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Help! - Bike Only Starts When Pushed

Help Bike Only Starts When Pushed.png

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!

- Paul
 

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Amazon

DIYMotoFix.com

 

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Another issue I have seen on high compression 4 strokes is the operators ability to kick start it hard enough. I had a 1978 Yamaha TT500 bored over to a 550cc that few people could kick start successfully.

 

 

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There is some good information,  but considering the audience for this post,  I feel it skipped a lot of "tuning" type issues that keep a bike from kickstarting that are NOT compression related.   In a forum like this you have so many different skill levels I can see people running to their dealer for an overhaul,  when all they need to do is fix their carburetor etc.   I do find that people who don't deal with 5-25 year old worn out and passed down motorcycles tend to over look those kinds of riders that end up with the old neglected rigs.  

 

Technique as described for the yamaha "550" above is one of those issues not even mentioned.    

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Trust an old salt with dozens of dirt bikes through the garage... if it's got gas, got spark, some compression at the kicker but still doesn't want to start, go first to the carb float bowl, unscrew the pilot jet, eyeball for crud, poke a tiny strand of copper wire thru. Get it good and clear. Has been the fix more times than I can count, 2T's and 4T's.

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I'm surprised nobody has mentioned timing yet, or possibly weak coil or even a crappy fouled out plug that's been overlooked. Ossagap is right, a lot of people might've read that and thought they needed major repairs when not necessary. 

I just got through having this very problem with my '90 xr200r. My friend and I got a chance to look at it again, and found that it wasn't timed right. Simple fix, the pulse generator was 180 degrees off, so it was timed on the wrong stroke. Which after turning it around, and adjusting the valves correctly, it's finally starting and running good.

If you're mechanically inclined and like doing the work yourself, get yourself a manual for the bike. I got mine used on ebay for  $10 and free shipping. It helped a lot 

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Having spark is not a tell all. It is possible to have weak spark. I had a 250 f with an aftermarket " lighting" stator. It had spark. But would not kickstart. A good hill would get her running  but would stutter under heavy load.  Stock stator solved all of it. 

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On 2/9/2017 at 1:00 PM, ossagp said:

There is some good information,  but considering the audience for this post,  I feel it skipped a lot of "tuning" type issues that keep a bike from kickstarting that are NOT compression related.   In a forum like this you have so many different skill levels I can see people running to their dealer for an overhaul,  when all they need to do is fix their carburetor etc.   I do find that people who don't deal with 5-25 year old worn out and passed down motorcycles tend to over look those kinds of riders that end up with the old neglected rigs.  

 

Technique as described for the yamaha "550" above is one of those issues not even mentioned.    

I agree with your comments and I probably could have framed this one better. My intention was to get beyond the carburetor and look at the next level of potential problems that not everyone thinks about. However, in the process I can see your point as to how beginners could get overly concerned and skip some basic checks. 

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On 2/13/2017 at 5:24 PM, Idahoyamahauler said:

Having spark is not a tell all. It is possible to have weak spark. I had a 250 f with an aftermarket " lighting" stator. It had spark. But would not kickstart. A good hill would get her running  but would stutter under heavy load.  Stock stator solved all of it. 

Thank you for sharing your experience and possible problem to consider during diagnosis.

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On 2/12/2017 at 2:03 PM, Daniel627 said:

I'm surprised nobody has mentioned timing yet, or possibly weak coil or even a crappy fouled out plug that's been overlooked. Ossagap is right, a lot of people might've read that and thought they needed major repairs when not necessary. 

I just got through having this very problem with my '90 xr200r. My friend and I got a chance to look at it again, and found that it wasn't timed right. Simple fix, the pulse generator was 180 degrees off, so it was timed on the wrong stroke. Which after turning it around, and adjusting the valves correctly, it's finally starting and running good.

If you're mechanically inclined and like doing the work yourself, get yourself a manual for the bike. I got mine used on ebay for  $10 and free shipping. It helped a lot 

Thanks for sharing your story and I'm glad you got your problem sorted!

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On 2/9/2017 at 10:34 PM, Holeshotjohnny said:

Trust an old salt with dozens of dirt bikes through the garage... if it's got gas, got spark, some compression at the kicker but still doesn't want to start, go first to the carb float bowl , unscrew the pilot jet, eyeball for crud, poke a tiny strand of copper wire thru. Get it good and clear. Has been the fix more times than I can count, 2T's and 4T's.

I completely agree that going to the pilot jet is a great first step. I should have perhaps broken this post into two parts, one for basic checks, and this one for more advanced issues. Thanks for your comments and advice.

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On 2/16/2017 at 4:24 AM, Paul Olesen said:

I agree with your comments and I probably could have framed this one better. My intention was to get beyond the carburetor and look at the next level of potential problems that not everyone thinks about. However, in the process I can see your point as to how beginners could get overly concerned and skip some basic checks. 

 

Trust that I have read your other presentations and  this is the first one that I thought needed editing to certain audiences. 

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Using the starter vs bump starting would draw down the battery voltage and make for a weaker spark on the transistorized ignition, so that may be the factor in making it only bump startable. Try a jump start with a good battery to see if it fires up then.

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On 2/16/2017 at 5:30 AM, Paul Olesen said:

I completely agree that going to the pilot jet is a great first step. I should have perhaps broken this post into two parts, one for basic checks, and this one for more advanced issues. Thanks for your comments and advice.

That would really help someone new to bike mechanics, whose 31 year old XR600 is extremely hard to kick start, especially when cold. 

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My 350 4 stroke would only start when pushed and after looking at a just about everything else, I disassembled the top end and found that it had a stuck compression ring.  So, it fits this discussion exactly.

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i have recently rebuilt a 2004 Crf250r.. i've got new Vertex piston, Factory Connection cylinder and Hot Rods crank. bottom end bearings ect. im sure the bike has seen many miles and revs beyond comparison by previous owners. i feel as if i may need to replace or change the head/valves as i have only successfully kicked it over 3 times now. i must always bump it in 3rd. monster compression.  but after the first few heat cycles i shimmed the head and found one ex valve had 0 clearance and another intake was very tight as well. never thought about the valves and the guides mating and sticking to each other. also could be carburation. its all been gone through however there is always room for error/improvement with a new rebuild.  Still worried i can barely kick the bike. its only seen one tank thus far.. needs more break in time to free up the new parts maybe?? 

 

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