Smoking CZ when the jetting is right, or is it?

[Kenneth S (222)]

Hi Tech Heads again,
This one is for the jetting experts,
We've been playing with this thing (74 CZ 250 pictured in my Dry Clutch thread) and I am determined to get it running it's best, whatever that is!
We are running it with Castrol TT at 33:1.
I have been adjusting the pilot air screw to find the optimum setting for the bike and with the current pilot jet I have been able to lean the screw out to the point where it won't start at all, no matter what I do, bump start, kick a million times, it just doesn't have enough fuel to kick. I have found that as I screw the screw in, make it richer, I can get the bike to a point where it will start quite reliably after the 2nd or 3rd kick, so I figure I must be pretty much in the sweet spot.
My question is why does it still smoke so much at idle? If I have the jetting just rich of too lean, then I thought the idle should clean up. Is this just a trait of these old bikes or is my understanding of how to tune the jetting missing something?

[stump magnet]

i had been having simular probs with my 360 sidepiper, was running great then started to show same symptoms getting worse and worse over the season, it was the primary side crankshaft seal allowing gearbox oil to be drawn in, only put two and two together when i was only getting half the gear oil out!
worth checking.

[gordon67]

Hi KJ222,
Get out onto the track and set up the carb in race speeds and make sure its getting out of the gate on clean revs and see if it is revving out clean on the main jet,...its no good messing about in the workshop making those adjustments,.. or get an expert rider to set it up for you before you hole a piston,...good luck

[BultacoMacca]

Ken, I dont know about your CZ but my Bully smokes a lot when idling, and thins out somewhat as revs rise and a race goes on, naturally. Like gordon says its what happens at the start gate and out on the track thats important, that it carburates well on the track, not that it 'smokes too much at idle
ps (it'll be good to see another in pre75!)
Mick

[matcho mick]

isn't that why simple 2smoke oil pumps are (usually) cablelinked to the throttle,buggerall at idle,ramping up on wot,think outboard multis were 200/1 at idle to pass emission laws,i'd worry if it stopped smoking  ,

[Noel]

Bump it up to 20:1 add a bit of castor oil and enjoy the smell
Noel

[Lozza]

Hi Tech Heads again,
This one is for the jetting experts,
We've been playing with this thing (74 CZ 250 pictured in my Dry Clutch thread) and I am determined to get it running it's best, whatever that is!
We are running it with Castrol TT at 33:1.
I have been adjusting the pilot air screw to find the optimum setting for the bike and with the current pilot jet I have been able to lean the screw out to the point where it won't start at all, no matter what I do, bump start, kick a million times, it just doesn't have enough fuel to kick. I have found that as I screw the screw in, make it richer, I can get the bike to a point where it will start quite reliably after the 2nd or 3rd kick, so I figure I must be pretty much in the sweet spot.
My question is why does it still smoke so much at idle? If I have the jetting just rich of too lean, then I thought the idle should clean up. Is this just a trait of these old bikes or is my understanding of how to tune the jetting missing something?

The engine isn't in phase or on pipe to get the temps to a point where all the oil is burnt. Who cares about what it does at idle anyway?

[Kenneth S (222)]

i had been having simular probs with my 360 sidepiper, was running great then started to show same symptoms getting worse and worse over the season, it was the primary side crankshaft seal allowing gearbox oil to be drawn in, only put two and two together when i was only getting half the gear oil out!
worth checking.

Stump, you hit the nail on the head. Opened up the top end and had a little sea of oil in the bottom of the crank cases. I think what did it is we put 600cc of gear oil in when it only required about 200-300cc. This bike has a dry clutch so we don't think it needs nearly as much gear oil. Time for a full rebuild I think.