Today I want to talk about a situation I hear all too often. Someone’s bike, whether it be a two-stroke or four-stroke, only starts when it is pushed.
Before I discuss potential causes for this scenario, take a moment to think through the situation yourself.
What mechanical factors would result in either a two-stroke or four-stroke only starting when it is bump started?
In either case, the reason the engine is able to start when it is push started is because it is able to build more compression than it otherwise could when it is kicked or the electric starter is engaged. More compression is achievable because the cranking RPM is higher than what’s possible with the aforementioned starting methods. With a higher cranking RPM for a four-stroke, more air will fill the cylinder on the intake stroke, and for a two-stroke the scavenging process will be improved. With this being the case we must look at reasons why the engine is struggling to build compression in the first place.
Starting problems specific to four-strokes:
1. Valve seat recession - When a valve seat wears out and recedes, the valve moves up towards the camshaft. This leads to diminished valve clearances and if left to run its course, the valve and shim will bottom on the camshaft’s base circle. This can prevent the valve from seating and make the engine hard to start.
2. The valve is bent - A valve with a serious bow to it may get jammed up inside the guide and not return all the way back to its seat. Bent valves typically result from an over-revved engine where the valves contact the piston. Valves can also bend to a lesser extent if they were mated to valve seats that were not cut concentrically to the guides, or they were paired with worn seats.
3. The valve stuck in the guide - This is usually due to the engine overheating. When the engine overheated the clearance between the valve and guide diminished which caused metal to transfer from one part to the other, ultimately ruining the surface finish on one or both parts. Once this happens the valve may be prone to sticking in the guide until the engine warms up.
4. The valves and seats do not seal well - Worn valves and valve seats can compromise the seal between them. Valve and seat wear is a natural part of running an engine but can also be accelerated by ingesting dirty air.
Starting problems specific to two-strokes:
1. The reed valve is worn - Reed petals that don’t close all the way, are chipped, or bent will not allow sealing of the crankcase and efficient gas flow up from the crankcase into the cylinder.
2. An engine seal or gasket has failed - A two-stroke engine requires a well sealed crankcase and cylinder in order for it to scavenge gases efficiently. A worn crank seal, leaky base gasket, or problematic power valve seal can all make starting more difficult.
Two and four-stroke problems:
1. The piston rings are worn - Worn piston rings will allow compressed gases to escape past them.
2. The head gasket or o-rings are leaking - Usually a leaking cylinder head will be accompanied by white smoke if coolant is being pushed into the combustion chamber, by coolant being blown out the radiator, or both.
I hope you found this rundown of potential problems useful for diagnosing bikes that like bump starting over a kick or the push of a button. Can you think of any other problems that would lead to lack of compression? If so, leave a comment and share them.
If you liked this post and want more technical info, check out my book, The Four Stroke Dirt Bike Engine Building Handbook. In it you will find over 300 pages of technical knowledge to help you get off on the right foot when rebuilding!