Carb tuning, larger accelerator pump nozzle?

-
When the clutch lets out, the load on the engine increases and the engine slows slightly. Any slowing results in lower vacuum. Lower vacuum means less pressure differential so as the clutch lets out it will be a little leaner than the setting in neutral. Its easier to see this on a automatic.
upload_2021-6-2_13-4-1.png

This going the other way, drive into park (neutral).



dano said:
Timing is 20 degress initial, 34 total. Have not check when it comes in.
Click to expand...
This could be messing with you.
Are you sure it doesn't drop below 20 degrees at when the rpms drop to 750?
I've not worked with that distributor but most have some method of adjusting the preload on the springs.
A typical Chrysler distributor, even a mopar performance one, will come setup so it start advancing around 750 rpm.
 
@mderoy340

I have had the initial as high as 30 degrees (I didn't reset the total). It did like it and would just buck back a tiny bit when starting hot.

@Mattax

At this point I'm going to see what is going on with the distributor and report back. Might be a few days until I get back to it.
 
Last edited:
dano said:
@mderoy340

I have had the initial as high as 30 degrees (I didn't reset the total). It did like it and would just buck back a tiny bit when starting hot.
Click to expand...
Yea. That's the only thing I disagree on if you intend to use vacuum advance but its good you experimented with that approach.
What an engine likes in neutral is not the same as what it likes with a high load.
And when the clutch is let out, that load is very high compared to the max power the engine can produce at these rpms.

I would work in small steps from where you are now.
I would go for 18* at 750 - 850 rpm and make sure there is enough tension on the spring to be sure its really the initial.

With a relatively rich mixure there shouldn't be a need to light it off early. I'm running 17* initial with a 230 @ 50 cam right now. heads, compresion, intake valve closuing point all have some effect so no one answer.

Don't worry about where the idle mix screws end up. That's what they are for. As long as they have effect. If you turn them and there is no response, then they are out of working range. Four corner idle is going to be more difficult. Try not to have to mess with the secondary IFR and IAB as that will impact how well the secondary opening responds.
 
Mattax said:
Yea. That's the only thing I disagree on if you intend to use vacuum advance.
I would work in small steps from where you are now.
I would go for 18* at 750 rpm and make sure there is enough tension on the spring to be sure its really the initial.

With a relatively rich mixure there shouldn't be a need to light it off early. I'm running 17* initial with a 230 @ 50 cam right now. heads, compresion, intake valve closuing point all have some effect so no one answer.

Don't worry about where the idle mix screws end up. That's what they are for. As long as they have effect. If you turn them and there is no response, then they are out of working range. Four corner idle is going to be more difficult. Try not to have to mess with the secondary IFR and IAB as that will impact how well the secondary opening responds.
Click to expand...

Thanks, kind of why I went back to 20 degrees. Probably will hook up vacuum advance but that will be down the road and factor I don't want in play right now. The idle mix screws are very responsive. I can lean it to die or choke you out pig rich with them. Seems to be they should be a finer thread for 4 corner carbs. Right now 1/2 turn from in closed sets it close enough to start and be a bit lean. One warm I set them with the air cleaner installed.
 
Just because the starter kicked back at 30 doesn't mean it will at 27. You can adjust the vacuum advance after you get the car to run good without it. Today's pump gas fuel needs more initial timing and your aluminum heads are dumping heat.
 
Transition issue aka pump shot, volume and its duration.
You'll figure it out.

Remember lean is when it cuts out momentarily and or jerks... while Rich is a bog which is deep & blubbery sounding but not jerky and missing...more like a garbage disposal fighting the chunks of meat....ghhrrrrrr..then it buzzes up in speed....or the mower over some gopher hills.
 
Back in post #39, I suggested reducing the MAB size to bring in the main system earlier & gets more fuel into the system. Because you could have a lean 'hole' from the idle cct declining & main system starting.
The original Holley book, written by Holley engineer [ later VP ] Mike Urich says this: "Decreasing the bleed [ mab ] size increases the pressure drop across the main jet to pull more fuel through the main system, giving a richer mixture".

Note that early Holleys did not have removable E jets & ABs because they worked just fine with fuel jet changes.

You could also look at increasing the size of the PVCR &/or power valve to a lower number. The PVCR supplies the additional fuel in the main system, when the engine is loaded. The PV controls when the extra fuel is added.
 
MOPAROFFICIAL said:
Transition issue aka pump shot, volume and its duration.
You'll figure it out.

Remember lean is when it cuts out momentarily and or jerks... while Rich is a bog which is deep & blubbery sounding but not jerky and missing...more like a garbage disposal fighting the chunks of meat....ghhrrrrrr..then it buzzes up in speed....or the mower over some gopher hills.
Click to expand...

Defiantly lean by the feel and confirmed by the wide band, which I know is a lagging indicator.

From driving it I don't even think I'm getting into much of the pump shot if at all.

Plan of action is:
1. Confirm distributor timing including how much and by when. Adjust if needed.
2. Back off primary idle and marginally open secondary idle and test.
3. Reassess after data analysis of 1 and 2.
 
dano said:
Defiantly lean by the feel and confirmed by the wide band, which I know is a lagging indicator.

From driving it I don't even think I'm getting into much of the pump shot if at all.

Plan of action is:
1. Confirm distributor timing including how much and by when. Adjust if needed.
2. Back off primary idle and marginally open secondary idle and test.
3. Reassess after data analysis of 1 and 2.
Click to expand...

Justin's basic carb 101-vac secondary

Here is what you do with a vac sec carb ...
Put the stiffest spring or setting on the secondary. You don't want it to open because you want to focus on the primary and not have the secondary interfere and throw you off. Then you take it out for a cruise..who cares about the acceleration for now... you want to lean the jets till it surges 'test anywhere from 45 mph n up to 60mph' creep up in speed to again insure the sec stays closed...then once it surges a hair.. jet up 2 sizes. Check your meter if ya want. Now start dialing the pump shot for a nice smooth roll of the throttle type acceleration 'from a stop'... tailoring the pump sho includes proper lash at the pump arm, '.050 from max travel 'then go out and try stomping on it... try a power valve that's about 2" less than the idle rpm... take it out and stomp on it again. Once you get it pulling smooth till it plains out ' due to late secondary- still mostly closed with the stiff setting/spring possibly ' ...then you can try a light spring, lightest even..n see if it then bogs or likes it. Too light will bog, or can.. maybe not with a smaller size carb like you have.. but in any case.. that potential bog could throw you off course and messing with the transition when you dont need to.
For mechanical carbs primary tuning you can either adjust your throttle cable loose so that max pedal is only max primary opening, leaving the secondary closed...or simply control your foot. From there you work on wit and part to wide open with the secondaries which is about the same or easier. Any questions can be asked as to not write a novel here.
Just remember that it's a carburetor and it will always show a hair richer during the pump shot.
Have fun.
 
Last edited:
MOPAROFFICIAL said:
Justin's basic carb 101-vac secondary

Here is what you do with a vac sec carb ...
Put the stiffest spring or setting on the secondary. You don't want it to open because you want to focus on the primary and not have the secondary interfere and throw you off. Then you take it out for a cruise..who cares about the acceleration for now... you want to lean the jets till it surges 'test anywhere from 45 mph n up to 60mph' creep up in speed to again insure the sec stays closed...then once it surges a hair.. jet up 2 sizes. Check your meter if ya want. Now start dialing the pump shot for a nice smooth roll of the throttle type acceleration 'from a stop'... then go out and try stomping on it... try a power valve that's about 2" less than the idle rpm... take it out and stomp on it again. Once you get it pulling smooth till it plains out ' due to late secondary- still mostly closed with the stiff setting/spring possibly ' ...then you can try a light spring, lightest even..n see if it then bogs or likes it. Too light will bog, or can.. maybe not with a smaller size carb like you have.. but in any case.. that potential bog could throw you off course and messing with the transition when you dont need to.

Have fun.
Click to expand...

Thanks. FYI this is a mechanical secondary carb and most of my driving hasn't even got into the secondary's yet.
 
Well, just to circle the wagon, boy I feel like a 1/2 idiot. Not full and I'll explain that later.

So genius here (that's me) install a brandy new flashy black Lokar throttle cable during carb install. Works great, looks good, does its job "if" the cable is 99.9% tight. Well, mine had about 3/4" in slop. As in it was too long and I had excess cable at the pedal. Fix that and it helped a lot but the lean bog remained, just not as pronounced. I chalked it up to me getting used to the diaphragm clutch and just driving the car being a new thing for me. Plus this is the only manual car I have now, so call me rusty using the left leg.

Other than moving the IFR down and plugging the middle emulsions holes, I returned the carb to box stock. Issue remained, so I used a white cam. Still off throttle bog. Installed the 37 Pump shot and when I returned the car just wanted to idle lean, which it hadn't before, checked for vacuum leaks, found none so I reset the idle mixture screws to 1/2 on all four and got it back to being ~14:1. but it ran odd, so I closed the secondary as far as it would go and not bind, idle dropped so I opened the primary to compensate and went for a dive. Holy crap, now this thing works like it should. No more bog, goes like hell but its rich on acceleration (not WOT, just moderate up to 4K) like 10:1. Cruse isn't bad at in the high 13s.
 
Last edited:
Sounds like your back on track to start tuning.
I forgot if you've mentioned the AFR unit. The Innovate stuff responds pretty quick. Examples here of cudafever, Hysteric's, and my 1/4 mile passes.
Proform 850 dialing it in closer
Proform 850 dialing it in closer
Proform 850 dialing it in closer (That stepwise change was probably the vac secondary spring but I haven't had a chance to verify)

The really good guys get the idle and off idle tuned in first. I'm not that good. I get it idling decent, then try to get a quarter mile pass or dyno pull to get the main jets and main air bleeds close. Then primary main jet for highway. Repeat as needed. Pump shot last (when possible).
--
For idle I want the least drop in rpm and vacuum shifting into gear.
For off-idle, no hesitation when very slowly accelerating from stop.
For wide open throttle I'm looking for MAB and e-bleeds that give a flat AFR in 3rd at WOT, as well as best over all power. Then adjust main jet for best power (mph in the 1/4 mile). Sometimes this takes a couple iterations.
For highway a jet size or two richer than when it lean surges at 60 or 65 mph steady driving.
If needed then change power valve restriction and retest at WOT.

Desired Carburetor AFR Characteristics At Different % Load
 
Last edited:
Regarding lowering the iFR ‘s I have only read one article (racingfuelsystems) where a poster mentions after relocating the restrictors to not plug the old holes. Is this correct?
 
Bewy said:
Don't see idle speed making a difference, because coming off the clutch you are well above idle speed [ whether 750 or 900 rpm ].
I would reduce the size of the main air bleed. This will bring in the main system earlier because it sounds like there is a 'gap' where the idle cct stops & main system starts.
Click to expand...


Just to clean this up, decreasing the size of the MAB will delay the start up of the mains and will make the fuel curve go rich at higher rpm. Increasing the size of the MAB will start the mains sooner and make the fuel curve get leaner at rpm. You have it backwards.
 
Rat,
No I don't have it backwards. You do. Read post 57, it explains it.
Making an air bleed, any air bleed, bigger weakens the signal to THAT circuit. It therefore takes more airspeed [ bigger throttle opening ] to generate enough pressure drop to pull fuel from that cct.
 
To add too my post above. From the Carter Carbs book.

" By varying the size of the bleed [ air ], the amount of suction [ vacuum ] reqd to start fuel flow can be altered. As bleed size is enlarged, the vac reqd to initiate fuel flow is increased. Conversely a reduction in bleed size reduces vac signal requirements & fuel flow is more easily initiated.
 
Bewy said:
Rat,
No I don't have it backwards. You do. Read post 57, it explains it.
Making an air bleed, any air bleed, bigger weakens the signal to THAT circuit. It therefore takes more airspeed [ bigger throttle opening ] to generate enough pressure drop to pull fuel from that cct.
Click to expand...


Sorry my friend. A bigger MAB will delay the start of the mains. That’s the simple result of it. It will also lean the fuel curve at higher rpm. I understand how air bleeds work. Again, a bigger MAB will delay the start of the mains. A smaller MAB will start the mains sooner, and will move the fuel curve richer at rpm.
 
Bewy said:
To add too my post above. From the Carter Carbs book.

" By varying the size of the bleed [ air ], the amount of suction [ vacuum ] reqd to start fuel flow can be altered. As bleed size is enlarged, the vac reqd to initiate fuel flow is increased. Conversely a reduction in bleed size reduces vac signal requirements & fuel flow is more easily initiated.
Click to expand...


Yeah, that is wrong too. It is the opposite of what you are saying.
 
Rat,
Can you read? Your statement, post 69: "A smaller MAB will start the mains sooner".
I said the same thing waaaay back in posts 39, 57 & 67.
And there is nothing wrong in my post 67, which you claim in your post 70. In fact you are contradicting yourself.
 
Bewy said:
Rat,
Can you read? Your statement, post 69: "A smaller MAB will start the mains sooner".
I said the same thing waaaay back in posts 39, 57 & 67.
And there is nothing wrong in my post 67, which you claim in your post 70. In fact you are contradicting yourself.
Click to expand...


My bad. You bark so much crap it’s hard to keep it straight. A bigger MAB will start the mains sooner and be leaner at rpm. A smaller MAB will delay the mains and make the fuel curve richer at rpm. It doesn’t matter who wrote the book, if you think a bigger MAB will delay the mains you’ve never tuned with an O2 sensor. You can watch it happen.
 
An O2 sensor doesn't defeat the laws of high school physics.....
What you said above in post 73 exactly contradicts what you said post 69.
 
You must log in or register to reply here.
-

Back
Top