Post by Mark Sullens on Oct 10, 2009 23:48:51 GMT -5
First of all, you must understand that your NOT tuning a GASOLINE Carburetor, E85 is a different fuel and must be tuned differently than Gasoline.
I’m sure that Phil or someone could chime in with the particulars of E85 vs. Gasoline and the physical differences….
One of the problems with tuning E85 is that you really never know if your lean or fat until you test it with an wideband O2, reading the plugs doesn’t always work because ethanol is a cleaning fuel and your plugs could look new, even after a few months.
Plus the fact that the fuel will change from E85 to E70 as it switches from summer to winter blends.
If you do not have a wideband O2 gauge then water temp has always been a good way to tune your jetting, and I suggest for Racing or street driving that the temperature needs to be at least 170* as the Ethanol requires a temperature of 172* to vaporize.
I have found that a 7-9* temprature rise in the 1/4 mile will net you the best and most consistant power.
So what about the carb itself:
Several different things control your idle; Idle feed restrictors, idle air bleeds, idle air mix screws and more.
The idle air mix screws in the sides of the metering blocks are used to fine tune the idle by allowing an air/fuel mix to enter the air stream of the venturi below the throttle blades.
While you may hear they need to be 1 ½ turns out, this is not the case, it is just a good starting point in a properly tuned carburetor.
It is important for your throttle blades to be set properly to achieve a good idle and prevent hesitations, usually you will want a small square in the transition slot [ ] as viewed from the bottom of the carburetor on the secondary side, and a secondary that is slightly more open with the primary slightly more closed will net you a better transition into the 4 barrel side of things.
The main jets controls most cruise & W.O.T. fuel metering.
The idle circuit under cruise conditions supplies most of the fuel and this circuit is easiest and most commonly tuned with the idle air mix screws.
If @ steady speeds (2200-2800 rpm / return road rpm) the engine surges it is too lean.
You can usually lean the idle mix screws down till you get this surge at 2200-2800 rpm & then richen it up to be safe.
If at this RPM you’re sluggish and lazy, you could be too large on your transfer restrictions
Winter Blend - The higher gas content in the fuel the lower the jet needs to be (E70),
Summer Blend - The higher Ethanol content the larger the jet needed (E85).
Because of the changing fuel and the weather changes these two should virtually cancel each other out and you should never have to go more than 1-3 sizes in a jet change
Typically on E85 1 jet size change, unlike gas, will make a difference in performance.
again....if you do not have a wideband you can tune your jetting for a water temp change of 170* to 177-179* in a 1/4 mile pass.
more than 9* rise and your lean on your main jetting.
less than 7* rise and your rich on your jetting.
and for the record, when I tuned my jetting with my wideband, my temprature went up 8* in a 1/4 mile pass.
Now its time for Power Valve tuning:
When you accelerate from cruise to W.O.T. manifold vacuum drops to almost 0" and the power valve opens increasing fuel flow to the main well and thus to the boosters.
This additional fuel is needed for high demand situations (full throttle) and the power valve restriction channels are the way to tune it (2 channels that are visible when you remove the power valve).
The Primary Main Jet (PMJ) + the Power Valve Channel Restrictions (PVCR) should always net the same flow as the Secondary Main Jet (some exceptions do apply)
In the carburetors I build I determine what flow I want from the rear of the carburetor then I decide how much fuel I want the PVCR to add, once I know these two a handy chart I have will tell me what PMJ needs to be in the carburetor…this will keep the carburetor balanced front to rear usually within 2% of flow.
So how do you know which Power Valve to use?
First I always suggest a 4-window high flow alcohol power valve, just because I don’t want restrictions in the power valve.
I try to get an idea of how much vacuum you have at idle in gear and I will drop the Power Valve opening rate by 1.5-2” depending on the type driving.
Changing the Power Valve to a higher or lower number to tailor when the added flow through the PVCR starts.
Bigger number will pull fuel sooner and add fuel to the middle of the power curve, while a smaller number could lean the curve…
This is where wideband testing really helps… .
Just because it says it will open at 6.5 doesn’t mean it will…. .
we DO NOT use a power valve in drag race engines or hot street cars.
What about MAX power and the top end?
People will sometimes tell you to jet for MPH…those people are WRONG, this is a torque fuel and mus be tuned for ET, not mph.
After you have the idle/off idle/main jet tuned and the PVCR tuned, then you can adjust the top end fuel curve by changing out the HSAB (inner bleeds on top of the carb)
Smaller air bleed............richer high and mid rpm mixture.
Larger air bleed.............leaner high and mid rpm mixture.
Smaller fuel jet...............leaner low and mid rpm mixture.
Larger fuel jet................richer low and mid rpm mixture
Typically with E85 you will want more fuel under higher cylinder pressures. A fuel curve that gets richer as rpms climb is desired.
Changing the float levels alters the amount of fuel in the bowl (reservoir) and the carburetor’s ability to feed the main wells through the jets. By raising the float level the engine’s response is quickened as you have raised the fuel level inside the main well allowing the fuel to be picked up by the booster quicker.
This is an adjustment frequently used to eradicate a lean stumble but is not always the answer to a lean stumble.
By lowering the float levels, the activation of the main metering circuit is delayed and consequently produces a leaner mixture coming off idle. For street applications the latter condition is more economical.
Off Idle Hesitation: Is one the most common problems w/E85 carbs especially when used on modified engines. If you experience an off idle stumble as soon as you accelerate from part throttle or idle check the Water temp first, cold engine can and will usually have a stumble or hesitation. (remember 170*)
dont forget to check the distributor for correct initial timing ….no seriously, I have had customers call me with issues and it ended up being a distributor issue.
(Inadequate timing will also cause a backfire through the carb under acceleration).
Still have a hesitation? check the pump circuit to make sure that the squirter produces fuel as soon as you move the throttle. If not make sure your linkage is set properly (there should be little (.015) to no play between pump arm and pump linkage at idle), and then if you have no pump shot trace the pump circuit back to locate the problem.
I have had the floating check needle stick closed before…. not a fun task to undo.
If you still have a stumble, as a test adjust the idle mixture screws about 1 turn richer from best idle & see if this cures the problem.
One thing to remember when it comes to pump shot is that you only want enough pump shot to cover the lean hole in the fuel delivery curve, any more can make the car lazy.
People tend to get crazy and I have heard of Nozzles as big as .067” with 50cc pumps…crazy, and a cover up to another problem at best!
I hope that I have helped you with some tuning questions, and I ask that if you would like to add to this then feel free to comment.
Be safe….go fast!