302 SBF stumble

[77cruiser]

Maybe he too smart for the rest of us here, or it's top secret & not to be handed down to us peons.

[statsystems]

Maybe he too smart for the rest of us here, or it's top secret & not to be handed down to us peons.

Nope. It's just like the 50* seat discussion. Real life isn't like school. Learn on your own.

I'm surprised how many think a VS is still worth while.

I'd keep doing what you all are doing. Makes me money fixing junk that people continue to do, even though it's been proven wrong...like dual plane intakes...VS carbs...90/10 shocks.


Yup. You do like technology.

[Frankshaft]

The one thing about those old carb selection formulas, they were based off of under 100% ve. These days it's easy to get well above 100% . That makes a difference. Also, back whenever, we had about 10 choices. Carbs have come A LONG ways. New carbs, even a off the shelf quick fuel carb works so much better than an old otb 4778 carb, the circuits jive so much better it's crazy. Not to say a sharp carb guy back when that's all there was, could make them work much better. The carb modification shops, understand how all the circuits jive SO MUCH better than they used to, not to mention the modern billet metering blocks, booster designs etc. The trick billet stuff is so good it's almost unbelievable. Not even close to the way they were. If you want to give up a TON of power and drivability, use the old formulas.

[statsystems]

The one thing about those old carb selection formulas, they were based off of under 100% ve. These days it's easy to get well above 100% . That makes a difference. Also, back whenever, we had about 10 choices. Carbs have come A LONG ways. New carbs, even a off the shelf quick fuel carb works so much better than an old otb 4778 carb, the circuits jive so much better it's crazy. Not to say a sharp carb guy back when that's all there was, could make them work much better. The carb modification shops, understand how all the circuits jive SO MUCH better than they used to, not to mention the modern billet metering blocks, booster designs etc. The trick billet stuff is so good it's almost unbelievable. Not even close to the way they were. If you want to give up a TON of power and drivability, use the old formulas.

Exactly. My points exactly. If a moron such as myself can figure it out, why can't the gurus here get it?

Even back in the early 1980's if you were careful you could run a bigger carb than that math suggested. The first engine I was paid to build was a 318 dodge. When it was all said and done, we used a 750 DP (way too big) a set of 1.750 headers (big torque killers) a Strip Dominator intake (the detested single plane RACE only intake) and DC 284/.484 cam. With a 3.90 gear and B&M Super Holeshot converter it went 12.40's in decent air and made the guy big money on the street. After a couple of years he sent it to someone else for a freshen up. Off came the intake. The cam was replaced with a Comp grind. The carb was replaced with an 1850 Holley because the carb was too big.The car was a consistent 13.00-13.20 car.

Luckily, he keep the intake and carb. Brought it back to me and I put the same cam back in it and the SD back on, bought the same cam and dropped the 750 back on and it went right back to the number.

To this day he goes nuts when people tell him his car needs a VS carb. Why? Because he runs a TR with 2 850 DP carbs. He really loves it when they tell him it would be faster with 2 edelbrocks.

[cjperformance]

Recently sold my Mustang to a friend of mine, its a fox body, carb'd 302 street strip type deal. 3 months ago when I got it finished up and sold it, it ran ok. Now it has developed a stumble/hesitation issue. It does it with 2 different carbs on it, the 650dp that was always on it, and a 600dp that we borrowed to try. If you get the engine up to operating temp and whack the throttle open you can hear a gurgle come up through the intake...like valve noise or something. then it will clear out and rev clean. Same thing will happen under load when you drive it...put it in 2nd gear and nail it about 20mph roll, same issue. Timing idles at 25 deg and totals out at 36 by about 4000rpm. What types of issues should I be looking for to get this fixed. It seems to be getting progressively worse, could it be a wiped exhaust lobe?

One very typical problem is when the accel pump arm sprimg collapses as the throttle is opened quickly. Moderate opening speed and all is fine but on quick opening the arm spring compresses as the throttle moves so the action at the pump diaphram arm does not keep pace in line with the arm that the pump cam operates, so on quick throttle you get a lean area and that gurgling intake sound you are hearing.
Bigger squirters cover this problem but make it over rich on moderate throttle opening speeds.
Watch the pump arm spring as you snap the throttle, that spring should NOT compress at all and unseat the adjuster nut off of the pump arm on either the primary or secondary. The spring is only there to make sure the pump arm can not mechanically jam the pump arm against the cam and cause the throttle to stick open at any point.
The fix is simple, remove the adjuster and use a small washer as a shim between the pump arm and the spring, readjust the arm and retry. Sometimes you may need an extra shim.
Once you get the spring and diaphram arm to follow pump arm movement on a quick throttle snap you can now adjust squirter size correctly.

[MadBill]

Craig, there has to be some compliance in the accel linkage, because with a sharp stab to WOT the full 3-5 cc. of fuel can't instantaneously be delivered and since liquid is incompressible, the linkage would be damaged.
Between the accel pump's operating cam indexing/lift and the range of possible squirter sizes, almost any engine's needs can be addressed.

[cjperformance]

Bill- im definetly not saying to remove the spring , it must be there, and i do agree that there must be compliance in the mechanism it the situation arises that throttle movement V pump circuit flow would mean a 'hydraulic lock' situation causing damage to the arm etc.
I find that for eg, using a 31 squirter in X application will give this lean area due to spring compression. I can change pump cam and actually get no more out of the squirter as flow V spring has peaked. I can then fit a 35 squirter and the extra flow stops the spring compressing and the lean area is fixed. But when not using max throttle opening speed the 35 squirter is a little rich in other areas. Yes i definelty can swap pump cam now and alter where the extra fuel comes in OR i can go back the the 31 squirter that was spot on everywhere at less throttle opening speed where the spring was not compressing , add a shim and gain some extra flow at sharper opening speeds. I find this to give better all round driveability at all throttle opening speeds as used in a street/strip car when compared to the bigger squirter.
I have never had a pump arm fail or even bend doing this, im not talking much, up to maybe .060/.070" of shimming, until the spring 'just' no longer compresses as the throttle is snapped, and well within the holley range of provided spring adjustment and certainly well withing the strength of the mechanism. I have had a few occasions where i have (no parts/shim washers available) simply slightly bent the pump arm to allow me to use the adjuste nut on the spring to adjust properly to fix this problem.

[MadBill]

Thanks for the clarification Craig. (and for the mini tuning seminar. )

[cjperformance]

Thanks for the clarification Craig. (and for the mini tuning seminar. )

No problem Bill . An interesting note is that i have had this occur on new carbs(and used/rebuilt), yes every engine is different, of the same list number and on fairly similar combos and then not occur on others. Sometimes with only a simple shim washer or arm tweek and readjust the pump circuit is then otherwise fine needing no further messing with. I have never bothered to compare springs from carbs that do or dont do it.
I have seen it so many times now that that when I get something that drives fine apart from a hesitation or bog when the throttle is mashed , that the 1st thing i do is whip the throttle open by hand with engine running and watch the spring, if it floats off of the arm i put my finger on top of the spring adjuster and apply a little pressure as i crank the throttle, if the symptom reduces or dissapears, i shim it, then retest and tune from there. Its easy on the primaries but a bit rev happy on the secondaries if a DP carb!! In which case i remove the carb, and wet test the pump over a container , make sure the spring does not float then go on tuning on car.

[groberts101]

Bill- im definetly not saying to remove the spring , it must be there, and i do agree that there must be compliance in the mechanism it the situation arises that throttle movement V pump circuit flow would mean a 'hydraulic lock' situation causing damage to the arm etc.
I find that for eg, using a 31 squirter in X application will give this lean area due to spring compression. I can change pump cam and actually get no more out of the squirter as flow V spring has peaked. I can then fit a 35 squirter and the extra flow stops the spring compressing and the lean area is fixed. But when not using max throttle opening speed the 35 squirter is a little rich in other areas. Yes i definelty can swap pump cam now and alter where the extra fuel comes in OR i can go back the the 31 squirter that was spot on everywhere at less throttle opening speed where the spring was not compressing , add a shim and gain some extra flow at sharper opening speeds. I find this to give better all round driveability at all throttle opening speeds as used in a street/strip car when compared to the bigger squirter.
I have never had a pump arm fail or even bend doing this, im not talking much, up to maybe .060/.070" of shimming, until the spring 'just' no longer compresses as the throttle is snapped, and well within the holley range of provided spring adjustment and certainly well withing the strength of the mechanism. I have had a few occasions where i have (no parts/shim washers available) simply slightly bent the pump arm to allow me to use the adjuste nut on the spring to adjust properly to fix this problem.

I shim most all my accelerator pump springs. Always thought they were a bit too soft and it's surprising how it can affect initial shot delivery volume.

[Geoff2]

Frankshaft,

No, the carb sizing formula was not based on less than 100% VE. It was based on 100% VE, page 7 of the original Holley book. Yes, VE has improved over the years, I said that back on page 1....

Yeah, carbs have come a long way. Holley discovered aluminium... And pretty colours too!! What has NOT come a long way is how carbs work. Same old, same old, no change there.

The OP stated that this car was to be driven on the street. That changes everything when it comes to carb selection. For a small-ish engine like a 302,
a vac sec carb will be best for overall performance & drivability.

[cjperformance]

DP and VS both have their places.

DP is great higher stall rpm convertors, and lower gears and for drivers of manual transmission cars that do realise you dont floor it at 1500rpm in top gear on a mild small ci/low tq engine.
Yes pump shot gets used to cover areas where you may hit WOT and not yet have the airspeed to pull the main circuit and that is perfectly fine and what the pump is for on a correctly sized and well matched combo and tune, this shot is tuned to engine demand over a given rpm acelleration rate V how quickly the throttle is applied.
The only hitch to this and 'street' driving is times when for reasons of extra weight, steeper hills etc that cause a change in acelleration rate, particularly when acel rate is slowed and pump shot is consumed before correct main signal is achieved, then you get a lean area and the associated bog in the tq curve.

VS does allow for a wider rpm range to be used that will see the driver holding WOT. The 'on demand only' nature of the VS secondaries is far better where taller gearing and or a lower stall convertor is used and smooth performance us required over a very wide range of conditions.

My personal experience and view on VS V DP is simply that a DP carb does have the edge over a VS (or any air valve type setup) in a more controlled environment, by that i mean race track or a strip/street (not street/strip! There is a difference) where given conditions are tuned for using a race type setup where typically higher rpms are used before WOT is applied and where the rpm band used is narrow enough that WOT can be maintained with the given rpm drop on shift to allow (particularly in the case of auto trans where rpm drops but throttle stays pinned open*no pump shot cover*) adequate main signal without needing a pump shot to cover fuel demand. So a 'less compromised' setup.
BUT, more combo compromises, less driver nouce, more differing driving conditions, etc the VS is great as it has the ability to cover for pretty much any misgivings thrown at it. Im not Carb Prejudice! I'll use Holley/Clones there of, Eddys, QJ, TQ, Autolite(still have a few original 4bls) and the clones like holleys toploader or summit's clone of holleys autolite clone!!
They are all great when used correctly!

[427dart]

Wouldn't mind trying a pair of 650 DP's but as you can see no space! Maybe smart Statman could build me a pair of 4160 double pumpers?

[cjperformance]

427dart - with some mucking around maybe you could mount 2 carbs sideways but yes you look very tight for room.
But, What you can do is use a 45 or thereabouts squirter, a 50cc acel pump kit and a cam to come in gently but pump to or almost to WOT then modify the secondary linkage to operate mechanically OR if a class states VS must be used (enter cheat here!) with some well disguised vac lines , elec over vac switch and a vac tank you can have Vac operated secondaries that will follow the primaries at whatever rate you like!