Hey Gents, new to the forum. Just rebuilt a 73 Norton 850 from the ground up. Just to give you an idea, here is what I have done: Cylinders overbored 10 thousandths with new pistons (hepolite) and rings. Valves have been re-seated. Boyer MKIII has replaced the old points. 12 Volt DinaCoil running new H.T. leads to the NGK 7ES plugs. New wiring harness (Main and headlight). New capacitor, New Stator, New Battery, New Fuse, gosh new everything! Also has a SINGLE Mikuni 34MM carb. Carb set up with a 260 Main, 2.5 slide (with 2 springs), 35 pilot, and a 159 P2 needle with a K&N round filter.
Now that you have the run-down, here we go. Timed the Boyer at 31 T.D.C at 5000 rpm. Had some trouble as the throttle would not hold at 5000. As soon as it gets to 5000 rpm, it drops back to 3000, then goes back to 5000 and so on. As the drop in RPM occurs, the carb spits fuel back through the air filter. Managed to get it timed at 31 degress before kicking down to 3000. I Messed with the idle air screw to get it to run good at 3000 rpm. Bike idles like a dream and runs smooth and beautiful until I get to 5000 on the road, nothing left. There is no power past 5000 with my setup. In fourth gear I can only max out at 60 MPH. No back fire at all, and plugs are brownish black. (Slightly Rich). Experimented with the E-Clip on Mikuni but still kicks RPM down to 3000 at a stand still. no matter how rich or lean. Keep in mind I do NOT have a Rev. Limter either. Starts first kick and idles nice, but I want and need the top end! Asked Boyer and they emailed me back and told me to check all blockages in carb (done). Unplug the charging stator and try running off battery (done). No contact from cam magnets with boyer stator either. All no help. All wiring is soldered and shrink tubed. Sorry for the book I wrote, but you need the full details. Any info would be appreciated.
#1. "RE: Top End Problems! Help!" In response to Reply # 0
I have no experience with Mikunis but this does sound like a carb problem. Presumably it's rubber mounted and free from air leaks ?
Boyers that are playing up usually cause problems at low revs or are a bit intermittent. If they're consistent then they usually give up altogether.
Are you confident that the valve timing is in order as this could cause spitting back and an inabilitity to rev. The instructions about "10 links" between the marks" are not always that obvious if you haven't done it before.
My advice would be to double check the valve timing and then, if the old components are available, try substitution. Even knackered old Amals will let you attain full revs (in fact full throttle is about the only setting where they will perform well !) Likewise try the old points or better still borrow a spare Boyer from someone (most of us have several!)