I recently discovered some information regarding the "slapping sound" that I, along with other VT enthusiasts have encountered.
I've been suffering the "slapping sound" for the most part since I purchased the bike some 9 years ago. Oddly enough, it has only gotten louder over time, but I can't remember accurately 10 years ago. It primarily occurs when the engine is cold, and as you rev up the engine, it is very pronounced at certain rpms (i.e. 2000 and 3300), but is noticeable though at any rpm until the engine warms up a significant amount. Then, like a bad cloud, it is gone and makes no slapping sound until you restart it as a cold engine.
In addition to the noise my engine has made, it has progressively been becoming more un-reliable for the past 3~5 years, worse each year. This year, it went from killing or faltering at idle (at normal operating temp, in the summer primarily), to not starting very well and running even worse at operating temperatures.
I spent an afternoon troubleshooting the bike and was surprised at what I found. After I got it up to normal temp (the fan kicked in once), it started to run very poorly. I checked the spark on each cylinder by using a EXTRA spare plug that I knew was good. The rear was fine but the front cylinder had little or no visible spark. I then disassembled the bike down to where I could access the harness and connectors involved with the ignition to the coil from the sending units at the crank. I cleaned each and every connection and applied DILECTRIC GREASE to all connections (to guard against moisture and oxidation) and tried it once again. No luck there. I then progressed to switching out the forward coil with a good coil from my Shadow 500 I'm restoring. I then had a nice, bright spark at the plug. It ran 100 times better, perhaps more than it ever has!
I installed the different coil (from the Shadow) and took the bike on a nice long ride. Boy did it run smooth and reliable! As I took off on the bike, I noticed something else (or should I say, I DIDN'T notice something...), the engine made NO slapping or other misc. noises while the engine was stone cold. I rode it again another time, at night to return some movies I had rented, and again, NO noises!
I'm very curious as to what caused the noises in the first place.?.? I'm suspecting that with a "bad" or going bad coil, it's inductance changes, perhaps enough that when it is cold, it fires at the wrong time, perhaps slightly delayed or even possibly advanced compared to the other coil's timing in relation to TDC (top dead center of the piston). Possibly, the noise was pre-ignition or a firing completely AFTER the piston has started it's downward movement. Or, perhaps it was the incorrect firing timing and the noises were the mechanical parts in stress during that split second. Either way, the problem seems to be resolved and I'm a proud owner of a fine VT500 engine once again! Time to get back on the road and enjoy some flawless miles on my Ascot!!!!
(April 2001) Well, after (the longest EVER) winter storage, I brought the Ascot out of the shed. It still had the Shadow's coil for the front cylinder. I was shocked at hearing the slapping sound again. However, it did fire right up, never that good before the different coil. Perhaps it was a fluke that the engine noises subsided in the still-yet warmth of the late fall. As soon as I can get on the road riding it to work and back, I'll have more data as to the reliability and noise results.
(June 2001) I've had my Ascot running pretty well lately, but like clock-work, once the engine warms up, it likes to die at stoplights. In addition, it sputters above 8000rpms. Otherwise, the bike couldn't run any more perfect down the road...good MPG, smooth operation, minimal throttle for highway speeds cruising. I've been putting up with this silliness since the snow has melted. I suspected ignition problems and sure enough...I replaced the REAR coil (Oct. 2000, FRONT coil change) with the last "spare" coil from my Shadow resto project. At the same time, I reseated all ignition connections and cleaned the battery terminals. Other than the bike running 100% better with the different coil, I didn't notice the slapping sound (gone again!) and it would keep idling, no matter what--very steady and solid idle sound. I can now take the engine up to and past redline and it sounds/pulls solidly, but I think I could go with larger jets yet since the power seems to fall off ever-so-slightly, like it's still thirsty. I'll play around with setting the float level perfectly for each bowl. So, in conclusion, I've replaced both coils to remedy performance and internal noises. I suspect that the rear coil suffered the same fate as the forward coil, thermal/age onset breakdown. It was just bad luck that it went only tanks of gas after the front. That was why the sound "reappeared" this spring--it was the rear's turn to do as the front did. ** as an added note, for my first ride (~82F ambient temp), the temp stayed down between the first notch (1/4) and the "thermostat" notch (1/3). Previously, with the bad coil(s), I was suffering overheating situations just casually riding around. I'll see how things stay or change over the next month or two.
I couldn't stand the 'ticking' (very LOUD ticking), despite how well my engine was running. At just under an original 40,000 miles, still poor performance above 8K, but economy was good, as was cold starting. I felt the ticking sound was perhaps due to a valve adjustment in dire need. After taking off the valve covers, I was amazed to find strange damage in the rear head's valvetrain. The valve stem was mushroomed and split (like you've been hammering a steel fence post with an 8lb sledge hammer all afternoon into some very hard ground), and the adjuster's tip was no where to be seen, worn flush with the rocker arm. Here, for quite some time, the rear cylinder's exhaust valve was stuck just a little bit open, not enough to cause havoc with the piston, but enough to allow exhaust to escape. It must have been due to the high compression ratio, that the engine ran quite well, smoothly mile after mile. Once you have an immovable object with an irresistable force, you get what I got. I immediately started removing the engine from the frame, and installed a good used engine from a Shadow 500 I was sitting on for many years prior. The Shadow engine ran beautifully, and have logged another 9K on the bike. I haven't taken the engine apart yet, but will keep you posted once I have the head off in my hand.
Check back every few months. -Scott