Topic: 78 CB750 Boring the pilot jet - whats the non-bored equivalent size?

[bollingball]

       Getting a 1% richer is common because the needle doesn't come all the way out of the tube at full throttle.

I know that now, .......but I wouldn't have known it just by reading the carb manuals, most of which say it only influences 1/4-3/4......which is my point....everything you read in "the book" is not always correct.

brewsky
 I found that out the hard way too I have a 1978K750. The manual says the pilot jets are not removable. So I cleaned everything but that. Then found out that you can pull them out.

Ken

[lucky]

Brewsky is the one guy that has actual test facts and records.
His A/F chats are VERY helpful.

Can you repost your chart of the A/F ratios Brewsky?

[brewsky]

This is still a work in progress, so no real conclusions yet........

I HAD Uni-Pods and Mac 4/1 with baffle to start with, and this sheet shows changes since re-installing stock airbox with K&N replacement filter and 4 addtl. 1" holes added to the rear of the box. It had been previously butt dyno tuned by a decent mechanic, but I could tell he had it on the rich side rich.

Changes noted are main jet size, slow jet size, needle shims added, idle mix changes, exhaust baffle mods.

The readings in the sheet are direct A/F readings, on the road, from the installed Dynojet Wideband Commander....except the first, which is from actual dyno run.

Hopefully I will be able to have another dyno run next weekend to see what the changes have done.

[Rigid]

I posted a drill size versus jet size chart on this site at one time but can't find it now

[anders288]

I have an Innovate LM-1 one of the best tools I have ever bought. Getting data in from real world riding conditions
seems to work better than on a dyno for me. 

[timdhawk]

What you have now:

1978CB75K
4 into 1 exhaust no muffler.
POD air filters.
idle jet #45mm drilled.
Main jet is #100mm
Mixture screws turned out 2.5 turns.

Actually, that's not correct. What I had at the time of the post was a stock idle jet drilled to approx .40mm and #122 mains. What I have now is #42 idles and 120 mains with approx 2 1/4 turns out on the screws. I still run better at1/8-1/4 throttle with about half choke.

[crazypj]

Whats your float level set at?
 You may find lowering it 1mm may help? (it's the real reason for the +/- 1mm in specifications, not so it can be set anywhere in range)

[lucky]

I have an Innovate LM-1 one of the best tools I have ever bought. Getting data in from real world riding conditions
seems to work better than on a dyno for me.

Why don't you tell the readers what a Innovate LM-1 is?

IS it a lunar module?

[lucky]

Whats your float level set at?
 You may find lowering it 1mm may help? (it's the real reason for the +/- 1mm in specifications, not so it can be set anywhere in range)

The float level is set so that the float bowl stays full of gas.
The -or+ is just so you know the tolerance.

[crazypj]

Sorry, no
It's a way to change mixture and stay in EPA spec
Correct float level is ALWAYS the number given, the +/- is for low/mid range adjustments if there is a running problem
Lower level on float raises level of fuel and richens transition and mid-range
The total fuel flow at/near max rpm stays the same as that is limited by the jet sizes

[anders288]

  Lucky check them out

            http://www.innovatemotorsports.com/

   This is the first time I have herd that you can ajust low and mid range by changing the flote level by 0.040 inch. Where did you come by this Crazypj?

[brewsky]

I have an Innovate LM-1 one of the best tools I have ever bought. Getting data in from real world riding conditions
seems to work better than on a dyno for me. 

I agree completely...
The reason for the dyno for me is to correlate the A/F readings between my meter and the dyno sniffer, and to see what effect the changes have on the power curve.

It is hard to detect a couple of horsepower on the butt dyno, but it is easier to feel the circuit transition changes as you watch the meter.

[anders288]

Trivia about butt dynos air force tested pilots and they couldn't accurately detect HP changes until it was over 20%
 

[crazypj]

  Lucky check them out

            http://www.innovatemotorsports.com/

   This is the first time I have herd that you can ajust low and mid range by changing the flote level by 0.040 inch. Where did you come by this Crazypj?

 It's a well know fact among PROFESSIONAL motorcycle techs
the number given is always the correct float height, the plus or minus is NOT a tolerance.
The problem is, people are so used to seeing a tolerance it's treated as such even though it's wrong in this instance
I've been doing this since 1968, there is an awful lot of well known stuff that has/is being forgotten
Anyone who has ever done FAST (Suzuki) training will know about it

[brewsky]

Lower level on float raises level of fuel and richens transition and mid-range

?
Lower float level=lower fuel level (unless the carb is upside down)

[crazypj]

Lower level on float raises level of fuel and richens transition and mid-range

?
Lower float level=lower fuel level (unless the carb is upside down)

 Uhhhhh, don't think so
You need to have a better understanding of how things work
For example, stock float height 17.5mm
Set float level at 15mm , it has to rise up further to shut off gas so fuel level is higher in float bowls (and emulsion tubes/jets)
set float level at 20mm it shuts off early so fuel level is lower in float bowl
 There is a lot of counter intuitive stuff going on with a bunch of mechanical parts
 I've been doing this stuff for over 43 yrs and was teaching it for more than 11 yrs

[brewsky]

Lower level on float raises level of fuel and richens transition and mid-range

?
Lower float level=lower fuel level (unless the carb is upside down)

 Uhhhhh, don't think so
You need to have a better understanding of how things work
For example, stock float height 17.5mm
Set float level at 15mm , it has to rise up further to shut off gas so fuel level is higher in float bowls (and emulsion tubes/jets)
set float level at 20mm it shuts off early so fuel level is lower in float bowl
 There is a lot of counter intuitive stuff going on with a bunch of mechanical parts
 I've been doing this stuff for over 43 yrs and was teaching it for more than 11 yrs

??
Uhhhhhhhhh............
Think about it some more.....
LOWER float = LOWER fuel level...period. (again, unless the carb is upside down)

I've been flushing toilets for more than 60 years, (I guess that makes me a professional flusher), and I can assure you that if you lower the float in the tank, the water level will be lower also. Same difference with a carb float....it's not that complicated.

Now IF you meant that a SMALLER measurement of float height (from the top measurement reference) raises the fuel level, you would be correct.

[Rigid]

Pictures anyone?

[crazypj]

OK, I guess you set float level with carbs on bike?
Nope, you invert them so they would be upside down
Lower float level means the float is closer to gasket surface, always has always will (as it's shown in all service manuals)
When you lower floats you have a smaller measurement of float height
 Rigid, your pics are right but nomenclature is wrong. higher number from gasket surface is called a high float level
 There are multiple procedures carried out on various components where up is down when fitted, (eg cylinder heads,transmissions built in top case, etc) I thought anyone who worked on bikes would understand this?

[brewsky]

OK, I guess you set float level with carbs on bike?
Nope, you invert them so they would be upside down
Lower float level means the float is closer to gasket surface, always has always will (as it's shown in all service manuals)
When you lower floats you have a smaller measurement of float height
 Rigid, your pics are right but nomenclature is wrong. higher number from gasket surface is called a high float level
 There are multiple procedures carried out on various components where up is down when fitted, (eg cylinder heads,transmissions built in top case, etc) I thought anyone who worked on bikes would understand this?

Actually, I have set floats with the carbs still on.

Have you never seen a carb with a float that pivots in the bowl itself?
How would you apply your explanation to that case?

Rigid is correct, .........your explanation is confusing to people who don't think upside down, and when there is room for misinterpretation, pics are best!

PEACE! and apologies to the OP for subject drift!!

[crazypj]

Yes, I have seen carbs with pivots in bowl, usually not on motorcycles though
 Even the Mikuni's with independent floats on pins use arm to shut off fuel that is measured from gasket surface.
 This is a motorcycle forum not automotive, correct explanation is the way I've given it.
High float level means high off gasket face, low float level is closer to gasket face.
If your using Harley or automotive terms that's where confusion comes in
HD just make #$%* up
WTF is a compensating sprocket? (actually I know, just wondering how many others have had dealings with HD?)
Except for a few years working on construction equipment, all I've ever worked on is motorcycles, from tech to service manager to small business owner to instructor

[brewsky]

Mikuni Carb Tuning Manual……….

“The fuel level in the bowl controls the amount of fuel in the fuel mixture. Too high a level allows more fuel than necessary to leave the nozzle, enriching the mixture. Too low a level results in a leaner mixture, as not enough fuel leaves the nozzle. Therefore, the pre-determined fuel level should not be changed arbitrarily”


Honda Common Service manual………….

“lean mixture…….float level too low”
“rich mixture…….float level too high”


Honda Common Service Manual………….

“As fuel is consumed and the level in the chamber falls, the float and float valve are lowered and the chamber is immediately filled to a specified level. A rise in the fuel level causes the float and its valve to rise, the valve contacts the valve seat, and the supply is cut off.


Crazypj………………..

“High float level means high off gasket face, low float level is closer to gasket face.”

“Lower float level means the float is closer to gasket surface, always has always will (as it's shown in all service manuals)”

“higher number from gasket surface is called a high float level”

Crazy, I have no doubt you know what you are talking about, you are just having a hard time explaining it in a way that people who don't think upside down can understand.

For example, you wouldn't be able to explain fuel overflowing the bowl by saying the float level is too LOW............or........the engine won't idle since the idle jet is not picking up fuel due to the float level being to HIGH...........it just doesn't make sense to the common man.

[crazypj]

It's been common practice  for longer than I can remember to have float and fuel levels 'opposite'
Although it's possible to adjust float levels with carbs on the bike the most common way to set them is on the bench, semi-inverted.
For a number of years, Kawasaki didn't even give a float height, only a fuel level (Suzuki did the same thing for a year or two)
The Honda manuals 'lost' something in translation, although two of the three quotes you supply say fuel level not float level
 I'm not trying to be a dick about this (it's just natural  ), but, I've pretty much only ever been a motorcycle mechanic/tech both as a hobby and professionally
It's a case of using correct terminology even if it is counter intuitive to self taught hobby mechanics.
(just wait until we get back onto fork swaps, rake and trail changes,etc  )

[anders288]

It's been common practice  for longer than I can remember to have float and fuel levels 'opposite'
Although it's possible to adjust float levels with carbs on the bike the most common way to set them is on the bench, semi-inverted.
For a number of years, Kawasaki didn't even give a float height, only a fuel level (Suzuki did the same thing for a year or two)
The Honda manuals 'lost' something in translation, although two of the three quotes you supply say fuel level not float level
 I'm not trying to be a dick about this (it's just natural  ), but, I've pretty much only ever been a motorcycle mechanic/tech both as a hobby and professionally
It's a case of using correct terminology even if it is counter intuitive to self taught hobby mechanics.
(just wait until we get back onto fork swaps, rake and trail changes,etc  )

    crazpj you keep referring to hobby mechanics I have gone to Honda tech centers for ten years have passed ASE master tech test.

Being a pro doesn't make you correct. I would like to see take on this in print.

[crazypj]

It's been common practice  for longer than I can remember to have float and fuel levels 'opposite'
Although it's possible to adjust float levels with carbs on the bike the most common way to set them is on the bench, semi-inverted.
For a number of years, Kawasaki didn't even give a float height, only a fuel level (Suzuki did the same thing for a year or two)
The Honda manuals 'lost' something in translation, although two of the three quotes you supply say fuel level not float level
 I'm not trying to be a dick about this (it's just natural  ), but, I've pretty much only ever been a motorcycle mechanic/tech both as a hobby and professionally
It's a case of using correct terminology even if it is counter intuitive to self taught hobby mechanics.
(just wait until we get back onto fork swaps, rake and trail changes,etc  )

    crazpj you keep referring to hobby mechanics I have gone to Honda tech centers for ten years have passed ASE master tech test.

Being a pro doesn't make you correct. I would like to see take on this in print.

Automotive and motorcycle are completely different terminology for a great deal of things
Do you use Sprauge clutches or sprag clutches?
Sprauge was a mathematician but the incorrect spelling of SPRAG has stuck in American and Japanese automotive/motorcycle terminology (sprag is of Scandinavian origin)
I'm registered with American Honda, Honda GB, American Suzuki Motor Corporation and Yamaha.
I have multiple manufacturer training, including some BMW from the early 80's and some Harley in 2009
I have posted pics of Honda 'passport' from Honda GB and have various other competed course certificates somewhere safe
What is it you wanted to know?