Topic: CB750F Jetting Question

[rockandride6]

Well, at least I THINK it is a jetting question.  I just finished rebuilding my 1975 CB750F to what I would call "restored+" with the goal of modifying it as someone might have done when it was new.  The only real engine change I made was pod air filters and I believe I have tracked my issue down to that.  With a 110 main (just rebuilt the carbs, so everything else is stock with the exception of being one position UP on the needle) I can't really open the throttle up past 1/8th without it bogging down

I thought it was too rich, so I switched back to the stock 105 main and it got worse.  Now, I understand (or think I do) it is running far too lean and STARVING for fuel.  I can run it pretty well with the choke on a bit, but if I open up the choke and try to ride, no go.  I have a set of 120 Main Jets, should I give that a shot? If I am running 120 mains, should I change anything else? (pilot jet?)

I really don't want to have to dive into it, but should I change the needle clip position?  Thank you all so much for any help!  Here she is in all her glory:

[flybox1]

pods make it lean.  you'll need to INCREASE fuel supply across the throttle range.
add your 120's, and be prepaired to change your mixture screw setting, increase pilot jet size, and (probably) make a needle clip change.
Only accurate plug chop tests can show you what is necessary.  Make only one change at a time and test. 

[flybox1]

I now see you still have the stock exhaust on the bike, hopefully you still do.
Change back to, or get a stock airbox to go with it.  Your bike will thank you. 

[RSchaefer]

Love the color!!!!!

[Cqyqte]

I have a '75 CB750F with 849cc pistons and I put a set of '77 carbs on because of the accelerator pump features.  My '77s run 125 mains 35 pilots with 1.75 turns out and K&N pods and I have yet had an issue with fuel mixture ratio.  I know the big bore pistons will demand more fuel flow but I would try the 120 mains or the 125 mains to get a read as to how rich or over rich they may make things.

[Cqyqte]

The accelerator pumps make a big difference in tuning the off idle transition with the pods.  So if you can can your hands on a set cheap jump at them.

[rockandride6]

Thanks for all the input and comments everyone!

I will try the new mains later this week and report back with the rest as well!

[Guido56]

I had the same problem with my K1
I solved with jets 130.
All are advised not to change the idle jet and the position of the needle.
 

[flybox1]

All are advised not to change the idle jet and the position of the needle.

why? 

[Guido56]

'Cause it's too hard to work on carburetors and get a good result.
There are too many variables.
If the carburetor and 'lean, better to increase the main jets without changing too much.
If I'm not mistaken, all carburetors Four of the jets have the minimum of 40.
The height of the needle, changing the fuel mixture at medium revs.
Sorry, I'm using the translator.

[flybox1]

'Cause it's too hard to work on carburetors and get a good result.
There are too many variables.
If the carburetor and 'lean, better to increase the main jets without changing too much.
If I'm not mistaken, all carburetors Four of the jets have the minimum of 40.
The height of the needle, changing the fuel mixture at medium revs.
Sorry, I'm using the translator.

actually, these carberators are quite simple to work on.
you have a few areas to make adjustments based on throttle position, and plug insulator deposit results from plug chops/dyno tuning
If you have a good stretch of road to load your bike, and safely do a plug chop, the process is simple.
Pods change A/F mixture throughout the throttle range. The OP still has to match main jet, slide needle height, pilot jet and mixture screw settings to the the pods on the bike.
Just increasing mains wont be enough..

[Guido56]

I did not say that It is impossible, only that 'complicated by failing to provide all the things you listed.
Definitely not 'thing to everyone

[martin99]

Rockandride, Flybox is giving you good advice. One step change, plug chop, and if all good move on to next step. I think you're going to end up with 125/130 mains, that's my experience anyway. Try the needle clip one position lower than stock - raising the needle. I run PDs, never found it necessary to increase the size of the pilots, although I'll readily concede that doing so would improve things further. It's acceptable to me, and that's the key with pods, reach a state of tune that you are happy with but don't expect it to run as well as the stock set-up. With time and patience though, you can get pretty close and it'll be a blast to ride, as long as it's not too windy and you've got deep pockets!