airdoor is most likely set too loose.

 

[remytherat]

Accelerator pump has a pretty strong shot, and I set the float level to spec before installing the carb

BTW on a 318/TQ, if the secondaries open in neutral, the Airdoor is most likely set too loose.

Good to know. I didn't realize that. Maybe wind it up 3 turns. 4? What about a vacuum restrictor?

 

[Bewy]

I gave you some tips in your other post. Did you check those out? Happy to help, but not when you advise folks to check things.....& they don't. That wastes my time & everybody elses'.
[1] off idle stumble. Engine idling, gently close the choke until the engine sound changes. Wire/lock the choke blade in this position. Test drive. If stumble is better or gone, mixture is lean.
[2] Sec bog. Does the dash pot [ pass side, rear of carb ] pull in at idle? If not, it is defective. It can still be defective if it pulls in if it is old. Rubber inside goes hard & is slow to release.
To test: remove connecting link & wind up AV spring to 3 turns. Test drive.

Report back with results.

 

[RustyRatRod]

Happy to help, but not when you advise folks to check things.....& they don't. That wastes my time & everybody elses'.

Get used to it, bucko. That's how it is around here. lol

 

[RustyRatRod]

Right now there are two issues I am facing: stumble right off idle and major bog when secondaries open. Idle mixture screws set for highest vacuum (21" Hg). Accelerator pump in either position did not affect situation. Metering rods adjusted 2 rotations from bottomed out. Air valve wound up about two turns. When given gas in neutral, throttle response is very good and secondaries open perfectly. In drive there are the above problems. Off idle stumble can be driven around by very slowly easing onto the throttle, after which the car acts normally. When the peddle is punched to the floor, the car dies and bump starts itself when foot is taken off the gas. When the secondaries are eased into over the course of 10 seconds or so it runs very well.

Engine info
6
Original unmodified 1969 318
Stock cam
1972 340 intake
1972 small primary thermoquad
Stock advance curve, ~12˚BTDC initial timing
2 1/4" h pipe exhaust

There is more than one hole in the accelerator pump arm that attaches to the top of the carburetor. Make sure the rod going to the throttle linkage is in the hole closest to the carburetor body. That increases the pump shot. Also, FWIW, 12 degrees initial I would treat as a minimum starting point. That engine is low compression and can stand a good bit of initial timing. Maybe pull in to 16 initial and drive it. As long as the engine continues to idle up as you pull in timing it's wanting more. You may of course have to readjust the idle speed when you raise the initial timing. It's needing more initial timing because it's trying to inhale a bigger gulp of air through a better flowing intake and carburetor.

 

[remytherat]

I gave you some tips in your other post. Did you check those out? Happy to help, but not when you advise folks to check things.....& they don't. That wastes my time & everybody elses'.
[1] off idle stumble. Engine idling, gently close the choke until the engine sound changes. Wire/lock the choke blade in this position. Test drive. If stumble is better or gone, mixture is lean.
[2] Sec bog. Does the dash pot [ pass side, rear of carb ] pull in at idle? If not, it is defective. It can still be defective if it pulls in if it is old. Rubber inside goes hard & is slow to release.
To test: remove connecting link & wind up AV spring to 3 turns. Test drive.

Report back with results.

I currently don't have the choke installed on the carb, although I could put it on for this. I went through the other tips on your post: no leak through the boosters, set timing to your recommendation for the moment. Sorry If I didn't make that clear

Maybe pull in to 16 initial and drive it. As long as the engine continues to idle up as you pull in timing it's wanting more. You may of course have to readjust the idle speed when you raise the initial timing. It's needing more initial timing because it's trying to inhale a bigger gulp of air through a better flowing intake and carburetor.

I was wondering why it seemed to want a lot more timing...I'll give that a try, although probably won't have time to mess with it till wednesday.

Thanks for the help everyone. No idea what I would do otherwise

 

[Bewy]

Agree, it could use a little more timing, but with 21" of vac, 12* init should be ok & I doubt more timing will fix the problem. That seems a veeeeeery high vac #. Is the idle dead smooth, it should be?

Is the off idle stumble just with a small amount of throttle movement...or when you stab the pedal?

 

[remytherat]

Is the idle dead smooth, it should be?

Yes. Maybe just the tiny cam?

Is the off idle stumble just with a small amount of throttle movement...or when you stab the pedal?

Sort of both: when you ease onto it there is a flat spot then it picks up. When you stab it from idle to about 1/3 or 1/2 throttle it starts to die then about a second later surges pretty hard. Steady cruising at 1/4 throttle and 55mph it does fine. Its only when I actively open the throttle, almost like the carb is catching up with my input

 

[CFD244]

Great ideas

 

[AJ/FormS]

Accelerator pump has a pretty strong shot, and I set the float level to spec before installing the carb

I know that TQ pumps do pretty good, but I'm not talking about hammering it.
Your carb has multiple tip-in problems, which at your vacuum level, should not exist. Your low speed system is too slow to get moving. The A-pump is usually slower yet when tickling the throttle.
Really, the problem boils down to the AFR dipping lean at Tip-in, so that means, as explained initially, the fuel is lagging.
So then, it could just be a low wet fuel level, or air getting into the idle well that was not designed to be there.

In addition to what I said in a previous post, I will now add this.
If your intake-plenum is getting air anywhere other than past the primarys plus a trickle from the PCV valve, then, to keep the rpm down, you will close the throttle with the speed screw. This has the unhappy result that the transfers also shut off. In response to the rough idle that then occurs, you will open the mixture screws, and then it idles just fine.
But when you step on the pedal, the transfers lag, the AFR goes lean, and you feel it as a hesitation.
But you say, the vacuum is 21 inches.
To which I say, yeah so? All that means is that your throttles are very closed. Reinforcing my point.
I am NOT suggesting that your intake is sucking air.
I'm suggesting that If the WET fuel level is correct, then, either the transfers are stalled, or unregulated air is getting into the intake from a more local; source; such as
> the secondary throttle valve is not shut tight, or
> the base gasket is not sealing, or
> the PCV is flowing too hard, or
> a throttle shaft is really baggy, or
> the booster is sucking air, or any other accessory is, or
and so on
BTW
stale gas will do a similar thing because all the light weight molecules have evaporated. This leaves only the heavier molecules behind which are not particularly eager to burn. Your gas must be clear. Faintly yellow can already be troublesome.

And finally, too much Idle-advance while making the low rpm snappy, has a side effect.
The extra advance increases the idle-power, which increases the idle rpm. In response, you will crank the speed screw out, which then reduces the fuel coming from the transfers, and there we are again chasing our tails.
NOT ALL carbs are able to deal with extra advance.

BTW; your metering rods do little to nothing to eliminate tip-in sag. Tip-in sag is strictly an Idle synchronization issue between; the transfers, the mixture screws, and the wet fuel level.
A bog after tip-in, or more like a flat spot, as the rpm increases under light acceleration, now that would be Power-circuit issue which does involve the Metering rods and specifically how far out of the M/Js the mechanical lifter has lifted the tree.
To solve a primary bog under more spirited roll-ons, with 21 inches of vacuum, yur probably gonna need a different metering-rod spring.

 

[GTX JOHN]

Try tightening Air door.

Bend accelerator rod to adjust timing of
squirt.

 

[remytherat]

So then, it could just be a low wet fuel level, or air getting into the idle well that was not designed to be there.

I will try raising the fuel level

In response, you will crank the speed screw out, which then reduces the fuel coming from the transfers, and there we are again chasing our tails.

To make sure I am understanding correctly, some transfer slot exposure at idle is desirable? Doesn't this mean I want less initial timing?

Bend accelerator rod to adjust timing of
squirt.

Ok sounds good.

 

[RustyRatRod]

WHEN does the pump squirter start to spray fuel? It should start immediately when the throttle lever is moved. If it does not, it should be adjusted correctly.

 

[Bewy]

The off idle stumble: you could have TWO problems.
[1] Just adding a little throttle to move the car
[2] When you stab the throttle.


[1] Remove the carb & check how much T slot is showing below the blades 0.020 - 0.040" is ideal. If NONE showing it indicates an air leak OR this carb has bypass air holes; these could be in the carb base, in the pri blades, or both. The symptom is a small flat spot because most of the t slot is above the blades & acts as an air leak, leaning the mixture. The fix is to plug the holes. If there are two sets of holes, plug one set & retest. Holes in the blades can be sealed with pop rivets [ may have to drill the holes to 1/8" ]. Holes in the base can be sealed with welding/coat hangar wire. Peen the alum top & bottom with a punch to hold in place.

[2] Large throttle opening/flat spot is leanness. Remove the met rods & test. This will add a lot more fuel.

[a] What is the carb model #? It is stamped on one of the mounting feet.
These carbs have a lot of internal passages/orifices that can be plugged with debris or get corrosion from sitting around. No way to really tell.
[c] Report back.

 

[AJ/FormS]

I will try raising the fuel level


To make sure I am understanding correctly, some transfer slot exposure at idle is desirable? Doesn't this mean I want less initial timing?

Ok sounds good.

1)I didn't say to raise the fuel level! It may already be correct!, Only change it if it is not correct,
2) absolutely yes the transfer slot should be exposed to be looking about square under the blades. and adjusted like that, the mixture screws should be about 2.5 turns out.
#) For highest vacuum, your engine will want idle timing of about 25 degrees.
With the T-slot synced up, obviously the rpm will be outta sight, and the trans is gonna BANG! pretty hard when selecting a gear; so you can't run it like that.
Since you cannot optimize the Idle-timing anyway, then you can run whatever Idle timing the engine will not bang the trans at and still not have the dreaded tip-in sag. Beyond that, the engine does not care what the Idle timing is.
If the engine is stock, you should always start your IDLE-tune, with T-slots synced and factory timing, which, on a stock 318 with a 2bbl carb was always zero to 5 degrees. Once the Tip-in sag is gone, THEN you can increase the timing. But if the Idle-rpm climbs too high then it will bang the trans. So you gotta find a happy medium.

Now; that happy medium will change with the Convertor stall. The higher the stall, the less it will bang. but; the higher the stall, the less important the lowspeed timing is. So if your convertor is stock, with a stock cam and compression engine you may have a 1700 stall, which means if the Idle becomes too powerful, because of extra timing, then the engine will be tugging to go all the time. Having to sit on the brake pedal All the time, gets old in a hurry. Whereas if you had a true 3000 stall, you could almost run the timing locked right out. So again, Idle-timing is a compromise.

The only good reason for a streeter to run a lot of Idle timing, is if the distributor has a straight slow timing curve and no vacuum advance. In this case, if you don't run the numbers then the low-rpm power gets sluggish.
Over the years I've been here, (not that many) @Mattax has published a lot of timing curves and a lot of smart advice.
But in a DD or rice burner, there are better ways of waking up the low-rpm power, than excessive Idle-timing.
In your case, with a stock convertor, a true 12* at 550/600 rpm in gear, is, IMO, already borderline too much Idle-timing.

Btw, I once had a manual trans 292/292/108 cammed 11/1 Scr 367, that would pull itself, with correctly synced T-slots, along at 550 rpm with the timing pulled down to 5 measly degrees of advance. I still have this combo minus that cam, but now with a 230/237/110 cam that will do the same thing, down to 500 rpm.
The point is this; once the Wet fuel-level, to T-slot, to mixture-screw setting are right, the engine does not need a lot of Idle-timing; AND the earlier your intake valves closes, the less Idle-timing she will want.
In your case the stock cam is advertised as 240/248/110 and installed straight up the Ica is just 50*. That is a pretty small number.
Performance street cams run in the range of 60 to 68/70 or so, and a weekend warrior might run up to the mid 70s, possibly a lil more. Each bigger cam will like a lil more idle-timing, but it's also gonna like a lil more stall.

 

[remytherat]

What is the carb model #?

6139SA

I will follow your steps tomorrow evening

 

[RustyRatRod]

Damn.

 

[Bewy]

AJ,
You are supplying contradictory info which is not helping. You say his engine will want about 25* of idle timing...& then in the same post you say 12* could be borderline too much? Here in Oz we call that having a bet each way!!
Idle timing is a compromise? No it is not. There will be a correct amount for each engine.

As for the engine tugging with a stock type c'ter.....lower the idle speed!. You can do that with this engine because of the stock, low duration cam. It makes more HP at idle rpms than long duration, lumpy cams which require higher idle speeds; this extra HP allows idle speed to be lowered & engine able to carry the c'ter load.

 

[AJ/FormS]

You are supplying contradictory info which is not helping.

Listen, I know you're a smart man, but if you can't then neither can OP.
So then, for the OP;
as to 25*,
this is easy, just tug on the distributor until the Idle speed no longer increases, and that is what she likes.
and as to Idle timing:
Take the carb off. Set the T-slot exposure to square, and the mixture screws to 2.5 turns out, then reinstall the carb. Start it up and back the distributor up until you get the idle speed you want, that doesn't bang into gear.
Now twiddle the mixture screws for best idle quality never mind what the vacuum gauge says.
Next, count the turns out.
If the screws are more than 2.5 turns out then the Transfers are not supplying enough put the screws back to 2.5 and increase the Slot exposure until the idle quality improves, then retard the timing to get the rpm back down. But
If the mixture screws are less than 2.5 turns out, then the transfers are sending too much, so put the screws to 2.5, and close the transfers a hair, then reset the idle speed with timing.
>now comes the test; looking for elimination of the tip-in sag, and reduction of transmission banging into gear.

 

[AJ/FormS]

Idle timing is a compromise? No it is not. There will be a correct amount for each engine.

I beg to differ.
I can set my t-slot exposure and sync it to the mixture screws, to eliminate the Tip-in sag, and then vary the timing anywhere between 5* to 20* just with my dial back electronic module, from the driver's seat, and with a manual trans drive it like this, without a tip-in Sag.
If I want to, I can mess the exposure up, and if the tip-in sag is not too bad, I can still drive it. Or
If I want to, I can fatten up the Idle mixture until my eyes cry uncle , and still drive it like that.
> at idle, you can mess the heck out of these adjustments, and still get it to idle.
Driving it is another matter totally.
With an automatic, the Tip-in sag has got to go. And if you use pump-shot to do it, or a rich mixture-screw setting, you will pay for that choice every time you fill the tank. Just bite the bullet and give the engine the Idle timing that the T-slot sync dictates.

Idle-timing is ALWAYS a compromise because,
the tuner is always trying to get to>>>>>>> whatever the maximum Power-Timing might be for your combo, as early as possible without getting into detonation .................. from; whatever your Idle Timing is.
If your Idle Timing is 10 degrees and your earliest Max-timing is 36* at 3600 rpm, then you mod the distributor to connect the dots. This comes to a curve of about @26* in 2600 rpm = 1*per 100, thus 10+8= 18 measly degrees of power-timing, and with this curve, the low rpm power might really suck.
The tuner might be able crank the idle-Timing up to 18 degrees, and now the curve is going to need to be, still 1*per 100, which means at an 1800 rpm stall, the engine will see 18+ 8= 26 magically delicious degrees.
But if this new number causes detonation, then back to the distributor machine you go, and with less Idle-Timing to start with.
This is how compromises are made.

 

[remytherat]

Honestly I think both of you are getting at the same point that the issue is with the lack of adequate transfer slot open at idle. I will take the carb out and set it to 0.30" with the mixture screws 2.5 turns out. Then i'll retard the timing to get the desired idle rpm. If it is too retarded that means there are bigger issues. Didn't notice any holes in the throttle plates but I haven't had the time to take the carb off. Will probably do tomorrow.

 

[RustyRatRod]

Damn.

 

[Bewy]

AJ,
Tug the dist until the idle speed no longer increases....& that is what the engine likes. Your words....& they are correct. In total agreement. It is also the correct timing for THAT engine. There is no compromise for the correct amount of idle timing. If the timing is changed, then it has been compromised...& it was a bad move.

Sorry, more misinformation. If you get detonation, you put stronger springs on the dist weights to delay ign advance. You do not reduce idle timing.

Reset idle speed with timing? Don't think so. The number of turns out on the mixture screws is going to be a product of idle timing, idle vacuum [ cam size ], engine size ...& the carb itself. Don't know where this magic number of 2.5 turns comes from. The number of turns will be whatever provides the smoothest idle. What is imperative is that 0.020-0.040" of t slot is exposed at the desired idle speed with the correct idle timing. This is how the carb is designed to operate & provide the correct see-saw action of T slot operation for a smooth off transition from the idle cct to main cct.
Some combinations will require bypass air to achieve this.

 

[remytherat]

Took the carb off. Took off the top and it turns out the floats were significantly too low. Fixed that. Then I set the idle screw to where there was about 0.020" of transfer slot. Tightened down the pipe plug over the vacuum port which may have been causing a vacuum leak.

Very slight stumble when cold, but nothing like what I was experiencing before. Once it warmed up it was almost completely smooth. I think I will shorten the accelerator pump linkage a small amount to dial it in and possibly go another 1/4 turn on the idle speed screw. It should be good to go in that aspect.

The secondaries have about a 1/2 second bog when I stomp on it but catch right up if I keep my foot pinned. Easing into them at 60mph feels great and 90 comes up real quick. I've never driven a spread bore carb car and the sound...wow. I swear the hood buckles a little. I might try shortening the dashpot linkage a little––I am already 2.5 turns wound up on the spring. What about a vacuum restrictor? Heard old uncle tony talking about that, although I know better than to do what he says without asking other people first. He seems to get away with stuff most of us don't lol

Thanks for the help everyone