I've been sorting out a 79 T5 for the last two months and finally got it running right. The last hurdle was particularly tricky.
The timing marks on this bike were missing - only the D and S present. At least they were in the right spot, though it's a guess where exact TDC might be. So I guessed and scratched a mark just above for TDC, then figured about a 1/2 tooth above that for 2 degrees. (the distance from the crest of one tooth to the crest of the next is 3.75 degrees)
The bike popped and spit at almost-closed throttle openings, but fine otherwise. It idled and started fine. I found and corrected problems in the ignition and carbs and somehow retorquing the heads helped momentarily. But the problem persisted.
Checking timing with the timing light, it was dead-on, but saw that revving the engine didn't change the timing. The popping would happen right as the throttle opened, but the mark didn't move.
Pulled the distributor and cleaned/lubed the advance mechanism. It made a little difference.
I'd read the section in Guzziology about advance springs several times over the years, but never really totally understood it. I figured I needed new advance springs so pulled out the reference to determine which springs I needed.
This time I got it. Close inspection of Dave's graph of distributor advance curves made sense. All of the tonti distributors from this era are identical but for the advance springs. The high compression (high performance) bikes got a different set of springs and their advance curves are different. But the low compression curve starts early - about 500 RPM. That means that at idle speed the timing should already be several degrees advanced from static.
Coupled with that, Richardson said he was always curious why the low compression bikes wouldn't advance to 33 degrees full advance, only about 28.
So, as a guess, I figured a conservative estimate would put static at 5 degrees to help reach closer to 33 full advance. Compensating for the advance already advancing at 500 RPM would make about 8 degrees at idle speed.
This cured the popping and spitting at very low throttle opening, but I'm still going to replace the advance springs with the lighter ones for the performance machines. Mark at Moto Classics said he always replaces them with the performance springs, and now I understand why.
I've always heard that the important timing mark is the full advance, so I checked and I'm hitting it. It's 8.8 teeth above TDC - I had to make a mark there also of course.
I've also heard that the way to time an engine is to advance until pinging, then back off a little. With the timing advanced as it now is from the stock setting there's no indication at all of pinging.
I hope this helps others since it's taken me a month or two to work through this one.
