No, leave mechanical advance alone for now.
Don't mess with air bleeds.
Not too expensive, but if you can, pick up a Mityvac and an A/F gauge.
Don't mess with air bleeds.
Not too expensive, but if you can, pick up a Mityvac and an A/F gauge.
Edelbrock 1407 off idle bog
- Thread starter Baker7888
- Start date
Can you post the cam number? From that I can pull the cam card.
When you assembled the engine, did you clay number one? Do you know your piston to valve clearance? If you are ok there, you can try 1.6 ratio rockers on intake, will give a little more overlap, essentially about 10 degree extra duration.
What intake manifold are you using? If it's dual plane, is it notched?
When you assembled the engine, did you clay number one? Do you know your piston to valve clearance? If you are ok there, you can try 1.6 ratio rockers on intake, will give a little more overlap, essentially about 10 degree extra duration.
What intake manifold are you using? If it's dual plane, is it notched?
Sounds like you are getting closer to resolving the issue and per your description it sounds like this issue is just on the primary side.
From memory... I think primary on the 750 is 1.44 inch and on the 600 is .98 inch. I Might be wrong..., the specs not in front of me at moment.
Yet, that said, maybe change to 600 CFM might be the key.
Some basic science below...
The way vacuum advance works is the following: The engine starts at base timing. Engine is running and creates vacuum. This vacuum draws fuel through the idle circuit at same time timing advances.
As you accelerate from idle to cruise vacuum drops, throttle plates are more open and now fuel flows from transition slot until plates open further and fuel is pulled through the booster in the venturi, which is part of the venturi effect.
As engine increases in RPM there is less vacuum pulling on the advance plate and you lose the vacuum timing advance. Engine increases in RPM but is losing timing. Combustion is poor, power drops off, pollutes more, etc...
So, there is a cross-over point, where mechanical advance takes over for the loss in timing and timing plate moves in the distributor and timing increases.
From how the distributor is setup depicts when you get full timing at specified RPM. Too soon you can get ping/knock... (I.E.: Detonation). Too late and engine labors it's called labouring, poor fuel economy, poor power, more pollution.
So all you need to do is find the sweet spot where vacuum advance drops off and mechanical advance takes over. This keeps timing smooth through all ranges.
Now the next is fuel and the similar applies. Engine starts and fuel through idle circuit is usually rich, but engine is cold and combustion process not very efficient. RPM increases and fuel runs through the slot plus a little from mixture screws. If too rich idle bleeds help with this to become a little leaner. As you get into cruise fuel flows through venturi.
Then, set by the OEM of the carb, is how much air and fuel can move and is physically limited. But just like timing there has to be a smooth transition through each process.
Then you have the basic mechanics. Piston goes up and down and pumps air, as the engine is nothing but an air pump. Cam sets how much air can move per RPM. This is controlled by valve lift and duration. This air movement is restricted by inflow and outflow.
Inflow restriction is the physical limits of the carburator, intake manifold and flow through the head. Outflow restriction is exhaust scavenging in the head, flow out of the head, flow through exhaust manifold or headers, pipes, mufflers and laminar currents under and around vehicle.
At each point you can measure and change items in this process to get a desired outcome.
So much for the science. In your case you need to just square up the timing and double check fuel/air flow. From cruise to WOT seems to run good as desired.
From memory... I think primary on the 750 is 1.44 inch and on the 600 is .98 inch. I Might be wrong..., the specs not in front of me at moment.
Yet, that said, maybe change to 600 CFM might be the key.
Some basic science below...
The way vacuum advance works is the following: The engine starts at base timing. Engine is running and creates vacuum. This vacuum draws fuel through the idle circuit at same time timing advances.
As you accelerate from idle to cruise vacuum drops, throttle plates are more open and now fuel flows from transition slot until plates open further and fuel is pulled through the booster in the venturi, which is part of the venturi effect.
As engine increases in RPM there is less vacuum pulling on the advance plate and you lose the vacuum timing advance. Engine increases in RPM but is losing timing. Combustion is poor, power drops off, pollutes more, etc...
So, there is a cross-over point, where mechanical advance takes over for the loss in timing and timing plate moves in the distributor and timing increases.
From how the distributor is setup depicts when you get full timing at specified RPM. Too soon you can get ping/knock... (I.E.: Detonation). Too late and engine labors it's called labouring, poor fuel economy, poor power, more pollution.
So all you need to do is find the sweet spot where vacuum advance drops off and mechanical advance takes over. This keeps timing smooth through all ranges.
Now the next is fuel and the similar applies. Engine starts and fuel through idle circuit is usually rich, but engine is cold and combustion process not very efficient. RPM increases and fuel runs through the slot plus a little from mixture screws. If too rich idle bleeds help with this to become a little leaner. As you get into cruise fuel flows through venturi.
Then, set by the OEM of the carb, is how much air and fuel can move and is physically limited. But just like timing there has to be a smooth transition through each process.
Then you have the basic mechanics. Piston goes up and down and pumps air, as the engine is nothing but an air pump. Cam sets how much air can move per RPM. This is controlled by valve lift and duration. This air movement is restricted by inflow and outflow.
Inflow restriction is the physical limits of the carburator, intake manifold and flow through the head. Outflow restriction is exhaust scavenging in the head, flow out of the head, flow through exhaust manifold or headers, pipes, mufflers and laminar currents under and around vehicle.
At each point you can measure and change items in this process to get a desired outcome.
So much for the science. In your case you need to just square up the timing and double check fuel/air flow. From cruise to WOT seems to run good as desired.
I have a vaccum pump. What were your thoughts? I will look at A/F kits today. The last thing that came to mind last night, was cranking vac advance.
Cam is Comp 08-432-8, card attached. I did not use clay or know piston clearance. I am using a spring called Bullet VS 2700 which is advertised to handle up to .575 lift. Idk if 1.6 rockers are an option.
Intake is a GMPP LT1 carb intake. It’s a dry intake. Yes it’s dual plane. It is not notched. It’s a pretty tall intake though.
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The below is a prime example of why there is so much confusion about the subject of ignition timing. Sadly this example is just one of many.
Contains a bunch of garbage and written by someone that probably never setup an engine or even, ever did a tune up.
Some how air injection is "antiquated", cruising highway speed at about 3000 rpm the throttle plates are barely open. And somehow as more air rushes into the manifold it creates pressure to move the vacuum can back to its original setting. The article further explains running leaner is cooler, and this was done because earlier engines ran "very, very hot" and could warp valves and "crack" heads. And all because vacuum advance wasn't used.
It then goes on to explain if you are running a Roots blower, to make sure you hook up the vacuum can "routed under the blower" and this is because between the carb and blower, there is "very little" vacuum when the throttle plates are closed. Yet the photo shows hooking the vacuum line to the intake manifold. Evidently they were not aware that what was "under the blower" is actually the intake manifold.
Sadly this is actually from a fairly respected source.
Contains a bunch of garbage and written by someone that probably never setup an engine or even, ever did a tune up.
Some how air injection is "antiquated", cruising highway speed at about 3000 rpm the throttle plates are barely open. And somehow as more air rushes into the manifold it creates pressure to move the vacuum can back to its original setting. The article further explains running leaner is cooler, and this was done because earlier engines ran "very, very hot" and could warp valves and "crack" heads. And all because vacuum advance wasn't used.
It then goes on to explain if you are running a Roots blower, to make sure you hook up the vacuum can "routed under the blower" and this is because between the carb and blower, there is "very little" vacuum when the throttle plates are closed. Yet the photo shows hooking the vacuum line to the intake manifold. Evidently they were not aware that what was "under the blower" is actually the intake manifold.
Sadly this is actually from a fairly respected source.
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Funny, i was just reading that last night.
Ok hear me out. We’ve discovered if i crank the advance (well beyond a safe limit) the take-off shudder is almost gone.
We’ve also discovered, at my normal timing settings, adding choke, the shudder is almost gone. During this, On the vac gauge the vac drops to around 5 instead of zero.
My thought was to leave my usual timing settings alone, and then crank vac advance to compensate. However by rights, under load (i.e. takeoff from a stop) that vac signal should disappear. So i am struggling to find a workaround for this.
That’s why my other thought was to crank base timing and further limit mechanical but it would mess up my whole curve. So then ultimately i am using timing to compensate for some unknown carb design flaw ?
Believe it or not, I was actually wrong, but in defense, I stated the specs were not with me at the time.
Specs: (Inches)
Bore.....................Primary........Secondary
1406: 600 CFM.........1.44..............1.69
1407: 750 CFM.........1.69..............1.69
So it could be change to 600 CFM might be the wise step. Or try first to open primary booster pickup to .041 as I noted below.
Spending 30 dollars might be better than another 500 if it fixes it.
Also the carbshop.com has a refurbished 600 for 250 dollars available.
1407
1406
Specs: (Inches)
Bore.....................Primary........Secondary
1406: 600 CFM.........1.44..............1.69
1407: 750 CFM.........1.69..............1.69
So it could be change to 600 CFM might be the wise step. Or try first to open primary booster pickup to .041 as I noted below.
Spending 30 dollars might be better than another 500 if it fixes it.
Also the carbshop.com has a refurbished 600 for 250 dollars available.
1407
1406
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Some other information around the Edelbrock Carb.
Around 1993 the carbs were leaned out and at a time where few suppliers were selling 10% Ethanol fuel. This was done for emissions purposes. Today most suppliers are 10% or more for pump gas.
This issue relates to the current known problems with off idle bog/stumble. A possible solution to this and noted on several forums is to open up the primary well pickup (jet) attached to the booster. Open up from OEM .035 to .041 and this has been reported as helping solve this.
Personally I would not do that until you have a second, maybe sacrificial set to drill out. This way you can always reset to out of box settings, in the case it does not work
A primary booster set runs about $15.00 USD 2023 prices.
Also re-jet kit, also running about $15.00 USD.
Mainly use current jet but thinner rod, or use larger jet with it's corresponding rod, or trial/error mix and match until you get desired result.
Next is get your baseline idle vacuum and divide by half and this will give you which spring to use. Spring kit comes with chart and springs are color coded to corresponding in/hg found value. Spring kit is also about $15.00 USD.
Seems like to get the Edelbrock to run right and not bog, is to add about $45.00 USD to the purchase price.
Around 1993 the carbs were leaned out and at a time where few suppliers were selling 10% Ethanol fuel. This was done for emissions purposes. Today most suppliers are 10% or more for pump gas.
This issue relates to the current known problems with off idle bog/stumble. A possible solution to this and noted on several forums is to open up the primary well pickup (jet) attached to the booster. Open up from OEM .035 to .041 and this has been reported as helping solve this.
Personally I would not do that until you have a second, maybe sacrificial set to drill out. This way you can always reset to out of box settings, in the case it does not work
A primary booster set runs about $15.00 USD 2023 prices.
Also re-jet kit, also running about $15.00 USD.
Mainly use current jet but thinner rod, or use larger jet with it's corresponding rod, or trial/error mix and match until you get desired result.
Next is get your baseline idle vacuum and divide by half and this will give you which spring to use. Spring kit comes with chart and springs are color coded to corresponding in/hg found value. Spring kit is also about $15.00 USD.
Seems like to get the Edelbrock to run right and not bog, is to add about $45.00 USD to the purchase price.
Last edited:
The Carburetor Shop
thecarburetorshopmn.com
Remanufactured Edelbrock 1406, 600cfm Electric choke carburetor
Remanufactured Edelbrock 1406, 600cfm Electric choke carburetorPrimary Jet: 98Secondary Jet: 95Metering Rod: 75x47Step up piston spring: Orange (5")Main Venturi Diameter1.19Body Height3.25Bore Diameter Primary Bore 1.44; Secondary Bore 1.69Appearance Grades:-------------------------"A”Grade...
thecarburetorshopmn.com
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