[mut]

I took my carbs apart today to start cleaning them. A couple of things became apparent. The carbs were progressively less dirty from right to left. That is, the carb closet to the clutch was most dirty and the one closet to the brake was cleanest.

Two of the carbs were very gummed up and the jets were clogged. When I first got this bike running it felt very sluggish. I imagine that the carbs were a big issue. Also, there were a few rubber gaskets on different screws that were dry rotted and the #3 carb was missing a spring for one screw. I am thinking about a carb rebuild kit to clean up the dry rot and I think i'll need to re-jet.

I live at 7000ft and am putting a 4-1 kerker exhaust and k&n pod air filters on it.

Is there a way to tell what size jets I have in it now?

Does anyone have any suggestions for jet sizes to try for the kerker, pod, 7000ft combination?

 

[kenessex]

H Mut,
There are 2 ways to tell the jet size. Keihin jets are numbered by the nominal diameter of the hole in the jet in millimeters. so a 120 Keihin jet has a hole .120mm in diameter. yo could get a set of fine metric drill bits and see which drill bit shank will fit through your jet. (Mikuni numbers heir jets based on the amount of theoretical fuel the jet can flow in milliliters per second.)

Or
You an read the number stamped on the jet. It s either on the side of the jet or on the flat end. If it i corroded or discolored it is very difficult to find or read. If t is not marked then it is not genuine Keihin and I would throw it away and get genuine Keihin jets n appropriate sizes.

As far as jetting for a particular pipe and altitude, you are going to just have to try and see. Are you going to keep the stock airbox and filter? Generally speaking you will need to probably go up a couple of sizes on the main jet and 1 size on the pilot to start. By a size i mean an increase of 5. i.e. from a 105 main to a 115 is two sizes.


Jetting is a bit of an art, but you can do it if you are patient and willing to take the time and effort. Good luck.

Ken

AHRMA 412
Vintage racing - old guys on old bikes

 

[pampadori]

the size is stamped in extremely small size on the sides of the jet. most cb750k's have 40 pilots and 105 mains. i am at 2000" and i added a short 4 into 2 exhaust will very little back pressure. i only changed the mains to 110. 115 might would do better but i don't have any to try. The bike runs well to 9k rpm with no dead spots.
my other bike has 130 mains and 40 pilots and has lots more done to it. it runs too rich until the top end, then runs fine to 10k rpm. then for some reason it will missfire and not rev any higher. it has the goods to go higher installed, and i really haven't figured this one out. maybe it has everything but valve springs. i know its got yoshimura rods and cam.
anyways, i would get some 115 mains and some 110 mains and see what it likes better.

'72 cb750
'75 cb750
'76 cb750

 

[mut]

Thanks for the responses.

If I am going to do the trial and error, should I be changing out just the main jet or all the jets, or does that depend on the symptoms?

 

[Geeto67]

@ 7000 feet I would start with the stock jets. At most with cb750s you usally have to only go one size up for the filters and pipes and that for me was at sea level.

 

[lucky]

A 120 main jet is actually 1.20mm =.047 thousandths

 

[lucky]

A 120 main jet is actually 1.20mm =.047 thousandths

 

[lucky]

A #40 pilot jet is .40mm=.015 thousandths of an inch.

Use a steel guitar string to clean it out.

 

[lucky]

A #40 pilot jet is .40mm=.015 thousandths of an inch.

Use a steel guitar string to clean it out.