Because pump E85 can vary seasonally in some geographic areas, it may be necessary to adjust jetting or air bleeds to maintain the best possible tune up if you do not run your own blend. In some areas the ethanol content increases in the summer time and the gasoline content increase in the winter. This change in fuel blend was intended to help with cold weather start up in flex fuel vehicles, but can adversely affect non-fuel injected cars converted to E85 if you are unaware.

Assuming your tune up is perfect to begin with, your engine will act richer when more gasoline is in the blend and it will act leaner when more ethanol is in the blend. In my experience the most I have had to change a tune up in one season (from richest to leanest E85) has been 4 jet sizes and .003 in the air bleed. This is with a blend change from E83 to E72.

How do you know when the E85 blend has changed and a "tweak" to your tune up is needed? There are a couple indicators.

The old reliable method is by reading your plugs. Yes, this still works with E85, just as it did with gasoline - you are looking for the same clues with some minor changes. First, E85 seems to color the plugs the same way gasoline does, it just takes longer and the color is less pronounced. A light-tan colored porcelain and a light carbon ring that goes 3/4 of the way around the end of the plug's threads usually indicates a mixture that is right in the ball park. Keep in mind that you should only read a fresh plug after one WOT pass; shut the engine off immediately and pull the plug to get an accurate WOT reading.

The best way I have found is to monitor AF ratios with a wideband O2 meter set to the Lambda scale. There are a few models on the market designed for gasoline that work just fine with E85. For WOT air fuel ratios, depending on the engine, most engines make best power somewhere between .77 and .85 Lambda. This is a little richer than gasoline. For idle, I seem to have best luck setting the idle mix screws for .85-.90 Lambda. And for cruise (this can vary widely depending on the camshaft used) anywhere from .80 to 1.1 Lambda can work well. Obviously, leaner will provide better mileage; however, sometimes the "good mileage tune up" comes at the expense of throttle response.

Here is an example of a tune-up sheet that I send with all of my new carburetors to assist the user with tuning. While some companies tune their carburetors on the rich side of the scale to provide minimal problems and customer call backs, I prefer to tune my carburetors for max power and consistency. Providing the information below helps the customer stay in the ball park if tweaking a tune up is necessary due to blend variations.

Once you are sure that you need to make a change due to fuel ethanol content or an engine modification you need to understand which circuit to change and then how much to change it. Below are some basic suggestions for tweaking your tune up to better match your engine and fuel.

Idle and Transition circuit tuning

The idle and transition circuit has the greatest effect on your cruise. When at light throttle below 2500-3000 RPM there are two orifices that can be tuned.

The Idle Feed Restrictor, or IFR, controls how much fuel is available to the idle and transition circuits. Small changes of .002" or .003" to the IFR can make a big difference. The low speed air bleed controls the air and can be used to fine tune easier than the IFR. A change of .004" will usually make a noticeable difference and is easily accessible from the top of the carb.

Here is an example of an E85 idle and transition tune up. As you can see, small changes should be done with the low speed bleed and large changes with the idle feed restrictors.

The low speed air bleed is also important to how an engine responds to WOT. The low speed bleed controls the sensitivity of the throttle slot. The small the LSB the more sensitive it becomes. With larger carburetors (especially on smaller engines) the low speed bleed may need to be set very small in order to achieve a transfer slot that is sensitive enough to provide that instant throttle response needed from low RPM.

A quick test to see if your idle feed restrictor and low speed air bleed are "in the ball park" is to adjust your idle mixture screws to provide an idle mixture of .85-.90 Lambda. This is about 1/4 to 1/2 turn richer than max vacuum at idle. After your mixture screws have been set properly, turn one of them in (clockwise) until it stops (don't force!). Count the number of turns it takes to close the screws from the proper idle setting. If the number of turns is between 1 and 2 you are in the ball park. If you are closer to 2 turns your idle and transition circuit is on the lean side; if you are closer to 1 turn it is on the rich side.

Main circuit tuning

Your Horsepower Innovations carburetor comes with the high speed bleed sized properly for your application. It is not recommended that you change your high speed bleed to fine tune WOT mixtures because the high speed bleed affects the "flatness" of your fuel curve. A smaller bleed tends to make the curve go richer at higher RPM and a larger bleed tends to make the curve go leaner at higher RPM. The high speed bleed also affects how quickly the main circuit starts up (what RPM and load) so again, be careful about changing high speed bleeds!

For fine tuning, jet sizes of 2 at a time are recommended. There is a tolerance with standard Holley jets that sometimes makes jet changes of only 1 size unnoticeable. Changes of more than 2 at a time can move a tune up from on the lean side to "dangerously lean" so be careful and always spot check your spark plugs!

It is also important to note that when changing jets it is important to maintain the factory "jet spread" from front to rear. All Horsepower Innovations E85 carburetors come equipped with a power valve, and the power valve channel restrictors (the actual metering orifices behind the power valve) are calibrated for your specific application. This allows your engine to cruise and stage at a good AF ratio and maintain a front-rear fuel balance at WOT. Only on rare (race only) occasions does Horsepower Innovations recommend removing and plugging the front power valve.

Accelerator pump tuning

E85 carburetors require more fuel from the accelerator pump for transitioning from stage or cruise to wide open throttle than gasoline carburetors. The viscosity of E85 is different than gas and because of this it flows differently - so you may notice that the accelerator pump calibration on your Horsepower Innovations carburetor is very different than your old gasoline carb.

Generally speaking, there are two primary ways to tune with the accelerator pump system. The pump cam controls the amount of fuel per pump shot. The bigger the cam the more fuel is delivered. The shape of the ramp will also affect how quickly the pump shot is delivered, especially when transitioning quickly from idle to part throttle. The pump squirters control the timing or how fast that volume of fuel is delivered. A large aggressive cam and a large squirter will deliver a big shot of fuel very quickly, where a smaller less-aggressive cam and a smaller squirter will deliver less total fuel over a longer period of time. A large cam and a small squirter will stretch that large volume out over a very long period.

If your engine bogs immediately on a quick hit of the throttle the most likely culprit is actually the ignition, TQ converter, rear end gearing, or camshaft! Tight converters, freeway gears, and a slow ignition advance curve are very hard to tune through with a pump cam! If these things are all in order try a larger squirter and a larger pump cam. If the engine responds initially, then bogs out for a split second, try a smaller pump squirter to lengthen the pump shot duration.

Probably the most important thing to check with an E85 carb that uses large pump cams is that the linkage is properly adjusted. The pump lever should begin to move as soon as the throttle lever is moved - no gap between the pump lever and the throttle lever. When at WOT the accelerator pump lever should not be bottomed out and jammed tight! There should still be room to mover the lever a tiny bit. Finally, make certain that the levers do not bend and are properly aligned - misalignment or flex can be a difficult problem to run down.