Topic: Jetting Carbs Question

[TwoTired]

First off... Sorry I should've mentioned what I have for an exhaust but I am not 100% sure what it is. Here is a photo of the bike with exhaust, any ideas? It does have baffles for sure.

Looks like a MAC.  It probably has similar muffler pressures as the stock 4 into 4 exhaust pre 77K.  (I'm just extrapolating here, as actual engineering data for all the exhaust systems is non-existent or non-publicly available.)
The stock 77-78 CB750K exhaust systems used higher pressures than earlier models.  This allowed two things, it allowed less fuel requirements at idle (an EPA goal) and it made the bike quieter, to help satisfy government noise pollution regulators.
The original 77-78 exhaust likely sacrificed a little power at RPMs near redline to satisfy regulators import goals unrelated to speed performance.

TwoTired
I would love to have adjustable needles, if I find a set of 77 F of K or a 78 F needles will the fit right into my slides with no mods.?

yes

Stock my carbs come with 110 jets, do you think my 115 jets will need to be changed? I have a whole set of all size jets.

I will guess that 115's will be OK with your set up.  I would also expect a shim under the needle clip will be better, and a 1/2 to 1 turn outward on the IMS.  But, I'm no soothsayer.
Should run OK, not so sure what the fuel economy will be. And I expect some testing would help refine the setting after some feedback from plug deposits are learned.  (Or, a dyno fuel map readout.)

I have also heard of people drilling extra holes in the bottom of there air box, what do you think about that?

Fine for the drag strip, where you need that redline and above power needed to have an edge.

In the rain, extra holes will let in more water.  Also extra holes will effect the carb throat pressures which govern much of the volume each and every fuel orifice flows in the carb.  Carbs are stupid and can't automatically react to what's been done to air inlet and exhaust channels of the machine.  They were set by a team of Honda engineers and mechanics for the characteristics of the stock set up.  Change that and the carbs need fuel orifice changes in reaction by the mechanic doing the alterations.  Without knowing the engineering changes impacted by style or "looks" alterations, accurate, predictive jetting recommendations are impossible.  Depending on skill level, one can make descent guesses.  But, the only true way to accuracy is with known component characteristics, or test data.  The only components well known, are the stock components and the carb settings for those are all provided in published data.

Cheers,

[livetoskate]

Posted by: bollingball
livetoskate That is a pollution control device called breather separator Look at the service manual 78 supplement section 1-11 around page 291.
The bike will run without it but your lungs will not like it. Also I think TT said it will cause a problem with out it but I don't recall what it was. I think something about pressure in the bottom end. Maybe he will see this and refresh my memory I ended up putting mine back on because of something he said. That was a few years back.

Ken

Thanks alot, guess I should try to find on or see if I got one. If it may cause an issue.

That's a Nice looking workspace and garage livetoskate  ...it looks like you also Live to Ride !

Haha Thanks a lot, needs a good spring cleaning after this winter.

[TwoTired]

From TT
I think if I had a 78 CB750K and a non-stock muffler system, I would beg borrow or steal needles from a 77 F of K or a 78 F that are adjustable.

From Ken I wish You could tell me just what carbs these are. By that I mean 42a or b or 41 a or b. I know they are not 42b that is what I have on my 78 750K I bought a rack of 42a carbs just because they were a good price and complete. It was my understanding they came off a 78 750f. Do you or anyone else know if they would have the adj. needles if they are stock. I have a lot going on and did not want to break them down until winter. PLEASE let me know what you think. I am not sure what the 77 K or F came with.

Ken

I'm not well versed in CB750 nuance.  (But, I read a lot.  )
I know there are errors in other areas of this chart (see link).  But, for the 77-78 CB750s it shows:
PD41a - 77K  (Adjustable needles)
PD41b - 77F  (Adjustable needles)
PD42a - 78F  (Non adjustable needles.)
PD42b - 78K  (Non adjustable needles.)
http://www.motorcycleproject.com/motorcycle/text/specs.html

I thought I took my 78 CB750F carbs apart (42a) and found adjustable needles in there (10 years ago  ).  But, the bike was old when I got it and I can't certify it was untouched since new (In fact, it was in sad shape when I trucked it home along with boxes of parts.)

So, if you are looking for adjustable needles, get some PD41 carbs or needles from them.

Hope this helps,

[TwoTired]

I was getting all the parts together for putting my airbox back in and then looked at the mico fiche and saw a couple parts I don't have nor do I know what they are. I am attaching a photo. The parts I am not sure I have are bordered by red. What exactly are they and are they all necessary  for the proper function of the airbox?
Thanks again

livetoskate That is a pollution control device called breather separator Look at the service manual 78 supplement section 1-11 around page 291.
The bike will run without it but your lungs will not like it. Also I think TT said it will cause a problem with out it but I don't recall what it was. I think something about pressure in the bottom end. Maybe he will see this and refresh my memory I ended up putting mine back on because of something he said. That was a few years back.

Ken

Ken is correct.  It was mainly put on by EPA mandate to reduce pollution.  However, it does more than that.  Primarily it takes noxious fumes and carcinogenic hydrocarbons from the engine crankcase and feeds then to the engine to be burned.  It also helps remove water condensation from the engine crankcase, extending service life.  And in theory, it applies a slight engine vacuum to the crankcase, which should aid in piston ring seal (reducing blow by).
Finally, if the device is not installed and the fitting at the top of the air box is not plugged, it sources unfilter air into the engine bypassing the filter and can change the carb throat vacuum which can effect air fuel ratios.

I think that covers it. 

Cheers,

[lucky]

Twotired
So I am going to go back to my air box, but I was thinkIng about getting the K&M filter... Is that worth it, good/bad idea?

That's what I use on my Cb750 F models.  My main requirement is economics, in that the filter is cleanable and reusable with the K&N cleaner and oiling kit.  I like not needing to buy another filter.
I didn't change any jetting for my carbs, but I do have the stock 4 into 1 and muffler (quiet, and probably more restrictive than any aftermarket 4 into 1 system.
I do have an 77 750 F with a K8 motor (77F carbs) in it and a kerker exhaust system on it. Sadly project overload has left that one languishing in my garage, protecting the floor from accumulating dust.

also I have a 4 into 1 exhaust would that make me skip up a jet size? Guess I'm
Just asking If you think I'll be good at the stock setting with a 4 into 1 and possibly a K&n filter?

Kinda depends on the 4 into 1 brand and design, and if it has baffles in the muffler/collector (they aren't all the same, you know).
Check out the stock settings for the 77-78 CB750 and compare them to the Cb750F for the same years.
http://www.motorcycleproject.com/motorcycle/text/specs.html

I think if I had a 78 CB750K and a non-stock muffler system, I would beg borrow or steal needles from a 77 F of K or a 78 F that are adjustable.  Or, I would put the non-adjustable needles in my lathe and add grooves to them to get the midrange adjustment acceptable.

I'd also expect to set the IMS to slightly more turns than book value, due to lower exhaust system pressures.

The main will be adjusted for a clean plug chop reading after a few 1/4 mile drag race simulations.  With my Kerker, I'll probably start at #115 and see what the plugs say.

If you want to avoid the recursive test and try, then getting a fuel map off a dyno test would save a lot of time and uncertainty.  But, many just won't spend the money.  And, the prowess and cooperation available at dyno shops has a rather wide quality variance.

Cheers,

OK...
Several questions here.

The STOCK 4 into one exhaust would be more restrictive I would think that a aftermarket exhaust. But I do not have the proof.
It is just that when most aftermarket exhausts are sold they usually say"less restrictive".

I gave my bike (1978K) as an example.
stock air box
stock filter.
#120 main jets.
#42 idle jet.
Cycle-X 4 into 2 into 1 exhausts with internal metal baffle,no fiberglass.
mixture screw 1 turn.

The fact that you have a STOCK 4 into 1 exhaust you would not need to rejet unless you change the intake system

Remember you have air coming in, and air going out
If you change either one of those you may need to make adjustments.

As far as the K&N pods with stock exhaust I do not know what that would do.
BUT you said that when trying to give it throttle it would bog. IF it was fully warmed up  and mixture screws adjusted for lean best idle, then I would say it needs a larger idle jet.

If the mixture screws will not solve the hesitation problem from idle, you need to increase the size of the idle jet.

On the last question

QUOTE.. "I have a 4 into 1 exhaust would that make me skip up a jet size? Guess I'm
Just asking If you think I'll be good at the stock setting with a 4 into 1 and possibly a K&n filter? "

4 into 1 exhaust (stock or aftermarket?)
K&N filter (possibly)

Not enough solid info to make a comment.

[lucky]

First off... Sorry I should've mentioned what I have for an exhaust but I am not 100% sure what it is. Here is a photo of the bike with exhaust, any ideas? It does have baffles for sure.



Lucky
Thanks for all the info, I am planning a road trip in about 5 weeks and don't have much time so I am going to stick my airbox on in stead of my K&N oval pods.
Plus my pods don't perform very well in the rain even when freshly oiled up, and I always get stuck in the rain on this trip
Your recommendation on changing my idle jets to #42 would be if I was keeping my pods ....Correct?
I understand that the airbox is actually restricting/choking my carbs of air but That isn't a bad thing is it? Seems like everyone thinks the 750's run great with the stock airbox. I wouldn't know because I have yet to put many miles on a 750 with an airbox.


TwoTired
I would love to have adjustable needles, if I find a set of 77 F of K or a 78 F needles will the fit right into my slides with no mods.?
Stock my carbs come with 110 jets, do you think my 115 jets will need to be changed? I have a whole set of all size jets.
I have also heard of people drilling extra holes in the bottom of there air box, what do you think about that?


I really appreciate all the help and knowledge. I am trying to absorb all I can!

It looks like the exhaust could be a MAC or stock headers and aftermarket muffler.
Just cannot tell from the photo.

But if you change to pods from the stock airbox you may have to make adjustments.
It might just need a mixture adjustment or it could need a different idle jet.

If it is fully warmed up and when you try to give it throttle from idle and it bogs or stalls or has to be "babied"  then it probably need a larger idle jet.
As far as midrange and the needle you would need to ride it and make sure it accelerated smoothly and check your plug readings.

For your trip I would put the stock air box back on if it was already running ok.

[lucky]

From TT
I think if I had a 78 CB750K and a non-stock muffler system, I would beg borrow or steal needles from a 77 F of K or a 78 F that are adjustable.

From Ken I wish You could tell me just what carbs these are. By that I mean 42a or b or 41 a or b. I know they are not 42b that is what I have on my 78 750K I bought a rack of 42a carbs just because they were a good price and complete. It was my understanding they came off a 78 750f. Do you or anyone else know if they would have the adj. needles if they are stock. I have a lot going on and did not want to break them down until winter. PLEASE let me know what you think. I am not sure what the 77 K or F came with.

Ken 

I'm not sure what carbs mine are I can look since they are already off my bike at the moment. Did the 1978 cb750K come with different carbs, I would assume they are all the same on that model...Right? Also I did thoroughly clean my carbs and actually just had them cleaned and completely rebuilt by Harisuluv.

The 1978's had the accelerator pump feature on the #2 carb. The CB750A(automatic) had it also.

From what I have seen Harisuluv does a excellent job for the price.
Just make sure that when the engine is OFF (float bowls full of gas) and you look into the intakes , turn the throttle a couple times you can see the nozzles actually squirt fuel. Use a bright LED flash light. Make sure all 4 of them work.

[lucky]

From TT
I think if I had a 78 CB750K and a non-stock muffler system, I would beg borrow or steal needles from a 77 F of K or a 78 F that are adjustable.

From Ken I wish You could tell me just what carbs these are. By that I mean 42a or b or 41 a or b. I know they are not 42b that is what I have on my 78 750K I bought a rack of 42a carbs just because they were a good price and complete. It was my understanding they came off a 78 750f. Do you or anyone else know if they would have the adj. needles if they are stock. I have a lot going on and did not want to break them down until winter. PLEASE let me know what you think. I am not sure what the 77 K or F came with.

Ken

I'm not well versed in CB750 nuance.  (But, I read a lot.  )
I know there are errors in other areas of this chart (see link).  But, for the 77-78 CB750s it shows:
PD41a - 77K  (Adjustable needles)
PD41b - 77F  (Adjustable needles)
PD42a - 78F  (Non adjustable needles.)
PD42b - 78K  (Non adjustable needles.)
http://www.motorcycleproject.com/motorcycle/text/specs.html

I thought I took my 78 CB750F carbs apart (42a) and found adjustable needles in there (10 years ago  ).  But, the bike was old when I got it and I can't certify it was untouched since new (In fact, it was in sad shape when I trucked it home along with boxes of parts.)

So, if you are looking for adjustable needles, get some PD41 carbs or needles from them.

Hope this helps,

Thanks TWO TIRED,

Bike Bandit parts diagrams are kind of worthless on this detail of the needles.
Many times they use the same photo for different models etc.,.

Better than the car parts world where now there are no photos or drawings just numbers!

Hey at least they got rid of all those good for nothing artists. (Joking)

[livetoskate]

Ok Thanks for all the info again!

Lucky
I actually had my K&N pods on and it was running good but not as good as it should of. I haven't ever had my air box on this bike in the 6 years I've owned it the previous owner gave it to me when I bought it because he had taken it off and put pods on.
I just want to have the air box on to see how much different it will run and mainly to be better off when I get stuck driving in the rain.

[livetoskate]

I will guess that 115's will be OK with your set up.  I would also expect a shim under the needle clip will be better, and a 1/2 to 1 turn outward on the IMS.  But, I'm no soothsayer.
Should run OK, not so sure what the fuel economy will be. And I expect some testing would help refine the setting after some feedback from plug deposits are learned.  (Or, a dyno fuel map readout.)

So I am getting a set of adjustable needles, I just ordered them. When you say shim under the needle you are talking about if I use my current needles that are non adjustable correct? If I have adjustable I can just adjust the needle to sit higher right?

[bollingball]

So I am getting a set of adjustable needles, I just ordered them. When you say shim under the needle you are talking about if I use my current needles that are non adjustable correct? If I have adjustable I can just adjust the needle to sit higher right?
[/quote]

I did not write that but you are correct higher or lower. How much were or are they??

Ken

[TwoTired]

I will guess that 115's will be OK with your set up.  I would also expect a shim under the needle clip will be better, and a 1/2 to 1 turn outward on the IMS.  But, I'm no soothsayer.
Should run OK, not so sure what the fuel economy will be. And I expect some testing would help refine the setting after some feedback from plug deposits are learned.  (Or, a dyno fuel map readout.)

So I am getting a set of adjustable needles, I just ordered them. When you say shim under the needle you are talking about if I use my current needles that are non adjustable correct? If I have adjustable I can just adjust the needle to sit higher right?

Yes, shimming raises a needle that only has one clip groove.  Adjustable needles allow moving the clip to a lower position, which raises the needle in the slide.  I suppose, you *could* do both, though.

FYI, I can assure your that the exhaust on your bike is most definitely NOT a stock one either F or K.

Cheers,

[mrrch]

If you have the correct thickness of shim you could adjust an adjustable needle by half a clip position instead.

[bollingball]

 From Lucky
The 1978's had the accelerator pump feature on the #2 carb. The CB750A(automatic) had it also.

From what I have seen Harisuluv does a excellent job for the price.
Just make sure that when the engine is OFF (float bowls full of gas) and you look into the intakes , turn the throttle a couple times you can see the nozzles actually squirt fuel. Use a bright LED flash light. Make sure all 4 of them work.

I would just add one thing to this. If the carbs are still off it is alot easier to check the nozzles, pump function and float level clear tube method while the carbs are on the test bench jig. That way if adjustments are needed it is a snap. About the only thing to do when on the bike is adj. IMS and sync.

Ken 

[lucky]

I would never try to do it while they are mounted on the bike.

[livetoskate]

FYI, I can assure your that the exhaust on your bike is most definitely NOT a stock one either F or K.

twotired
Yeah I knew the previous owner bought the new exhaust a few months before I purchased it and it I think I do remember him mentioning it being a Mac.

I would just add one thing to this. If the carbs are still off it is alot easier to check the nozzles, pump function and float level clear tube method while the carbs are on the test bench jig. That way if adjustments are needed it is a snap. About the only thing to do when on the bike is adj. IMS and sync.

Ken

Ken
I have tested all the nozzles that the accelerator pump controls and the are all working, I have also adjusted the float heights with my float tool.
Would you recommend testing them another way?

[livetoskate]

I did not write that but you are correct higher or lower. How much were or are they??

Ken

Well I probably paid to much but I paid $20 for a used set but the looked straight and clean. Guess 5$ per needle isn't to bad.
Was there somewhere I could've gotten them new?

[lucky]

I will guess that 115's will be OK with your set up.  I would also expect a shim under the needle clip will be better, and a 1/2 to 1 turn outward on the IMS.  But, I'm no soothsayer.
Should run OK, not so sure what the fuel economy will be. And I expect some testing would help refine the setting after some feedback from plug deposits are learned.  (Or, a dyno fuel map readout.)

So I am getting a set of adjustable needles, I just ordered them. When you say shim under the needle you are talking about if I use my current needles that are non adjustable correct? If I have adjustable I can just adjust the needle to sit higher right?

Exactly.. Using shims under the needles is problamatic and has limitations.
Adjustable needles would give more consistent results. Make sure those adjustable needles look just like the non adjustable needles when put side by side.
Make sure the grooves start at the same place on the needle.
The needle with more grooves may be longer. Don't know.

I may try that myself soon. I need to ride mine more to find out.

[livetoskate]

Off topic but I am planning on ordering a new battery since I need one for my 1972 project anyways. Is there any certain battery that anyone may recommend? Higher cranking amps good/bad brands or type? Also I have heard a lot about the regulator rectifiers in one unit,  are those worth getting and can the be used with an electronic ignition?
Thanks sorry for the off topic reply

[livetoskate]

I was just going to buy a gasket for my air cleaner and realized I might have a older style air box from maybe a 1976. I didn't realize they were different

If I use the older style air box would I run into any problems?

[grcamna2]

You're bike is a K..I think it would be OK to use an older K Airbox along with  K carb rubbers in front & behind the carbs...I got this from a forum member here who Knows.

[TwoTired]

I was just going to buy a gasket for my air cleaner and realized I might have a older style air box from maybe a 1976. I didn't realize they were different

If I use the older style air box would I run into any problems?

76 model did not use PD carbs.  If they are anything like the 550's the rubber couplers will be different length and size.  I don't know personally, though.

Cheers,

[lucky]

Twotired
So I am going to go back to my air box, but I was thinkIng about getting the K&M filter... Is that worth it, good/bad idea?

That's what I use on my Cb750 F models.  My main requirement is economics, in that the filter is cleanable and reusable with the K&N cleaner and oiling kit.  I like not needing to buy another filter.
I didn't change any jetting for my carbs, but I do have the stock 4 into 1 and muffler (quiet, and probably more restrictive than any aftermarket 4 into 1 system.
I do have an 77 750 F with a K8 motor (77F carbs) in it and a kerker exhaust system on it. Sadly project overload has left that one languishing in my garage, protecting the floor from accumulating dust.

also I have a 4 into 1 exhaust would that make me skip up a jet size? Guess I'm
Just asking If you think I'll be good at the stock setting with a 4 into 1 and possibly a K&n filter?

Kinda depends on the 4 into 1 brand and design, and if it has baffles in the muffler/collector (they aren't all the same, you know).
Check out the stock settings for the 77-78 CB750 and compare them to the Cb750F for the same years.
http://www.motorcycleproject.com/motorcycle/text/specs.html

I think if I had a 78 CB750K and a non-stock muffler system, I would beg borrow or steal needles from a 77 F of K or a 78 F that are adjustable.  Or, I would put the non-adjustable needles in my lathe and add grooves to them to get the midrange adjustment acceptable.

I'd also expect to set the IMS to slightly more turns than book value, due to lower exhaust system pressures.

The main will be adjusted for a clean plug chop reading after a few 1/4 mile drag race simulations.  With my Kerker, I'll probably start at #115 and see what the plugs say.

If you want to avoid the recursive test and try, then getting a fuel map off a dyno test would save a lot of time and uncertainty.  But, many just won't spend the money.  And, the prowess and cooperation available at dyno shops has a rather wide quality variance.

Cheers,

Many people including myself purchased dyno kits and there was not much in  the kit but there was a coupon to get a dyno test. Problem is the dyno places that have those dyno jet dynos said if they do any adjustments it can cost up to $300.00!!!

So like many other people, my coupon sits at home in a drawer.

And another thing .....Do I really want some very young guys that grew up with fuel injection and lap tops, work on my carburated bike of which they may know NOTHING.
I don't think so.

[bollingball]

FYI, I can assure your that the exhaust on your bike is most definitely NOT a stock one either F or K.

twotired
Yeah I knew the previous owner bought the new exhaust a few months before I purchased it and it I think I do remember him mentioning it being a Mac.

I would just add one thing to this. If the carbs are still off it is alot easier to check the nozzles, pump function and float level clear tube method while the carbs are on the test bench jig. That way if adjustments are needed it is a snap. About the only thing to do when on the bike is adj. IMS and sync.

Ken

Ken
I have tested all the nozzles that the accelerator pump controls and the are all working, I have also adjusted the float heights with my float tool.
Would you recommend testing them another way?

I am just saying do everthing on the bench while it is much easier to get to.
As far as float level I like the clear tube method better and I do that on the bench and put the fuel level 2 to 3 mm below bowl gasket. I think I have a photo. Look at carb #4 you can just see the fuel level. May need to click on the picture to enlarge. The test stand is easy and cheap. I c-clamp it on the bench and then store it away when not needed. The board the rack is mounted on swivels so you can get them level.

Ken

[Gearheadgreg]

Lucky + 1 here on that Slow jet!! I have a stock 78 SS factory headpipe 4-2-1 with a aftermarket slip on,,Just the decreased back pressure from that one Mod was enough to Lean out the idle circuit, Bad hesitation!! and popping on Decell, I have the stock airbox and have the 78 PD NON adjustable Needles,,,I put a shim under the clip and went from a 35 to a 38 Slow jet.....SHAZZAMMMM Smooth as glass and mid range is strong No flat spots.....SLOW JET should be the FIRST change..considering 90% of cruising is NOT the MAIN circuit,,  and i have access to a Dyno with a Sniffer around the corner seen the results and the numbers Got the Youtubes with some Dyno runs LOL..