2: Poor Mid-Range Performance

Possible Causes:

1.	Carburetor Tuning
2.	Exhaust system
3.	Too much cam
4.	Ignition
5.	Low compression pressure

Carburetor Tuning:

Typically, mid-range performance is controlled by the jet needle/needle jet combination. This is because the majority of mid-rpm operation is at low throttle settings or on the highway at cruising speeds of 50 - 70 mph. The HSR42 or HSR45 can deliver enough air/fuel mixture to support these speeds with throttle openings between 1/8th & 1/4, where the straight-diameter part of the jet needle controls fuel flow.

Mikuni supplies four different jet needle sizes to accommodate tuning requirements in this range, one set of four for the HSR42, four for the HSR45 and another set for the HSR48. They differ only in the diameter of the straight section of the needle. The leanest is J8-8DDY01-98 (HSR42 example part number) and the richest is J8-8CFY02-95 (HSR45 example part number). We commonly refer to these needles by their "dash" number (-95, -96, -97 or -98).

Flat throttle response in the mid-rpm range is seldom caused by either an over-rich or overly lean condition. Flat mid-rpm performance is more likely due to the effects of the cam or exhaust design. If the needle size is incorrect, it will normally reveal itself as poor mileage (too rich), slow warm-up (too lean) or light detonation when accelerating moderately from around 2500 to 2900 rpm (again, too lean).

A typical FXD (either engine type) motorcycle will deliver around 45 mpg at 65 mph on a flat, windless road. A heavy touring machine (FLHT- series) may be down a few mpg from that standard. Fuel mileage in the 30s indicates a rich condition.

Please refer to the tuning manual, available on the Manuals page for instructions on diagnosing and tuning.

Note:

Confusing symptoms is one of the most common errors in diagnosing carburetor tuning inaccuracies. For instance, low power at 60 mph (2500 rpm) in top gear may have one or more of several causes: The exhaust system may not work well at that rpm, the cam design may not work well at that rpm, the ignition timing could be incorrect for that rpm, or, --- the carburetor could be set too lean or too rich at that throttle opening.

Notice that when the carburetor was mentioned above, it is the throttle opening we refer to and not the rpm. This is an important difference.

While the performance of other engine components depend, to a large extent, upon rpm, the carburetor only responds to the position of its throttle valve (slide) and the amount of air flowing through it (and sometimes the direction of that air flow).

One of the most valuable carburetor tuning aids is to change rpm (down or up shift) while holding the same road speed. An example: The engine gives poor acceleration from 60 mph (2570 rpm) in top gear. If you maintain the road speed and down shift to fourth gear, the throttle setting will remain essentially the same but the engine rpm will increase 20%. If the poor top gear acceleration is due to, say, poor exhaust system performance at that rpm, then, the problem will either go away, get better or at least change its character. If, on the other hand, the problem is carburetor tuning, the poor acceleration will remain the same because the carburetor throttle opening is the same.

Exhaust system:

Straight pipes:

Open straight pipes perform poorly in the 2500 to 3800 rpm range. If they are 34" or longer, they do not perform really well at any rpm.

Symptoms include missing, backfiring through the carburetor, reversion (fuel dripping out of the air cleaner) and poor acceleration.

Open mufflers:

"Gutted" mufflers with stock (or stock-like) header pipes tend to perform poorly in the same rpm range as straight pipes and exhibit similar symptoms.

Long thin mufflers:

Long, small diameter mufflers with full-length baffles often exhibit the same symptoms as straight pipes, although their over-all performance may be better.

High performance 2-into-1 systems:

These systems are often poor performers in the 2000 to 3000 rpm range. Most 2-into-1 exhaust systems deliver a significant torque dip at 2500 which is slightly less than 60 mph in top gear for most stock Harley Big Twins.

Header pipe diameter:

The great majority of Harley engines, of any displacement, do their best work with 1-3/4" diameter exhaust pipes. Larger pipes tend to suppress mid-rpm performance and, for that matter, seldom deliver the best power at high rpm either.

Header pipe length:

The stock header pipe is about 30". Multiple tests, made by several groups, confirm this length as being very nearly the best for all-round performance. Shorter (less than 27") and longer (over 32") header pipes significantly reduce peak power, throttle response and over-all performance. An exception to this "rule" are a couple of the high performance 2-into-1 systems which work very well with longer (and un-even) header pipe lengths. Stock Harley header pipes are near-perfect in diameter and length.

Muffler size:

It is not possible to make a muffler quiet, small and powerful at the same time. One can choose power and small, quiet and small but not all three. The reason stock mufflers are poor performers is because they are small and quiet.

However, small and loud is not a guarantee of performance. In general, small mufflers with large straight-through, perforated tube baffles (looks like a tube with many holes drilled in it) make the most power and the most noise. An exception to this rule (there may be more) are the popular H-D Screamin' Eagle (and Cycle Shack) small slip-on mufflers which perform very well yet are not straight-through designs. The popular louvered core baffles restrict flow at full throttle & high rpm and reduce power a bit as a result. 

Too much cam: 

The most important cam timing event is when the intake valve closes. The intake closing point determines the minimum rpm at which the engine begins to do its best work. The later the intake valves close, the higher the rpm must be before the engine gets "happy."
 
High rpm cam designs often perform poorly in the rpm range associated with ordinary riding. The problem with such choices is that the engine seldom spends time in the rpm range favored by such cams. Unfortunately, in the quest for maximum power output, many-too-many Harley owners choose a late-closing, high-rpm cam for their engine.

A majority of any Harley motor's life is spent in the mid-portion of is rpm limits, between 2000 and 4000 rpm. At open-road cruising speeds, that range is more like 2500 to 3500 rpm. With current Big Twin gearing, top gear at 2500 rpm returns a road speed of 60 mph and 3500 delivers 84 mph. Riders sometimes "putt" around at 2000 or less. Even when accelerating to cruising speed, few of us use more than 4000 - 4500 rpm as a shift point. Very seldom, in day-to-day use, do our engines get near 5000 rpm, let alone 6000.

Even the mildest of Harley-Davidson's aftermarket cams (Evo or Twin Cam) do their best work above 3000 rpm. At 2000, the majority these cams seldom perform as well as the stock cam(s).

The rpm at which a Big Twin gets "happy" can be predicted by the closing point (angle) of the intake valves. The angle is expressed as the number of degrees After Bottom Dead Center (ABDC) that the valves reach .053" from being fully seated.

30   degrees = 2400 rpm 35   degrees = 3000 rpm 40   degrees = 3600 rpm 45   degrees = 4000 rpm 50+ degrees = 4500 rpm

These relationships are approximate but should hold true to within 200 rpm or so. They also assume that all other tuning factors, exhaust, ignition, etc., are operating correctly.

If you have one of the late-closing cam designs installed, say one that closes the intake valves later than 40 degrees, then you cannot expect excellent performance at 2000 rpm. No carburetor adjustment, ignition adjustment or exhaust system can change this.

Ignition:

Stock H-D Evo Big Twin ignitions have two advance curves ---- a quick advance curve for part-throttle, light load running, and, the very slow advance curve for mid to full-throttle running. It is this second curve that determines the ignition timing when accelerating even moderately. While not the most common reason for 'soft' or 'flat' acceleration in the mid-rpm range, the stock Evo ignition doesn't help.

The Screamin' Eagle Evo ignitions have the same full throttle advance curve as the stock ignition. The only difference between the two is the rev limiter rpm which is 5200 for the stock unit and 8000 (much too high) for the Screamin' Eagle ignition.

Ignitions with quicker advance curves, such as the CompuFire (curves 6,7 or 8) or Dyna 2000 (#1 curve only) have aggressive advance curves and improve throttle response and part-throttle performance in the mid-rpm range, especially below 3000 rpm. These two examples are that only; there are other after market ignitions that also contain quicker advance curves.

Stock Twin Cam ignitions are more complex than the earlier Evo type. They use a manifold pressure/engine revolution rate system for choosing ignition timing for any combination of rpm and throttle setting. We have no reason to recommend non-Harley ignitions for the Twin Cam engines.

Low compression pressure: 

The higher the pressure within the combustion chamber when the air/fuel mixture is ignited, everything else being equal, the more power your engine produces and more efficiently it runs. However, if the pressure it too high, detonation (pinging) may occur which can destroy an engine.

Each combustion chamber design has an upper pressure limit above which serious, damaging detonation is likely. With modern American 92 Octane lead-free gasoline, a reasonable upper pressure limit is 180 psi for the Evo Big Twin and 190 psi for the Twin Cam. A well-tuned motor should not suffer detonation with these pressures.

The standard method for determining the compression or cranking pressure of an engine is to remove the spark plugs, install a standard compression gauge into one of the spark plug holes and, with the throttle full-open, crank the engine over with the starter motor until the pressure gauge needle stops rising. This usually takes 4 - 8 compression strokes. Both cylinders should be tested.

Stock Evo and Twin Cam motors develop cranking pressures in the 150 psi range. If a late-closing cam is installed, with no other changes, the cranking pressure will go down. The reason high compression ratio pistons and racing cams are so often associated is because the higher compression ratio pistons (and/or milled heads) are needed to regain even the normal moderate cranking pressures, let alone raise them for more power and efficiency.

Low cranking pressures (because of late closing cams and stock pistons) can significantly reduce performance in the mid-rpm range.