Thread: Help to get off the line please!

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First post here, please bare with me !

So I built one of the 7 engines in the article "7 Hot Engine Combos - Mild to Wild" from Chevy High Performance magazine August 2003, I think it was the Muzak Mouse but I do not recall, specs of my build are as follows:
-350 SBC, 4 bolt mains, 9:1 compression
-Cam is an SSI from PAW: 214/224 duration (280/290 adv) with .443/.465 lift @50
-GM Cast Iron Vortec Heads
-Edelbrock Performer RPM Air Gap Intake
-GM Ram's Horns manifolds, 2-1/2" (clearance issue or I would have gone with headers)
-600 CFM Carb (mine is a Carter AFB)

Engine runs GREAT! It is in my '59 Chevy 1/2 Ton pickup with a Turbo350 trans and a 10 bolt corporate wtih 3.42 gears. I believe my torque converter stall is around 2100-2400 but the trans was a freebie so I didn't ask too many questions at the time!

Part throttle and WOT performance are awesome but transitioning from idle to the primary circuit is an issue. I have called and worked with Edelbrock Tech Support but alas, no avail.

Engine has 18 in of vacuum idling in park between 800-850 RPM and drops to 600-650 RPM in gear, not too much lope either; it takes off smooth with minimal throttle. Distributor is stock HEI with 20 degree weights and stock springs, for now, vacuum hose disconnected, for now (possibly forever), I am all in with 36 degrees of advance around 2800 RPM (16 degrees initial timing).

PROBLEM is, truck cannot get off the line... It bogs or stalls if you floor it.

I would appreciate any insight or advice from the experts out there or anyone that has a similar build; I know this combo will work, I just have not coaxed the genie out of the bottle yet. Thanks in advance

Welcome to CHR. Sounds like you've got a great combination on the engine build. Cam spec are reasonable as are your gears and stall speed (I really like the Vortec heads) and your vacuum is pretty good as well.

My first thought is that you you may be rich on the primary jets/metering. The Edelbrocks are a bit rich out of the box and if you had a 1406, I would point you to their (Edelbrock's) web site for the instructions on how to diagnose and then replace the jets/metering rods. I'm assuming that you have a true "AFB".

While I suspect that your Carter may be a jet size bigger than it needs to be, a quick check can confirm. If you have a good shop near by, they can sniff the exhaust and tell you if that's the problem. I'd try that first.

Others will chime in, I'm sure.

Again welcome to CHR,
Glenn

Transition circuit is a step or two fat... As Glen said, most are fat out of the box... If you don't have the Edelbrock calibration kit, you'll need to get it then follow the chart in the book (or on the Edelbrock site) to get the carb dialed in... Once you get the calibration right on the carb, you may want to reconnect the vacuum to the ported side of the carb, then work on the vacuum canister to slow down the advance rate a bit.....

Originally Posted by glennsexton

My first thought is that you you may be rich on the primary jets/metering. The Edelbrocks are a bit rich out of the box and if you had a 1406, I would point you to their (Edelbrock's) web site for the instructions on how to diagnose and then replace the jets/metering rods. I'm assuming that you have a true "AFB".

Good info... I have both Carter and Edelbrock manuals, which look suspiciously identical, I guess I should get the Tuning Kit that has the assortment of rods and jets. Yes my Carter is a true AFB

While I suspect that your Carter may be a jet size bigger than it needs to be, a quick check can confirm. If you have a good shop near by, they can sniff the exhaust and tell you if that's the problem. I'd try that first.

Not sure if any local shops have smog sniffing setups for hotrodders, most all focus on smog here is for registration and environmental - or are you suggesting a dyno shop?

Originally Posted by Dave Severson

Transition circuit is a step or two fat... As Glen said, most are fat out of the box... If you don't have the Edelbrock calibration kit, you'll need to get it then follow the chart in the book (or on the Edelbrock site) to get the carb dialed in... Once you get the calibration right on the carb, you may want to reconnect the vacuum to the ported side of the carb, then work on the vacuum canister to slow down the advance rate a bit.....

I have gone over the chart, don't really understand it yet, and have called Edelbrock tech spt, they pointed me towards step up springs and accel pump rod holes - I tried ALL they suggested and have not seen any significant improvement at all.

From what you two gentlemen are suggesting I might want to change metering rods to one with a smaller rich-step and/or go to a leaner main jet - did I get that right?

Thanks to both Glenn and Dave

sorry I disagree I think the transition circuit is too lean because the float level is too low when the throttle plate opens and uncovers the the trasition port the manifold vacuum drops rapidly and the the t port relies mostly on ventures pressure difference to flow fuel called Bernelle's law especially at w.o.t. just my opinion I could be wrong .....ted

Originally Posted by glennsexton

My first thought is that you you may be rich on the primary jets/metering. The Edelbrocks are a bit rich out of the box and if you had a 1406, I would point you to their (Edelbrock's) web site for the instructions on how to diagnose and then replace the jets/metering rods. I'm assuming that you have a true "AFB".

While I suspect that your Carter may be a jet size bigger than it needs to be, a quick check can confirm. If you have a good shop near by, they can sniff the exhaust and tell you if that's the problem. I'd try that first

All good info... I have both Carter and Edelbrock manuals, which look suspiciously identical, I guess I should get the Tuning Kit that has the assortment of rods and jets. Yes my Carter is a true AFB

Originally Posted by Dave Severson

Transition circuit is a step or two fat... As Glen said, most are fat out of the box... If you don't have the Edelbrock calibration kit, you'll need to get it then follow the chart in the book (or on the Edelbrock site) to get the carb dialed in... Once you get the calibration right on the carb, you may want to reconnect the vacuum to the ported side of the carb, then work on the vacuum canister to slow down the advance rate a bit.....

I have gone over the chart, don't really understand it yet, and have called Edelbrock tech spt, they pointed me towards step up springs and accel pump rod holes - I tried ALL they suggested and have not seen any significant improvement at all.

From what you two gentlemen are suggesting I might want to change metering rods to one with a smaller rich-step and/or go to a leaner main jet - did I get that right?

Originally Posted by ted dehaan

sorry I disagree I think the transition circuit is too lean because the float level is too low when the throttle plate opens and uncovers the the trasition port the manifold vacuum drops rapidly and the the t port relies mostly on ventures pressure difference to flow fuel called Bernelle's law especially at w.o.t. just my opinion I could be wrong .....ted

Just for argument sake, based on my very finite understanding of carburetion, would we be able to diagnose too lean/too rich at transition by how the engine reacts to flooring the pedal? For instance, too lean and the RPM would increase and stall due to fuel starvation vs. too rich the engine would flood and bog and possibly stall. Just thinking out loud here...

I can and will check/set float levels when I inspect/change jets, but am not sure this is part of the problem, no fuel starvation at WOT so far, but I haven't check fuel pressure yet either...

Thanks to Ted, Glenn and Dave!

Well – sorta:

While it seems complex, the carburetor consists of a “barrel” (or a series of “barrels) that provide a means of ambient air to pass through a venturi in route to the intake manifold. The venturi narrows in section and then opens again, causing the airflow to increase in speed in its narrowest part. The throttle butterfly rotates in a manner that ranges from nearly no restriction (WOT) to nearly closed (idle) controlling the flow of air through the carburetor throat and thus the quantity of air/fuel mixture delivered, which regulates engine speed.

Fuel is introduced into the air stream through small holes at the narrowest part of the venturi. Fuel flow is in response to a particular pressure drop in the venturi and is adjusted by means of calibrated openings in the fuel path typically called jets.

When the throttle butterfly begins to open (from the fully closed position), it exposes additional fuel delivery holes where there is a low pressure area created by the butterfly blocking air flow; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, enabling the transition to metering fuel flow through the regular open throttle circuit.

As the throttle continues to open, the manifold vacuum drops (since there is less restriction on the airflow), reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e., as the velocity increases, pressure falls). The venturi raises the air velocity, creating high speed and thus low pressure, sucking fuel into the air stream through a nozzle or nozzles located in the center of the venturi. Sometime carburetors use booster venturis (annular discharge) within the primary venturi to increase the effect.

When the throttle is closed, the airflow through the venturi drops to a pressure that cannot maintain this fuel flow, and the idle circuit takes over.

Bernoulli's principle, which is a function of fluid velocity, has a dominant effect for large openings and large flow rates. Since fluid flow at small scales and low speeds is dominated by viscosity, Bernoulli's principle is ineffective at idle or slow running. The idle and slow running jets are typically placed after the throttle valve where the pressure is reduced partly by viscous drag, rather than by Bernoulli's principle.

My two cents worth - excerpted from some old archives rattling around my brain. Hope it helps.

Regards All,
Glenn

Great info Glenn

In reading the manual (carb prinicpals) as well as your post it seems richening up the primaries may not be the fix for me, I am looking at the secondary metering calibration since the transition seems to be handled by both the secondary aux system as well as the pump system - perhaps why Edelbrock suggested moving to the longer accelerator pump stroke to deliver more fuel.

Edelbrock's tuning sheet for the 1405 carb (very similar to my AFB) suggests upgrading secondary main jet which is located in the float bowl so perhaps I can kill two birds at once when I go checking/setting float levels...

Do one adjustment at a time. As suggested it could be the float level. Under hard acceleration, the fuel could move to the back of the bowl and cause the bog. I little more fuel in the bowl could fix it. If you do more than one adjustment at time you run the risk of not knowing what the problem was.

Originally Posted by Mikej

Do one adjustment at a time. As suggested it could be the float level. Under hard acceleration, the fuel could move to the back of the bowl and cause the bog. I little more fuel in the bowl could fix it. If you do more than one adjustment at time you run the risk of not knowing what the problem was.

Not too sure it is a fuel issue, my problem happens when standing still and flooring the pedal, the truck just bogs and will stall if you stay on it. Anything you ask it to do over 1500 RPM happens without hesitation.

The really vexing part of this is 1 out of 20 times the truck WILL launch, I just haven't figured out what is different between when she does and when she doesn't - which is why I am looking at the carb first - of course 4.11 gears couldn't hurt but let's not go there

How much part could operating temperature play? My cooling system is apparently SUPER efficient with an electric fan and aluminum radiator, engine runs around 160 all the time unless I turn the fan off...

Thanks Mikej!
BTW that is a sweet ride you've got there

Originally Posted by gearGrinder

Not too sure it is a fuel issue, my problem happens when standing still and flooring the pedal, the truck just bogs and will stall if you stay on it. Anything you ask it to do over 1500 RPM happens without hesitation.

The really vexing part of this is 1 out of 20 times the truck WILL launch, I just haven't figured out what is different between when she does and when she doesn't - which is why I am looking at the carb first - of course 4.11 gears couldn't hurt but let's not go there

How much part could operating temperature play? My cooling system is apparently SUPER efficient with an electric fan and aluminum radiator, engine runs around 160 all the time unless I turn the fan off...

Thanks Mikej!
BTW that is a sweet ride you've got there

operating temp has a major role. You cant go off of anything if ts not at temp.

Originally Posted by BigTruckDriver

operating temp has a major role. You cant go off of anything if ts not at temp.

I just got an adjustable thermo fan relay in yesterday, will install tonight - the existing one was just on all the time.

The adjusto turns off the fan 10 degrees lower than it starts it so what temp range should I be looking at?

Thanks for this, I assumed cooler was better...

Thanks to all who responded to my post I am happy to report there is progress!

Installed new thermo fan relay, set ON temp around 180-190
Found a possible vacuum leak at vac can on trans, fixed that
Played with springs in dizzy, went with blue/yellow from my crane vac adv kit
-lighter combo than stock springs
Played with initial timing
-16 degrees advanced was good all around
-20 was better low end but killed top end
-18 is actually a good compromise of decent low and still fairly aggressive top end
-it is set now somewhere between 16 and 18 degrees adv
-did several runs including heavy load (1 mile steep hill) at partial and full throttle, no knock at all
I didn't check it after the last run (was 2 AM) but mechanical advance is all in by 2000 RPM or sooner
Total timing is around 37 degrees
Played with mixture at idle, went 3/4 turn richer
Truck launches OK now! It doesn't spin 'em every time but that's wasteful anyway right?
Did 100 MPH on the way home on a flat section of fwy with no problem (no ping, hesitation or starvation) backed off to 90 and at partial throttle noted the tach just under 4000 RPM with about 16" manifold vacuum
-didn't want to take my eyes off the road in power mode

So the good news is that was a successful evening, the bad news is it seems like my torque converter stall is around 1400, not as high as I previously had determined
Now I am on leak patrol, everything except the brakes and coolant is leaking, I am going to go through all the engine/trans bolts today (ALL of them) and re-torque. The trans is leaking out of both ends so I will consider swapping torque converters when I pull the trans to re-seal it.

I am getting an HEI adjustable vac can today so I can play with adding vacuum but the mechanical only curve I have right now feels pretty durn good...

I will try to find a place to dyno/sniff for further carb tuning

Again thanks for all the help/advice!

Another tip is to get a oxygen sensor wired in to the system. It will tell you if you are running rich or lean at idle or through out the rpm range. I think edlebrock sells one for about 100 dollars. Or you can buy one and wire it up to a volt meter and it will do the same. Should get you right on with your fuel mixture. You can also tune your idle mixture with the vacuum gauge.

What a difference a day makes, or in this case a weekend

Changed spark plugs over the weekend, WOW what an improvement! The plugs that were in there were not seating all the way in and there was fouling on the threads but not the electrodes, the spark was happening IN the plug hole rather than in the cylinder and leaking as well... Not sure how the obviously-wrong plugs got in there but I decided to try swapping plugs and I'm sure glad I did.

Now this thing launches and runs like a champ, and boy did I need to change my timing, now my initial timing is around 8-10 degrees advanced otherwise she pings under load - even with the vacuum disconnected which leads to another topic and thread...

Originally Posted by BigTruckDriver

Another tip is to get a oxygen sensor wired in to the system. It will tell you if you are running rich or lean at idle or through out the rpm range.

This is a good idea, looks like AutoMeter sells a kit with sensors, bungs and gauge for around $350 - ouch

Originally Posted by BigTruckDriver

You can also tune your idle mixture with the vacuum gauge.

This is something I need to learn how to do. One thing I did notice, the vacuum dips a bit during idle then returns, not sure why.

If I had to guess I'd say there were a valve timing problem allowing intake and exhaust valves to overlap too much? My cam does overlap the valves but only by 2 or 3 degrees if I remember correctly - next issue on the list to chase...

Thanks again to all who chimed in with advice and suggestions!