85 vt500c won't start when hot

[JeffL1985]

Ok so its a 1985 vt500c first start up of the day fires right off runs fine unless you try to go full throtle then it starts missing unless you back down to about 3/4. Take about 15min ride shut the bike off wait 3-4 min bike will not start bike cools off 20-30min fires right up. Brand new battery shows around 12.5 when key is off checked the spark plug boots found the two straight ones to be bad replaced all the wires with custom accel wire kit, checked the coils when cold between the two primary connector lugs ohms out at 2.6-2.7 specks say should be 2.0 but run the bike around the block for 10min check them again front coil 2.9-3.6 ohms rear coil ranges wildly from 3.3-70+ ohms does this sound like the coils going bad.
also just synched the carbs, checked the voltage output at 2k-3k rpm stays around 13v and rebuilt the starter any body know whats wrong here I,m lost. Thanks

[John Hopkins]

If the battery reads about 12.5v that is a bit too low..but if it reads about 13v at 2k-3k that means it is charging ok...Probably wouldn't hurt to charge the battery with a charger capable of about 4amps..

It's not easy to fault find when the bike is not here but I would remove the air filter and check that is is fairly clean..

Changing the fuel filter is very cheap and easy to do, then saw the old one in half and see how clean it is.

The rear coil sounds strange but it is easy to say replace it when it might just be a dirty contact between the meter and the coil so check the connections before you rush out and buy a new one.

Using an extra spark plug to check for a spark when it won't start is an old trick..Just remove the plug cap and fit it to the spare plug and press the spare plug thread against the engine and watch for a spark..do this on all four plugs...If you have a good spark the fault lies with the fuel.

John.

[Zerzil]

Originally Posted by John Hopkins

If the battery reads about 12.5v that is a bit too low..but if it reads about 13v at 2k-3k that means it is charging ok...Probably wouldn't hurt to charge the battery with a charger capable of about 4amps..

My old Shadow, 13.6 was the magic number. 12.5 would be about where everything appeared to be turning on fine, but when I hit the start button, it would just all turn off and look like it died, or just wouldn't turn over. Hard to believe that 1 v would make all the difference, but it does. Also make sure the connection on the battery and wires are clean and sitting tight - the smallest amount of crud, or loose will affect this too.

Originally Posted by John Hopkins

Using an extra spark plug to check for a spark when it won't start is an old trick..Just remove the plug cap and fit it to the spare plug and press the spare plug thread against the engine and watch for a spark..do this on all four plugs...If you have a good spark the fault lies with the fuel.

... just remember to hold the rubber part - holding anything metallic will hurt (you'll know if you do it wrong)

HTH

Z.

[John Hopkins]

Originally Posted by Zerzil

... just remember to hold the rubber part - holding anything metallic will hurt (you'll know if you do it wrong)

HTH

Z.

I was going to keep quiet about that..but I use a piece of wood to press the plug against the engine..sometimes the voltage will jump across to your fingers if they are too close..

John.

[csmbt]

I had a similar issue on my vt750 1983 .... turned out the starter motor needed to be rebuilt, mine also would fire up cold but then not restart when hot

also you might want to make sure it's not a fuel flow problem of some description

[JeffL1985]

Finaly figured it out after charging the battery, rebuilding the starter, two of the plug boots were bad and it was that rear coil after it would get hot its resitance would sky rocket so just took it for a ride around town and out to the country with a few stops started right up every time but you still can't crank the throttle wide open but thats an issue for another day.
Thank you for the post.