Friday, December 22, 2017

Inside Rat Racer - Episode #44

By David L. Morrill
@ Mototique

2017 Sons of Speed Practice @ New Smyrna Speedway
Jim Dohms - Dohms Creative Photography
In the previous episode, I shared some inside details of Billy Lane's Sons of Speed races held @ New Smyrna Speedway during 2017 Daytona Bike Week & Biketoberfest. In this episode, I'll share some of the updates made to my 1921 Harley-Davidson blanked off single cylinder racer, known affectionately as "Rat Racer."

Rat Racer - 1921 Blanked Off Harley-Davidson Single
Michael Lichter @ 2017 True Grit Vintage Bike Show

Between the Bike Week race, and the Biketoberfest race, the only real changes made to the chassis, were to add a set of brass handlebar risers from Faber Cycle, along with a set of  Billy Lanes Choppers Inc. 1" board track handlebars. This allows me to adjust both the handlebar height & angle for comfort.

I also added a kill button, and a compression release, to the left side of the bars. A Biltwell throttle assembly from J&P Cycles along with a modified stock throttle cable gives a self closing throttle for safety.


My 21J motor is numbered in the mid 3400s of a total production of 4526 - 1921 Model J 61ci V Twins. It has been blanked off by removing the rear cylinder creating a 30.50ci-500cc single race motor. I uses a 1924 iron piston & rings.

Left Side
I shared the basic set up of the bottom end of this motor in Episode #8.  The female 21J rod was replaced by 1924-29 model J/JD drilled rod, which is longer than the stock 1921 J model rod.

Add caption

The longer rod had another advantage. While not changing the motor's stroke dimension, it does push the piston up to with 1/16" of the top (photo below) of the cylinder bore at top dead center. The stock 21J rod stops piston travel about 3/8" below to top of the cylinder bore, which greatly limits the compression ratio and performance. The longer late model rod solves that problem.

Through the Intake Valve Pocket
Exhaust valve at the top, Piston Crown at the bottom
I am currently running a standard 21J - 4 lobe cam, with a set of modified cam followers for a blanked off single found by Terry Marsh. I hope to have one of Matt Walksler's race cams installed for the next race.

Standard 21J Cam with Modified Cam Followers


My motor uses a standard 21J Intake Pocket , which has been slightly ported to remove the sharp edges, which impede gas flow. It has a stock intake valve, with a 1923-29 conical intake spring & collar supplied by Competition Distributing. The Intake valve spring is shimmed 40 thousands of an inch to increase spring pressure. A stock exhaust valve was used, along with a stock style exhaust valve spring also from Competition distributing. I am in the process of replacing the exhaust valve spring with a heavier spring from Matt Walksler of Period Modified.

Hood Intake Tower - Left           1923 Conical Spring Vented Intake Tower - Right

I have two different intake towers I've used. One is after market prototype make by George Hood, and the other is a later model vented tower for the conical spring. Both towers use the ball end JDH style intake push rod. 


Carburetion has been the single biggest challenge of this project. I went through several period Schebler & Linkert carburetors, with little success, and one carburetor fire. Most were for twin cylinder bikes, and could not be leaned down enough to perform well through the whole throttle range.

When I toured Billy Lane's Choppers Inc. and examined his Sons of Speed racers, I found he was using modern carburetors, so I decided to give them a try. The first Mikuni VM32mm was too large, and lacked mid range performance. I then settled on a round slide Mikuni VM26-606 carburetor setup for a small bore 4 stroke dirt bike.

Rubber Mounted Mikuni VM26-606 with K&N Air Filter

While tuning is still a work in progress, the current carburetor specs are:

190 main jet, 22.5 pilot jet, Air Screw 1.2 turns out, 1.5 slide, O-O Needle Jet, 5E75 Needle on the middle notch.

With this setup, the motor starts easily, idles smoothly , picks up rpm, when the ignition is advanced, and pulls well through the rev range. The carburetor is rubber mounted with a piece of high temp water hose, and has a K&N air filter installed.

The stock 21J cast gear case cover/oil pump was replaced with the stamped steel gear case cover/cast oil pump introduced in late 1922.

1922 Stamped Steel Gear Case Cover & Cast Oil Pump

I have used a total loss battery powered ignition system for many years now. It used a 1925 timing case with original points, powered by a small 12 volt Security System sealed battery. My most recent update was to replace the original points with a Gotronic Electronic Ignition. The points were replaced by a magnetic sensor, that triggers a small control box. A Dyna 12 Volt Single Wire Coil provides the spark. Ignition timing is set with the timer at full advance 9/16" - 40 degrees Before Top Dead Center.  The small green LED light on the top of the control box signals the triggering of the ignition as the timing lobe passes the sensor.

Modified 1925 Timer & Gotronic Ignition Control Box

These are the modifications I've used over the past few years. All seem to have worked out well on the street & track, so far!

A short ignition test video below: 


Biltwell Inc. -  J&P Cycles Destination Daytona - Ormond Beach, Florida

Choppers Inc. - Billy Lane - Daytona Beach, Florida

Competition Distributing - Sturgis, South Dakota

Gotronic Ignitions - Gelu Olaru - Romania

Period Modified - Matt Walksler - Waynesville, North Carolina

Terry Marsh - Klamath Falls, Oregon

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