Hello, this is my first post, but I’ve been reading this forum for a few years now. Thanks for being so helpful! I apologize for the length, but I’m trying to anticipate some of the questions folks will ask.
Situation: 1974 CB350F runs very rich from idle to ~2K. Moving to a smaller pilot jet didn’t help. Adjusting the air screws (all the way from the specified ⅞ turn out to 6+ turns out) doesn’t seem to affect idle speed. New/clean plugs are covered in soot after starting and running in the driveway and even when it’s taken out on some 45 mph streets nearby (trying to run it at 5-8K to clean them off) for 5 minutes. Whitish-smoke coming from the exhaust. If I pull the plugs after just running it in the driveway, they sometimes have a bit of shine and the smell of fuel on them.
When I get it out on the street and warmed up, it runs fairly well from ~2K on up, except for when I pull the clutch in and let off the throttle (e.g., when approaching a stop sign), it will die instantly, until I release the clutch and the rear wheel starts it up again or if I keep the throttle rev’d a bit it won’t die. When I adjust the idle screw (the knurled thumb screw) enough so that it won’t die at every intersection, it is "idling" at ~3K.
This is now a brat style bike that was effectively stock when I got it. It was running well then and has had this rich idle problem ever since I got the engine put back together. The previous owner (my uncle) ran it at ~5100’ of elevation and I’m currently at ~4800’.
Current set-up and Diagnostics thus far:
Top-end rebuilt:
- New rings, cylinders ball-honed, new gasket/o-ring kit, valves brought to sparkling clean, new cam chain guide and slipper installed.
- Valve lash set to the looser side of 0.002”-0.003”.
- Leak-down test showed all four cylinders between 1.1% and 2.2%.
Carbs rebuilt:
- Ultrasonic cleaned/baking soda blasted, every passage sprayed through with carb cleaner.
- All new o-rings and float bowl o-ring.
- Original brass cleaned and reinstalled: 75 mains, 18234 needles (I think these are original), emulsion tubes.
- New brass installed: new Keyster float needle valve assemblies (fixed some leaking valves when I used the old brass), new 32 pilots (down from the stock 35s I ran for awhile) from Jets R’ Us.
- Float height set to 24mm (higher than spec’d, did this to attempt to lean it out).
- A little smear of fuel-resistant gasket maker on the main o-rings before pushing them into place. There is definitely some corrosion of the “towers” that the main jets push into and the main jet o-rings don’t feel all that hard to push into place, so I wondered if I had fuel leaking around the o-ring and decided to ensure it couldn’t.
Air-in:
- Currently using the stock setup: airbox, airbox-to-carbs piece, and airbox “snorkel”.
- I tried using pods, but decided to take that variable out of the equation after reading many forum posts.
- Put in brand new intake insulators (the carb-to-intake rubber boots).
Air-out:
- Delkevic 4-into-1 headers with Cone Engineering “Quiet-er Core” Reverse cone slip-on muffler
- Before I tore it all down, it had the stock 4-into-4 exhaust
- All 4 headers are hot after running for a bit
Electrical:
- Rewired with NWT Cycletronics X21 controller (basically an M-unit competitor) and hand-built wire harness, all new connectors or soldered connections.
- Moved to Dyna S electronic ignition, as I was wondering if I just couldn’t figure out points; they are new to me and seem to be shrouded in black magic
- New Magna high-output coils, plug caps, plug wires, and many sets of new spark plugs have been tried (both the standard D8EA and a set of hotter D7EA plugs, as I live in town and there aren’t many chances to get it rev’d up for any real period of time).
- Lithium Antigravity AG-801 battery and a lithium-specific regulator/rectifier.
- Battery is in good shape, system is charging it (when the RPMs get high-enough, of course).
- Resistance measurements of stator, coils, spark plug wires, and caps all within spec.
- I have spark, but I don’t know if it’s good enough or “weak” (I’m just not experienced enough to say)
Any ideas about causes or diagnostics I can try would be much appreciated. I haven’t worked on motorcycles for ~25 years (and those were all dirt bikes) so while I’ve learned a lot over the past couple of years and am fairly mechanically-minded, I could still be making a dumb mistake (or five!). As you can see from the list, I've tried a lot, and I'm starting to second-guess whether it will ever run correctly. Thanks for reading this far and for your thoughts!
Mike