Carb troubleshooting.

166 Views 5 Replies 2 Participants Last post by Doc_Rot, 10 d ago

Doc_Rot

Looking for some help/input on diagnosing a carb issue.
1999 Bandit 1200 engine - stock carbs.

Here is the chain of events:

I put a inline fuel filter in.
go for a ride - bike is running very well. 30 min in the bike started bogging off idle - Look down and the carbs are dripping fuel. I've had these Keyster float needles hang up before and leak fuel so I decided to replace them with OEM.
Find a lot of debris from fuel filter glue in carbs. Dont buy Emgo filters! I figure this is what is hanging up the float needles.
Clean out the carbs
Install OEM float needles.
Install new fuel filter (different brand)
A similar bog/miss is still present but is worse. 10-30% throttle it will not accelerate.
Triple checked the float heights - 14.6mm +/- 1mm
new o-rings on float assembly.
check diaphragms for holes - all good
Switch back to the Keyster float needles. The worse bog/miss is still there. carbs are not leaking.

I don't understand what could have changed since it was running so well before the floats started leaking. It starts and idles well, at 10-30% throttle it misses and won't accelerate unless I go nearly wide open and then it will pull past the bog. This is something I would associate with float height/needle but everything checks out. I used a clear plastic tube to verify the fuel levels were the same across all carb bodies.

I am completely baffled. Could the choke plunger be leaking? what else should I look at?

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alexei

· #2 · 13 d ago

The fuel filter glue may have gummed up some passages in the carb. I know you cleaned the carbs, but in this case, have you verified that all the air passages are pristine? Air jet inlet, emulsion tube (needle jet), and if there is any accelerator pump circuit for this carb that may be clogged.

Check vacuum hose, TPS sensor connection.

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Doc_Rot

Discussion Starter · #3 · 12 d ago

Thanks for the reply.

all vacuum ports are closed. There is no TPS on these carbs.
Double checked the needle circuit today, cleaned out with carb cleaner and blasted with air. Checked all other jets and circuits. they are super clean.

Im fairly confident its a carbueration issue, but for the sake of ruling it out I also checked these things:

compression - 155PSI on all cylinders
plugs have 1000 miles on them and color looks good.
checked voltage while running - 13.4 at idle - 14.7 at 3krpm
13.2V to coils.
carb boots are new - checked they are tight.
coils are new
petcock and filter freely flow fuel
no vapor lock
pilots set at 3.5 turns out as per Holeshot directions.

One thing to note: with choke half on the miss seems to go away as if its too lean? Should I try lowering the float level to 13mm?

These are not complicated carbs! WTH is going on?

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alexei

· #4 · 11 d ago

@Doc_Rot
Thanks for the post, it is very easy to follow. Appreciated.

How old is the fuel?

With the choke half open, I'd think it's lean, too. I'd ask if the pilot screw if it's air screw or fuel screw?

If it's located before the slide (facing air box), it's an air screw; or after the slide (facing the engine), it's a fuel screw.
try richen the pilot 1/8 turn at a time

Normally, I'd mark the location of the pilot screw turn before taking it apart for service, and start from there after reassembly.

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Doc_Rot

Discussion Starter · #5 · 10 d ago (Edited)

I lowered the float level 1 mm to 13.6 and went on a short test ride and the stumble appears to be gone.

I was reviewing the instructions from Holeshot on their jet kit and there is a note "remove and leave off 2 rear carb vent tubes. They are no longer used and can cause tuning headaches if not routed properly." I had previously removed the hoses, but when I put in the new floats I turned the vents to face down - previously they were facing up. I did this for no real reason other than to not catch debris.

This time I was in there I rotated the vents to face up again and the stumble is gone with the float height change. I am now wondering if this small difference was allowing turbulence to pressurize the bowl very slightly to lower the fuel level. I would be surprised if 1mm fuel height difference would make such a difference on its own. The stumble wasn't noticeable at low speeds but anything over 25mph. I haven't had a chance to test this theory yet. I will report back.

I don't want to call it too soon, but I hope I am finally out of the carb dungeon here. Had the carbs on/off at least a dozen times at this point.

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Doc_Rot

Discussion Starter · #6 · 10 d ago

alexei said:
How old is the fuel?
Fuel is Fresh

With the choke half open, I'd think it's lean, too. I'd ask if the pilot screw if it's air screw or fuel screw?

If it's located before the slide (facing air box), it's an air screw; or after the slide (facing the engine), it's a fuel screw.

try richen the pilot 1/8 turn at a time

Fuel screw.

Normally, I'd mark the location of the pilot screw turn before taking it apart for service, and start from there after reassembly.

Factory set is at 1.5 turns. with the jet kit the new setting is 3.5 turns. I may do another 1/4 turn out to richen it up further.

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