The carbs have been completely overhauled. I removed them and gave them a through cleaning as follows:
- Complete disassembly (minus butterfly shaft removal)
- Soaked in heated ultrasonic bath with detergent for two 480-second cycles
- All passages blown out with compressed air
- Passages confirmed clean with gauge wire
- New Genuine Keihin jets installed (#42 pilots, #110 mains)
- New float valve installed and valve seat polished
- Float heights set to 17mm above gasket surface (found at 16mm)
- New enrichment plungers installed
- New slides & diaphragms installed w/ 3.0mm bleed (originals showed excess wear on the engine side)
- New float bowl gaskets installed
- Pilot screws removed, cleaned and installed with new O-rings
- Pilot screws set to 2.5 turns from seat
After the carburetor cleaning, I installed new intake rubber boots, new vacuum caps and new intake-to-head O-rings. I also installed new spark plugs, gapped to 0.9mm and a new Triumph air filter. I finished by synchronizing the carbs using a manometer.
After all this work, the bike was started on full-choke and observed: The idle was smooth and consistent for the first 15 to 20 seconds, before dropping and becoming rough. At that time, the right exhaust popped a few times, almost sounding like a backfire or miss. As the bike warmed up, though, the idle smoothed out and remained consistent.
When riding, the revs no longer hang and the bike pulls well all the way from idle to full throttle. The first 10 to 15 minutes of riding, once warm, are great. The bike idles well, pulls well and runs as it should. Then, after 20 to 30 minutes of riding, it still idles well, but develops a stumble/misfire right off idle that causes the bike to chug and lurch before it will smoothing out and running great. If I run the RPM's a bit higher and slip the clutch, the stumble/misfire doesn't happen. The surging noted previously is still present at lower rpm, lower-throttle, constant speed cruising but much reduced.
I suspected that, perhaps I missed something in the carbs, so I pulled them off and re-cleaned them. I found no gunk, no partially clogged passages, nothing. It's clean. I also checked all the boots and caps for an air leak and found none. Since I'm certain the carbs are clean, I've begun to suspect other issues may be causing the problem; perhaps ignition. I've read a multitude of issues caused by a faulty pickup sensor or failing Gill coil, so I've ordered a new pickup sensor from Triumph and Nology coil, hoping this may help.
Last night, I tested the TPS (according to this procedure
) and it seems to be set appropriately. I also set the idle speed to 950 rpm using an induction tachometer on the left plug wire. I then went out and performed some troubleshooting.
The bike is warmed up, choke off. It idles well and runs beautifully. This continues for approximately 10 miles. Then, when making a right-hand turn, I downshift to second gear and roll on the throttle. Engine speed is low, say 2000 rpm or so. As I let the clutch out, the bike stumbles and bucks and stumbles. As I continue to turn the throttle, it picks up and runs beautifully again, through the remaining gears.
Turned the pilot screws in 1/4 turn. Exhaust note sounds less-crisp but the bike feels fine until that low rpm, second-gear roll on and then it bucks and stumbles, just like the first run. During the stumbling, I pulled the choke out a bit to see if it would change the behavior and it did not; the stumbling continued.
Turned the pilot screws out 1/4 turn. Same story. Bucks and stumbles at low engine RPM in 2nd gear. Noticed that it seems to have a minor misfire in higher gears at low engine-speed cruising (say fourth gear at 30 or fifth gear at 50).
So to recap:
Stumbling in second gear at low engine speed throttle roll-on. Only happens at low engine speed and regardless of pilot screw settings or choke position. Happens only when the engine is warm; after approximately 10 miles of riding.
Based on this information, I'm certain this is not a fueling issue but an ignition issue. Given that the bike seems to run better on the left-hand cylinder and given that it pops on cold-startup on the right cylinder, I suspect the coil is faulty on the right lead.
Measuring the resistance of the resistor caps, both are with an acceptable margin of 5kohms. The left is spot-on at 5kohms and the right measures 4.6kohms. The leads have good continuity and very low resistance. They are good. Moving on to the coil and measuring across the two posts yields an open circuit. No continuity. A sure-fire indicator of a failed secondary winding. This morning, I swapped leads between the right and left-hand coils. Upon cold start, the misfire problem follows the right-hand lead. The right-side of the coil isn't functioning properly. So I've ordered a replacement coil from Nology. It should be here tomorrow. We will see if that solves the problem. More to come.