jeting

[pcbwnmf4u]

ive got a 78 cb 750 it my first build
 i put custom pipes un baffeld an volcity stacks can any one suggest what size jets i should use

[shultz_1978]

Every one has their own opinion about this.

1) you will never get the bike to run as good as the stock air box. The volcity stocks just like pod filters let in a lot more air.

In my apinion if you let in more air you need more gas. I would try  larger idle gets and raising the main jets. Then turning in your mixing screw.

Some have just put in larger idle jets and run the mixture screw out. It is hit or mis till you fine the sweet spot you like how it runs.

Good luck

[Lucky 1]

Jetting not jeting.

With your set up on the 1978 CB750:


Accelerator pump system and nozzles must be working.
All other tune up issues must already be completed.

Use #120 mains.
#42 idle jets. #45 is too big.
The non adjustable slide needles will need to be changed to 1976 slide
needles (the easy way) OR you will have to shim the needles with shims(washers).

There is a pocket in the inside bottom of the slide.
The washer/shim should sit on top of that pocket in the bottom
of the inside of the slide and one .020-022  under
 the needle will give you .032 thousandths raising of the slide needle.
When you buy little washers from the hardware store they need to be
measured and the burr removed by sanding with a finger nail file brass shims.
Stainless washers are more precision but difficult to file (if needed).

Mixture screw about one turn.

I just saved you weeks of work.

It just so happens that when the floats on the 78 are measured and adjusted that they are even with the top of the idle jet.


The shim if brass must not be so large that it can get stuck like this.
You may have to file it a little.
But it still must be larger than the pocket it covers.


When the washer gets stuck, this is what can happen.
 Then the needle will get worn on one side.


Make sure the spring is centered at the bottom of the slide or there will be problems.
Like I said if you get some 1976 needles and just lower the clip two notches over the stock setting of the 1978 needle then your life will be a lot easier.

[Lucky 1]

It will run just as good.
The rumor that it will never run right is not true.
But the slide needle HAS to be delt with. No main jet and idle jet combination is going to make it run correctly.

The air filter on those bikes was like a giant choke,
restricting the air and the idle jets restricted the fuel to match.
 The air was restricted to about 30% of the intake area.

[shultz_1978]

If you feel like reading. I was going through some problems with my carbs. This is what I went
through. http://honda-cb750-s.456789.n3.nabble.com/1978-cb750-carb-swap-td3911046.html#a3917291 good luck

[shinyribs]

Lucky's recommendation on jetting is exactly what I did on my 76 with pods and a straight 4-1. Worked perfect as far as pods are concerned.

[barelycompetent]

Check out this website.  Its the kit I used in my 81 with pods and 4-1 exhaust.  It comes with everything specific for your bike, and if you still cant get it running right, you can ship your carbs to them and they will dial it in.  Before anyone asks, yes my 81 is running like a champ, no issues with the pods.  For 80 bucks you have nothing to lose.

http://www.6sigmajetkit.com/

[Lucky 1]

Of course the 1981 is a completely different situation.
It has two main jets. That looks like a good company with a good product.


There are dyno places that know exactly what you need, but none of them are going to help you until
You give them your Dyno certificate you got with your kit (worth $100) then more money to change any parts.

Count on $100 +

[barelycompetent]

True lol, thanks Lucky but my point is that website will be specific to your year, and model and any specific mods you have done.  Heck they even want to know your elevation above sea level lol.  They include everything you need to rejet the carbs in the kit.  

[Lucky 1]

YES. I went to the website and the product is the first that talks about all the important things.
Thanks.

[barelycompetent]

Welcome, its a good kit, I havent had any issues with it and very straightforward instructions, and it was even my first set of carbs to rejet!  If I can do it, then it must be simple lol!

[seestheday]

Starting a hijack... [Sorry]

Barely competent, that is very interesting.  Was your bike running well before you put pods on, or did you put them on at the same time that you fixed up a lot of other stuff?

Once I am finished the top end work I'm doing and the bike is running great I plan to tackle the more cosmetic stuff.  Pods are something I've been interested in for quite some time, and if I can avoid buying a full set of mikuni's for more than I paid for the bike I'd like to explore it.

[barelycompetent]

Sees the bike was running before I did the pods, but I had other mods such as the 4-1 exhaust, and things to fix all along, so I had planned on rejetting anyways.  I had a buddy that turned me onto that site because he had done the same thing to his honda and his bike runs great.  I had to be talked into the pods by the same buddy because I had read all of the negative stuff about them and the cv carbs.  I have to say that with that kit and my exhaust, I havent had any problems with them yet.

[seestheday]

Very interesting.  So just to confirm, you have smooth acceleration all the way through your power curve?  You aren't bogging or hesitating at all at idle, 1/4, 1/2, 3/4 or full throttle in any gear or load?

EDIT: which stage kit did you end up getting?

[seestheday]

Shiney, I thought that you recently put your airbox back on and noticed a significant performance improvement?

[barelycompetent]

Im not at home so cant post a video of it running right after I did the carbs, but I have no bogging or hesitation at idle.  As for the other throttle positions I will have to get back to you on that after I really put her thru her paces this weekend.  On the low end I would say no I havent noticed any, thats using my butt dyno, but thats just running it around locally, I honestly havent taken her on the highway to really open her up and check the high end.  Now you have me wondering lol, cause its been running just fine around town.  Im also going to be paying more attention to her in the low end now too.  As long as the weather holds for me this weekend I will run her thru her paces and get back to you.

[barelycompetent]

As far as the stage kit I just went with 1, and gave them all of the information on the bike, year, exact model, type of exhaust, pod air filters and where I live for elevation above sea level.  They designed the kit for the bike based off of that.

[seestheday]

Cool, I'm asking those questions based on what I've read here:

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

Need to combine it with some more info/tips from here: http://home.everestkc.net/malsin/Motorcycle/carb/mc_carb_theory_101.htm

[barelycompetent]

I completely understand sees, that kit comes with everything, multiple size pilot jets, different size mains, spacers, everything. Instructions if it runs lean and if it runs rich.  Very straightforward. I followed them step by step and didnt have any issues, and like I said just running around town I havent had any issues with it.  If you really check out that website it explains alot, and they have great customer service, per my buddy, as I didnt need to call them!  Hope that all helps, let me know what you decide.

[shinyribs]

seestheday wrote

Shiney, I thought that you recently put your airbox back on and noticed a significant performance improvement?

Correct. Low speed power and midrange throttle roll-ons increased dramatically.It did kill my top speed dramatically as well.About 15-20 mph lost with the airbox vs the pods.Pods are top end screamer devices.It's just that simple IMO.

But the the tuning Lucky mentioned is the same tune I used when I did have the pods on the bike. It wasn't very clear,but that's why I worded my reply that it was a good tune ''as far as pods are concerned''.