Author Topic: carb jet question  (Read 913 times)

0 Members and 1 Guest are viewing this topic.

Offline Scott

  • 72 CB750
  • Expert
  • ****
  • Posts: 833
carb jet question
« on: October 04, 2017, 11:03:12 AM »
Hi there

I am trying to get to the bottom of what I think is a jetting issue.

I bought a set of standard CB750 carbs a few years ago that had been rebuilt for a "race engine" I told the bloke rebuilding them that the carbs would be used on a engine bigger than standard specs.

When the carbs arrived I couldn't help but bolt them on to my K2 to see what would happen although they were not intended for this engine.   It ran fine above idle, it even seemed to go better mid range, but at idle it ran rough and if you blip the throttle it pumped out black smoke.

Fast forward to now and I have fitted the "race engine" to my bike and thought great I can try these carbs on the engine they were built for.  Well it does the same thing.  Idle is lumpy and as soon as you twist the throttle it pumps out black smoke.

I pulled them from the bike to check jet sizes the slow jet is #40 the main jet is #108 and the needle is 27201 with clip 2nd from bottom.

I checked this against my old carbs which have the same slow jet, the same needle with the clip in the same position, but the main jet in the old carbs is #110

I guess after work today I will bolt on the old carbs to the new engine to see what happens, but for now I would like to know how much difference there is in a 108 (new carbs) to a 110 (old carbs)

I have always had the air box in play when testing either set of carbs.

Thanks for any thoughts on this.
Scott

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,301
Re: carb jet question
« Reply #1 on: October 04, 2017, 11:32:32 AM »
IIWM (assumed timing/advance are correct)...3 items asap.
1. Clear tube test the bowls for adequate fuel levels, and adjust till correct.
2. Idle plug chop with new plugs, and IMS adjustments as necessary until plugs are dark tan.
3. Vacuum sync.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Scott

  • 72 CB750
  • Expert
  • ****
  • Posts: 833
Re: carb jet question
« Reply #2 on: October 04, 2017, 12:11:14 PM »
Timing set spot on and advance working but not acuratly checked.

I can't believe I never checked float (fuel) level. I just assumed it was OK.

 I will get the new carbs back on and check with clear tube method.

I am going to put the jets from old carbs into rebuilt carbs as an experiment and out of curiosity.

Thanks for your input
Scott

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,301
Re: carb jet question
« Reply #3 on: October 04, 2017, 12:39:21 PM »
Timing set spot on and advance working but not acuratly checked.

I can't believe I never checked float (fuel) level. I just assumed it was OK.

 I will get the new carbs back on and check with clear tube method.  Leave the carbs off.  Do this on the bench as they most likely will need adjustment.  Easier to rig an aux fuel source than take the carbs off/on a few times.  Only when all are correct, put the carbs back on the bike.

I am going to put the jets from old carbs into rebuilt carbs as an experiment and out of curiosity.

Thanks for your input
Scott
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Scott

  • 72 CB750
  • Expert
  • ****
  • Posts: 833
Re: carb jet question
« Reply #4 on: October 04, 2017, 12:53:46 PM »
Makes sence. No worries.

I'm at barstard work now so I can get back into it this sarvo.