Topic: low speed jets 78 750F

[nteek754]

Hey all cleaning up some carbs on a 78 750 F took out the low speed jets (they were all plugged)   what determines the actual size the end that sticks out?  side holes were fine  can I just run a drill bit through them or will that changed the size or integrity of the jet

[Bido]

I have used a .35 mm or #80 drill bit to clean the idle jets you should be carefull not to enlarge or score the jet.  I would let them soak in carb cleaner for a while first and try to blow them out with air. You can find the drill bits at a hobby shop. I have seen them on EBAY if you want to replace.

Good Luck
Bido

[TwoTired]

Inside the barrel of the jet body is a precision orifice, or restriction.  This diameter is what determines the size.  Each side of the orifice proper, also has a ramp to blend into the larger tube size.

In my opinion, drills are far too aggressive for cleaning purposes, with a danger of changing hole sizes and causing damage to the blending ramps inside the jet body.  The ramps are there to minimize turbulence, which can effect flow at different fuel velocities.

Cheers,

[nteek754]

Hey all. well I learned the hard way  Ive been messing with the Ks for 30 plus years on the low speed jets I opened up  on this 78 Fand the jets were plugged so, took some out of a parts set (they were plugged also) . Like a dumb as I put a 1/16 inch drill bit through the original set. Then opened another set up and hmmmm these were not plugged. so I took the carbs back off and replaced the ones I screwed up. Back together now and run a whole lot better. Ive been out of town and out of touch with this forum. Next time I will post before I really screw something up thanks a bunch all. Craig in Maine

[BobbyR]

Inside the barrel of the jet body is a precision orifice, or restriction.  This diameter is what determines the size.  Each side of the orifice proper, also has a ramp to blend into the larger tube size.

In my opinion, drills are far too aggressive for cleaning purposes, with a danger of changing hole sizes and causing damage to the blending ramps inside the jet body.  The ramps are there to minimize turbulence, which can effect flow at different fuel velocities.

Cheers,

I would use a strand to lampcord wire or something similar in size. I accidently punched out the orifices and I was spewing unburned gasoline vapor out of the pipes. I mean a white mist of fuel. I could have lit it. You simply want to loosen deposits so the can be washed or blown away.

[eurban]

You can get new #35 pressed in pilots relatively cheaply from Sirius Inc. . . . . You should be able to clear your stock jets with just a bit of work.  Soaking in carb cleaner, boiling in water and then going at it with a single strand of copper wire should typically do it.  A few years ago I actually bought a micro drill bit sized to aproximate a #38 opening in an attempt to enlarge a set of stock 35 pressed in pilots.  I was able to enlarge the holes by simply twisting the bit by hand.  Not sure if this would do much damage if you used the correct bit for a #35 opening but probably better to stick with copper wire.  BTW I was under the mistaken impression that my cammed big bore engine would require a larger pilot.  In the end, clean stock 35s work nice! 

[bryanj]

http://www.siriusconinc.com/search/show_result.asp?searchtype=model&curp=2&searchvalue=cb750&category=0#

And look for the 750 slow jets--only one size though

[nteek754]

Hey all, still no better (only run on choke) . Now I had some other parts carbs BUT they were to a 78 K so are the slow speed jets a differant size in the F models got a new set coming, just gonna be really upset if this dont fix the problem. The last set I put in  I could suck on the jet ( no wise cracks) and get air through them all and the holes at the bottom were clear. Wow 30 plus years  with the Ks and never had to mess with the slow jets. Im still running my original set off my 74 33 years ago running them on my 70 836 engine and they are great I like them old style so much better. I have been searching the web but cant find the specs on the differances in the  Ks and Fs  HELP!!!! Craig in Maine

[nteek754]

I got air to go through but never anything like a piece of wire Im hopeing this is my problem that the jets just aint big enough  or cleaned up well.

[joeb]

Have you cleaned your primary jets , and is your accellerator pump working. my carbs work fine with the stock low speed jets with a 836 kit and cam ,M.R head.   sounds like more compressed air and carb cleaning is in order.     

[nteek754]

everything else looks good they are nice and clean and as for that it is running great on the mains, just idle and on up tp like 3000rs (with the choke on)  It comes alive and sounds good its a stock 78 supersport Im figuring out that it is the low speed circut somewhere, I have  put air through all areas just wish I could find out if they used differant slow jets in the Ks verses the Fs they do list them in two differant rebuild kits so that kinda leans me towards there is something differant and I know its not the mains they are 105s  intake boots are solid so no leakage there after 30 some years dazed and confused Craig in Maine

PS this is not my bike I would of blown it up by now   just kidding  grrrrr

[joeb]

Did you take out the pilot screws that are in front of the float bowls that control your air flow and did you put them back together properly and did you clean out that circut.

[livetoskate]

This may help you know what is in the stock carbs,helped me.
http://www.motorcycleproject.com/motorcycle/text/specs.html
Good Luck

[nteek754]

Hey all, thanks a bunch at least now Im headed in the right direction. and not quite so lost. Dont know what Im gonna do if the new slow jets dont smarten it up. But I feel its gotta help. Thanks agin Craig in Maine

[eurban]

Slow jets are the same size for the Ks and Fs with the accelerator pump carbs (77 and 78 bikes).  Did you observe the function of the accel pump yet?  Take the air filter off and whack open the throttle.  You should see a squirt of fuel shooting down the throat of the engine from each brass nozzle located at the bottom of the carb bore (next to the choke plate)  Pumps got to work for these carbs to function properly.

[nteek754]

Ok put the new jets in  NO differance, so now I am gonna check out the acc pump closer. Would putting he choke on over ride the acc pump the reason I ask is witht he choke on this bike is right on sounds great and pulls its revs  but I know that aint right . Im nott home out on a trip with my own 750 frig workin on someone elses just kidding Jake  (my cousins bike) I know I sound like a rookie but like I said before 33 years and never messed with these style carbs. thanx a bunch Craig somewhere in the states.

[Bido]

Check for leaks around the intake boots had a similar problem and that was the trouble lack of vacuum to the carbs.

Good luck
Bido