My 1975 CB400F restoration project

[Cb750r]

I chased my tail jetting my 750 for quite some time, what really frustrated me is a lot of carb tuning states to get the main jet sorted first and work your way down…
In my scenario to get it to run right wide open was too lean at idle and to get it to idle the main was super rich, I played with countless needle positions and float heights and in the end a bigger idle jet sorted things out. 

I will second what others have said about non genuine brass, even in sport bikes I’ve jetted I try and avoid non genuine jets/needles.

Just another thought, where are you elevation wise? We have from sea level to fairly high base elevations in the pacific west coast to deal with, and small bikes have less hp to deal with jetting that’s off for your ‘base’ elevation. My snowmobile actually had a dial to lean out jetting as I climbed to keep it running optimally as I climbed. I’ve also had dirt bikes with ‘mild jetting’ changes from stock that are widely used elsewhere but would foul plugs at my elevation. Maybe your bike was ‘ok’ or slightly rich at sea level and you’re too high for it to run well now?

I am at or near sea level
 Seattle Washington
My plan is to get everything back to rather stock settings and jet. #s then go from there. I have 4 OEM #75 jets on the way

Well then it’s not elevation related unless you went straight up snoqualmie pass on your test loop! It’s crazy how sensitive some bikes can be to changes. That dirt bike I got to run great, but with a spark arrestor would still foul plugs so I needed to be leaner than stock for the elevation and modifications I had to ride in the woods here.

[eem46]

Hi, great looking bike! Which clutch basket rubbers did you use? Which dealer/brand? Kind regards, Emiel

[ozzybud]

i ordered these from the UK

https://www.ebay.com/itm/156080700060?itmmeta=01HSHKFBZTS2FDQZR7835J6AZ1&hash=item2457227a9c:g:2MgAAOSwNetloepT&itmprp=enc%3AAQAJAAAA8JuGvduuE6KzzeGdWVTIKHG8yM1kafSVMCQtNMKEGWBgih9OeKWIuYacbVWT0D4wjNvC7ntLpzpXneD%2BIiiIpd69UTE4MlOKGuVr9D2Q%2F0pc2S7qr5nfJeGBAPc%2F7EOeR5Ykxsw0Yd%2Bt3s8R9BLeLo%2B7b6XiV0gM26rpeGRTcTiEYzdYLOIs%2BkpJ2S0I8n2APopOvN9oqOER57G66FHYX04uKEKlygdL5NJCuebGPZWTitWfMFy45NYRs2yiFuq2KqeXxSNtyKQYT8TH05PFDHeeW0WaS5gSilz4an9bvIO6vxptT1XIBn7gMz9zEccrRA%3D%3D%7Ctkp%3ABk9SR4jAvbPMYw

Mine were type "B". it is my understanding that Most 408 cc bikes use Type "B"  and the 398cc motors use Type "A".

[ozzybud]

I finally received the rest of the correct Keihin brass from CMS yesterday. I got the carbs assembled and  setup on the bench as close as i could get them synchronized.

As I suspected the previous owner put in some larger Jets and messed with the Needle position. There were #90 main Jets #44 slow and the needle clip was at the bottom groove. pulling the needle up to full rich. I replaced with #75 main #38 slow and clip in the middle position .

Took it out yesterday between showers. Wow what an amazing little bike. ran like a Swiss watch.

[Cb750r]

I finally received the rest of the correct Keihin brass from CMS yesterday. I got the carbs assembled and  setup on the bench as close as i could get them synchronized.

As I suspected the previous owner put in some larger Jets and messed with the Needle position. There were #90 main Jets #44 slow and the needle clip was at the bottom groove. pulling the needle up to full rich. I replaced with #75 main #38 slow and clip in the middle position .

Took it out yesterday between showers. Wow what an amazing little bike. ran like a Swiss watch.

So glad to hear you sorted out the jetting! Congrats!

[K2-K6]

Nice work to get it back on track.

As many of the threads indicate on this topic, these carb are deadly accurate for jetting, with such importance in really, really clean installation air passageways.

Assuming no mods to motor and intake, then they carburate with great accuracy.

Another consideration if needed for minor adjustments, the size of the idle jet and subsequently the ultimate airscrew setting cover a very significant range of response and quite high up into the rev range on anything much below 3/4 throttle opening.

As with this original jet specification fitted by PO, a larger main jet is often fitted which completely stuffs the overall running scheme.

As you find though, a real delight to ride when optimum setup is done.  A lovely small,capacity four indeed.

[ozzybud]

Thank you..

I have been taking the bike for shorts rides around the neighborhood doing test n tune and feeling it out before i go any distance.

I waited a few rides to get the RPM's up and go a little faster. i noticed On WOT it was bogging at about 6500. so i ordered some slightly larger main jets #78 They still have not arrived. I noticed it was getting a little worse and doing it at about 5000 .

Not sure if anyone has had this happen? For 1 the rear brake light was stuck on. The battery was Not getting charged with my short little rides.I charged The battery overnight.

The first test ride it made it to redline and smoothly accelerated in all RPM ranges! Rode it for about 20 miles. all the plugs are a nice Amber. So I will not Be Yarding off the carburetors again to re jet.