Problem with 247 Cota

[Denis]

I have a '71 247 Cota which is having an issue I cannot track down. On level ground it has plenty of power and runs evenly, never quits, and does fine. However, if I put it under load or try to pull up a hill it loses power, knocks and often stalls unless I rapidly twist the throttle back and forth and feather the clutch lever in and out. I've disassembled, inspected and cleaned the clutch, installed a new cable and a new Amal trials throttle, the carb has been cleaned and it's getting fire and fuel.

I removed the head and checked for side-to-side play on the piston and rings: they're tight. I've removed the right side cover and there is no up and down movement on the crankshaft: it's fine. I don't think there are any crankcase leaks either. Would it run erratically even on level ground if there were crankcase leaks?

The points seem to be set correctly.

Suggestions so far are:
1. Running lean.
2. Still not getting enough fuel.
3. Timing is off.
4. Weak coil or mag.
5. Clogged exhaust.

Any ideas? This thing is driving me nuts.
Thanks!

[David Lahey]

My suggestions to check are
Fuel flow rate from tank to float bowl (blocked tank vent/restricted fuel tap/restricted fuel filter inside banjo on carby/water inside fuel tank)
I know you have said that the carby has been cleaned but the symptoms also sound like a tiny drop of water moving around inside the float bowl (assuming it still has the round bowl AMAL Mk1 carby). With that carby, the water only blocks the main jet when the front of the bike is higher than the rear or under steady accelleration and it moves away from the main jet when the bike is level again.
If it only happens once the motor is hot, it may be the ignition capacitor - the timing jumps all over the place due to sparking at the points.

Crankcase leaks affect the bike the same at any angle and generally have the biggest effect at small throttle openings, mainly causing the idle to be fast and lean. Some crankcase leaks change as the motor changes temperature.

[montynut]

The fibreglass fuel tank can be affected by modern fuels and cause the fuel tap gauze filter to block so fuel flow could be an issue. Weak spark (points, condenser, coil or bad earth circuit), clogged exhaust or blocked air filter? Doesn't narrow it down much does it  .

[OSSA PHANTOM]

 How are the petcock screens? check the float level, check the carb to cylinder connection to make sure you are sucking air and leaning out your mixture.

[Denis]

Thanks all, great suggestions!

More facts which may rule out a couple of mentioned items: 1) the tank has been thoroughly cleaned and lined with Caswell's liner, 2) the petcock is new (the old one leaked and was shot), 3) the airfilter is new and probably has less than 10 hours on it (and I do not live in a dry dusty area).

I'll remove the petcock and inspect the screen, and the screen at the carb, which is the stock one (Amal 620 or 647 - bike's not in front of me at the moment).

Now, I noticed Sunday that the carb-to-intake manifold looks wet in several places. I will change that part.

If water or condensation is getting into the carb, I wonder where the hell it's coming from? The bike stays inside when I'm not riding it and the tank filler o-rings are tight. If there is water in there, it makes sense that it blocks the main jet when the front of the bike is higher than the rear.

I noticed Sunday that it had more power going up the hill ONE time than other times on the same hill the same day. Maybe ignition then! I'll also check that.

Thanks all!