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Discussion Starter · #1 ·
I'm starting to run out of ideas for this. So before I do something terrible to this bike, I thought I'd try asking.

I'm riding a '78 Yamaha sr500. It's got a brand new battery and plug in it. When I get up in the morning, it starts strong on the first kick. When I get off of work it does the same thing. But if I stop anywhere after driving it for less than a few miles I'm looking at a wait of thirty minutes to two hours betwen trying to get it started and the engine flooding.

This only happens about 40% of the time, but that's far too much for my taste. After it happened last night I walked it home a few miles and put it on the charger to see if I had somehow killed the battery trying to start it. Full charge.

So if anyone has an idea or would like to see a video of me setting the whole thing on fire, let me know.
 

Shantytown Mayor
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As cool as that would be let's hold off on that.

First you need to know why it won't start.You need 4 things to make an engine run: fuel/air mix, spark, compression and it has to happen at the right time so timing also. I would say the timing is ok being it does run ok when it does get running.

See if it is getting spark first. To do that when it won't start take the plug cap off the plug and stick a screwdriver into it. Hold the metal part of the screwdriver about 1/8" away from the cylinder head while you hold onto the handle and someone cranks the engine over. You should see a nice fat blue spark. If you don't then your problem is ignition related.

Coils and condensors will sometimes fail when they are hot but work ok when cold. Condensors are pretty cheap and you may as well replace the points along with the condensor. You already replaced the spark plug so you should be good there. Coils on the other hand are a little bit more money. You can have it tested but that won't tell you much sometimes if it only malfunctions when hot. So give it a full tune up (plug, points, condensor) and clean or replace the air filter, it probably needs it anyways and it don't cost too much.

If you have spark then the next thing to check is see if you are getting fuel. You said it floods so I assume it's getting fuel. As long as it's not getting to much you're golden there and from what you say I think you're ok in that department.

Now we will move to compression.When things get hot stuff expands and you could be loosing compression. The fuel air mix needs to be compressed enough or it won't light off and explode like it should. I would check the valve clearance. There are caps on the cylinder head that you need to remove to check that. You should really pick up a manual at this point because it can get somewhat complicated until youve done it a few times. First you'll need the engine on the top of the compression stroke. Then you need to run a feeler gauge between the tappet and tip of the valves, both intake and exaust. This is called checking valve lash. The intake is generally around .003" of clearance and the exaust is a little more at .005" clearance because the exaust valve gets hotter than the intake and expands more. These are ballpark figures, hence why you need the manual for exact specs. Usually you can get by with those specs I gave ya but try to be exact as possible. If they are off you'll need to set them. There is a lock nut and set screw ont he end of the tappet.
 

Shantytown Mayor
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Next you'll want to check compression. This is done with a compression gauge. You take the spark plug out and thread the gauge hose into the hole. Crank the engine over about 5 times and record that reading. Around 120 PSI is the minimum I think for good compression for a 4 stroke single again a manual would have the exact spec. Next unthread the hose and pour about a teaspoon of oil down the plug hole and turn the engine over slow a few times. Retest and record that reading. If it goes up most likely the piston rings are worn. If it stays the same you have a leaky valve or something else.

I would guess it's something fairly simple like the valve lash or condensor or maybe just a hogged up air filter letting it get too much fuel.

Here is a site that has parts for your bike. Their prices are pretty good also. I've used them many times in the past. Just click on the Yamaha tab and find your model.

http://www.partsnmore.com/
 

Shantytown Mayor
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Anytime.

I would clean the air filter first and check the valve lash. Those don't cost a cent to do other than buying a set of feeler gauges and somethin' to clean the filter with if it's a foam one. Still I would give it a full tune up either way being it don't cost a whole hell of a lot. Probably hasn't been done in awhile.
 
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