Tx650 advanced timing

jayel · Posted: April 16, 2006, 11:11 am Post subject: Tx650 advanced timing

I have a 1974 Yamaha TX650A twin with breaker points ignition they are four(?) lines used for timing, the F mark is between two lines and the T mark to the right of that, when using the strobe at idle both cylinders fire at the F but when advanced they go way past the fourth mark to the left, which I assume is the advanced timing mark. Questioning whether should I adjust the bob weight stops to bring the advance back to that fourth mark? My Haynes book doesn’t say anything about where it should run advanced. Thanks jayel

grizld1 · Posted: April 16, 2006, 1:14 pm Post subject: Re: Tx650 advanced timing

Your assumption is correct. You've encountered a classic engine-killer, and I congratulate you on checking both advanced and retarded positions. When the advance governor mechanism wears, it throws wide, resulting in an advanced high-rpm setting when the idle's dialed in. I've seen a number of cases in which engine failures due to holed pistons were directly attributable to that condition.

First, here's what does not work. Don't even think about the caveman fix of tapping/bending the advance rotor tabs in. A lot of idiots who somehow keep jobs turning wrenches for a living will tell you to do that. If you do, the tabs may hold for 1,000 miles before they fall off and leave you DOR, if you're lucky. They're highly tempered and very brittle. Also, don't try to cheat the idle timing back to compensate. You'll create a bad throttle lag till rpm's pick up enough for the advance to kick in, and may create a start-up backfire that'll pop the carbies right out of the boots.

There are a number of cures. Some guys report success repairing the slot in the rotor that's pinned to the camshaft, getting the slop out of the system that way. Some kind of thin shim folded over the slot might do it--don't try welding that piece. My preferred repair is to weigh the advance weights, lay a thin layer of braze in the notch where they contact the rotor tab, and file 'em in to spec; then reweigh 'em and, if they're heavier than the original weight, take material out of noncritical areas to return 'em to spec. If you're thinking replacement, a used unit will eventually give you the same grief; the Yamaha advance rotor sold for over $350 when I priced one 14 years ago; and a Boyer Micro-Digital electronic ignition from 650 Central or Pro-Flo (not the newer Micro-Power, which is more troublesome and doesn't buy ya much, IMHO) will run ya in the neighborhood of $160. It's what I've used for 14 years.

jayel · Posted: April 16, 2006, 2:26 pm Post subject: Re: Tx650 advanced timing

ok good, I did find a picture in the haynes the shows it lined up with the fourth mark at full advance MikesXS.com has the ATU for 6 bucks so its getting replaced, this was a barn bike so its in need of a lot of cleaning and care they did drain the carbs before it sat for about five years it started right up but I knew the timing wasn't right, the biggest problem so far was the front brake - cleaning out the funky old fluid and scale, chasing around replacing missing flats and lockwashers also had fun rewiring this beast lots of cut and twisted back together wires previous owners must not have owned a soldering gun, cleaned the switches and everything works now, ready to road test now before I was getting one lean or hot plug and one cold so hopefully fixing the advance will cure that

grizld1 · Posted: April 16, 2006, 5:32 pm Post subject: Re: Tx650 advanced timing

You may need what's in Mike's kit, but it won't help with the wide advance throw. What Mike sells is a repair kit with new springs etc., not the whole unit. It's useful if you need new springs because they're too weak to pull the rod back to retard position, but that isn't your situation. There's no six-dollar bolt-on fix for your problem. Your advance governor defect won't cause one side to run hot and the other cold.

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