Scooter won't start when hot
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Author:  ordinal [ Sun Jun 06, 2010 6:10 pm ]
Post subject:  Scooter won't start when hot

Alright, so my Z50 has been running perfectly for several months now. It has the stock cylinder, tecnigas Tek pipe, 4.3g rollers, malossi clutch and bell, malossi yellow torque spring and a new belt.

I rode it for half an hour the other day and then parked it. After getting back to it 5 minutes later, it refused to start. I kicked and kicked and kicked for an hour but nothing. I had a fat spark, and the plug was getting wet. A guy living across the street from where I parked came over and lent me a compression gauge, and I got 90 PSI. So I thought it was toast: Either the rings, reeds or a gasket.

I come back the next day with a truck to pick it up, and as soon as I get there it starts on the first kick! Runs perfectly again. I stop it and start it, not a problem. I ride it home for half an hour (runs well) and stop it. Then, it acts the exact same as the other day, it will not start under any circumstance. I don't own a compression gauge so I do not know what it was, but I am assuming that it was, once again, low.

What is my problem here? Does having compression while cold but not while hot indicate something specific? I will check the reeds first, and if it's not those I'll do a top-end rebuild, most likely. Anyone experienced this before?

Author:  DerHahn [ Sun Jun 06, 2010 6:41 pm ]
Post subject:  Re: Scooter won't start when hot

i had an issue like this on my suzuki years ago. but it had nothing to do with the compression. it actually had a hairline crack in the intake. when cold it was fine. but when hot, the crack opened up allowing too much air in and it wouldn't fire.

drove me nuts for weeks. you may not have the same issue, but it's something else to think about.

Author:  ordinal [ Sun Jun 06, 2010 9:23 pm ]
Post subject:  Re: Scooter won't start when hot

I do not think it's a fuel ratio issue due to the compression reading I got the first time it happened (90PSI). Thanks for the advice though.

Author:  speedandstyle [ Sun Jun 06, 2010 9:28 pm ]
Post subject:  Re: Scooter won't start when hot

It is related in a physics way! When cold it has enough compression to run but as soon as it gets hot enough then it can't. Metals expand when hot so in a way it could be related. If your compression{hot or cold} reads at 90psi then it is time for a new top end.

Author:  ordinal [ Sun Jun 06, 2010 9:33 pm ]
Post subject:  Re: Scooter won't start when hot

I was thinking that myself, and I only discounted the idea of the intake manifold having a crack because that wouldn't affect a compression test. There could easily be a hairline crack in the cylinder or a pinhole in a gasket, so based on that same principle there could be a variety of problems.

I was just wondering if difficult hot starting is a symptom of FUBARed rings or if there could be something more complex at work.

Author:  DerHahn [ Tue Jun 08, 2010 4:35 am ]
Post subject:  Re: Scooter won't start when hot

did you recheck the compression since getting home? this was a compression gage that belonged to someone else. how do you know it was correct? best to test again to be sure before tearing down the engine when it doesn't need to be.

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