1996 Shadow VT500 won't start when warm

[Zephans]

I bought a 1996 Honda Shadow last year. It came with a peculiar problem. It starts and runs fine when the engine is cold. However If it warms up for a couple minutes or more then turned off it will not start until it cools off. (Exception: It used to start up again if I tried within 20 seconds of turning off.)

When the problem occurs the starter can barely turn over the engine. It is like there is a huge vacuum or blockage of some sort keeping the engine from turning over easy. I haven't tried compression start when trying to start. I have tried using ether to get it moving as a fuel diagnostic step but starter could only turned over a few times and ether appeared to do nothing.

I am thinking that it is vapor lock issue of some sort in the fuel line. Unfortunately I have no idea how to diagnose, isolate, and/or fix the problem. (I had someone tell me to insulate the fuel line, but that doesn't seem very feasible given tight tolerances.)

I will note that this model has a spiffy fuel on/off/reserve switch that is also has a vacuum hose connected to the engine head. Fuel won't go through the switch unless the engine is turning over.

Related story: The vacuum gas shutoff feature caused me some brief pause when I opened sediment bowl and turned fuel ON to drain tank… only to find no gas would come out. When I turned over engine it actually started (with sediment bowl off!) making me seriously wonder why my motorcycle will only run when I have sediment bowl off :o. Of course the sediment bowl was draining gas well (into a turkey roasting pan) but it was still hilarious after having problems starting with everything hooked up but running fine while pouring gas out. I closed bowl and drove around the block a couple times. I had several big pauses when trying to accelerate but otherwise ran fine… until I returned home and turned off and tried turning on again. I have since drained gas and put in 1.5 gallons of fresh gasoline. No change in warm start problem :-).

I need to find and purchase a manual for this vehicle (and a road tool kit since it has none). I feel pretty ignorant about how this motorcycle works and steps to maintain and fix it right now. I also need someone's kind help to get me past this warm start block. Please reply!

Thanks in advance! -Zephan

[mute]

If you wanna make sure it isnt a vapor lock, run it, carefully, without the cap on the tank, or so loose that it wont stop gas from spilling out. If it runs better and restart, buy a new gas cap for it. If that doesnt stop the problem, may want to try a different set of coils, not sure if your local honda shop has a way to check em.

[Zephans]

Hi Mute. Thanks for the reply and suggestion. I've actually opened the gas cap after the motorcycle has warmed up but it did not affect the real hard starter turnover condition. Perhaps running with gas cap open would do different (I haven't tried this yet).

Someone gave me another theory that merits consideration. I was told that a defective starter motor can increase resistance when hot. This would exactly match my symptom which is starter trying really hard to barely turn over engine when motorcycle is warm.

Vapor lock symptom would be "out of gas" even when gas tank has fuel. Unless someone tells me that vapor lock or other problem can induce a vacuum or blockage that would overwork the starter motor I am going to pursue the faulty starter theory. Please post if you have heard of this or have any helpful comments. My next task is to get price quote from local Bent Bike for starter, find out if it is a DIY project for novice mechanic, and go from there.

FYI: Ran my kids around the block today several times. My Shadow ran perfectly until I was done and turned it off for the evening... then tried starting it just to confirm the warm start problem. Yup, still there.

-ZephanS

[mute]

Yep, didnt even think of the starter being the culprit. Im so used to havin no mechanical problems on the metrics.. lol. You whould be able to change it yourself, all you need is a pretty good memory if you have to remove other stuff to get to it.

[CB750F]

You may want to try replacing the battery cables also. When they get hot the resistance goes up and current cant flow through them fast enough to turn the starter.