Topic: #3 carb not quite right

[KeithB]

1974 CB750K
All main and pilot jets are new.
Carbs have been cleaned
Accurate vacuume carb balance done and "air screws" set the same.
All float levels checked and the same and needles at the same clip setting.
All plugs new and coils good and wires and caps clean an correct.
Timing is strobe accurate.

When I start the bike, it needs choke for a short time and then will idle with no choke.
#1-2-4 cyl seem to be running right and the exhaust pipes heat up more or less the same.
 #3 is a bit reluctant and pipe doesn't heat up the same.
If I take 2 fingers and put them over the intake to the carb and get it to pull a bit more fuel, #3 runs well  for a short time.
Not sure what the cause is but it seems to be lean at idle.
Thoughts?

[Yamahawk]

Hi Keith,
  What you describe is a lean condition on the #3 carburetor. Whether it is from jets, passages, or an air leak is the conundrum...
If you are sure that you carb passages are clean, (internally) then I would search for an air leak at the manifold using starting fluid or carb cleaner, spray it at each connection and see if there is a rise in rpm. I am sure you will figure it out. If it isn't air or fuel, the next thing is the coils and wires, for that #3 cylinder. There is always a chance that the wire isn't making good contact, or is arcing out on the frame.
Charlie

[KeithB]

I'm 100% sure it's not wiring as I am very comfortable with electrics and I have done a very comprehensive diagnostic.
The air leak is something I will double check.
there is always a chance that a bit of something didn't come out of a passage with the cleaning.

[Yamahawk]

Yep, did you actually install new plug wires? They do break down over time, and can arc out. If you can, at night or in a dark garage look at the wires with the engine running to see if there are any sparks to ground.
There is always that chance there might be a speck of dirt somewhere in that circuit, or in the pilot jet too. I have seen my own pilots get narrowed through sediment that is very hard to clean out, and gives a tiny peep hole that isn't as large as when it was new.
Charlie

[KeithB]

Yep, did you actually install new plug wires? They do break down over time, and can arc out. If you can, at night or in a dark garage look at the wires with the engine running to see if there are any sparks to ground.
There is always that chance there might be a speck of dirt somewhere in that circuit, or in the pilot jet too. I have seen my own pilots get narrowed through sediment that is very hard to clean out, and gives a tiny peep hole that isn't as large as when it was new.
Charlie

While I take your point, trust me when I say I have covered the electrical part of this problem.
I suspect you are on point with potential debris remaining in the low cct passage.

[HondaMan]

A slightly-lower float level in the #3 bowl will cause this, too. You can test this a bit by turning the #3 screw outward about 1/4 turn to see if it gets better: if it does, then the bowl is running slightly lower than the others. This can happen from the infamous "dimples" that appear after a while on the float tangs, where they push the float valve shut: it makes a slight latch that tends to prevent the float valve opening as the levels starts dropping, and also tends to make it overflow momentarily (and intermittently) if new float valves were installed, but the dimple was not removed.

[KeithB]

A slightly-lower float level in the #3 bowl will cause this, too. You can test this a bit by turning the #3 screw outward about 1/4 turn to see if it gets better: if it does, then the bowl is running slightly lower than the others. This can happen from the infamous "dimples" that appear after a while on the float tangs, where they push the float valve shut: it makes a slight latch that tends to prevent the float valve opening as the levels starts dropping, and also tends to make it overflow momentarily (and intermittently) if new float valves were installed, but the dimple was not removed.

I was suspecting something like a lower level and could not think of an easy way to test.
thanks.

[Sellsme]

I had sort of the same thing, number 4 for me. after disassembling  many times no luck. THEN I found it. one of the needle jets was a smaller size than the rest. The needles themselves were marked and correct but the jets were not marked. I  had no way of knowing what size the jets were. Until I removed every little part and slid the needle jet over the needle. #4 jet would not slide all the way down the needle but the other 3 would would. Take nothing for granted, over 40 years of others messing with these carbs.

[KeithB]

I had sort of the same thing, number 4 for me. after disassembling  many times no luck. THEN I found it. one of the needle jets was a smaller size than the rest. The needles themselves were marked and correct but the jets were not marked. I  had no way of knowing what size the jets were. Until I removed every little part and slid the needle jet over the needle. #4 jet would not slide all the way down the needle but the other 3 would would. Take nothing for granted, over 40 years of others messing with these carbs.

I did check the jets and they are marked the same but next time I have them off the bike I will take a close look at the needle to jet fit.
Thanks for the tip

[KeithB]

A slightly-lower float level in the #3 bowl will cause this, too. You can test this a bit by turning the #3 screw outward about 1/4 turn to see if it gets better: if it does, then the bowl is running slightly lower than the others. This can happen from the infamous "dimples" that appear after a while on the float tangs, where they push the float valve shut: it makes a slight latch that tends to prevent the float valve opening as the levels starts dropping, and also tends to make it overflow momentarily (and intermittently) if new float valves were installed, but the dimple was not removed.

Tried adjusting the screw but it made no difference. All the boots are very tight but I will do a leak test. Propane from a torch should work.
I am going to try raising the float level a mm to see if that works.
Another thought I had was allowing a little more vacuum on #3 when doing a balance with vacuum gauges.
Otherwise, the carbs come off (again!) for a complete inspection.

[DV Red Herring]

I'm just going to throw this out there. I had a similar situation happen, and after chasing all the rabbits down the hole... come to find out I had a faulty spark plug. Even brand new NGKs can be bad out of the box.

[HondaMan]

I'm just going to throw this out there. I had a similar situation happen, and after chasing all the rabbits down the hole... come to find out I had a faulty spark plug. Even brand new NGKs can be bad out of the box.

More than one! Especially if the parts dealer dropped them once (or more)...

[KeithB]

As noted in a previous post, I have carefully exhausted the possibility of bad plugs, caps, wires and coils.

But let's explore this.
How would  choking a carb and making the mixture rich would "solve" a bad plug problem?

[evinrude7]

my votes are leak at boot between carb and motor or low/stuck float. 

[KeithB]

my votes are leak at boot between carb and motor or low/stuck float. 

I did pull the carb bowls a number of times to check the floats and the fuel level.
The leak is something I will check out this coming week.

[KeithB]

Update...
Checked for leaks in carb boots. All tight.
Raised the float level just a touch. No difference.
Changed "air screw" position. No difference.
It does backfire a bit and spits fuel back out of the carb intake  when rolling on the throttle.
#3 pipe never seems to get as hot as the other 3.
Can't really do a plug chop as it's still not good weather to get out on the road.
Maybe re-check the valve lash?
Other than this one problem, the bike starts and runs fine.

[evinrude7]

Update...
Checked for leaks in carb boots. All tight.
Raised the float level just a touch. No difference.
Changed "air screw" position. No difference.
It does backfire a bit and spits fuel back out of the carb intake  when rolling on the throttle.
#3 pipe never seems to get as hot as the other 3.
Can't really do a plug chop as it's still not good weather to get out on the road.
Maybe re-check the valve lash?
Other than this one problem, the bike starts and runs fine.

i had a problem once where a float was bent.  the two sides of it were splayed outward from one another and it was rubbing on the bowl sides, getting stuck.  not sure what carbs you have.  maybe this can't even happen with your floats. 

[KeithB]

Update...
Checked for leaks in carb boots. All tight.
Raised the float level just a touch. No difference.
Changed "air screw" position. No difference.
It does backfire a bit and spits fuel back out of the carb intake  when rolling on the throttle.
#3 pipe never seems to get as hot as the other 3.
Can't really do a plug chop as it's still not good weather to get out on the road.
Maybe re-check the valve lash?
Other than this one problem, the bike starts and runs fine.

i had a problem once where a float was bent.  the two sides of it were splayed outward from one another and it was rubbing on the bowl sides, getting stuck.  not sure what carbs you have.  maybe this can't even happen with your floats. 

Good idea.
Already checked that floats are working correctly.

I'm out of ideas.

[evinrude7]

sorry keith.  hope you get it sorted soon. 

[KeithB]

One of these things is not like the other (3) so if I keep at it, I'll figure it out.