1973 100 G5 2 Stroke. Why won't it start?

Did you check for a spark? Remove both spark plugs. Put them back into the boot covers and hold them one by one against the cylinder or any clean metal part. Ignition on and turn the engine over. You should get a fat blue spark?

Put your finger in the plug hole, turn the engine over. The compression should blow your finger out.

The plug should be wet with fuel if the carburetor is working.

Engines only need three things to start
Compression
Fuel in the right dose
A spark at the right time, just before top dead centre on the firing stroke of the 4 stroke engine.

Hopefully our Harley Guru and ex dealer Greg Mahin will be along soon with manuals and more advice.

Lots of advice here

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Did you check the timing? If the timing is off there will be a reduction in performance and power will be greatly reduced. If the engine is "pinging" or "knocking" under heavy acceleration, your timing is advanced, and pre ignition is occurring (that's when the spark has ignited the fuel in the combustion chamber prematurely), resulting in a knocking sound. When the piston is compressing the air/fuel mixture within the combustion chamber, the spark plug produces a spark from current it receives from contacts closing in the distributor cap. (Where all the spark plug wires are plugged into) if that spark come BEFORE the piston finishes it up stroke or compression stroke, the piston is then being slowed down from the kinetic energy produced by the exploding fuel. Thats where we loose our power. Idealy, the spark should come AFTER the piston has rotated over the top of its stroke, It has reached its limit in compression and is on its way back down then the spark should ignite the compressed fuel, thus giving the piston an additional "shove" back down to start its "Power" stroke, once its reached the bottom of the power stroke (stroke #2 of 4) it is then carried back up for the "exhaust" stroke pushing the burnt fuel out of the combustion camber and into the exhaust manifold, one it has reached the top of that stroke it begins its journey back down to start stroke number 4, the Intake stroke. Drawing in air/fuel mixture to again be pushed back upward to compress the air/fuel, and repeat the process over and over again. Now that I have thoroughly bored you to tears, Yeah, check the engine timing, if its not the timing it may be a vacuum leak.

Often it's the easiest thing that stump us. Try changing the spark plug first. Sometimes a spark plug just won't fire under load, i.e., while it's in the cylinder but will spark if you have it removed. There are checkers for this, but it's a lot less expensive to just replace the spark plug. Do that first and let me know.

Do you have compression, good spark and is fuel getting into the cylinder.?

posible problem is the starting capacitor of the motor,,?? better replace it asap....

there are three things that engine needs to start fuel,spark and compression you stated that fuel and spark but what about compression if no compression could rings or valves!

Whilst I cannot offer a definite solution to this problem, it is worth disassembling the carburettor looking for dark brown material that collects if you don't drain the fuel tank after each use. I have started running all of my petrol engines dry to avoid this problem of fuel evaporating leaving coagulated oil in the carburettor as a membrane. You won't damage a good motor by doing this since as you should have lubrication for the piston rings right up until the agonal stroke.

The exhaust valve opening at the beginning of the compression stroke is designed to make the engine easy to crank.
Since you know the exh. valve is opening, it means you have torn into the engine (maybe ?) because you don't really see it any other way. Did you put it back together correctly ?
IF an engine has spark, proper timing, compression and fuel it will run.
Since yours runs for a bit, I say you have a carb/fuel delivery problem.

if its a 2 stroke then you need fuel spark and air to get the bike to start have you got compression? check it by removing the spark plug and put your finger over the hole air try and start the bike your finger should be pulled into the hole if not then a piston ring is either cracked or broken

open the air box and check the filter then clean the carb thats all you need for a 2 stroke

if its a 4 stroke then again check the air filter and compression remember that a 4 stroke doesnt have the oil with the fuel it goes in a separate chamber

hope this helps


John

if it has a kill switch make sure it is on, check that you are getting spark at the plug and have the right fuel in the motor.
Remove the air filter from the carbi and squirt some 2 stroke fuel directly into the throat of the carbi and try to start it.
hope this helps