Example, for a 10 tooth driver, 88 tooth driven and a 34.5
in. roll out – OR = 39.37 x 88 / 10 / 34.5 =10.04 rev/meter
Note that the higher the ratio is its numerical value is lower and vice versa.
When trying a different gear ratio you want to evaluate
its result on your performance. The stopwatch may tell the story, but you also
probably want to know how your top speed was affected. If you use a tachometer
with a maximum recall you can use this to calculate your top speed. The formula
for speed is:
V (km/h) = RPM x Driver teeth x Roll out / Driven teeth / 656.2
So for the above example at 15000 rpm the speed is – V = 15000 x 10 x 34.5 / 88 / 656.2 = 89.6 km/h.
As a general rule I believe its best to find the lowest
ratio that does not cause a loss of top speed or over revving of your engine, as
this will give the best jump out of the slow corners. Don't ignore the stopwatch
either. For quick reference I like to use a chart that shows the overall ratio
and maximum speed for the typical ratios you would use and your typical maximum
RPM. This can be done manually or on a spreadsheet program. I limit my charts to
the gears I have in my toolbox and sort it by overall ratio. Here is a sample
you can expand on.
|Overall Gear Ratio Chart||
Speeds shown are km/h at -
Changing valve springs at
Or anywhere else for that matter. Although I have never seen a valve spring break, they do start to take a permanent set (collapse) pretty quick. So we put fresh springs in every 2 or 3 races. The springs are pretty cheap (about $3 each) and easy to change without removing the head. So it is good insurance to change them often.
Here's the way I do it. Remove the valve cover and spark plug, then turn the engine over until the piston is at top dead center on the compression stroke (Both valves are closed). I use a screwdriver in the spark plug hole to feel the piston at top position. With a screwdriver in one hand, push the valve spring keeper down enough to turn the rocker arm to one side of the valve with the other hand, thus exposing the spring keeper. Remove the valve rotator "pill" from the exhaust valve stem. Unpack the new springs and have them sitting upright within reach, ready to use. Fish the bent end of an "L" shaped Allen Key into the spark plug hole and position it so that it holds the valve closed by levering it against the piston top with light force (don't force piston down). While holding the valve shut with the Allen Key in one hand use the other hand to remove the valve spring keeper and spring, install the new spring and re-engage the spring keeper. Replace the exhaust valve "pill". Press the spring keepers down as before and engage the rocker arms. Check and adjust the valve lash. It should not have changed, as we did not touch the adjuster nuts. This process only takes 5 minutes, but don't rush it, and don't be interrupted while you are doing it. You don't want to drop the valve into the cylinder. Note that the two spring keepers are different, as only one accepts the exhaust rotator pill, so don't mix them up. If you accidentally drop a valve, it may still be engaged in the valve guide. You can usually fish it back up with the Allen Key or by carefully raising the piston if it has gone down a bit. Otherwise you have to remove the head to recover the valve (very rare).
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Race prep your Honda engine
To get a race prepared Honda you've got a few options. You
can buy a new race ready engine form one of the several engine builders in your
area (in the Vancouver area I suggest Red Head or Bulldog. You can buy a used
motor from another racer. If you buy a used motor, you should go through it or
have it freshened up before it is run. Or you can buy a new Honda and do the
race prep yourself. This is the route we took. It saved us some money, but the
greatest reward is when you beat some of the pro-built engines. I'll go through
what you should do to get going with a Honda GX200 (6.5 hp) or GX160K1 (5.5 hp)
Start with a copy of the ASN Canada rule book and a repair manual that covers these engines (Haynes Small Engine Repair is fine). The modifications that we will make to the engine are as follows.
Install a pulse driven fuel pump. The main object here is to minimize the opportunity to get oil transferring from the crankcase through the pulse tube to the pump diaphragm. If the pump diaphragm cavity fills with oil it will impede the pumps operation. I installed my fitting in the case side cover just behind the top rear bolt. Locate the hole so that it is inside and clear of the internal rib structure of the cover. Drill and tap the cover for a 1/8" NPT fitting. Install a 1/8 npt to 1/4" barb fitting with thread sealant in the hole. Mount the pump over the engine and connect the pulse hose between the engine and pump. The pump and hose should be arranged so that any oil in the line will drain back to the crankcase. Install a small fuel filter in the line from the tank to the fuel pump. I also mount a squeeze bulb primer pump in the line from the tank, which makes it easier to start the engine if the carb was dry. Keeping the fuel cool is a good idea, so consider incorporating an insulator in the pump mounting.
Lap the valves and set valve lash. Just
like piston ring seal, valve sealing is soooo important to top performance. Hand
lap the valves with a fine lapping compound, followed by a fine metal polishing
Valve lash also plays a role in how your engine will respond. Generally speaking we are looking for a tight lash setting to increase valve lift and open duration. A good place to start is 0.002" cold for both intake and exhaust. When the engine is hot the cylinder and barrel, being aluminum, will grow more than the valve linkage, resulting in a looser setting. If you set the lash to tight when the engine is hot, then there may be no clearance when it is cold. A tight setting will benefit power at high rpm while a looser setting will be better at lower rpm and produce more torque when exiting corners. Because we have no gearbox the engine must produce good power across a wide range of rpm. What's best at high rpm and peak output may not be the hot ticket overall if it takes too much away in the midrange.
A couple of pointers for adjusting valve lash. First loosen and re-snub the adjuster jam nuts just so that the adjuster can still be turned with moderate effort, similar to the effort of turning a stover lock nut. Then adjust the valve lash by turning the adjuster nut (the jam nut should follow). Done. No need to tighten the jam nut further. Next time you do an adjustment, all you have to do is turn the adjuster nut. The jam nut only needs to be tended to if the adjuster turning effort is too light. Do not set the valve lash with the jam nut loose, as when you tighten it, it will shift the backlash in the adjuster threads and throw your setting off.
Altering the cam timing. Advancing or
retarding the cam timing will have an effect on the power curve and where on the
curve power is maximized. In considering the timing of four events, intake
opening, intake closing, exhaust opening and exhaust closing, it is generally
accepted that the point at which the intake valve closes has the most effect on
performance. By the time the intake valve closes the piston has completed its
downward travel and is beginning to come back up on the compression cycle. even
though the piston is moving upward, the inlet air/fuel charge is continuing to
flow into the cylinder due to the flow inertia, or ram effect. As the piston
moves up, pressure increases in the cylinder. At the point where this pressure
equals the ram pressure, intake flow reversion begins to occur. Having the
intake valve open beyond this point of reversion is just wasteful and hurts
compression and cylinder filling, thus causing a loss of power. Conversely if
the intake valve closes before he reversion point then a lost opportunity to
completely fill the cylinder with air and fuel is realized, and power is not
optimized. The problem is that the reversion point varies with engine speed. At
higher rpm the piston is further up on the compression stroke before reversion
occurs. For this reason a retarded cam will work better at high rpm while
advancing the cam will produce more torque at low rpm. As we are dealing with
Honda GX160K1 and GX200 engines here, and they each have quite different cam
profiles, my recommendations are specific to our experience with each engine. On
the GX160K1 we have had good results by retarding the timing 4 degrees from the
stock position. This is for an engine that is running without any restrictor
plate. We have not run with a restrictor, but my guess is that the restrictor
would hurt the ram effect and advance the reversion point, so I suspect he stock
cam timing would work better. On the GX200 the cam profile has a much later
intake closing event than he GX160 so we have been happy with leaving the cam
timing as delivered from Honda.
The cam drive gear is simply a press fit on the crankshaft, so to retard the timing it is removed, turned 4 degrees clockwise (viewed from the drive end) and pressed back on. Sounds simple, but how do you know if it has been moved the correct amount? First we must take a reading of the cam's initial position. Install and zero your degree wheel. Rig up a dial indicator with an extension that extends down the intake pushrod hole and bears on the valve lifter. With the degree wheel at TDC on the compression stroke, zero the dial indicator reading. The dial indicator reading is set to zero, but the pushrod should still have some extension travel available. Turn the flywheel clockwise (viewed from the flywheel end) until the dial indicator reading is descending to 0.020" and record the degree wheel reading at this point. This reading should be close to 215 degrees ATDC. Note, always turn the flywheel clockwise for these readings to avoid any errors caused by backlash in the cam gears. If you don't have a dial indicator then you can get by with a feeler gauge. In this case, first set the valve lash at 0.025" at TDC. Then using a 0.005" feeler turn the flywheel just until the rocker pinches the feeler (0.020" before valve close) and record the reading on the wheel. Repeat several times to ensure repeatability of the reading. Scribe alignment marks on the crank gear and the crankshaft for visual reference when re-setting the gear. Scribe a second mark on the crank about 0.040" to the right of the first. Using a gear puller, pull the gear and stop pulling just short of releasing it from the shaft. Now scribe alignment marks on the back side of the gear and the shaft. These marks are easier to reference when starting the gear back on the shaft. Now remove the gear, rotate it the 0.040" or 4 degrees clockwise and press it back on. To press it back on I use a piece of 5/16 UNF treaded rod in the end of the crank, a short piece of tubing or pipe, a washer plate and a nut. Warming the gear helps too. Re-install and zero the degree wheel and re-check the reading at 0.020" before intake closing as done before. The new reading should be about 219 degrees ATDC if you were lucky. If the reading is not what you expected then you can decide to fix it or try it on the track. It's your call.
I don't have a gear puller, so I made up a Mickey Mouse puller that works fine. I used three 3/8" carriage bolts with their heads hooked over the gear, a plate over the end of the crank drilled for the three bolts, and a hose clamp to hold the bolts against the gear. Progressivley tighten the bolts to pull the gear off.
ratio by decking the block. To play this game you will need a 50 cc burette
to measure the combustion chamber volume. The measuring procedure is described
in the ASN Canada rule book under section 36.2 of the Technical Regulations. The
"Marvel Mystery Oil" they refer to as the testing fluid is a gasoline additive
available at Canadian Tire stores. To avoid being disqualified, you must leave a
little room for carbon build up. I suggest making these adjustments after you
have run the engine for 2 or 3 races to build up some carbon. You can then
measure the volume with carbon and after cleaning it out to determining how much
allowance is needed. For reference, each 0.10 cc volume reduction requires a
0.0011" deck cut on the block.
To shave the deck I use a piece of 1/2" thick plate glass with a sheet of sandpaper taped to it. I tape the glass down to my work bench and completely strip the block. Then I sand the deck down by working it in a figure eight pattern on the sandpaper. I start with 320 grit and finnish with 600 grit. I measure progress with a depth gauge in the head bolt holes. Proceed slowly and check your progress carefully, as going too far is a disaster. So far the GX200s that I have checked don't need much cut, so even if you don't do this it probably won't make much difference. I am currently running an engine that is not cut, and it is competitive.
On the GX160K1 the rule allows quite a bit of improvement. Fortunately most, if not all of it, is achieved by replacing the GX160 head gasket with the thinner GX200 gasket. This takes off about 0.032", or 2.91 cc. So even if you don't have a burette, I would make the gasket change, and you should be safe. In either case you will have to clean the carbon out regularly to avoid possible disqualification.
Modify the throttle linkage. A couple of points to consider here. Whatever you use for throttle linkage, you want to make sure that it won't damage the carburetor throttle lever or butterfly shaft with linkage over-travel. If these parts are damaged, they are not listed seperately as service parts by Honda and would require carb replacement or an alternative repair. And secondly make sure you have adequate return springs in the system. I use 3 return springs, one directly on the butterfly shaft lever, one on the relay lever on the engine, and one on the accelerator pedal. My linkage system uses a relay lever that is mounted on a pivot bolt that is threaded into the governor shaft hole on top of the case. From this lever I use a pushrod made from a bicycle wheel spoke to actuate the carb throttle lever. I have a bend in this pushrod so that when the carb lever reaches its stop the pushrod will bend, or buckle, and not put excessive force on the carb lever. The pushrod is such that it returns to its normal shape when the accelerator is released. The relay lever also reverses the direction of travel so that the accererator cable can extend forward directly towards the pedal. In my layout I mount the accererator cable to one of the front fuel tank ears. Another method of achiving over-travel safety, is to insert a tension spring between the cable end and the lever or pedal. For a throttle cable, I use a bicycle long shifter cable (the light variety) which I buy without a sheath. For a cable sheath I use 3/16" nylon tubing with compression fittings for end attachments. The cable is quite loose in the tubing and doesn't bind. To mount the sheath to the motor I drill and tap the front of the fuel tank mount ear 1/8" NPT for the compression fitting.
Install Header and Muffler. Firstly
you can build your own header, as I do, if you have a MIG welder. I have a local
fab shop plasma cut the flange plate and pre bend the tube. I use 1" OD x 0.040"
wall 4130 (chrome-molly) aircraft tube. This tube is a bit tricky to bend
without buckling, but if you use a large radius and limit the bend angle it will
turn out fine. Do not try bending it without a good tube bender. To have flanges
cut you can download the following CAD and Acrobat files. The fab shop can use
the CAD file to program their cutting machine.
FLANGE CAD FILE (.dxf)
FLANGE ACROBAT FILE (.pdf)
It is a good idea to weld a small stopper to the tube to locate the muffler, as the tube must insert at least 1" into the muffler to be legal. The stopper will give you an easy inspection reference and prevent the header from going too far into the muffler and thus hurting performance.
Install air filter and filter adapter. Rules affecting air filters and air filter adapters have changed periodically. Make sure you understand the current rules and use components that meet them. There may be some parts, new or used, out there that do not conform to the current rules and should be avoided.
Install clutch. My comments here apply
to the "Max Torque, Draggin' Skin" type of clutch, but may also apply to other
brands as well. The clutch may be installed with the drive sprocket inboard or
outboard, whichever way works best for your situation. The clutch is retained
with a 5/16 UNF bolt and flatwasher in the end of the crankshaft. When installed
the clutch will have some "end play" on the crankshaft and can float in and out.
This is OK. I tried clamping the clutch tight with no end play, my fist time,
and only damaged the clutch drive hub. Your clutch should be serviced after each
race. I clean and de-glaze the drum and shoes, and clean and re-oil the bushing
with two drops of oil. Do not over oil the bushing as it will bleed onto the
shoes and prevent lockup.
|Parts required to build and install a Honda GX200 engine.|
|GX200QH||1||Honda 6.5 HP engine|
|99101-ZF50780||1||#78 main jet, used during break in period.|
|99101-ZF50750||1||#75 main jet.|
|GX140 emulsion tube (nozzle)|
|12391-ZE1-000||0||Valve cover gasket|
|12251-ZL0-003||0||Head Gasket (shim style)|
|11381-ZB2-800||1||Crankcase side cover gasket|
|18381-ZH8-800||0||Exhaust header gasket|
|13331-357-000||0||Flywheel key (file to advance ignition timing).|
|13013-ZL0-001||1||Piston ring set, 0.25 mm oversize (grind to 0.006" gap)|
|14751-883-000||2||Valve spring, G200|
|BPR6ES||1||NKG spark plug.||Auto Supply|
|1||Fuel pump from a Honda GC160|
|#6-32 allen head set screw, 3/8" long||Industrial Supply|
|325-4A or 125-4A||1||1/8 npt to 1/4" barb fitting (pump pulse)||Industrial Supply|
|1||RLV-B91-1" muffler||Kart Shop|
|1||Honda 1" x 12" header pipe.||Kart Shop|
|1||1/8 x 3/4 steel flatbar 16" long for muffler brace. (should be supplied with header)|
|1||Stainless hose clamp 1" (muffler to header)|
|1||Stainless hose clamp 2-1/2" (brace bar to muffler)|
|10||Header wrap tape 1" wide.||Auto Supply|
|1||Air filter adapter|
|1||Air filter||Kart Shop|
|1||Engine mounted chain guard.||Kart Shop|
|1||Dragon Skin Clutch Kit wit 13, 14, & 15 tooth drivers and black shoes||Kart Shop|
|1||Dragon Skin 10 gram weight kit.||Kart Shop|
|1||62 tooth #35 split rear sprocket.(plus other sizes as required)||Kart Shop|
|1||#35 chain, 36" long||Industrial Supply|
|1||"Rust Check" spray Chain lube)||Canadian Tire|
|1||3/16" fuel hose (yellow that stays soft).|
|5||1/4" fuel hose (yellow that stays soft).|
|1||Fuel primer bulb, 1/4"||Canadian Tire|
|0||4 cycle racing oil (14oz per fill)||Kart Shop|
|0||or - Amsoil 0-30w synthetic (14oz per fill)|
|1||5-30w motor oil for break in only then switch to synthetic||Canadian Tire|
|1||Throttle cable (lightest bicycle shifter cable)||Bicycle store|
|1||Throttle cable sheath, loose fit.(bicicle brake cable sheath or 3/16" nylon tubing)||Bicycle store or Industrial Supply|
|Cable sheath end compression fitting, 68-3A (1/8" NPT to 3/16 tube)||Industrial Supply|
|1||Throttle cable end spud|
|1||Motor mount, 4 cycle||Kart Shop|
Need a Degree Wheel? Make your own. Just download my degree wheel drawing, print it off on as large a sheet of paper as you can, cut it out and glue it to a piece of aluminum or masonite. Drill the center for your 5/16 clutch bolt, and your done. Make up a pointer that bolts to the fuel tank ear on the block and file or grind a chisel point on it. The larger your degree wheel is the easier it is to get accurate readings.
Download my "Engine build report" sheet in Microsoft Office format or Adobe Acrobat format. Or the Cam Profile Spreadsheet.
Here's a handy chassis setup and timing record sheet that we use.
Some other links:
Do it yourself karting - Jamie Webb's self help site has lots of good tech info.
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