Weak idle on Timing Side

74 850 Mk2 is missing at idle on the timing (right) side, the exhaust has a light chuffing sound with somewhat erratic pulses, also exhaust is noticeable cooler than the primary side. Primary side runs nicely steady pulses, throaty tone. The bike holds 1000 rpm idle. Runs strong at at speed 2500 rpm and up, smooth without missing, pulls nicely sounds good.

- relatively new Amal carbs, pilot circuits cleaned and blown out. Intake manifold O rings sealed with gasket rtv, manifolds trued using sandpaper /glass technique, carefully tightened. Tested for air leaks with propane, still have eyebrows , no change in running.

- Older Boyer ignition, battery fully charged, tested coils and ignition wires with ohm meter, new plugs. Plugs and wires swapped, no change. Pulled both plugs and checked spark by turning ignition on/off, decent spark, surprisingly stronger on the timing side. Primary plug colour nice tan, Timing side plug sooty black.

- Timing side runs on its on with primary side plug grounded out, throaty sound no chuffing. Primary side runs on its own with timing side grounded out , timing side chuffs the same as when not grounded out.

I am going to set up each side individually, (used the Bushman technique originally) and adjust idle and see what happens.

What else should I be checking?

Might swap the coils left to right and see if it follows.....
Jim H.

Oh cool someone else's impossible mystery. One set of new plugs ain't enough to know for sure, unless switching them does nothing different, which eliminates 1/2 of possible plug misfire question. Wasted spark ignitions can have ~15% less energy on the reversed polarity electrode, ie: electrons are jumping from blunt hook to sharper center post, so maybe the plugs are too cold for both to fire well low down. Recent similar mystery was solved by a correcting weak ground which apparently was enough to make unequal exhaust temps. Peer into carb to see if spray tube still standing proud as the other. Needle clips have been know to fall down. Maybe some low rpm pressure lost down the tunnel that's not enough to bother faster pressure cycles? Might open oil cap or breather hose to sense that.
A chipped slide flashes me back but you'd of corrected already. You did say an older Boyer, which means all tuning bets off until upgraded trigger leads or triple checking current ones. Maybe a float is trapped on gasket lip. Sometimes just the worlds minds focused on an issue solves it like magic for me.

The trigger leads have been upgraded and I cleaned the Kill Switch last week as any downward movement on the signal light switch killed the bike... I read the recent problem fixed by checking grounds checked the obvious but haven't gone through all of the wiring. Its almost like the ignition is strong enough to run the cylinders individually but not both.

Lord, this sounds familiar... see my post from last week on something similar.

Have you tried swapping coils?

If you can get hold of a ColorTune gizzmo it can give you a wealth of info.

drc

Nagra, just know we feel your frustration, if that helps enduring the turmoil w/o a ride to work it off. The biggest mystery is what can only affect one side in a wasted spark ignition, if ignition good that only leaves mechanical items and passages. So with that in mind I'd immediately continue to focus on electric grimlim only revealed below isolastic isolation rpms, which implies wires making breaking as already found in one area. Might put a volt meter inline with Boyer and see if it jumps around low rpm but steadies up on rev's. Weird things can happen behind the trigger plate too that stumped 6 famous experts all night long till I passed around white lightening then an interested onlooker notice a subtle vibration in the trigger plate, then we found terminal clipping the magent, but only after finding and fixing 6 other thing that each one did exactly the same symptoms all by itself so no change at all in the horrific backfires and cheery red to muffler pipes on each mean start up - after resietting timing the spike to the ski so only ran at hi rpm on open headers. Bad items I can remember were a coil, the fuse, internally, not blown or broken visually, loose battery terminal, and also permanently fouled R type plugs replaced with non R which then turned my LED signals into syncopated timing lights. Which reminds me its sometimes revealing to look around after sun down.

by maylar » Tue Jul 31, 2012 3:31 pm

Lord, this sounds familiar... see my post from last week on something similar.

Have you tried swapping coils?

If you can get hold of a ColorTune gizzmo it can give you a wealth of info.

Click to expand...

I read your posting with great interest as I seem to have the same symptoms. I've cleaned up the Boyer ground, and get 0.6 ohm reading between the ground and the + battery terminal and the ground on the timing side Z plate. Shot lots of carb cleaner and air thru the pilot circuit. Its a little better but still missing. Runs without missing on its own with the primary side grounded out. I haven't completely bypassed the Kill Switch as the PO soldered and heatshrunk all of the connectors but cleaned it. I'll keep eliminating before pulling the connectrs apart. I guess I'll check the magnets next. Found myself pricing Trisparks...

Decided to take her out today anyway and run some gas through her in the nice 34c weather. Fired up and had the same chuffing and missing at idle. Rode about 10 km, stopped and tweaked the timing side pilot screw and both slide screws evenly to pick up the idle, a little better. Rode another 10 km into Niagara on the Lake and did the same, the missed disappeared. Rode on home another 10 kms or so. No miss, nice steady idle at just under 1000 rpm at all the stops and when I arrived. Not quite sure why but I'll take it. To answer the Hobits pilot screw question; primary 1 1/2, timing 1 3/8 turns out.

That my good sir is what we call a Hillbilly tune up in Dixieland, one of the most satisfying acts of good fettering when it works out better than expected. Good gas and good throttle to ya.