Topic: Tuning for Intake and Exhaust Mods.

[MechSJ]

First off, my 77 cb750k came with pods on it already and 120 mains... previous owner couldn't get it to run right, i picked it up for a song. I found a float installed upside down, fixed that, still wouldn't run right, so I cleaned the accelerator pump circuit and the primary jets and got curious about the needle position with the 120 mains, needle was on the leanest setting, well that explained the lean misfire at 1/8-1/4 throttle, clip to middle of five position choices... reassembled the carbs and it ran great, even started with no choke cold... I ran it for a bit, it idled rich so I turned the idle mixture screws in and in some more, leaving them 1/2-3/4 turn out from seated. So, the bike ran strong, good low end and strong mid-range, but choppy and rich at idle, and I had a bit of a hanging idle as well, it took a few seconds for it to settle at a stop sign or between shifts. Turns out I has installed the slides backwards... (anyone else done this? be honest.) I have since flipped the slides back around and I'm still to lean on the pilot jets, I have some on order from Sirius, the only place to get the friction fit primary jets. Oh yah and I'm back to three and a half turns out on the mixture screws. I'll up-date on my progress in seven to ten business days.

[brewsky]

I would go in this direction:
 http://item.mobileweb.ebay.com/viewitem?itemId=270998348183

OOPS! thats the wrong link....there is a 78 on e bay now about the same price. You will need a 77 or 78K model year

[lucky]

I would go in this direction:
 http://item.mobileweb.ebay.com/viewitem?itemId=270998348183

OOPS! thats the wrong link....there is a 78 on e bay now about the same price. You will need a 77 or 78K model year

I agree because that stock air box and filter is like a giant choke. It richens the mixture. But with pods you can still make it run right.

Two clip positions and that larger idle jet will work.
But Brewsky you drilled 4 one inch diameter holes in your stock air box didn't you?

[aperry]

Quick question:  On the older carbs I don't think can you insert the slides backwards because there is a slot in the slide that lines up with a pin in the housing.  Does this not exist in the 77 carbs?  Or am I completely wrong?  I want to know because I still have one carb that's acting a little funky.  Sometimes I notice the hanging idle too, but it's not too bad.

Aaron

[brewsky]

First off, my 77 cb750k came with pods on it already and 120 mains... previous owner couldn't get it to run right, i picked it up for a song. I found a float installed upside down, fixed that, still wouldn't run right, so I cleaned the accelerator pump circuit and the primary jets and got curious about the needle position with the 120 mains, needle was on the leanest setting, well that explained the lean misfire at 1/8-1/4 throttle, clip to middle of five position choices... reassembled the carbs and it ran great, even started with no choke cold... I ran it for a bit, it idled rich so I turned the idle mixture screws in and in some more, leaving them 1/2-3/4 turn out from seated. So, the bike ran strong, good low end and strong mid-range, but choppy and rich at idle, and I had a bit of a hanging idle as well, it took a few seconds for it to settle at a stop sign or between shifts. Turns out I has installed the slides backwards... (anyone else done this? be honest.) I have since flipped the slides back around and I'm still to lean on the pilot jets, I have some on order from Sirius, the only place to get the friction fit primary jets. Oh yah and I'm back to three and a half turns out on the mixture screws. I'll up-date on my progress in seven to ten business days.

You didn't mention cleaning the slow jets?
What did you set the float height?
A stock airbox will be a lot easier to set the carbs up with.
here's one:
http://www.ebay.com/itm/Airbox-CB750-750-4-honda-1978-sohc-/110940419643?_trksid=p2045573.m2042&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D1735175317663753817%26pid%3D100033%26prg%3D1011%26rk%3D3%26

[brewsky]

I would go in this direction:
 http://item.mobileweb.ebay.com/viewitem?itemId=270998348183

OOPS! thats the wrong link....there is a 78 on e bay now about the same price. You will need a 77 or 78K model year

I agree because that stock air box and filter is like a giant choke. It richens the mixture. But with pods you can still make it run right.

Two clip positions and that larger idle jet will work.
But Brewsky you drilled 4 one inch diameter holes in your stock air box didn't you?

I did, but that was because I wanted take advantage of the K&N filter, and to play with my A/F meter!
The stock box also has built in velocity stacks to shape the airflow, and the pods don't, or at least the Emgo's don't.

[brewsky]

Quick question:  On the older carbs I don't think can you insert the slides backwards because there is a slot in the slide that lines up with a pin in the housing.  Does this not exist in the 77 carbs?  Or am I completely wrong?  I want to know because I still have one carb that's acting a little funky.  Sometimes I notice the hanging idle too, but it's not too bad.

Aaron

I don't recall any slide pins in the PD carbs.
That does allow them to be installed backwards.
The raised (cutout) side should be toward the air filter.

[MechSJ]

my slides really where in backward, ran great from 2,000 through 6,500 too, just to much black soot at idle, I had trouble in town and at stop signs. I wounder if anyone had done this as a racing mod? and yes I did clean the pilot circuit and the accelerator pump circuit as well. I have pilot jets on order and bigger mains as well, though it is quite ridable now, but it is still a little lean.

[lucky]

Please let us know when you get the larger idle jets. Thanks.

It will NEVER run right with the #35 stock idle jets.

All of the carbs with that mechanical linkage and shafts and lifter
 arms can have the slides put in backwards.

[754]

 I would run k&N over Emgo pods..

[MechSJ]

If my cb had come to me with the stock air box, I would be looking at ways to modify it to have less negitive pressure on the filter side and retain the volicity stacks, but I was not do fortunate. And I too would not choose to run Emgo's.

[MechSJ]

Got my self some #42 primary and #130 main jets in the mail, if all goes well I may upgrade my Emgos to K&Ns. I'll keep you all posted on my results.

[sethdhawkins]

Put the slides in backwards on a cb175 once. Spent all day trying to figure out the carb issues and it came down to my own stupidity.

[gsb]

Hi !
I have f1 stock air box .I have K&N filter to put it in .How to mod the stock air box -to drill holes .Any pics about this mod?
Thanks gsb 

[brewsky]

Holes on K8 box:

[exf5003]

I run K&N pods (def recommend them over Emgos) and have had pretty good luck with them, but I am curious, do you guys rejet or keep stock with drilled air boxes with K&N filter?

[gsb]

Hi !
Thanks  ,i have Hondaman book ,but i could not figure out where the holes are drilled. Yours are 4 x 2,53cm diameter ?
gsb

[brewsky]

I run K&N pods (def recommend them over Emgos) and have had pretty good luck with them, but I am curious, do you guys rejet or keep stock with drilled air boxes with K&N filter?

I re-jetted up slightly, but I also have a 4/1 exhaust.

[brewsky]

Hi !
Thanks  ,i have Hondaman book ,but i could not figure out where the holes are drilled. Yours are 4 x 2,53cm diameter ?
gsb

Yes....4@25.4mm each

I have no idea if that is what is referenced in Hondaman's book or not, since I don't have one.......yet!

[lucky]

I run K&N pods (def recommend them over Emgos) and have had pretty good luck with them, but I am curious, do you guys rejet or keep stock with drilled air boxes with K&N filter?

I re-jetted up slightly, but I also have a 4/1 exhaust.

CAn you please re post your graph showing your testing results Thanks LUCKY.

[lucky]

Got my self some #42 primary and #130 main jets in the mail, if all goes well I may upgrade my Emgos to K&Ns. I'll keep you all posted on my results.

One change at a time bro.

[brewsky]

I run K&N pods (def recommend them over Emgos) and have had pretty good luck with them, but I am curious, do you guys rejet or keep stock with drilled air boxes with K&N filter?

I re-jetted up slightly, but I also have a 4/1 exhaust.

CAn you please re post your graph showing your testing results Thanks LUCKY.

...
Here it is.......

Note this is a K8 and will not necessarily hold true for other years/models as the carb specs are different.

Also, I believe the #42 pilot is too large even though the chart doesn't show it, as it goes way richer off idle. I have some #40s to try and may add one shim to the needle

Dyno for for the last 2 entries here:
http://forums.sohc4.net/index.php?topic=83748.25
At the bottom of the page

Still a work in progress.