Hello!  I have a 1968 YG5T 80 (73 cc) bike.  I have it running fairly well, but am trying to wring the last bit of top end out of it. 

This bike has an advance and is spec'd to be statically timed at 1.8mm before TDC at full advance.  I have done this carefully.  When doing the static setting, I also adjusted the timing pointer to the timing mark so they line up at the 1.8mm before TDC.  Basic steps I used:

• Set point gap at widest opening to .013"
• Dial indicator in the spark plug hole and find TDC
• Back crank to 1.8mm before TDC. (Checked TDC and 1.8mm before TDC multiple times to confirm accuracy)
• Set adjustable timing marker to line up with rotating timing mark at 1.8mm before TDC
• Disconnect condenser and install static timing light with sound.
• Fully advance the advance mechanism and hold open (with a clip I made).  I confirmed advance if forced to full.
• At full advance, adjust points to just open at 1.8mm before TDC when all screws tight.  Verified this setting to confirm accuracy.
• Reconnect condenser

The bike starts and runs well - but if I bring the engine up in revs while using a dynamic timing light (strobe) and view the timing marks previously set, the timing looks a few degrees too advanced. Hmm.

Has anyone seen this issue?  I.e. A discrepancy between static and dynamic timing?  My only theory is that the points spring (not advance springs) is weak and possibly an aggressive cam is "throwing" the points open at higher RPM before the actual top of the lobe.  This would sort of be like a points float issue, although the points are closing - i.e. there isn't any engine missing.

I'm tempted to leave the timing as is, having used the static timing method which seems to be the only valid "official" Yamaha instruction.  Just another mystery I thought someone in the group may have thoughts on.

Thanks!
-Adam

Break on points is where spark is produced, not well after the fact.

Study kettering iggy.

cliff

Hi Adam,

It has a battery ignition, right?  Any way you can post a picture?

Read like you're doing it pretty right--i think. So if its lacking top end stop looking there & [dare i say it] clean the exhaust baffle. By memory there quite a big removable one on those. But not only the baffle, check right through & even up the port as can grow right at the piston/port. You say lacking 'A Bit' of top end, just explain that a little more please. Does it fall off the perch or sort of try to rev. Here's some pics, just hit to make bigger. Left mine in the log fire overnight & the rest of the pipe in the bonfire overnight.

Plus showing crank seal pushed out. Mine was a magneto version hence the visible stator coil. Won't be your problem, mine wouldn't run at all--all of a sudden. Had me head scratching for a bit untill i found that.

Just a quick update.. I rechecked the timing and I think there's just some "slop" in the points mechanism that makes a super accurate setting not really feasible. Meaning, there's probably about a +/- .05 mm piston position error range to the adjustment. Essentially, this means I have the points opening at 1.75mm ~ 1.8mm BTDC. Anyway, I also checked dynamic and it looks okay now.

This bike is the US alternator version with electric start. I will try to take some pictures, but I'm riding the motogiro this weekend and have been working to get everything ready for that, so I won't have time for pics until after that ride. If anyone's going to be at the giro let me know!!

I actually have a new exhaust baffle in, so I don't think there's an exhaust restriction, but I will check.

Finally, after leaning the carb needle out, I've reverted to one needle position richer from center (one groove down from center on the needle) and gone to a 125 jet. This seems to make the bike a little more responsive and have a slight higher top speed - although getting it into the high 45+ mph range is about all it's got. On a long straight without too much headwind, it may touch 50mph.

The spec claims 50 ~55 mph, so I expect it is a little underpowered, but this bike has led a hard life and probably has somewhat low compression (I haven't tested recently).

Thanks again for the input!

- Adam

Sounds like you have a handle on it. 

Just so you know, 1960's "specs" were extremely optimistic.  I think my MG1-T (very similar engine) lists an impossible top speed and it's supposed to get like 150 miles per gallon. 

P.S. It's technically a generator not an alternator.  Yamaha did have bikes with alternators that look similar.. like the RD350/400.