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Discussion Starter #1
I bought an '83 VT500C this summer. I Rode it through November and one morning as I entered a highway I found myseld stuck at about 40mph, and I couldn't get the RPM's up. The problem cleared itself and the bike ran great for the 20 mile trip I took. A few blocks from home on the return trip the same thing happened. Days later I started it up and found that it idled fine but I couldn't get the revs up reliably. Today after taking the carbs apart, cleaning them, replacing the jets I could buy (Slow and Main. I had to re-use the needles) I put it back together and it started right up and idled great. Revving it was smooth and easy for a few minutes, and then it started smoking badly out the exhaust (normal for this bike) and then revving became spotty again. I was told by the guy I bought it from it was running too rich. I don't see anything in the Clymer manual about adjusting the mixture. If anyone has any ideas I would appreciate your input.
 

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hows your air filter? Pull apart the electrical connectors on your spark units and CDI's and coils. Clean them with contact cleaner or brake cleaner and a brush.

How many miles on your shadow? What colour smoke? I had an engine with over 90k on it and it didn't smoke.
 

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Discussion Starter #3
Thanks for your reply. The air filter appears to be clean, and the housing had no dust in it at all when I opened it. I'll try your other suggestions though. As for the mileage, I would have to call it unknown. The odometer shows a little under 18,000, but the guy I bought it from said he bought it showing 11,000 miles and the speedometer was not functioning at the time due to a bad cable. I'll post my results.
 

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i believe there is an adjusting screw between the carbs which regulates the mixture. not sure which direction youshould be turning it but you can figure it out aftera couple trials. first, i would count the turns it takes to put it all the way in, that way you know where it was if you decide to put it back in the same spot or "start over."
 

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Discussion Starter #6
So just to post an update on my problem. Before I started taking apart the connections, it occurred to me I had added oil the day before my problems started. I pulled the plugs and they were all oil fouled. I had only put them on about a week before and the Bike had been running great. I guess I overfilled it. The old ones were just carboned up. I did an oil change and I cleaned and re-gapped the plugs and put them back. While putting on the front deep well wire (coil wire) the boot detached from the wire, so now I have virtually no spark. I'll try to repair/replace that this weekend, and see where things go from there.
 

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While putting on the front deep well wire (coil wire) the boot detached from the wire, so now I have virtually no spark.

You should still have spark on the other spark plug. You wont notice much if any difference in your bike if one of the two spark plugs fails. Clean those connections. If it doesn't fix this problem, it will prevent the next one from happening.

I never bother cleaning spark plugs. Always replace.
 

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You should still have spark on the other spark plug. You wont notice much if any difference in your bike if one of the two spark plugs fails.
Hmmm, I can kill a cylinder on my bike by pulling one plug boot.

The plugs are in series, by the way the coil is wound. If you disconnect one plug, the other dies with it.

--Justin
 

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This is true since most plugs foul out and are a short circuit, still allowing the plug to conduct electricity, although not produce a spark.

Disconnecting a plug is a whole different story.

--Justin
 

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Discussion Starter #11
So I cleaned all the contacts, adjusted the choke cable, charged the battery, and now the bike starts on the first try every time, but it's still smokey, and I've only been able to rev it up above 4000 RPM a couple of times. Is the fuel mixture an issue here or am I still looking for an electrical problem? Thanks.
 

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So your sure that your air filter is clean and new? Really sounds like its choked out. Also sounds like you may have a bad slide diaphragm.

Could be electrical, a cracked coil will arc more at high rpm's have you checked over your coils for cracks?
 

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Discussion Starter #13
Thanks, I'll check the coils more carefully, and replace the air filter anyway. What is the slide diaphragm? Is that the diaphragm attached to the vacuum cylinder?
 

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Yes under the top caps. You may not have it completely sealed to the top cap. They will shrink a little over time and are hard to get sealed. With the carbs off or the airbox disconnected you can tell if they are sealed ok by lifting up on the slides. If there is a slight suction resistance to movement than you likely have them sealed ok. IF they are not they will move up and down with little to no resistance

This is no good however for finding small pin holes in them.
 

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If the smoke is blue? If the plugs are oil fouled? Then it's time to do a compression test IMHO. Maybe a stuck ring? A pint of ATF in the crank case might help if that's the problem.

If in fact the engine is burning oil and you don't want to rebuild it, you can go up one or two heat ranges on the plugs and keep it from oil fouling the plugs as often.
 

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Discussion Starter #16
Thanks again. Based on the timing of the problem, I'll wait for the new air filter to arrive and change that first. Then I'll try the ATF, and then recheck the vacuum cylinders. I'm saving those for last because the problem started before I rebuilt the carburetor.
 

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Discussion Starter #18
Changed the air filter tonight. No difference. I've never done a valve adjustment, but I've only had the bike since August. Tonight it started easily, and idled fine. I tried accelerating sowly and it slid easily up to about 2000-2500 RPM and then started sputtering. I'm going to check the coils as planned and if they check out, I'll check the valve adjustment.
 

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Discussion Starter #19
I tried to adjust the valves today. The Clymer manual is ambiguous on which gap I'm supposed top be measuring. It says teh space between the adjuster and the Stem. To me the adjuster is the stem. The photo is worthless. Can anyone give me some details on that? I'm done until next weekend. Thanks.
 
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