Improved Uniflow Mod

Many Savage owners have noticed the tendency of the engine to lean out as the level in the fuel tank drops. Amongst Savage Hobbyists, this effect is commonly know as the Half Tank Lean (HTL) problem. Tuning is always a compromise between running too rich at the beginning of a tank of fuel and running too lean near the end of a tank of fuel. Engine performance is sluggish with a rich mixture and engine damage can result from a lean mixture due to lack of lubrication and detonation as the engine begins to run too hot.

This article describes a modification to resolve this problem which benefits from low cost, ease of embodiment, and the retention of the stock fuel tank. Alternate modifications are available which may allow further benefits, such as improved Center of Gravity and better Fuel Tank protection. Check the Modifications page for alternate choices.

Contents 1 Theory 2 Modification 2.1 Parts Required 2.2 Tools required 3 Procedure 3.1 Tank Preparation 3.2 Installation 3.3 Re-tuning the Engine 4 Tips 5 Helpful Links

Theory

Concerning the common HTL term, the engine doesn't really lean out at exactly the half tank point. It leans out constantly as the fuel level drops. It's because it gets noticeable at around the half tank level that the problem gets its name. HTL is caused by "at least" these two problems:

• Static: As the fuel level drops, the pressure of the fuel to the carburetor drops. This is not a large change, yet is a significant contributing factor. The shallower the tank, the less of a problem this is.

• Dynamic: As the fuel level drops, it takes longer for the pressure line to build pressure in the tank when you go from idle to wide-open-throttle (WOT). Reducing the restriction of the pressure line helps a great deal, therefore shortening it is one answer that will make a noticeable difference.

The standard way to minimize changes in engine performance due to fuel level changes is to pressurize the fuel tank using the exhaust, therefore minimizing the hydro-static effect of varying fuel levels. With respect to Figure 1, it can be seen where exhaust is routed to the fuel tank via a Pressure Line helping force fuel to the carburetor via the Fuel Line.

Figure 1 - Standard fuel system

This modification involves making changes to the system which includes a one-way check valve, some fittings and the minimizing of Pressure Line length. As can be seen in Figure 2, pressure from the exhaust at point “A” now enters the tank at the same level as the Fuel Pick-Up, point “B“. The exhaust pressure at any specified rpm will now be consistent near the bottom of the tank at the Fuel Pick-Up. The one-way valve is closed as the pressure in the exhaust area of the tank is lower than the pressure near the bottom of the tank due to the weight of fuel in the tank.

When the throttle is released and the engine rpms decrease down, the pressure in the exhaust system drops. The one-way valve now opens as the pressure at “C” is less than the pressure at “D”. Exhaust gas pressure in the tank is reduced from the top of the tank instead of fuel being forced back out of the tank from point “B” towards point “A”.

Figure 2 - Fuel system with uniflow mod

A more pictorial view can be seen below in Figure 3.

Figure 3 - Fuel system with uniflow mod

Modification

Parts Required

T-fitting: Robart P/N 221

Fitting: 8-32 thread, Du-Bro P/N 539

One-Way Valve: Yamada P/N YS0405 (see alternate under Tips)

Extra fuel line

Ty-Wrap, small

Tools required

8-32 Tap and Tap Handle

Portable Drill and drill bit

X-Acto Knife

Procedure

Tank Preparation

1. Disconnect all lines from the fuel tank and remove it from the vehicle.
2. Referring to Figure 4, drill and tap the tank for the fitting using the 8-32 tap. Drill the hole smaller than you would if tapping metal and only partially tapped the hole (tapered tap not all the way through). The fitting needs to make its some of its own threads to prevent leaks.
3. Install fitting and gasket. The fitting should get start getting snug before the gasket becomes compressed.
4. Re-install fuel tank in vehicle.
5. Connect the fuel line
6. Connect the pressure line to the newly installed fitting

Installation

1. Cut the pressure line at a point between the air filter and the top of the tank.
2. Cut a new piece of tubing and connect one end to the fuel tank lid nipple.
3. Connect the straight-through section of the “T”-fitting between the tank lid connection and the exhaust pressure line connection.
4. Cut the pressure line between the fuel tank lid and the “T”-fitting and install the one-way valve. Make sure it is installed so that air will flow from the fuel tank to the “T” fitting.
5. Connect the remaining pressure line from the newly installed tank fitting to the third nipple of the “T”-fitting.
6. Shorten all pressure lines for a minimum best fit. This is important.
7. Use a Ty-Wrap to loosely hold the line to the air-filter intake tube so it won’t be near the spur gear. Be careful not to pinch the pressure line.

Figure 4 - photo of modified fuel system

Re-tuning the Engine

1. Richen the high-speed-needle (HSN) about a ¼ of a turn (CCW) to be safe.
2. Fill the tank with fuel and start engine.
3. Once fully warmed up, re-tune engine.

Tips

• Pre-clean your One-Way Valve. It should only take a gentle puff of air to open it. This is called the "cracking pressure", and is commonly referred to as “stiction”. Manufacturing oils on the internal silicon flapper valve and housing must be cleaned. This is particularly easy with the Yamada valve as it can be taken apart, cleaned with alcohol, the parts patted dry with a tissue. After a lot of use, the one-way valve may become a little sticky, and should be cleaned again with alcohol.

• It is not considered wise to allow an engine to run out of fuel while operating at WOT or under heavy load. Once this mod is embodied, there will be little indication the engine will be about to run out of fuel. It may be best to bring the vehicle in to check the level of the fuel once running for a considerable time, and either take it easy from the point where the fuel is very low, or shut the engine down and re-fill the fuel tank.

• Although the aluminum bodied Yamada valve is highly recommended, an alternative plastic valve is available. It is made by Fourmost and is P/N 138. They aren’t recommended as they require far too much "cracking pressure", however they can be used with slight modification. It’s fairly easy to do, and only takes a few minutes. Refer to the instructions below and the sketch in Figure 5, Once done, the valve should operate just as well as the Yamada valve.

Fourmost Valve

1. First, take the valve apart. They aren’t meant to be dissembled, so to do this, use a small jeweller’s flat bladed screwdriver. Work it around the valve where the white and red plastic housings meet. This will relieve the pressure of the white part where it is swaged over the red housing. Once you’ve gone around once, start going round again, but from time to time, pry gently with the screwdriver until you see the two parts start to separate. Then you can just pull it apart by pulling on the two barbed ends.
2. Next, clean the oil off the parts. Use alcohol and pat dry with a tissue.
3. Now you modify the white housing. Using a sharp x-acto knife blade, insert the tip into the air-hole on the inside of the white housing and rotate it gently to make a chamfer. Do not use a drill bit, as it will gouge and ruin the housing. Make the top of the chamfer about twice the diameter of the original hole. This will allow more air-pressure against the flapper valve and increase the force against the flapper valve by a factor of 4, so much less air-pressure is needed to open the valve. Additionally, since less flapper valve rubber to white housing contact is now there, less “stiction” due to exhaust residue will be present.
4. Now the tricky part. You must remove any flash around the edge of the chamfer. A flat-bladed jeweller’s screwdriver in combination with your x-acto knife will do the job. A perfectly flat inside of the white housing is required for the flapper valve to seal.
5. Clean out the white housing again, and re-assemble. Note that the flapper valve has a step in it. The step goes towards the red-housing. The flapper must be fitted into the red housing first, and goes in until flush. Then snap the two housing back together.
6. Now test it. A gentle puff applied to the white end should flow freely through the valve. Now try the red end. At no time should there be any air-leak. If there is one, you must have missed some flash, and you need to take the valve apart again.

Figure 5 - Fourmost valve modification

Helpful Links

Uniflow Theory