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Author Topic: 1978 cb750k pilot jets  (Read 2553 times)

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Offline Mikejk

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1978 cb750k pilot jets
« on: July 21, 2018, 07:20:03 AM »
Hi everyone! Re-jetting the carbs on my cb750 to accomodate pods and 4 into 1 exhaust. Ordered 120 mains which seem good but the pilots are very different... still push in but the stock ones are two pieces, one threaded in to the other. The new jets would not be in the bowl nearly as far. I also noticed that (since i first accidentally ordered) older style threaded jet will thread in to the press in part of my original jets. I'll attach some pics for clarification. Anyone able to shed some light on this and recommend what configuration would be best? My thought is if the original press in part would accomodate older style threaded jets that would be an advantage if it ever needs to be re-jetted, but I'm not sure if it would be too low in the float bowl.
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Offline brewsky

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Re: 1978 cb750k pilot jets
« Reply #1 on: July 21, 2018, 10:45:13 AM »
Are you the Ichiban Moto guy?
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66 CA77
78 550K
78 CB750K
02 FZ1
09 GL 1800

Offline Mikejk

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Re: 1978 cb750k pilot jets
« Reply #2 on: July 21, 2018, 11:05:41 AM »
Quote from: brewsky on July 21, 2018, 10:45:13 AM
Are you the Ichiban Moto guy?

Lol! No, can't say i am. Sounds interesting though...
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Offline brewsky

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Re: 1978 cb750k pilot jets
« Reply #3 on: July 21, 2018, 11:28:16 AM »
Then he must have done the carbs before you!

No threaded jets are used and they are not stacked!

Use push in slow jets only
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66 CA77
78 550K
78 CB750K
02 FZ1
09 GL 1800

Offline madmtnmotors

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Re: 1978 cb750k pilot jets
« Reply #4 on: July 21, 2018, 11:37:14 AM »
Quote from: brewsky on July 21, 2018, 11:28:16 AM
Then he must have done the carbs before you!

No threaded jets are used and they are not stacked!

Use push in slow jets only

+1

Looks like that slow jet was modified.

This has always been one of my favorite Ichiban "How-To's":

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Offline Mikejk

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Re: 1978 cb750k pilot jets
« Reply #5 on: July 21, 2018, 12:52:42 PM »
Hmmm. Both pieces look to be keihin parts with the little star like "k" on them. But upon closer inspection, the push in part is 35 and the thread in part is 38... pretty anxious about how much of a difference this will make. Bike didn't run bad before, just hit a flat spot around 5k rpm under load. For the record, this must have been a PO's (Ichiban Moto guy?) doing, I've never had the carbs apart.
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Offline brewsky

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Re: 1978 cb750k pilot jets
« Reply #6 on: July 21, 2018, 01:33:35 PM »
What size are your new push ins?
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66 CA77
78 550K
78 CB750K
02 FZ1
09 GL 1800

Offline Mikejk

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Re: 1978 cb750k pilot jets
« Reply #7 on: July 21, 2018, 07:34:03 PM »
Quote from: brewsky on July 21, 2018, 01:33:35 PM
What size are your new push ins?
42, why do you ask?
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Offline Mikejk

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Re: 1978 cb750k pilot jets
« Reply #8 on: July 21, 2018, 07:44:33 PM »
I got everything back together and took the bike for a spin, definitely much more responsive and feels more powerful. I haven't yet messed with the mixture screws, i just put them all 1-1/2 turns out, nor have i re-synced the carbs. However, the reason i originally decided to up size the jet was because the bike wouldn't pull past 5k rpm and it still seems to be doing the same thing... not nearly as bad, just under heavy acceleration, whereas before it was under any acceleration as soon as it hit that 5k mark. I should also mention i was planning on shimming the needles but it was already done by the PO. I also noticed a couple of the floats were off a little so i fixed those too. Just ran out of time tonight to tinker with it any more but would welcome some suggestions for tomorrow.
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Offline brewsky

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Re: 1978 cb750k pilot jets
« Reply #9 on: July 22, 2018, 02:51:07 AM »
Quote from: madmtnmotors on July 21, 2018, 11:37:14 AM
Quote from: brewsky on July 21, 2018, 11:28:16 AM
Then he must have done the carbs before you!

No threaded jets are used and they are not stacked!

Use push in slow jets only

+1

Looks like that slow jet was modified.

This has always been one of my favorite Ichiban "How-To's":


Classic!
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66 CA77
78 550K
78 CB750K
02 FZ1
09 GL 1800

Offline brewsky

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Re: 1978 cb750k pilot jets
« Reply #10 on: July 22, 2018, 03:20:57 AM »
Quote from: Mikejk on July 21, 2018, 07:44:33 PM
  would welcome some suggestions for tomorrow.

OK, here are mine:

Lose the pods and get a stock airbox...….cut 4   1" holes in the rear....add K&N filter

112.5 mains, 42 pilots, 1 shim under the needles

If your 4/1 is a MAC, pull the baffle and drill a few extra holes and re-install

Make sure your accel pump is working

This setup is the result of tuning with onboard A/F meter and verified by dyno, and gives good power and no flat spots anywhere in the rpm range

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66 CA77
78 550K
78 CB750K
02 FZ1
09 GL 1800

Offline Mikejk

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Re: 1978 cb750k pilot jets
« Reply #11 on: July 22, 2018, 06:42:32 AM »
Quote from: brewsky on July 22, 2018, 03:20:57 AM
Quote from: Mikejk on July 21, 2018, 07:44:33 PM
  would welcome some suggestions for tomorrow.

OK, here are mine:

Lose the pods and get a stock airbox...….



I should've seen this one coming  ::) I don't necessarily have an affinity for pods but I don't have the stock airbox and don't want to spend the time or money tracking one down.

I replaced the accelerator pump and confirmed it was working.
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Offline flybox1

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Re: 1978 cb750k pilot jets
« Reply #12 on: July 23, 2018, 08:43:23 AM »
Quote from: brewsky on July 22, 2018, 03:20:57 AM
Quote from: Mikejk on July 21, 2018, 07:44:33 PM
  would welcome some suggestions for tomorrow.

OK, here are mine:

Lose the pods and get a stock airbox...….cut 4   1" holes in the rear....add K&N filter

112.5 mains, 42 pilots, 1 shim under the needles

If your 4/1 is a MAC, pull the baffle and drill a few extra holes and re-install

Make sure your accel pump is working

This setup is the result of tuning with onboard A/F meter and verified by dyno, and gives good power and no flat spots anywhere in the rpm range

^^^ this is a great place to start +1!!

my jetting is below..
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'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 Mikejk

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Re: 1978 cb750k pilot jets
« Reply #13 on: July 23, 2018, 02:49:31 PM »
Took it for a spin today and I'm pretty happy with it. But the throttle is a little sticky now for some reason. I've had the bike for a long time, never rode it much, and have to say this is the best it's ran since I've had it. Would it run better with the stock air box? Sure. But I don't have anything to compare it to so I don't know any better... 😊
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Offline flybox1

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Re: 1978 cb750k pilot jets
« Reply #14 on: July 25, 2018, 11:25:03 AM »
it'll run well if its tuned and jetted properly.
better?  not really....easier? (to tune and jet properly) for sure!

for that sticky throttle..
remove both cable ends from their carb attachments and seats.
actuate teh throttle bell....do the slides snap closed under the weight of the return spring?
No? - Clean carb exterior and slide chambers.
Yes?  twist the throttle.  do the cables and throttle sleeve move freely?
No?  find out why.  clean and lube throttle sleeve and cables.
Yes? - attach the pull cable.  twist the throttle.  Does it move freely?
No? - find out why.  check for proper cable routing. retest.
Yes?...attach the push cable.  twist the throttle.  Does it move freely?
No?  Find out why.  if throttle cable sleeve lengths are not right, this can bind cables.
Yes?....go ride...
« Last Edit: July 25, 2018, 12:11:58 PM by flybox1 »
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'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 Bankerdanny

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Re: 1978 cb750k pilot jets
« Reply #15 on: July 25, 2018, 11:42:12 AM »
It will also run better if it gets used more.
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Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200
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