Topic: Pilot Jets 77 CB750 Help! who has 40's or larger???

[CB750R]

Where do I get a set of 40 pilot jets or larger?? Sudco doesn't list pressed in kehien jets??

OK I have uni foam pods, a D&D can, on stock 4into1, I have 137.5 jets I have the needle shimmed with a washer to raise it higher than stock, I have the pilot screws out three turns and I still get lean popping coughing in carbs at partly opened throttle openings.  I have two sets of carbs both have sets of 35's as pilot jets.  I've seen other posters making due with 40 pilots, but I have 35's and need bigger!! I am at sea level. 

[eurban]

Where do I get a set of 40 pilot jets or larger?? Sudco doesn't list pressed in kehien jets??

OK I have uni foam pods, a D&D can, on stock 4into1, I have 137.5 jets I have the needle shimmed with a washer to raise it higher than stock, I have the pilot screws out three turns and I still get lean popping coughing in carbs at partly opened throttle openings.  I have two sets of carbs both have sets of 35's as pilot jets.  I've seen other posters making due with 40 pilots, but I have 35's and need bigger!! I am at sea level. 

Your post is just about identical to one a replied to (in the SOHC bikes section)yesterday . . .You might need larger pilots but I doubt it. In my experience the stock pilots (when clean) are more than adequate even on a modded bike. Are you certain that your current pilot jets, idle fuel and air passages are clean. How about the idle mixture screws? Your symptoms are typical of a bike with an impaired idle circuit or air leaks.  Did you remove the pilots to clean them and the passages underneath?.  You can order new #35 pressed in pilots from Sirius Inc, so you could ask them about larger options.  You could also drill your stock pilots with a micro drill bit.  A while back there were also some posts about installing threaded type pilots in the pressed in hole.  . . .Bigger isn't always better for performance BTW.  Your mains seem a bit large.  One thing that years of carb tuning has beaten into my head is that it is always a complete waste of time tuning a set of carbs that aren't actually functioning correctly.

[CB750R]

I am at sea level.  and there is another poster, using the same size mains as my self right now, who's reported good success. I have fully rebuilt these carbs, they don't leak, and aside from the coughing my bike pulls really hard, and idles pretty darn well too. I have used fine wires to clean the air jets/pilot jets, replaced rubber on air/fuel screw.  cleaned emulsion tubes.... I'd eat out of my float bowls! I know that I haven't seen any other posters shimming the needles but I have a 75 engine, and 77 bike/carbs, so I'm running a bit of an odd ball set up.  Thanks for the heads up on the sirius for jets, I might end up trying them.

[eurban]

If you didn't actually pull your pressed in pilot jets out, clean their lengthwise and crosswise passages completely, and then properly clean the fuel and air passages in the idle circuit (made possible by the idle jets being removed) then you may likely still have idle circuit impairment.  Did the jets come out?  If you haven't confirmed already, check the function of the acel pump too.  Air filters off, throttle whacked, fuel should squirt out of the brass nozzles down the throat of the engine. . . Shimming the needles is actually fairly common.  I bought a set of new needles specced for the 77s (Somewhere in 77 Honda switched to the single clip position type needle) from David Silver Spares that has the multiple clip positions.  In my 78K with a cammed 836 and pods I run the needle one up from the bottom (richest) position. Prior to my needle purchase I used various combinations of small electronics washers as shims.  Shims can also be useful as a half step between clip positions.   BTW I also run stock 35 pilots at a little less than one turn out. . . .   Your main jet size should be determined by WOT performance.  You can compensate for a lean main jet needle position with a larger main but you will get better performance with a main jet sized for WOT and  the needle position adjusted for best 1/2 to 3/4 throttle performance.  I would be very suprised if the 138s weren't significantly oversized for optimum performance in your application.

[CB750R]

thanks for the replies, I did pull the jets, and can see through all the side holes, through length of the jet, I soaked each carb body in this super toxic carb cleaner, then proceeded to clean poke and prod with the strands of a bicycle brake cable then get in everywhere. but good to know that your seeing success with 35's.  sirius has expanded there inventory since I rebuilt my carbies, I noticed that they have some 42's for another carb that look just like the pressed in units for the 77's carbs... so I emailed them to see if they can get there hands on various sizes of said jet!!   I'm getting great mid to almost full throttle response then it seems to fall flat WOT,  It felt worse with the 130's and 135's hence bumping up to 137.5's  with the 135's and lower, if I pulled the choke out slightly at WOT the bike would accel confirming a slightly lean condition, yes/no?  I think my tune is getting very close, just trying to get rid of the cough!!

[eurban]

Is your accelerator pump working?  These carbs require its function to accelerate smoothly.  Also do you trust the condition / air seal of the intake boots? Presumably you used the boots from a 77/78K bike? The 77/78 F boots are designed for the larger late f engines intake spiggots and the pre 77 750 boots dont fit the 77/78 carbs properly. . . .  Assuming things are funtioning correctly, The IMS screws on these carbs have a smallish range of mixture dominance so the slide cutaway profile (which you aren't going to be able to change easily) and the main jet needle position / profile will become players at relatively small throttle openings.  Considering you went up at least 12!!! steps from a stock 77K setting (at most a 115, f model with 4-1 was as low as 105) in these carbs in main jet size, a single shim on the needles is pretty mild.  Even though switching needle positions is a PIA you might want to try a couple of shims on each needle.  Just make sure that whatever you are shimming with is of the same thickness.  Many store bought washers will vary considerably in thickness. . . .  If the choke helps performance at a particular throttle setting then yes that would indicate a lean condition. 

[mmtsquid]

+1 on checking for an air leak - with the bike idling, spray a little carb cleaner around the intake boots - if the RPMs come up, you've got an air leak.

[CB750R]

hey guys, I think I may be chasing my tail a bit, in fact things appear too rich on idle now that the needles are shimmed.  I have got things pretty dialed down on the carbs, infact I've leaned out the pilot screw setting and getting it started is easier, I've got a carb sync gauge on route to help dial things down a little further, but for now the occasional sneezing, is pretty mild, the bike pulls great and part throttle response is good too. the plugs look good, just down to fine tuning! my boots are firm but dont appear to be cracked anywhere, and I might try replacing stock clamps with screw type gear clamps to lock em down firmer. Thanks for the input, guys!

[eurban]

Glad to hear it is coming around.  It is a bit tricky to find clamps in the correct width and diameter. I would be interested in your source if you find them.  Which rubbers did you use BTW; hopefully it was a set from a 77/78K.  Otherwise no clamp will help, at least not for very long.  Also, without the stock airbox, you will need to support your carb rack in some fashion.   Good luck . . . .

[lucky]

Is your accelerator pump working?  These carbs require its function to accelerate smoothly.  Also do you trust the condition / air seal of the intake boots? Presumably you used the boots from a 77/78K bike? The 77/78 F boots are designed for the larger late f engines intake spiggots and the pre 77 750 boots dont fit the 77/78 carbs properly. . . .  Assuming things are funtioning correctly, The IMS screws on these carbs have a smallish range of mixture dominance so the slide cutaway profile (which you aren't going to be able to change easily) and the main jet needle position / profile will become players at relatively small throttle openings.  Considering you went up at least 12!!! steps from a stock 77K setting (at most a 115, f model with 4-1 was as low as 105) in these carbs in main jet size, a single shim on the needles is pretty mild.  Even though switching needle positions is a PIA you might want to try a couple of shims on each needle.  Just make sure that whatever you are shimming with is of the same thickness.  Many store bought washers will vary considerably in thickness. . . .  If the choke helps performance at a particular throttle setting then yes that would indicate a lean condition.

I deleted this info because I found out it was all wrong.

[lucky]

I deleted this info because I found out it was all wrong.

[lucky]

I deleted this info because I found out it was all wrong.