Need a little help diagnosiing my problem

[Zhi]

Well, I have been told that when I blip my throttle, a huge puff of black smoke comes out of the tail pipe.
It could be somethings as simple as the filter since I've yet to change it since I got the bike back in 2000. However, I do clean it and oil it a tad.
What do you guys think would be the most likely cause, assuming it is not the filter?

Carbs are synced and clean
Dynojet jet kit with 100 jets installed
needles #2 slot from the top
Full flowing exhauted
mixture screws 2 and 1/2 turns out
Float height is 16 mm from the deck when tilted slitely
Also, the recent mention of needle jets reminded me that mine's are slightly worn and probably needs replacing.
Everything else on the bike is basically stock.

[Red01]

What do your plugs look like?
Sounds like a case of carbon buildup.

[Zhi]

Yep, the plugs are sooty black. The only clear parts are the areas where the spark hits the grounding prong.  I am thinking it may be a low end richness problem that is causing the plug buildup but am not sure how that extra fuel is getting there. Well, first thing next week, I'll grab some new plugs and a new filter, but I am a little doubtful that this will solve the problem.

[b4cruz]

test how healthy the spark is with the new plugs
before you close it all up

besides the smoke did it at least run well?
good gas mileage?
no dead spots or hesitation?

maybe you need to clean out all the carbon
if there is a lot of build up on your valves

[Red01]

maybe you need to clean out all the carbon

I agree! IMHO, clean out the carbon deposits BEFORE you put fresh plugs in.

One way to do this is to run up to operating temp, then run the bike at ~2-3K RPM with the airbox off (makes the following a lot easier). Take a squirt bottle of water and squirt it down the carb throat until the RPM's drop/stumble. Try not to spray so much water that you kill it, but if you do, just restart it. Continue this process with each carb/cylinder until no more black crap comes out the tailpipe. What happens is the carbon gets really hot once the engine is warm (if it's really bad it can glow and cause preignition - a bad thing [tm]) it can be shocked loose from the piston and combustion chamber by the water-cooled intake charge.
I've never had to do this on a bike engine, but I've done it on lots of car engines.

[b4cruz]

woudn't carb cleaner be better than water?
and after it'd need a brand new oil/filter
and fresh oil

[Zhi]

Will do with the spark, spray and water. As for the running of the bike, it runs very well except for the dead spot at or around 4300rpm and mpg is at about 38.

[JmuRiz]

I think most of our bikes have a dead-spot around there, no worries.  I'm sure tweeking the carbs could make it less noticeable, but I'm not a pro at carb tuning so I don't bother.
Let us know how the de-carbonizing works.

[gsxr400 racer]

maybe you need to clean out all the carbon

I agree! IMHO, clean out the carbon deposits BEFORE you put fresh plugs in.

One way to do this is to run up to operating temp, then run the bike at ~2-3K RPM with the airbox off (makes the following a lot easier). Take a squirt bottle of water and squirt it down the carb throat until the RPM's drop/stumble. Try not to spray so much water that you kill it, but if you do, just restart it. Continue this process with each carb/cylinder until no more black crap comes out the tailpipe. What happens is the carbon gets really hot once the engine is warm (if it's really bad it can glow and cause preignition - a bad thing [tm]) it can be shocked loose from the piston and combustion chamber by the water-cooled intake charge.
I've never had to do this on a bike engine, but I've done it on lots of car engines.

HUMMMMMMM! Verey interesting

[Red01]

woudn't carb cleaner be better than water?
and after it'd need a brand new oil/filter
and fresh oil

Most carb cleaners are flamable, so they don't shock the carbon off.

You're consuming the water in the combustion chamber, no need for an oil change due to this process. Water isn't getting into the oil.

[PitterB4]

I think most of our bikes have a dead-spot around there, no worries.  I'm sure tweeking the carbs could make it less noticeable, but I'm not a pro at carb tuning so I don't bother.

Yup... and FWIW, to fix it, I went one notch higher with my needles and fouled the plugs in just a few hundred miles.  Can I buy stock in NGK???  

I GOTTA do this water thing.  I think I finally have my carbs pretty well dialed in but have a ton of carbon build-up.  My poor daughter (she's 5) loves the B4 but always walks behind it when it's warming up and gets splattered.

[Zhi]

Poor kid.

[PitterB4]

LOL!  She never knew she had freckles!

[b4cruz]

You're consuming the water in the combustion chamber, no need for an oil change due to this process. Water isn't getting into the oil.

WOW
that makes so much sense RED
So you just keep doing it till the
exhaust isn't black anymore
GENIUS!

[Red01]

So you just keep doing it till the exhaust isn't black anymore

Yup, that's the idea.
It's a trick my Auto Shop teacher taught me back in high school.

With an engine that has downdraft carburation/injection, you don't even need a squirt bottle. You can use a cup and just tip it slightly so it feeds slightly faster than the engine can suck it out of the cup.

The only downside is on high mileage engines (or otherwise in bad shape), the loss of carbon buildup can make the engine run worse as the carbon can seal up weak rings and shady valve seats - as I learned with my Metro.