testy

Author Topic: Jet sizing help CB350F  (Read 323 times)

0 Members and 1 Guest are viewing this topic.

Offline Genesee

  • Full Member
  • *
  • Posts: 28
Jet sizing help CB350F
« on: December 05, 2018, 02:39:29 pm »
I’m getting back into my CB350F after riding something newer for the past couple years. It started right up on the first crank and ran just how I remembered after sitting about 2 years.

I think I’m going to tackle rebuilding the carbs soon as there’s some gas leaking from the bowls and surely there’s a little crud built up in there. When I rebuild the carbs I want to fully disassemble and clean/inspect/replace everything as needed and I want to replace the main jets for a couple reasons.

First off the motor is stock with the exception of the exhaust. Back in the 80’s my dad took the stock exhaust, cut off the mufflers and welded some tips on in place of the mufflers. So it’s a 4 into 4 exhaust with little to no restriction.

When he did this, he drilled out the main jets to compensate for the less restrictive exhaust. So reason 1 for replacing the jets is they were drilled and lost any of the flow characteristics that non-drilled jets supposedly have. Reason 2, the bike runs good in its current configuration with the drilled jets, but it COULD run better I think. I don’t know if the drilled jets are causing it to not run optimally or not... but maybe there’s a tiny bit of performance to be had with proper jetting.

So I want to use genuine Keihin jets when I do the carbs, but at $6.50 per jet, I don’t want to buy 3-4 sets of different sizes right out of the gate. Hopefully I can get really close with the first set and maybe go up or down a size as needed. So considering the exhaust is the only mod from stock, what would you suggest for a starting jet size?


Sent from my iPhone using Tapatalk
My Inherited 1973 CB350F Project:
http://forums.sohc4.net/index.php?topic=109351.0

Offline Deltarider

  • Really Old Timer ...
  • *******
  • Posts: 5,642
  • "Tremendous progress has been made... "
Re: Jet sizing help CB350F
« Reply #1 on: December 06, 2018, 01:52:17 am »
You have the stock airfilter? Then just go standard size main jets and forget about restrictive this and that. It's from the books and has no meaning for your CB350.
Money is like manure. It's not worth a thing unless it's spread around.

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,292
Re: Jet sizing help CB350F
« Reply #2 on: December 06, 2018, 07:08:21 am »
35 pilot jets and 78 main jets.
1 turn out on your mixture screw.  No needle adjustment.

www.jetsrus.com
'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 Genesee

  • Full Member
  • *
  • Posts: 28
Re: Jet sizing help CB350F
« Reply #3 on: January 09, 2019, 06:08:33 am »
So I finally got the carbs back together and fixed a leak with genuine needles and proper clear tube testing.

I ended up with 40 pilot and 80 main jets.  Although I have the stock air filter, I recently realized that the air box lid is missing (was never there since I've owned the bike) and the intake tube has a few holes cut in it (I think because the old seat was modified and "sat" right on the intake restricting flow).

In any case, I started out with 35 / 80.  The bike appeared to run way too lean.  Revs would hang around 2-3k rpm idle and the engine was slow to rev down after hard pulls.  Swapped out the 35s with 40s and idle and rev down is nice and smooth now.

The old drilled out main jets I pulled looked spot in between the new 75s and 80s I had on hand.  I know spotting the hole diameter by sight is not exactly scientific, but I'd say they were drilled out to about 78s.  I might try 78s next time I have the carbs off, but for now it's running great.

I any case, I ended up with 40/80 and 1.5 turns out on the mixture screws.  Idle is solid at 1200-1300 rpms.  Rev response feels incredibly improved over the previous jets.  It feels like a totally new bike now, pulls hard all the way to redline.  I need to do some plug chops sooner rather than later to see exactly where I'm at.

I'm trying to think of a good spot to do the plug chops.  There's a handful of backroads around me, just gotta find the right stretch.  Can't do it in the neighborhood I live in, bike is WAY to loud for that.
My Inherited 1973 CB350F Project:
http://forums.sohc4.net/index.php?topic=109351.0

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,292
Re: Jet sizing help CB350F
« Reply #4 on: January 09, 2019, 06:43:32 am »
What are your air screws (IMS) set at?
What type of exhaust do you have?
For a mostly stock bike  40's are too rich (even with your airbox cover off - there is not enough vacuum to make you excessively lean at idle w 35s)....which makes me think what else could cause the lean-ness....air screws.
80 mains probably would be OK with no airbox top and an open exhaust.
'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 WhyNot2

  • I'm finally an
  • Expert
  • ****
  • Posts: 1,244
Re: Jet sizing help CB350F
« Reply #5 on: January 09, 2019, 07:13:59 am »
So, do you have a secret on getting the carbs on and off.

Sounds like you've taken them off quite frequently, to hassle with the way I've been doing it.

Takes me almost all day to take them off, and put them back on, just to find out it won't run.

Mine has stock everything cept new delkevic exhaust.

Only 2900 original mile.
If it ain't raining, I'm riding.....~~{iii}?~~prost

If it sounds like I know what I'm talking about, it's because I cut and pasted from someone else.

Offline Genesee

  • Full Member
  • *
  • Posts: 28
Re: Jet sizing help CB350F
« Reply #6 on: January 09, 2019, 07:24:09 am »
So, do you have a secret on getting the carbs on and off.

Sounds like you've taken them off quite frequently, to hassle with the way I've been doing it.

Takes me almost all day to take them off, and put them back on, just to find out it won't run.

Mine has stock everything cept new delkevic exhaust.

Only 2900 original mile.
My secret?... Doing it over and over again. Practice makes perfect, haha.

Actually the biggest help was replacing the carb to air box rubber and carb to engine rubber with new parts. The old rubber was hard and brittle making the process a pain. New rubber and they slip on and off with no fuss. And the piece of mind that the rubber is new and seals better than the old rock hard pieces.


Sent from my iPhone using Tapatalk
My Inherited 1973 CB350F Project:
http://forums.sohc4.net/index.php?topic=109351.0

Offline zturb0

  • Full Member
  • *
  • Posts: 32
Re: Jet sizing help CB350F
« Reply #7 on: January 09, 2019, 01:33:14 pm »
So, do you have a secret on getting the carbs on and off.

Sounds like you've taken them off quite frequently, to hassle with the way I've been doing it.

Takes me almost all day to take them off, and put them back on, just to find out it won't run.

Mine has stock everything cept new delkevic exhaust.

Only 2900 original mile.


Just about, have my 76 400 complete.   ;D

In the last few weeks have had the carbs on and off severel times.

What I use, is a lever action tie down strap.

Get one that has about a  2'' wide strap (cheap,,, at harbor freight )

Hook both ends over the rear blinkers, and the full width of the strap over the 4 open mouths of the stock air box outlet tubes.

Crank, and adjust seating on the tubes, as needed,, while tightening.

Spraying the head side intakes with wd 40 helps install and removal as well

Got to where, I could have them off, and back on, in less then an hour. ;)

76 CB400F
98 VALKYRIE
00 ZX12R
97 BANDIT 12
89 PACIFIC COAST
91 ELIMINATOR 250

Offline WhyNot2

  • I'm finally an
  • Expert
  • ****
  • Posts: 1,244
Re: Jet sizing help CB350F
« Reply #8 on: January 09, 2019, 03:40:35 pm »
So, do you have a secret on getting the carbs on and off.

Sounds like you've taken them off quite frequently, to hassle with the way I've been doing it.

Takes me almost all day to take them off, and put them back on, just to find out it won't run.

Mine has stock everything cept new delkevic exhaust.

Only 2900 original mile.


Just about, have my 76 400 complete.   ;D

In the last few weeks have had the carbs on and off severel times.

What I use, is a lever action tie down strap.

Get one that has about a  2'' wide strap (cheap,,, at harbor freight )

Hook both ends over the rear blinkers, and the full width of the strap over the 4 open mouths of the stock air box outlet tubes.

Crank, and adjust seating on the tubes, as needed,, while tightening.

Spraying the head side intakes with wd 40 helps install and removal as well

Got to where, I could have them off, and back on, in less then an hour. ;)

Can you show some pix?
If it ain't raining, I'm riding.....~~{iii}?~~prost

If it sounds like I know what I'm talking about, it's because I cut and pasted from someone else.

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,292
Re: Jet sizing help CB350F
« Reply #9 on: January 09, 2019, 03:52:38 pm »
he's using the ratchet straps to pull the airbox plenum to the rear, giving more space to tilt the carbs up and down, and finally out the side.
'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 carnivorous chicken

  • Expert
  • ****
  • Posts: 828
Re: Jet sizing help CB350F
« Reply #10 on: January 09, 2019, 05:30:02 pm »
That's what I do as well.

Offline Deltarider

  • Really Old Timer ...
  • *******
  • Posts: 5,642
  • "Tremendous progress has been made... "
Re: Jet sizing help CB350F
« Reply #11 on: January 10, 2019, 12:05:42 am »
On my CB500 I can do all my carb maintenance including removing and fitting all jets with the carbs in situ. Once you know how it looks like inside, it's quite easy actually. I've replaced the 16 floatbowlscrews by allen (see pic). Maybe also feasable on a CB350F?
Money is like manure. It's not worth a thing unless it's spread around.

Offline zturb0

  • Full Member
  • *
  • Posts: 32
Re: Jet sizing help CB350F
« Reply #12 on: January 10, 2019, 07:34:45 am »
So, do you have a secret on getting the carbs on and off.

Sounds like you've taken them off quite frequently, to hassle with the way I've been doing it.

Takes me almost all day to take them off, and put them back on, just to find out it won't run.

Mine has stock everything cept new delkevic exhaust.

Only 2900 original mile.


Just about, have my 76 400 complete.   ;D

In the last few weeks have had the carbs on and off severel times.

What I use, is a lever action tie down strap.

Get one that has about a  2'' wide strap (cheap,,, at harbor freight )

Hook both ends over the rear blinkers, and the full width of the strap over the 4 open mouths of the stock air box outlet tubes.

Crank, and adjust seating on the tubes, as needed,, while tightening.

Spraying the head side intakes with wd 40 helps install and removal as well

Got to where, I could have them off, and back on, in less then an hour. ;)

Can you show some pix?

Sorry, no I can't. :-[

Didn't think, it was pic worthy, at the time.  ;)
76 CB400F
98 VALKYRIE
00 ZX12R
97 BANDIT 12
89 PACIFIC COAST
91 ELIMINATOR 250

Offline WhyNot2

  • I'm finally an
  • Expert
  • ****
  • Posts: 1,244
Re: Jet sizing help CB350F
« Reply #13 on: January 10, 2019, 09:53:36 am »
So, do you have a secret on getting the carbs on and off.

Sounds like you've taken them off quite frequently, to hassle with the way I've been doing it.

Takes me almost all day to take them off, and put them back on, just to find out it won't run.

Mine has stock everything cept new delkevic exhaust.

Only 2900 original mile.


Just about, have my 76 400 complete.   ;D

In the last few weeks have had the carbs on and off severel times.

What I use, is a lever action tie down strap.

Get one that has about a  2'' wide strap (cheap,,, at harbor freight )

Hook both ends over the rear blinkers, and the full width of the strap over the 4 open mouths of the stock air box outlet tubes.

Crank, and adjust seating on the tubes, as needed,, while tightening.

Spraying the head side intakes with wd 40 helps install and removal as well

Got to where, I could have them off, and back on, in less then an hour. ;)

Can you show some pix?

Sorry, no I can't. :-[

Didn't think, it was pic worthy, at the time.  ;)

That's ok man, I'll figure out a way to mess it up trying.
If it ain't raining, I'm riding.....~~{iii}?~~prost

If it sounds like I know what I'm talking about, it's because I cut and pasted from someone else.

Offline zturb0

  • Full Member
  • *
  • Posts: 32
Re: Jet sizing help CB350F
« Reply #14 on: January 10, 2019, 02:06:53 pm »
   

 Best I can do, is say,  this is the tie down I used. 

And after that, yer on,,, yer own...   ;)



« Last Edit: January 11, 2019, 04:42:31 am by zturb0 »
76 CB400F
98 VALKYRIE
00 ZX12R
97 BANDIT 12
89 PACIFIC COAST
91 ELIMINATOR 250

Offline whizzer

  • Full Member
  • *
  • Posts: 53
  • Be safe
Re: Jet sizing help CB350F
« Reply #15 on: January 10, 2019, 06:47:21 pm »
Another method, wooden broom handle.  Fit between carbs and engine to pull off, wiggling carb rack up and down. Reverse to install carbs with broom handle behind carbs and push on to lubricated rubbers.  Air box is pushed back as much as possible, allowing carbs to slide out the side.
72 cb350f
66 cb77 sold
72 Suzuki gt750 given away
59 Vespa given away
06 Whizzer sold

Offline Genesee

  • Full Member
  • *
  • Posts: 28
Re: Jet sizing help CB350F
« Reply #16 on: January 11, 2019, 05:25:58 am »
Another method, wooden broom handle.  Fit between carbs and engine to pull off, wiggling carb rack up and down. Reverse to install carbs with broom handle behind carbs and push on to lubricated rubbers.  Air box is pushed back as much as possible, allowing carbs to slide out the side.
This was exactly my method before replacing the rubber. I always fully removed the air box each time though, just easier having it out of the way.


Sent from my iPhone using Tapatalk
My Inherited 1973 CB350F Project:
http://forums.sohc4.net/index.php?topic=109351.0

Offline Deltarider

  • Really Old Timer ...
  • *******
  • Posts: 5,642
  • "Tremendous progress has been made... "
Re: Jet sizing help CB350F
« Reply #17 on: January 11, 2019, 07:25:33 am »
Have you tried my method? I remember I've done it on a CB350F.
Money is like manure. It's not worth a thing unless it's spread around.

 

;