Topic: Leaking Carb and Heavy Smoke, are they related.

[Ace Blackwell]

All,

I’ve got a smoking condition with my bike.  I have a 77 CB550F.   I rebuilt the carbs last year with no real road time on them.   The carbs have been clear tube set but have not been vacuum sync'd.  Also, the bike starts with choke on but within 15-30 seconds I have to turn to half or off.   The smoking described below came around a few months after rebuild installation.  It wasn’t there initially after build.

When starting my bike it pops(backfire) & misses.  It gets better as it heats up.  The idling increases to about 2,500 rpm.   I pull the gas and release to get it back to 1,100-1,200 rpm.  Over time the popping stops but the random miss still there.  While warming up the bike starts smoking, getting progressively worse.  At 8 mins it’s so thick I can’t see my house from the building in the back yard. Once I forgot & left it running for about 15 mins.  It’s a wonder the neighbors didn’t call the fire station.  It didn’t get any better.  I cut the bike off.  I noticed a black wet spot about the size of a dinner platter near the muffler to tail pipe joint.  I thought it was oil but the wetness evaporated in a few mins.  The floor was stained but not coal black or slick/wet. 

Initially when the bike starts there is no smoke.  At 2 mins smoke is visible but you have to be watching for it.  2-4 mins it’s about like a cigarette.  4-6 about like a normal outdoor grill.  Between 6-8 it get very noticeably thick, grease fire.   

Yesterday, to confirm if the gas spot was coming from the tail pipes / muffler or not.  I routed the over flow tubes off the driver side of the engine. I fired up the cold bike. It started and ran as previously described.   There was leaking from some of the overflows.  Carb 3 & 4 didn’t leak.  Carb 1 on/off dribble.  Carb 2 dribbled, then flowed, then blasted, then stopped.  The cycle took about 15-20 seconds and would start again. Dribble, light flow, a single water hose blast, then nothing.  What I notice was after about 8-10 mins the bike hadn’t started the super thick smoking.   It got as bad as a normal grill maybe but nothing as heavy as before.   My theory, my original routing of the overflows were restricting the heavy leak of carb 2 causing it to back up into cylinder and eventually out the tail pipe, if that’s possible.

So I said all of that to ask, any one has any idea on carb 2?  I’m also thinking the remaining smoke could be from lack of syncing.  Sorry so long but was trying to be thorough.

[ekpent]

 The overflow tubes also act as air vents but obviously gas should not be coming out of them especially while it is running.  What color is the smoke  ? You may be burning out some gas remnants that make it to the pipes from carbs leaking and if that is the case your oil is probably toast with gas also. Might want to take a whiff off the stick. Whatever is going on it sounds like it is smoking way to much. Usually out on the road black smoke is too rich,blue smoke oil and white smoke water condensation.

[TwoTired]

Have the carbs at least been been mechanically sync'd?

Have you checked the engine breather is clear to breathe?

Cold engines atomize fuel poorly.  This is why there is a choke to enrich the cold mixtures.  Cold engines aren't as efficient as warm ones.  So, a mechanical idle setting for a warm engine, will be inadequate for a cold engine, meaning the slides must be open more during the cold.  The choke ONLY changes the idle mixture, not the idle speed setting.  On this bike YOU have to make the compensation either by holding the throttle more open, or twiddling the big idle speed knob.  If you do the latter, then expect to tweak the knob again after the engine is warmed up and operating more efficiently, as it is completely normal for the engine to gain speed when warm.

It is NOT good to run the engine on full choke after starting.  The enriched mixture washes oil off the cylinder walls, and makes piston ring blowby excessive, contaminating the oil supply with abrasive carbon particles (soot).  After the engine starts YOU must reduce the choke to the minimum the engine needs to keep running while you hold throttle high enough to sustain operation.  As it warms, you can progressively reduce choke til it is fully off.  It should then idle as it did previously when warm, or if the idle knob was fondled, reset again.

Are you operating a fan on the stationary engine run (air cooled motor), or allowing it to overheat for 10-15 minutes?

Have you checked your oil supply for signs of gas contamination?

Have you checked for compression both cold and hot?

What do the spark plug deposits look like?  Running on choke too long can soot up and foul spark plugs.

What does your air cleaner look like?

Have you cleaned you points?

Do you know your condensers are good?

Overflowing carbs need to be addressed immediately, as if they overflow into the cylinders, the oil is compromised.  And piston rings and cylinders need oil to endure the punishment they receive.  They are no longer new as soon as they starve for lubrication.

Good luck!