help nailing down a curious issue

acadian · Jul 2, 2013

I've got the bike back together and, after sorting through a dozen niddly little problems, she's running well. I'm having trouble nailing down one mind bending issue... idle. The short version:

- plugs indicate rich idle and she puts out some black smoke
- bike responds as if lean at idle (surging, racing idle etc...)

All other throttle positions she's great, pulls hard, plug chops all clean and proper... I've tried a smaller pilot, bike idled way lean.

I've already ruled out:

- float levels
- timing
- leaks
- compression
- ignition system (plugs, leads, boyer etc...)

Carbs are twin PWK 32mm

Rohan · Jul 2, 2013

What happens when you screw the pilot air adjustment (idle) needles in or out ?

Some modern petrols give black plugs, without necessarily being rich.
Black smoke may be a different matter.

Amal carbs are famous for the pilot airway drillings gumming or blocking up, so it idles (roughly) only on the needlejet.

1up3down · Jul 2, 2013

What happens when you screw the pilot air adjustment (idle) needles in or out ?

a LOT with those flat slides is the answer

the air screw have a fast or steep taper so just a little turn makes a pretty big difference

the standard air screw setting is to start by screwing them in to fully seated and then backing them out about 1/4 of a turn, yes only one quarter of one turn, plugs are brand new or freshly cleaned for this

then start and ride and what you are looking for is popping of the motor on deceleration, keep turning the air screws out until popping, that then is too lean on the air screws so turn them clockwise in just a tad until the popping just disappears, then you got it

sent you a PM on this, acadian

christulin · Jul 2, 2013

Black smoke sounds like you're burning oil, which will blacken plugs in a hurry. Sounds like you're lean at idle. Rich mixture usually gives a whitish smoke and the exhaust will smell like gas. Don't trust plug readings with modern fuels. Warm up the engine, then put in clean plugs and set the idle to best tick-over and low speed running. I like to use my old Color-tune to set idle mixture to start off with then make tiny little adjustments at a time.

Hobot · Jul 2, 2013

Mostly Smells to me like boyah blues voltage drop or trigger wire internal breakage or some other shorting smoke out location. Normal other item, Worn out slides and bores if suspect at all could be diagnosed via some grease on em. Loose bowl or cap screws or balance tube. If engine idle is responsive to air screw reasonable turn out - then float level is good enough to not screw with idle stability. Maybe float trapped a bit so over fills at idle flows? Of course we assume your chokes removed are out of this equation. One miserable time waster I've run into was once tuned right the plugs were fouling form the bad idle etc so no joy till putting in new ones, or ugh, even going back to trashed ones to solve the mystery. Mind stretching pastime you've taken on.

acadian · Jul 2, 2013

christulin said:
Black smoke sounds like you're burning oil, which will blacken plugs in a hurry. Sounds like you're lean at idle. Rich mixture usually gives a whitish smoke and the exhaust will smell like gas. Don't trust plug readings with modern fuels. Warm up the engine, then put in clean plugs and set the idle to best tick-over and low speed running. I like to use my old Color-tune to set idle mixture to start off with then make tiny little adjustments at a time.

Isn't that the other way round? Black smoke = rich, blue/white smoke = oil?

Hobot · Jul 2, 2013

yep fuel excess tends to make dark soot while oil tends to fog white but both cases may appear if poor ignition. Jets can back out and needles fall too. Hm kill switch and key switch just floated to mind too.

B+Bogus · Jul 3, 2013

Could it be a float/float needle issue?
Perhaps the fuel level is too high - this may explain the overly sensitive pilot settings?

A friend of mine had a T120 which never ran right. I bought it off him, stripped the carbs and could find nothing wrong, except the needle jets looked odd.
The numbers checked out OK, but when I compared them to another set the holes were more than twice as big - I think they were 2-stroke jets.
I'm assuming all your carb internals are 'correct'?

Bernhard · Jul 5, 2013

B+Bogus said:
Could it be a float/float needle issue?
Perhaps the fuel level is too high - this may explain the overly sensitive pilot settings?

A friend of mine had a T120 which never ran right. I bought it off him, stripped the carbs and could find nothing wrong, except the needle jets looked odd.
The numbers checked out OK, but when I compared them to another set the holes were more than twice as big - I think they were 2-stroke jets.
I'm assuming all your carb internals are 'correct'?

Some one might have dirlled them out bigger :?:

crusadersports · Jul 5, 2013

christulin said:
Black smoke sounds like you're burning oil, which will blacken plugs in a hurry. Sounds like you're lean at idle. Rich mixture usually gives a whitish smoke

I think you'll find as arcadian says, that's bilge old chap! If you put oil in the combustion chamber, you'll get white smoke. If you're burning more petrol than the engine needs, it goes black because not all of it is burned. Take the sunglasses off, old boy!

help nailing down a curious issue

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