r75/5, underpowered, double timing marks

[stucknarut]

I'm restoring my first airhead, a 1973.5 (LWB) R75/5 and having issues with throttle response/power. I've adjusted the valves, checked the points, replaced plugs, set timing, carbs cleaned and rebuilt with new o-rings/diaphragms, and carbs synced. The bike starts and idles with no problems and runs solidly. It feels underpowered though, slow throttle response and speed tops out around 70mph. At idle, I see the timing mark moving around a bit, and a double mark sometimes. To keep idle down to 1100-1200, I'm at a half-turn out on the idle mix, vs the 3/4 recommended by Bing.

Any suggestions for what next? I haven't checked advance timing, will do that tonight.

[Pauls1150]

The R75 was not a power demon, and it had a heavy flywheel, but it should do a bunch more than 70 easily.

Too many variables to troubleshoot long-distance, but more things to check should include:
obstruction in airbox, filter, or plastic intake tubes
obstruction in exhaust
main jet wrong size or still clogged
needle clip in wrong groove or worn, or needle jet worn (but these affect midrange more than top end)
valves too tight, set 'em a hair loose (but this would be more noticeable at lower rpm)

Bouncing or double timing lights often mean the points aren't set right, or the advance springs are bad or the weights are sticking (not able to move out freely to advance the plate).

The difference between 1/2 or 3/4 turn isn't a worry, but you should use the throttle stop screws to set & sync the idle, not the mixture screws.

[stucknarut]

Too many variables to troubleshoot long-distance, but more things to check should include:
obstruction in airbox, filter, or plastic intake tubes
obstruction in exhaust
main jet wrong size or still clogged
needle clip in wrong groove or worn, or needle jet worn (but these affect midrange more than top end)
valves too tight, set 'em a hair loose (but this would be more noticeable at lower rpm)

Thanks, will check that stuff tonight. On the exhaust side can I just remove the mufflers without re-jetting, at least for a test? The main jets are both 135s and pretty clean, I dipped/blew out all carb parts and poked a wire through the smaller holes. Clip was on #3. I checked points gap, and lubed the felt and advance cam. I lubed the advance unit and the weights are moving but haven't replaced the springs, not a bad idea. Does anyone know degrees of advance for 3500RPM btw?

[20774]

Snowbum has a full article on the advance unit:

http://bmwmotorcycletech.info/ignitionsingleplug.htm

But he says the following:

NOTE that an even later automatic advance unit was made, carrying the Bosch number ending in -012, and was used on the late /6 and /7 to 1978, still with 78?? Dwell. Units ending in Bosch numbers -007-012 had advance limited to 34?? BTDC. Only the 005 unit had 39?? BTDC. The early advance looks the same but the holes in the cross plate allow the advance to swing a bit more open.

[stucknarut]

Ok...airbox, filter, and intake tubes look good. I re-checked the points and advanced the timing a bit since it was behind on full advance. A little bit better but I get an occasional backfire at full throttle, thinking maybe new needles and jets aren't a bad idea too. On the timing, still seeing bouncing and a double mark. Snowbum's site suggests the timing chain and sprocket might be the problem, and "hearing" it. Anyone know what a bad timing chain sounds like?