Topic: Main Jets for CB750K 1978

[42indian]

I have 4 into 2 exhaust and i need to replace the original main jets with 135 from reading the information here.  Mine have the type that push in not screw in.  Does someone have a part number for the main jets so i can hunt the internet or call the local shops?

[mlinder]

Why do you think you need 135's? Thats very, very large. Any other changes to breathing characteristics? A 4 into 2 (most of them, anyway) would require, probably, no jetting changes, alone.

[Spanner 1]

Stock main jets for your bike ( PD 42B carbs ) is 110's, so your already 10 bigger @120's.!

[mlinder]

The Hondaman article on carb jet changes describes goint to the 135 for the exhaust change.
I presently have the stock 120 main in now.

Can you link to that post, please? I don't like to disagree with Mr. Paris, and have a feeling you may be misreading some part of his recommendation.

[GammaFlat]

I've had discussions with HondaMan about jetting and what you're saying is pretty inconsistent with our discussions.  I'm with mlinder on this one.  I'm real curious to see a link to the article. 

Also... the jets on a K7 or K8 that are press-in are the idle jets.  The mains are screw-in. 

Don't fret... we'll get this figured out.

[mlinder]

Again, 110's should be fine with a 4 into 2. Most of the aftermarket 4 into 2's flow no better than the stock 4 into 1.
Have you made any other changes? Pods? Velocity Stacks? Removed air filter from stock airbox?

[Inkscars]

My bike had 115s in it with a 4-2 exhaust.
They are correct, idle jets are pressed in, main jets are screwed in.
Mine also has pd carbs.

[Inkscars]

It sounds to me like you need to start from ground zero instead of adding Insult to injury on those carbs.
Maybe You got this bike from someone who tinkered too much and gave up when it wouldn't run?
I'm guessing this thing is NOT running a stock air box.
If it were me, I would reset all my Air mix screws, unscrew the idle screw until no contact is made, pull apart the carbs and take note of my needle height. I haven't heard you mention what that is set at.
Fact is, You could jet all day long, but if your main needle isn't set properly, it's not going to do you any good.

[HondaMan]

Keep in mind that comment about 135 mains was restricted to the Yoshimura roadracing pipes and other equipment, together. That is all-out race stuff, circa 1970s. On the street, you'd be out of the RPM band far too much to be running those sizes of jets, and your sparkplugs would be forever fouling for lack of RPM.

On the other hand, if you're going to Bonneville with the bike....

[mlinder]

Yep, as I suspected.

Stick with the 120's, and get the propper idel jets for your carbs. You should be golden.

[eurban]

You are coming up with some odd numbers for jets in 78 750K carbs.  What are stampings on the carb body??  Should be PD42B.  We are talking about a 750 right?? Stock main jet is a SCREW IN #110 and the pressed in pilot (idle) jet is a #35.  Assuming you are talking about a set of the PD42Bs, I would be very surprised if the previous owner was able to source a set #20 pressed in pilot jets and can't envision why on earth they would have wanted to downsize so dramatically.  I think you need to take another look and make sure you are providing accurate info.  If you indeed have #20 pilots then you will need to source a set of #35s (sirius inc sells them) If you do find that the pilots are actually the stock #35s then you will need to confirm that they and the passages behind them are clear.  Removal is pretty much required.  On the 78 carbs, the main jet needles are not adjustable but it is possible to shim them to richen the mixture.  You will need to remove them and take a look if you suspect your carbs have been tinkered with.  Also for the carbs to function correctly, the accelerator pump must be properly functioning so you should check for that as well (remove air box, whack open the throttle quickly and observe whether or not streams of gas are being shot down the throats of the carbs from the 4 small brass nozzles located next to the choke plates)  If not you must fix.  Float needles must work properly and the float level must be adjusted.  Carbs should be at least bench synchronized (slides adjusted so they all are open the exact same amount at a given throttle setting) , preferably followed by a gauge snych.  Idle mix screws should at least be set to the factory baseline and then really should be adjusted according the the procedure in the manual (down load the factory manual here http://www.mediafire.com/?5ttzdg9yyuz) Now assuming that all your bikes other systems are up snuff then you are ready to start working on fine tuning (main jet size and needle shiming) the carbs to your intake and exhaust systems.  You are absolutely wasting your time on carb tuning if you don't have the basics covered first!

[medic09]

What he said.

eurban, did you say all that in one breath?!   

[lucky]

I think there was poor communication from the very beginning of this thread.
The main jet was not correctly identified.

It does NOT have push in main jets.
It DOES have push in idle jets.

The idle jets were #35's NOT 135's