Topic: Jetting fun! 77 CB750F Pd42a carbs.

[harald]

I have the same problem!
I have a cb750  1977 F2, bigbore 836 with webcam 41,
The jets are
125 main
40 pilots KN filter in standard filter casing
Needles are f2051f at next lowest position

The bike runs so lean from 1/4 throttle that I allready  have had two mild piston siezures. The motor gets very hot.

I am going to put jettings to
135 main
needle moved to lowest position
Bored 4 holes in standard casing

But I doubt that this will help, because I coming to the conclusion that the carbs are not capable of doing the job.
The taper on the needle is the problem.

Would it help changing to cb750 K2 carbs?

Has anyone used k2 carbs on their cb750 f2 with succes?

[martin99]

Enrichening the pilot circuit will have no effect on the main jet.


Pilots standard
IMS screws two turns out

Runs well, though isn't as smooth through the carb circuit transitions as a standard bike.

Perhaps a larger pilot would help?

Yes, you do have a point there I'll concede. The problem isn't that bad though, not enough IMO to warrant the cost of new jets..

[CB750R]

So a little update,

I've drilled up the pilots, ensured the accel pump is working perfect.

Turned in the idle screws to one out from three and a half

Removed the washers used as shims under needles

Dropped main jet down from 137.5 to 135

Results, the dyno isn't running at the moment, so the but dyno says it may still need main jet to go down to closer to the 120 most others report success on. That or I need to go one step leaner on the needles.

The idle jets did make a significant change on the entire setup.

[lucky]

I'm running a cb750f2

The jets are
137.5 mains
35 pilots
Needles are f2051f shimmed with a washer underneath and at lowest clip position.

Dude says there's a huge lean spot than corrects and may be a tad rich on he top end.

I'm already maxed out on the needle. And seem to be running very large jets.

Wtf is going on???   It pulled 55 hp

My only mods are a Tintop air cleaner dyna s ignition and coils.  And some very mild clean up of ports when I rebuild my engine.

Dyno guy said its Waaaay off bike runs pretty nice on the road...

So new needles?? Or more main jets?

What year is this bike?
What kind of exhaust?
What kind of intake?

Forget that needle number. Does not matter.
You did not even tell us the thickness of the shim under the needle.
Did you measure the shims/washers?? They can vary as much as .004 thousands each.


"A tad rich on top" Yes I would say so. Those are huge main jets for this bike.
I would use a 125 main jet.

It is hard to help you with so little information.

IF you get some 1969-70 carbs you will not have all of these problems.

[CB750R]

I'm running a cb750f2

The jets are
137.5 mains
35 pilots
Needles are f2051f shimmed with a washer underneath and at lowest clip position.

Dude says there's a huge lean spot than corrects and may be a tad rich on he top end.

I'm already maxed out on the needle. And seem to be running very large jets.

Wtf is going on???   It pulled 55 hp

My only mods are a Tintop air cleaner dyna s ignition and coils.  And some very mild clean up of ports when I rebuild my engine.

Dyno guy said its Waaaay off bike runs pretty nice on the road...

So new needles?? Or more main jets?

What year is this bike?
What kind of exhaust?
What kind of intake?

Forget that needle number. Does not matter.
You did not even tell us the thickness of the shim under the needle.
Did you measure the shims/washers?? They can vary as much as .004 thousands each.


"A tad rich on top" Yes I would say so. Those are huge main jets for this bike.
I would use a 125 main jet.

It is hard to help you with so little information.

IF you get some 1969-70 carbs you will not have all of these problems.

Seriously?

Did you read what you quoted?

77 cb750 f2
Intake tin top
Only other mods dyna s and coils
I have a different silencer on the stock header.

Did you read my last post?

Shims removed! So the thickness is 0.00000mm
Still too rich will try 120s.

Don't be a dick and tell me I'm not providing enough info, because your too lazy to read.

At this point I'm providing info to the group, and hope to have this as a guide for future members in a similar situation, your infinite wisdom suggests I buy new carbs, it I choose to go that route, I will end up with some CR specials like I also mention in an earlier post.

[flybox1]

Hey CB750R...might want to get your hands on OEM pilots.  these bikes really do prefer OEM jets. 
I'll never drill or buy aftermarket crap again.
the drill method can be a bandage as you search for a jet range, but is higly inaccurate unless you can chuck the bit and the jet for a steady job.
If youre still really rich on top, drop all the way to 130's. 
I think you'll find yourself lean in the mid range(main not kicked in yet), still, and need to put the shim back in...
Go right to 42's for your pilots. mixture screw at 3/4 to 1 turn out...

[CB750R]

Hey CB750R...might want to get your hands on OEM pilots.  these bikes really do prefer OEM jets. 
I'll never drill or buy aftermarket crap again.
the drill method can be a bandage as you search for a jet range, but is higly inaccurate unless you can chuck the bit and the jet for a steady job.
If youre still really rich on top, drop all the way to 130's. 
I think you'll find yourself lean in the mid range(main not kicked in yet), still, and need to put the shim back in...
Go right to 42's for your pilots. mixture screw at 3/4 to 1 turn out...

Cheers flybox,
I will order up some jets from jetsrus I bought the .40 and .45 drills and a pin vise before the other responder posted the link to them.

Honda doesn't seem to have any inventory for pilots any more, but proper keihin jets are more consistent than the random aftermarket stuff.

[lucky]

I'm running a cb750f2

The jets are
137.5 mains
35 pilots
Needles are f2051f shimmed with a washer underneath and at lowest clip position.

Dude says there's a huge lean spot than corrects and may be a tad rich on he top end.

I'm already maxed out on the needle. And seem to be running very large jets.

Wtf is going on???   It pulled 55 hp

My only mods are a Tintop air cleaner dyna s ignition and coils.  And some very mild clean up of ports when I rebuild my engine.

Dyno guy said its Waaaay off bike runs pretty nice on the road...

So new needles?? Or more main jets?

What year is this bike?
What kind of exhaust?
What kind of intake?

Forget that needle number. Does not matter.
You did not even tell us the thickness of the shim under the needle.
Did you measure the shims/washers?? They can vary as much as .004 thousands each.


"A tad rich on top" Yes I would say so. Those are huge main jets for this bike.
I would use a 125 main jet.

It is hard to help you with so little information.

IF you get some 1969-70 carbs you will not have all of these problems.

Seriously?

Did you read what you quoted?

77 cb750 f2
Intake tin top
Only other mods dyna s and coils
I have a different silencer on the stock header.

Did you read my last post?

Shims removed! So the thickness is 0.00000mm
Still too rich will try 120s.

Don't be a dick and tell me I'm not providing enough info, because your too lazy to read.

At this point I'm providing info to the group, and hope to have this as a guide for future members in a similar situation, your infinite wisdom suggests I buy new carbs, it I choose to go that route, I will end up with some CR specials like I also mention in an earlier post.

I am trying to help you. Sincerely.

I do not think you need new carbs.
It might be easier with different carbs though.

The term "TIN TOP intake" I do not understand.
I wanted to know if you are using the stock air box and stock air filter.

I would really like to know what the number is on these carbs.
That would explain a lot.

Stock header with muffler and silencer, or a silencer in the end of the header pipe?

I think it was good to remove those shims.
The groove that the clip is on is in question, unless we know what Number those carbs are. I would need the exact number of those carbs.

If you drill out the idle jet with a .45mm drill they will not work correctly.
Actually that is .017 thousandths. That will be too large. You will need a the next size down .42mm= .016 thousandths.
But those small drills are usually sold as numbered drills.
You would need a #78 drill which is .016 thousandths.
A model hobby supply has them.

You can just use a pin vice and twist it with your fingers.
It removes very little material.

[CB750R]

Sorry Lucky, I've had a lot of frustration regarding these carbs. With them only being used on two model years there is very little in the tuning department.

A forum member named Tintop made an intake with one large foam filter for the four carbs.

I purchased both .40mm and .45mm bits from kbc tools. They specifically listed micro bits in millimeter dimensions.

I've drilled them to .40 for now. They are the pd42a carbs. I'm on the lowest clip on the needles.

As mentioned the next step is to take the mains down again.

[CB750R]

Here's Tintop's antipod

[brewsky]

Sorry about the vague info from the dyno, he's a bit of a crusty guy, and said it was so far off on the air fuel it wasn't worth me spending any more time on the dyno till I got the needles corrected

It had a huge lean spike at 4000 k way above the line where 13 afr is. The. Dips low corrects around 12 for the remainder.

The lean spike could be the result of the operator whacking the throttle open.
You could ask him to hold the throttle at given openings to read afr at steady throttle.
Once the throttle is whacked open, the sniffer is no longer reading "low end" or "mid range".......mostly main jet effects at lower rpms.

[PeWe]

Stock engine... Why not stock carb setup? That's the big question....

[Retro Rocket]

Here's Tintop's antipod

What CB750R should also mention is that the filter sports modern rolled lip velocity stacks and is made by a company that makes formula one filter systems, its very free flowing....

[CB750R]

Stock engine... Why not stock carb setup? That's the big question....

Really that's brilliant analysis.... Do you think I'd be chasing down jetting if it ran fine on the stock jetting???

[flybox1]

Stock engine... Why not stock carb setup? That's the big question....

Really that's brilliant analysis.... Do you think I'd be chasing down jetting if it ran fine on the stock jetting???

Definitely not.    but IIRC, you have an aftermarket exhaust can.  since its not stock, you'll have to jet accordingly.
I think you are close to a jetting resolution for your current exhaust and anti-pod.  Stick with it....

[754]

I would be tempted to go back to needle one notch up, and 120 or maybe 125 jet.
 The idle srew being several turns out, to me indicates its out of the range.
 I have some parts carbs with slow jets here, but they got to be pulled out.

[CB750R]

Cheers Frank. I don't think i have a set of 125s in the carb box will have to dig a bit!!

Will call Ya.

Flybox. I see you have a similar setup, I appreciate your input. Do you have the needles with multiple clip positions? Or is yours the one with only only position on the needles?

[754]

Do the older style fit, may have some..
.my obeservations on the jetting are back from when 3 of us were running 811/836,s.
In fact one of the guys went smaller on the mains after it had run in... Even on motors bigger than 836,  they never seemed to use huge jetting..or shall I say..did not have to go as big as you would think.I Should ask 1080 what he ran in the 1080....

[flybox1]

Cheers Frank. I don't think i have a set of 125s in the carb box will have to dig a bit!!

Will call Ya.

Flybox. I see you have a similar setup, I appreciate your input. Do you have the needles with multiple clip positions? Or is yours the one with only only position on the needles?

Yes, our setups are similar, and i think you'd be pretty darn close with 130/42's, and one notch or clip position lower thank stock.
My needles are the latter, one-position needles. 

[lucky]

1/4 throttle to 3/4 throttle is controlled mostly by the slide needle position.

I agree that if your mixture screw is 3 turns out that it is out of range and needs a larger idle jet. Like a .42mm

If you turn the mixture screw in does it start popping on decell when it is revved up?

[andrewk]

JetsRus carries OEM press-in pilots up to 42.  I just bought a set of 38's there earlier this season.  Sucks that they're 15 a piece, but you gotta pay to play when you change them up.

That it responded well to drilling the jets is a good sign.  My next step (after getting the main jet right) would be to drill the pilots to 42, re-test, and order whichever setting ran better.  Some guys will tell you a hole is a hole, but I've experienced the difference more than a few times between drilled jets and new jets of the drill size. 

[redrider736]

So I've been searching without much luck. Where are folks sourcing said press fit style pilot jets.

Sirius inc is out of stock.
Dynoman has nothing on site.
Vintagecb750 no dice
Z1 ... even ebay is a strike out.

Give Mike a email at JetsRus. He has OEM Keihin press fit pilot jets in stock

[nightpoison]

I just purchased some jets from CB750cafe.com today. if you can't find the jets elsewhere, cb750cafe has them.

[CB750R]

1/4 throttle to 3/4 throttle is controlled mostly by the slide needle position.

I agree that if your mixture screw is 3 turns out that it is out of range and needs a larger idle jet. Like a .42mm

If you turn the mixture screw in does it start popping on decell when it is revved up?

Yes I had mentioned that when I installed the drilled out pilots I returned the mixture screws to one turn out. I'm getting a lot of popping on decel now, I assumed that's because it's still rich or I didn't do a good job re syncing. Next step is to try some 125 mains. I'm traveling for work the next two weeks, so you'll all have to live in suspense until then.  Thanks for your input.

[754]

An exhaust leak will pop on decel.
 FWIW. 1080 ran 130,s on his 1080 motor, think he said he tried 135 but went back to 130 . That was with cam, flowed head K1_6 roundtop carbs
 He also ran Kawasaki  smoothbore Mikunis, with 127.5 mains.