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

[timdhawk]

I bored my pilot jets according to the tech tip on hondachopper.com. I have stock 35's and bored them out with an  0.45mm bit.
What would this make it in the equivalent jet sizing? 40? 42?

I am running the dreaded pods and a damn near open 4-1 exhaust. With the pilot jets bored, my 0-1/4 throttle studders and will run "better" with upwards of 3/4's choke; which tells me i need more gas. I'm supprised i still need more fuel...

Oh, and my screws are out 2 1/2 turns. I've played with them from 1 turn to 3 turns and it didn't seem to help.
thanx

[bollingball]

F or K model 42a or 42b carbs? Is your accelerator pump working on all FOUR nozzles? Need more info. The pump has to work on these bikes.

Ken

[timdhawk]

F or K model 42a or 42b carbs? Is your accelerator pump working on all FOUR nozzles? Need more info. The pump has to work on these bikes.

Ken

42A K model. cards are very clean and diligently checked, rechecked and rebuilt. all components tested and work as they should. all 4 pumper jets squirt. 

[TwoTired]

A drill make a larger hole than whatever size it is marked.
The Jet sizes are in mm increments.  #100 main is 1.00mm in diameter.    A .45mm bit likely makes a #47 (or larger) hole.

If you still have off idle stumble with pumper carbs, I'd say your pump system isn't up to peak performance.

My pump system didn't work well enough until it would make the squirt jets put out 6-8 inch streams.

[timdhawk]

My pump system didn't work well enough until it would make the squirt jets put out 6-8 inch streams.

I woke up this morning wondering how much of a "squirt" constitutes a "properly working" accel pump...
If a 6"-8" squirt is normal... then I think you are correct and that would be my issue. Last year when rechecking my  rechecks, all 4 squirted and would get to the throttle valve but not with 6"-8" worth of force.

[TwoTired]

My pump system didn't work well enough until it would make the squirt jets put out 6-8 inch streams.

I woke up this morning wondering how much of a "squirt" constitutes a "properly working" accel pump...
If a 6"-8" squirt is normal... then I think you are correct and that would be my issue. Last year when rechecking my  rechecks, all 4 squirted and would get to the throttle valve but not with 6"-8" worth of force.

The check valve balls must seat/seal fully and allow fluid only one direction of travel.   The pump diaphragm's Micky mouse ears, must have holes the same diameter as the holes they mate to in the bowl and pump body.
 The holes in the squirt posts are actually slots.  Of course they gotta be clean too, but if they are enlarged or altered by aggressive cleaning tools, they actually squirt less distance.

It's always the details with carbs, it seems.

Cheers,

[timdhawk]

well, all 4 nozzles are squirting with force (more than i remember anyway). When I hit throttle hard it gets up and scoots... all my studdering seems to be when I hold constant throttle between 1/8th and 1/4.
After putting the carbs back on, i started with 1 turn on the screws and I am now at 3 full turns out.  It did improve a bit but still studders some.  3 1/2 is about the max turns i can get, right?

[TwoTired]

when I hold constant throttle between 1/8th and 1/4.

 

According to Honda, that is the domain of the throttle valve cutaway.  Via theory, making the cautaway larger would increase the velocity , thereby the vacuum that is pulling in the fuel.

After putting the carbs back on, i started with 1 turn on the screws and I am now at 3 full turns out.  It did improve a bit but still studders some.  3 1/2 is about the max turns i can get, right?

No.  If you stretch the spring some, you can turn them out farther without much fear of them falling out.

Tell me about the idle jet emulsion tube holes.  Are these stock jets or aftermarket?
Have the air jets been changed?  Main or pilot circuit?

Pods and open pipes are going to need a lot of fiddling with these carbs I'd guess, as the pods and pipes aren't helping the carb throat vacuum or fuel requirements at low speeds.

[lucky]

I bored my pilot jets according to the tech tip on hondachopper.com. I have stock 35's and bored them out with an  0.45mm bit.
What would this make it in the equivalent jet sizing? 40? 42?

I am running the dreaded pods and a damn near open 4-1 exhaust. With the pilot jets bored, my 0-1/4 throttle studders and will run "better" with upwards of 3/4's choke; which tells me i need more gas. I'm supprised i still need more fuel...

Oh, and my screws are out 2 1/2 turns. I've played with them from 1 turn to 3 turns and it didn't seem to help.
thanx

Unfortunately this may not fix the problem. The 1978 carbs cannot be rejetted.
I have spent months working on them.

A#35 jet is .35mm or .013 thousandths of an inch.
A #45 jet is .45mm or .017 thousandths of an inch.

You CAN put in a #120 main jet instead of the 110 main jet if you have a 4 into 1 exhaust.

But if you try to run the engine with pods or velocity stacks you will never get the right combo.
Tapered needles might help but which ones??? May not even be available.
The accelerator pump system is another wild card in the mix.

I am sorry if you read one of my recent posts about jetting the 1978 PD carbs. I never could get it to work.

With .058 (.044 shims and .012 pocket depth inside the bottom of the slide), shims under the needle and #45 idle jets it ran good until the plugs were completely fouled with carbon.

I gave up on the project. unless you want to start trying different needles from other carbs.

[crazypj]

I've been measuring some precision drill bits.
They are all undersize to drill correct size hole (don't remember if it was TT or Hondaman who pointed it out a year or so ago?)
0.45mm is pretty big for a pilot jet.
 Can you fit adjustable needles?
Probably want to change the taper to get it running right at cruise

[scottly]

 The 1978 carbs cannot be rejetted.
I have spent months working on them.

A#35 jet is .35mm or .013 thousandths of an inch.
A #45 jet is .45mm or .017 thousandths of an inch.

Any carb can be re-jetted. A lot of errors are caused by failure to understand jet sizes. A #45 jet will flow 60% more fuel than a #35 jet. That's a big jump! Add a couple thou for using a drill bit instead of a proper ream, and you're more than 80% richer than the #35s..

[crazypj]

You need to get a set of micro drills and 'pin vice' to hold them.
I have drills from 0.3mm to 1.00mm in 0.05mm steps.
I've used metric micrometer to check sizes to nearest 0.001mm and they are all 'undersize'
Of course,unless your doing a lot of carb mods on a regular basis it's probably cheaper to buy jets than the stuff I've mentioned 

[timdhawk]

Well, I bought some 42's to try since I still run better with some choke applied to the range in question. I do have a set of adj. needle carbs off a 77 as well as some true needle shims for my 78 bank to try. I've read about taking 0.020 off the beveled portion of the slide to help that range also.
Which would hit that spot best: needles up or shave the slide?

[timdhawk]

TTT

[lucky]

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.


My solution for this bike:

1978CB75K
4 into 1 exhaust no muffler.
POD air filters.
idle jet #42mm-NOT drilled.
Main jet #120mm
Mixture screws turned out 1 turn.
1977 slide needles raised by lowering the clip.
Compare 1977 and 78 needles to get the clip in the equivalent stock position on the 1977 needle ,then lower the clip one notch on that 1977 needle.


The main jet is too small. Stock main jet is #110
Would like to know how far above sea level you are.
Also check to see if the fuel distribution hoses from the accelerator
 pump to each carb have the restrictors in them as this helps raise fuel pressure.

[lucky]

I've been measuring some precision drill bits.
They are all undersize to drill correct size hole (don't remember if it was TT or Hondaman who pointed it out a year or so ago?)
0.45mm is pretty big for a pilot jet.
 Can you fit adjustable needles?
Probably want to change the taper to get it running right at cruise

I have actually already done this exact jet change on the same year bike for the same set up except,... the open pipes.
The #45 idle jet and two .020 shims on the needle sitting over the .012 pocket in the slide at the bottom got excellent results. Still rich and a #42 idle jet worked better.
Two .020 shims sitting down in that pocket would probably work better.
But since I was missing the accelertator pump restrictor tubes in the pump distribution hoses. I would just get 1977 needles and move them up one increment.

Forum member Brewsky drilled different amounts of holes ( three designs) in his exhaust baffle and his A/F ratio meter showed no difference as far as the mixture was concerned.

There was another person on the internet with similar set up that did some dyno runs and there was a dip in the HP curve at mid curve and said the solution would also be different slides but did not elaborate and once you start grinding the slides they are ruined if you are wrong. Twotired mentioned that also in this thread about slide modification.

The person that got this dyno run that was told to modify the slides is probably close to a solution, but where would you get new slides? And could you even get them?

[crazypj]

I have actually already done this exact jet change on the same year bike for the same set up except,... the open pipes.
The #45 idle jet and two .020 shims on the needle sitting over the .012 pocket in the slide at the bottom got excellent results. Still rich and a #42 idle jet worked better.
Two .020 shims sitting down in that pocket would probably work better.
But since I was missing the accelertator pump restrictor tubes in the pump distribution hoses. I would just get 1977 needles and move them up one increment.

Forum member Brewsky drilled different amounts of holes ( three designs) in his exhaust baffle and his A/F ratio meter showed no difference as far as the mixture was concerned.

There was another person on the internet with similar set up that did some dyno runs and there was a dip in the HP curve at mid curve and said the solution would also be different slides but did not elaborate and once you start grinding the slides they are ruined if you are wrong. Twotired mentioned that also in this thread about slide modification.

The person that got this dyno run that was told to modify the slides is probably close to a solution, but where would you get new slides? And could you even get them?

 That could possibly have been me?
 I've posted dyno runs with F/A readings and talked about slide modifcations, I have been doing this for a while (40+yrs  )

[lucky]

I have actually already done this exact jet change on the same year bike for the same set up except,... the open pipes.
The #45 idle jet and two .020 shims on the needle sitting over the .012 pocket in the slide at the bottom got excellent results. Still rich and a #42 idle jet worked better.
Two .020 shims sitting down in that pocket would probably work better.
But since I was missing the accelertator pump restrictor tubes in the pump distribution hoses. I would just get 1977 needles and move them up one increment.

Forum member Brewsky drilled different amounts of holes ( three designs) in his exhaust baffle and his A/F ratio meter showed no difference as far as the mixture was concerned.

There was another person on the internet with similar set up that did some dyno runs and there was a dip in the HP curve at mid curve and said the solution would also be different slides but did not elaborate and once you start grinding the slides they are ruined if you are wrong. Twotired mentioned that also in this thread about slide modification.

The person that got this dyno run that was told to modify the slides is probably close to a solution, but where would you get new slides? And could you even get them?

 That could possibly have been me?
 I've posted dyno runs with F/A readings and talked about slide modifcations, I have been doing this for a while (40+yrs  )

Actually it was Brewsky. But your experience interests me too.

Did you actually get a solution for the 1977-78 CB750??

[kball]

Good post, I was just going to ask about this.

[brewsky]

According to Honda, that is the domain of the throttle valve cutaway.  Via theory, making the cautaway larger would increase the velocity , thereby the vacuum that is pulling in the fuel.

I usually get this backwards, but doesn't making the cutaway smaller increase the velocity, and therefore make the mixture richer?

[TwoTired]

According to Honda, that is the domain of the throttle valve cutaway.  Via theory, making the cautaway larger would increase the velocity , thereby the vacuum that is pulling in the fuel.

I usually get this backwards, but doesn't making the cutaway smaller increase the velocity, and therefore make the mixture richer?

I didn't explain this very well.  And, in truth, I have not done any experimentation to prove my analysis.  But, here is my thinking in the matter.  Just ignore it if you wish.  I think the prior conclusion I had was correct, but the details of how, are different than what I previously posted.

The bottom of the slide is wedge shaped, and the cutaway is the size or ramp angle of the wedge.  It forms a venturi when the slide is nearer the bottom of it's stroke.   The velocity is determined by the differential pressure between the inlet and outlet of the carb throat and the bottom most portion of the slide, or where the smallest orifice exists. The wedge shape serves to compress air as it passes with velocity, making it more dense.   A larger wedge makes more air available for compression, leading to a higher density.  This denser air, given the same velocity, exerts more pull jet exits placed in the carb throat as there are more molecules involved.

It's just theory, though.  I'm not sure I care to prove any of it.  But, the wedge behaves as a velocity stack would, to pack more molecules in a given cross section, albeit without strict attention the laminar flow.

Cheers,

[anders288]

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2The larger the cut away the leaner off idle. A slide marked 3 is a 3mm high cut out. And it's affect is mostly when slide is moving up.

[brewsky]

According to Honda, that is the domain of the throttle valve cutaway.  Via theory, making the cautaway larger would increase the velocity , thereby the vacuum that is pulling in the fuel.

I usually get this backwards, but doesn't making the cutaway smaller increase the velocity, and therefore make the mixture richer?

I didn't explain this very well.  And, in truth, I have not done any experimentation to prove my analysis.  But, here is my thinking in the matter.  Just ignore it if you wish.  I think the prior conclusion I had was correct, but the details of how, are different than what I previously posted.

The bottom of the slide is wedge shaped, and the cutaway is the size or ramp angle of the wedge.  It forms a venturi when the slide is nearer the bottom of it's stroke.   The velocity is determined by the differential pressure between the inlet and outlet of the carb throat and the bottom most portion of the slide, or where the smallest orifice exists. The wedge shape serves to compress air as it passes with velocity, making it more dense.   A larger wedge makes more air available for compression, leading to a higher density.  This denser air, given the same velocity, exerts more pull jet exits placed in the carb throat as there are more molecules involved.

It's just theory, though.  I'm not sure I care to prove any of it.  But, the wedge behaves as a velocity stack would, to pack more molecules in a given cross section, albeit without strict attention the laminar flow.

Cheers,

Interesting theory, and I could swear I've even found opposite statements in different carb manuals regarding the cutaway effect in the past, but I can't find them now.

All the manuals I can find right now say bigger cutout = leaner, smaller = richer (like anders notes)

I have replaced a smaller cutout with a larger in the past to help cure an overall rich condition, but that was not the only change done at the same time, so I'm not sure which change had the most effect.

After installing the A/F meter and playing around with differing jets and settings, I've seen some results that contradict published charts and general statements. For example, simply raising the needle one notch resulting in a full 1% richer mix at WOT on one bike.

At any rate, timdhawk's condition may be mostly the result of the open exhaust, as that can wreak havoc at certain rpm's. And all the overlapping of the circuits at lower throttle openings really makes it interesting to tune.

[anders288]

According to Honda, that is the domain of the throttle valve cutaway.  Via theory, making the cautaway larger would increase the velocity , thereby the vacuum that is pulling in the fuel.

I usually get this backwards, but doesn't making the cutaway smaller increase the velocity, and therefore make the mixture richer?

I didn't explain this very well.  And, in truth, I have not done any experimentation to prove my analysis.  But, here is my thinking in the matter.  Just ignore it if you wish.  I think the prior conclusion I had was correct, but the details of how, are different than what I previously posted.

The bottom of the slide is wedge shaped, and the cutaway is the size or ramp angle of the wedge.  It forms a venturi when the slide is nearer the bottom of it's stroke.   The velocity is determined by the differential pressure between the inlet and outlet of the carb throat and the bottom most portion of the slide, or where the smallest orifice exists. The wedge shape serves to compress air as it passes with velocity, making it more dense.   A larger wedge makes more air available for compression, leading to a higher density.  This denser air, given the same velocity, exerts more pull jet exits placed in the carb throat as there are more molecules involved.

It's just theory, though.  I'm not sure I care to prove any of it.  But, the wedge behaves as a velocity stack would, to pack more molecules in a given cross section, albeit without strict attention the laminar flow.

Cheers,

Interesting theory, and I could swear I've even found opposite statements in different carb manuals regarding the cutaway effect in the past, but I can't find them now.

All the manuals I can find right now say bigger cutout = leaner, smaller = richer (like anders notes)

I have replaced a smaller cutout with a larger in the past to help cure an overall rich condition, but that was not the only change done at the same time, so I'm not sure which change had the most effect.

After installing the A/F meter and playing around with differing jets and settings, I've seen some results that contradict published charts and general statements. For example, simply raising the needle one notch resulting in a full 1% richer mix at WOT on one bike.

At any rate, timdhawk's condition may be mostly the result of the open exhaust, as that can wreak havoc at certain rpm's. And all the overlapping of the circuits at lower throttle openings really makes it interesting to tune.

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

[brewsky]

       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.