Topic: Jetting Carbs Question

[livetoskate]

Need some help/opinions on jetting. The bike is a 1978 CB 750k with K&N oval pods.
So I bought a jet kit because my bike would bog down when I gave it gas to quickly, I was assuming it was getting to much air and not enough fuel.
Common when you put pods on your bike...Right?
Finally got around to taking the carbs off yesterday and realized that the carbs had already been re jetted (at least I think so) I believe the 78 750k came with 110 main jets..Right?
My bike has 115 jets in the carbs now. I guess my question is: should I put the next size jets in 120's, or is that to big? Just hoping someone might have some thoughts/experience with
the same problem.

Thanks in advance

[bollingball]

At what throttle position does this bog happen?
Do you have 42a or 42b carbs?
Is your accelerator working at all four nozzles?
Have you cleaned the carbs and removed the slow jets?
Need more information.

                                 Ken

[grcamna2]

It'd be good to completely clean out the tank..get all the rusty dust out of it first. Then what Ken was saying...completely clean out all you're carb jets and passages & those 4 accelerator jets...they can be a real pain to fully clean,but WELL worth it  ; what color are you're plugs ? They're a very good indicator of how your jetting is.

[lucky]

Need some help/opinions on jetting. The bike is a 1978 CB 750k with K&N oval pods.
So I bought a jet kit because my bike would bog down when I gave it gas to quickly, I was assuming it was getting to much air and not enough fuel.
Common when you put pods on your bike...Right?
Finally got around to taking the carbs off yesterday and realized that the carbs had already been re jetted (at least I think so) I believe the 78 750k came with 110 main jets..Right?
My bike has 115 jets in the carbs now. I guess my question is: should I put the next size jets in 120's, or is that to big? Just hoping someone might have some thoughts/experience with
the same problem.

Thanks in advance

I have more up to date results that would pertain to this situation.
Go further down and read the up to date post.

[livetoskate]

yeah I thought about putting my air box back in but just don't like the look or the pain it makes taking carbs off and on.
Thanks for the help

[livetoskate]

Any suggetions If I was to put a K&N filter in my airbox with my 4 into 1 exhaust with my 115 main jets?
I have read a lot of post about it but none with my set up.

[killersoundz]

main jets only have an effect at the last 1/4 of the throttle when it's wide open. Try raising the clip position on the needle...

[TwoTired]

yeah I thought about putting my air box back in but just don't like the look or the pain it makes taking carbs off and on.

You are just trading one pain for another bigger one.

... and a poorer running bike.

Next time you pull off a hangnail, smack your toe with a hammer.  You won't even notice that ugly finger pain.

If you put the air box back on, you only have to put the carbs on once using stock settings.  With pods, you'll be doing it 25 times or more until you get bored and decide the carbs work "good enough".
But, you'll learn a lot about how carbs work in the process.   
 (Or, sell the bike to someone clueless and drooling over "the look".) 

Cheers, and good luck!

[grcamna2]

yeah I thought about putting my air box back in but just don't like the look or the pain it makes taking carbs off and on.

You are just trading one pain for another bigger one.

... and a poorer running bike.

Next time you pull off a hangnail, smack your toe with a hammer.  You won't even notice that ugly finger pain.

If you put the air box back on, you only have to put the carbs on once using stock settings.  With pods, you'll be doing it 25 times or more until you get bored and decide the carbs work "good enough".
But, you'll learn a lot about how carbs work in the process.   
 (Or, sell the bike to someone clueless and drooling over "the look".) 

Cheers, and good luck!

The Airbox gives the Best all around performance w/ great midrange & low end pull.
They flow Plenty w/ a K&N stock replacement style filter.

[livetoskate]

Haha thanks a lot I here what your saying, I had my carbs on and off 8-10 times and then did exactly what you said and just thought the bike ran "good enough"

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? 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?

Thanks again for the help

[livetoskate]

Oh I have 15 size main jets in my carbs and I believe everything else is stock.
Where are the #' on the others I can't seem to see them?

[lucky]

Need some help/opinions on jetting. The bike is a 1978 CB 750k with K&N oval pods.
So I bought a jet kit because my bike would bog down when I gave it gas to quickly, I was assuming it was getting to much air and not enough fuel.
Common when you put pods on your bike...Right?
Finally got around to taking the carbs off yesterday and realized that the carbs had already been re jetted (at least I think so) I believe the 78 750k came with 110 main jets..Right?
My bike has 115 jets in the carbs now. I guess my question is: should I put the next size jets in 120's, or is that to big? Just hoping someone might have some thoughts/experience with
the same problem.

Thanks in advance

You did not tell us which exhaust you have or the muffler type. Very important.

Put a #42 idle jet in it. You can get the push in style jets from Sirius consolidated for $24 for a set of 4 jets.
To remove the jets remove the float so you do not damage it, and then grab the jet with pliers or very small vice grips and twist and pull at the same time.
Put a little WD on the new one with your finger.
When you put the new size jet in, start it with your fingers and push - then you can take a little brass block or punch and LIGHTLY tap it in.

One turn out on the mixture screw.

With K&N oval filters the big issue will be the midrange.

Make sure the accelerator pump and jets work. With the( engine off) float bowls full, look into the intake of the cabs with the filters off and give it a couple of twists of the throttle. USE a LED flashlight, you should see a squirt of gas from the brass tube sticking up in the venturi. Make sure all 4 work.

IF you have the bike idleing and you quickly turn the throttle, if it bogs or stalls it is still too lean.
Use a #120mm main jet just to make sure it will not be too lean on the high end.
The #115mm may be allright too but you will need to check the plugs after you get the idle issue solved.

As soon as you put in the #42mm idle jet  you will see how good the throttle response is THEN sync the carbs.

Now you can move on to the other issues.

[lucky]

main jets only have an effect at the last 1/4 of the throttle when it's wide open. Try raising the clip position on the needle...

This carb has a non adjustable type needle.

[lucky]

I have this same situation going on with my 1978 CB750.

One thing about the stock airbox on this bike is this,
The opening on the box is only 30% of the total area of all 4 carbs inlets.(about 10.67 Sq. inches). The air box and stock filter is like a giant choke richening the mixture and limiting the air.

On my bike with the stock air box and stock filter. #120mm main jet,and no shims on the needles and 1.5 turns out on the mixture screws it would not idle with the stock#35mm idle jets. As soon as I put in the #42 idle jets BANG instant good idle, good throttle response etc.,. Why? Because the ONLY non stock part was the 4 into 2 into 1 performance exhausts with internal metal baffle is letting more air flow out, and so the engine wants more gas coming in.

It cannot get more air through that 3.7" X 1" inch intake hole in the stock air box.

Can you please report back and tell us which exhausts you have, Thanks.LUCKY

[lucky]

I just want to let forum members know that on these jetting issues I try to be very specific about ALL of the variables.

I do not want you to spend your money and waste your time on experiments that take time and effort and get you no where.

If you tell me ALL of the facts then I can tell you what I know.
I started working in a Honda dealership when I was 16 and now I am 62 and still working on motorcycles. All kinds of motorcycles. Many motorcycles.
So I do not know everything about every bike in detail but if we share info we can all benefit.

[TwoTired]

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,

[livetoskate]

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!

[livetoskate]

Wow just realized the photo I posted in the last reply is from 6 years ago when I bought this bike (my first street bike) and also the first week I had my garage!
Here is what 6years has done to me

[livetoskate]

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

[brewsky]

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?

Sure looks like a MAC, same as mine.

If so, 115's are probably too rich with stock box unless you are below sea level.

Mine is a little too rich with 112.5's and that is with 4  1" additional holes in the stock box with a K&N.

Now if you were to remove the baffle from the muffler, it could change. The MAC 4/1 baffle is pretty restrictive (unless you have the straight thru perfomance version).

I have 42 pilots also, and wish I had gone with 38 or 40's instead.

[bollingball]

 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 

[bollingball]

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

I found this but it does not mention the F models for 77 or 78

Ken

[bollingball]

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

[grcamna2]

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

[livetoskate]

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.