A recent request by one of the members of this site to post more information on the American Whizzer motors and performance upgrades are covered below. There will be additional comments at the end of the post to cover durability as part of the process. This post will also cover average costs, and will vary from different Whizzer collectors.
There are many modifications to enhance the vintage Whizzer motor. The "H" motor is slightly more difficult to extract power because of the smaller ports.
Mill .060" from the head. Remove the "island" in the combustion chamber. Open the intake and exhaust ports. Because of cylinder design the exhaust on the "H" cylinder can't be opened as much as later versions. Install a slightly larger carburetor, such as the Carter N703, N665, or Tillotson MT12A.
Install the larger valves from the "300" or later cylinder, the "H" has 3/4" valves, whereas the later motors used 7/8" valves. If possible replace both valves with the larger units, or at minimum upgrade the intake to the 7/8" size.
There are Weber heads, camshafts, and special carburetors to "hop" up the "H", however the larger valve, reworked combustion chamber, milled head, and larger carburetor will offer similar results for a lot less money.
You can also replace the original head gasket with a .010" custom copper gasket for even more compression.
The average "H" will travel at 35 MPH with an average rider [225 pounds]. The average "300" or later motors will easily exceed 45 MPH in stock form. Modified vintage motors have been known to hit 70 MPH on Dyno tests, however they were stroked, bored, special tri-pattern head [165 pound compression], special lifters, hi-lift camshaft, electronic ignition, Cushman MT series carburetor, and radical fuel blends.
I have built several hi-performance vintage motors, and the results were better than expected.
Average cost to mill head & rework combustion chamber $50.00
Re-cut valve seats, lap valves, deck cylinder, hone cylinder, rework intake and exhaust ports, paint cylinder with black cylinder paint Average cost $100.00
If you have the head milled & reworked and the cylinder ported, decked, etc, you should easily reach speeds of 45 MPH.
If you upgrade to the larger valves, hi-compression head, high lift camshaft [Weber], and a larger carburetor, the top speed should easliy hit 55 MPH.
The biggest gain will be in top end speeds, however the power increase will show up throughout the entire power band. The amount of additional torque will allow the Whizzer to easily climb most hills.
The hi-compression head will also need to be milled and a little chamber work.
Because of the outrageous cost of vintage Whizzer racing carburetors [NOS Tillotson/Weber $350.00] it might be wise to consider using the Carter N665 or N703 because of the larger venturi. The larger Carter units were used on the later "300" series motors with the bigger valves.
Best to concentrate on the cylinder, valves, porting, etc. A lot of power can be pulled from the motor by just making it "breathe" better. Increasing the size of the intake valve would make a large difference in getting the motor to rev higher [more top end]. Adding another 500 RPMs will add another 5 MPH to the top end.
The camshaft for the vintage Whizzer is a very costly item, as an example the NOS "Weber" competition version is worth from $250.00 to $300.00. It is possible to pull a lot of power from the motor with the stock camshaft.
Cylinder work with larger intake valve $165.00 to $175.00
Mill head & rework $45.00.
Rebuilt Carter carburetor $175.00
Plus gaskets & shipping.
Speed costs, how fast do you want to go???
Average costs are:
"J" cylinder with larger intake valve & port work $235.00
"300" cylinder with large valves, large ports, ported $300.00
NOS Tillotson "Weber" ML3 carburetor $350.00
NOS "Weber" high lift camshaft $275.00
Ported Cushman MT63 Carburetor $200.00
Several important items to consider in the performance upgrade are as follows:
The stock crankshaft will hold up well, but only to a certain point. Once the compression ratio is increased past 130 pounds, the German needle bearing crankshaft should be used. The tall oil breather should also be installed to allow 2 more ounces of oil in the crankcase.
To keep the "H" & "J" durable and use the original crankshaft the upgrades should be limited to milling the head, removing the island in the head, thinner copper head gasket, larger intake valve, and port work. A larger carburetor may also be considered, but cost wouldn't justify the end results. These upgrades will keep the cost around $200.00 [not including the larger carburetor], and produce enough power to make the average "H" or "J" motor to cruise at 45 MPH or better. Question is......... is it worth $200.00 to go 10 MPH faster [I think so, but everyone knows I like speed].
Hope this information is helpful.
Have fun,
There are many modifications to enhance the vintage Whizzer motor. The "H" motor is slightly more difficult to extract power because of the smaller ports.
Mill .060" from the head. Remove the "island" in the combustion chamber. Open the intake and exhaust ports. Because of cylinder design the exhaust on the "H" cylinder can't be opened as much as later versions. Install a slightly larger carburetor, such as the Carter N703, N665, or Tillotson MT12A.
Install the larger valves from the "300" or later cylinder, the "H" has 3/4" valves, whereas the later motors used 7/8" valves. If possible replace both valves with the larger units, or at minimum upgrade the intake to the 7/8" size.
There are Weber heads, camshafts, and special carburetors to "hop" up the "H", however the larger valve
You can also replace the original head gasket with a .010" custom copper gasket for even more compression.
The average "H" will travel at 35 MPH with an average rider [225 pounds]. The average "300" or later motors will easily exceed 45 MPH in stock form. Modified vintage motors have been known to hit 70 MPH on Dyno tests, however they were stroked, bored, special tri-pattern head [165 pound compression], special lifters, hi-lift camshaft, electronic ignition, Cushman MT series carburetor, and radical fuel blends.
I have built several hi-performance vintage motors, and the results were better than expected.
Average cost to mill head & rework combustion chamber $50.00
Re-cut valve seats, lap valves, deck cylinder, hone cylinder, rework intake and exhaust ports, paint cylinder with black cylinder paint Average cost $100.00
If you have the head milled & reworked and the cylinder ported, decked, etc, you should easily reach speeds of 45 MPH.
If you upgrade to the larger valves, hi-compression head, high lift camshaft [Weber], and a larger carburetor, the top speed should easliy hit 55 MPH.
The biggest gain will be in top end speeds, however the power increase will show up throughout the entire power band. The amount of additional torque will allow the Whizzer to easily climb most hills.
The hi-compression head will also need to be milled and a little chamber work.
Because of the outrageous cost of vintage Whizzer racing carburetors [NOS Tillotson/Weber $350.00] it might be wise to consider using the Carter N665 or N703 because of the larger venturi. The larger Carter units were used on the later "300" series motors with the bigger valves.
Best to concentrate on the cylinder, valves, porting, etc. A lot of power can be pulled from the motor by just making it "breathe" better. Increasing the size of the intake valve would make a large difference in getting the motor to rev higher [more top end]. Adding another 500 RPMs will add another 5 MPH to the top end.
The camshaft for the vintage Whizzer is a very costly item, as an example the NOS "Weber" competition version is worth from $250.00 to $300.00. It is possible to pull a lot of power from the motor with the stock camshaft.
Cylinder work with larger intake valve $165.00 to $175.00
Mill head & rework $45.00.
Rebuilt Carter carburetor $175.00
Plus gaskets & shipping.
Speed costs, how fast do you want to go???
Average costs are:
"J" cylinder with larger intake valve & port work $235.00
"300" cylinder with large valves, large ports, ported $300.00
NOS Tillotson "Weber" ML3 carburetor $350.00
NOS "Weber" high lift camshaft $275.00
Ported Cushman MT63 Carburetor $200.00
Several important items to consider in the performance upgrade are as follows:
The stock crankshaft will hold up well, but only to a certain point. Once the compression ratio is increased past 130 pounds, the German needle bearing crankshaft should be used. The tall oil breather should also be installed to allow 2 more ounces of oil in the crankcase.
To keep the "H" & "J" durable and use the original crankshaft the upgrades should be limited to milling the head, removing the island in the head, thinner copper head gasket, larger intake valve, and port work. A larger carburetor may also be considered, but cost wouldn't justify the end results. These upgrades will keep the cost around $200.00 [not including the larger carburetor], and produce enough power to make the average "H" or "J" motor to cruise at 45 MPH or better. Question is......... is it worth $200.00 to go 10 MPH faster [I think so, but everyone knows I like speed].
Hope this information is helpful.
Have fun,
