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1985 VT500C won't start when hot

8849 Views 48 Replies 10 Participants Last post by  David C.
I bought this bike back in Feb. of this year. It had a bad rod knock, so we tore it down and replaced the crankshaft, push rods, and push rod bearings. It is running fine. I can start it, drive it wherever I want to go, but if it's hot, it does not want to re-start. After a bunch of reading here, I decided that the fuel system might be creating a vacuum, so I started poking around. I noticed that the fuel shutoff valve has this vacuum line;

The new fuel shutoff valves do not have the vacuum line;

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Does anyone have experience with this problem, or should I be checking another part of the fuel system?
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Ok. Admittedly I have never heard of this kind of issue resulting from what you did. I rebuilt my p/c. With that said, You should reinstall the correct p/c. They make rebuild kits for them. So square that issue away first. If it still does the same thing then you have coil issues. I sus[ect that the p/c flaaper valve is not opening. Only problem is it makes no sense to me that it only does it when the bike is hot. So that brings me too


Do not mistake a weak spark, for good spark. These 1st gens are very dependent on a FAT spark. Remove the coils from the bike entirely and check for cracks and ohm the primary and secondary sides out as was suggested by Chuck. Check the manual for the specs. I forget off the top of my head.

Next advise is very important too. Disassemble the plug wires.
Clip about 1/4" from the plug and coil side of the spark plug wires to get to new conductors. Use dielectric grease on the plug and coil side to ensure a good electrical saturation is running through the ignition system. While your there regap the plugs too.
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Does it sound like it cranks a bit slower when hot? Like the starter is getting tired and drawing more current hot and then the whole system suffers from too much voltage drop? Maybe jump it with a strong battery to eliminate that.
Yes, it does. Almost like riding it is drawing the battery down. I just put a new Yuasa battery in it thinking that might be the problem, but it didn't help.
Must be the coils. I already ordered 4 new plugs since that was the cheapest thing to do, they will be here soon.
This is very telling. If it sounds like the starter is dragging every time its hot, then I agree with Swifty. You need to rule out the charging system. Heres a couple videos on how to check the stator and R/R.

If they check out then go to the coils as said before. Let us know what you find out.
congrats. was there any noticeable issues with the pugs. cracked or worn down.
I hear. Mine is too. I install 1 range hotter plug in the winter time and she fires right up. Mine likes choke in either temp, hot or cold. Less choke when its hot out.
As Eagle stated and as I stated a few pages back. I told you exactly how to test the coils (test them when cold, btw) I also explained how to clip the plug wires back. No need to buy new wires. Honda made awesome wires on the 1st gens.

I thought it was suspect when u came on here and said it fixed itself by changing the plugs. I even expressed my concerns then, when you posted that.

I am and have been in agreement with Eagle all along. Please go back and read my instructions. You do not have a fuel problem. Sorry if i sound crass, i don't mean
NP Dave.
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