Author Topic: Jetting (Read 3329 times)

Johnny340 · « **on:** March 13, 2018, 04:58:17 PM »

I have searched the multiple threads about jetting and don't want to get into a debate about airboxes vs pods. I have bikes with airboxes and this one doesn't.

I haven't read about a starting point for my setup, which is:
836 with Wiseco pistons.
K&N pod filters.
Carpy 4 into 1 open exhaust.

I currently have this jetting (stock is in brackets):
Main jets: 125 (105)
Pilot Jets: 45 (40)
Needle Clip position: Bottom (middle)
Air Screw: 1/4T (1 & 1/2T)

There is a slight hang at 2300RPM before settling into idle below 1000RPM that I'd like to get rid of.
Any constructive suggestions (besides putting the airbox back on) are welcomed. I know I'm not the first one to run an SOHC CB750 with this setup. Thank you!

Paintedseat · « **Reply #1 on:** March 13, 2018, 08:04:17 PM »

Are you sure the pilot jets were 40 from the factory? I would turn air screws out 1.5. Are you saying you need to blip the throttle when you come to a stop?

Yamahawk · « **Reply #2 on:** March 13, 2018, 08:04:29 PM »

Usually, when a bike hangs at a higher idle before dropping to the idle setting, it is because of a lean condition. Now this is the lower rpm range, so it could be due to the idle circuitry being partially blocked with crud, or the needle jet itself having emulsion holes blocked. If that isn't the case, I remember Hondaman's FAQ's stating that you can enlarge the upper holes in the pilot jets, and perhaps also in the needle jet. The pilot jet has 8 holes on them and you enlarge the 4 upper holes for a better emulsion, and it is supposed to get rid of that flat spot, if i remember correctly. You will have to look at his 'Thoughts' in the FAQ section to read what I am referring to.
Charlie

Johnny340 · « **Reply #3 on:** March 13, 2018, 08:10:39 PM »

Quote from: Paintedseat on March 13, 2018, 08:04:17 PM

Are you sure the pilot jets were 40 from the factory? I would turn air screws out 1.5. Are you saying you need to blip the throttle when you come to a stop?

I believe they were 40 stock. Yes, I had to blip the throttle mostly because the idle was set too low (to confirm that wasn't responsible for the idle hanging up). It ran worse with the air screws at 1.5T..

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Johnny340 · « **Reply #4 on:** March 13, 2018, 08:14:25 PM »

Quote from: Yamahawk on March 13, 2018, 08:04:29 PM

Usually, when a bike hangs at a higher idle before dropping to the idle setting, it is because of a lean condition. Now this is the lower rpm range, so it could be due to the idle circuitry being partially blocked with crud, or the needle jet itself having emulsion holes blocked. If that isn't the case, I remember Hondaman's FAQ's stating that you can enlarge the upper holes in the pilot jets, and perhaps also in the needle jet. The pilot jet has 8 holes on them and you enlarge the 4 upper holes for a better emulsion, and it is supposed to get rid of that flat spot, if i remember correctly. You will have to look at his 'Thoughts' in the FAQ section to read what I am referring to.
Charlie

Thanks Charlie. I agree on too lean down low. I'll pull the carbs again and confirm pilots and emulsion tubes clear. The enlarging idea sounds clever. I don't have his book though.. There was some crud from the tank, but I thought I cleaned it all out. Off come the carbs again.. I getting good at that!
Thank you for the suggestions and if anyone can detail that enlarging procedure I'd really appreciate it!

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Yamahawk · « **Reply #5 on:** March 13, 2018, 08:15:33 PM »

It's in the FAQ section here on the forum

Charlie

Johnny340 · « **Reply #6 on:** March 13, 2018, 08:15:59 PM »

I will search the 'Thoughts of A Honda man' but I don't recall reading that part before.

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seanbarney41 · « **Reply #7 on:** March 13, 2018, 08:22:37 PM »

exactly what carbs do you have? 086a? I have a mostly untested hypothesis that 086a have the best needle profile for pods/short/open pipes. So if you have 086a and the needle is already on the bottom clip position and it's still too lean?...this is why you run an airbox

Johnny340 · « **Reply #8 on:** March 13, 2018, 08:28:34 PM »

Quote from: seanbarney41 on March 13, 2018, 08:22:37 PM

exactly what carbs do you have? 086a? I have a mostly untested hypothesis that 086a have the best needle profile for pods/short/open pipes. So if you have 086a and the needle is already on the bottom clip position and it's still too lean?...this is why you run an airbox

I have Keihin 064A carbs.

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Yamahawk · « **Reply #9 on:** March 13, 2018, 08:29:39 PM »

http://forums.sohc4.net/index.php/topic,7401.0.html
Reply #13 for the emulsion tube mods...
Charlie

seanbarney41 · « **Reply #10 on:** March 13, 2018, 08:40:04 PM »

Quote from: Johnny340 on March 13, 2018, 08:28:34 PM

Quote from: seanbarney41 on March 13, 2018, 08:22:37 PM
exactly what carbs do you have? 086a? I have a mostly untested hypothesis that 086a have the best needle profile for pods/short/open pipes. So if you have 086a and the needle is already on the bottom clip position and it's still too lean?...this is why you run an airbox

I have Keihin 064A carbs.

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hold up on my claims here...I might have my numbers mixed up. Gotta go look some stuff up.

Johnny340 · « **Reply #11 on:** March 13, 2018, 08:41:09 PM »

Quote from: Yamahawk on March 13, 2018, 08:29:39 PM

http://forums.sohc4.net/index.php/topic,7401.0.html
Reply #13 for the emulsion tube mods...
Charlie

Thanks Charlie, got now it!

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seanbarney41 · « **Reply #12 on:** March 13, 2018, 09:00:01 PM »

Quote from: seanbarney41 on March 13, 2018, 08:40:04 PM

Quote from: Johnny340 on March 13, 2018, 08:28:34 PM
Quote from: seanbarney41 on March 13, 2018, 08:22:37 PM
exactly what carbs do you have? 086a? I have a mostly untested hypothesis that 086a have the best needle profile for pods/short/open pipes. So if you have 086a and the needle is already on the bottom clip position and it's still too lean?...this is why you run an airbox

I have Keihin 064A carbs.

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hold up on my claims here...I might have my numbers mixed up. Gotta go look some stuff up.

ok, I am gonna stick with the 086a's being the best needle...according to this chart
www.hondachopper.com/garage/carb_specs/carb_specs.html I think you mis read 069a as 064a which supposedly have the same needle as 086a. However, no absolute confirmation on the total accuracy of this chart. Honda was very slippery on what parts they used on what bikes plus things have been swapped around so much over the years. But, please note, only three different needles were available throughout the model run. Also note there is NO information on needle jet size and emulsion tubes to be found anywhere. Compare that to Mikuni VM's which offer dozens of needles and needle jets and guys still have limited success tuning those for modified intake/exhaust. So this is why so little really useful info can be found.

I have had luck adding a small straight through glasspack...seems to trick your pipe into thinking it is longer.

Johnny340 · « **Reply #13 on:** March 13, 2018, 09:12:31 PM »

Quote from: seanbarney41 on March 13, 2018, 09:00:01 PM

Quote from: seanbarney41 on March 13, 2018, 08:40:04 PM
Quote from: Johnny340 on March 13, 2018, 08:28:34 PM
Quote from: seanbarney41 on March 13, 2018, 08:22:37 PM
exactly what carbs do you have? 086a? I have a mostly untested hypothesis that 086a have the best needle profile for pods/short/open pipes. So if you have 086a and the needle is already on the bottom clip position and it's still too lean?...this is why you run an airbox

I have Keihin 064A carbs.

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hold up on my claims here...I might have my numbers mixed up. Gotta go look some stuff up.
ok, I am gonna stick with the 086a's being the best needle...according to this chart
www.hondachopper.com/garage/carb_specs/carb_specs.html I think you mis read 069a as 064a which supposedly have the same needle as 086a. However, no absolute confirmation on the total accuracy of this chart. Honda was very slippery on what parts they used on what bikes plus things have been swapped around so much over the years. But, please note, only three different needles were available throughout the model run. Also note there is NO information on needle jet size and emulsion tubes to be found anywhere. Compare that to Mikuni VM's which offer dozens of needles and needle jets and guys still have limited success tuning those for modified intake/exhaust. So this is why so little really useful info can be found.

I have had luck adding a small straight through glasspack...seems to trick your pipe into thinking it is longer.

You're right, I misread that number. Looks like JN (jet needle?) is different.. 2 vs 4. Thanks for the link.
I'll try that emulsion tube hole enlargement next.

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seanbarney41 · « **Reply #14 on:** March 14, 2018, 03:55:12 AM »

Also, make sure your timing advancer is working properly and do a clear tube test for float bowl fuel levels. Otherwise you will just be chasing your tail.

Johnny340 · « **Reply #15 on:** March 14, 2018, 10:32:17 AM »

Quote from: seanbarney41 on March 14, 2018, 03:55:12 AM

Also, make sure your timing advancer is working properly and do a clear tube test for float bowl fuel levels. Otherwise you will just be chasing your tail.

OK I did not misread that number. 064A for sure which, according to this chart is correct for my bike ('75 F model):
http://www.globalarray.net/user/mike_in_phoenix/motorcycle/text/specs_revised.html
I'm not sure how the needle compares to the one you prefer Sean.

The timing advancer is perfect. I will do the clear tube method to test the actual float levels. I looked that up but of course the photobucket images are not available

These carbs have the separate drain screw so I suppose I'll be looking for some brass fitting that threads in.. Edit: Just found Flybox's heated tube method. Cool. I'm going away for a few days and will tackle this as well as the Pilot Jet emulsifier tubes when I get back. Thanks again for the constructive comments!

seanbarney41 · « **Reply #16 on:** March 14, 2018, 01:16:57 PM »

that is interesting...the guy that made that chart certainly knows his #$%* although he himself notes that it contains/ed several errors. Pretty sure you are right about 064a being correct as I now remember that 069a was for the 550's. I have owned 2 '75 750f's and they both had 086a but their is probably more evidence of them being swapped than there is for them being original...but what I am getting at is they both had pods when I got them and ran well without even a good carb cleaning. I changed both to Henry Abe breadboxes, made a couple mainjet adjustments and had them both running and performing as good as with a stock airbox. Interestingly, the 2 bikes wanted very different main jets with very similar induction/exhaust.

setdog · « **Reply #17 on:** March 14, 2018, 07:34:13 PM »

those carbs looks nice. generally your problem sounds pretty mild, so id be looking for a very slight adjustment somewhere.

are you using keihin jets or aftermarket? i think keihin is better way to go. id first just go after the jets with some wire and clean the hell out them

i may get some cringes for what im about to say, but i sometimes think too much time is spent getting the jetting size and combination right, when its just
cleaning jet holes and mixture screw settings that need attention. small adjustments to mixture screws can yield good results.

ill leave out the episode where i failed to put back my vacuum screws and nearly went crazy.

Johnny340 · « **Reply #18 on:** March 17, 2018, 01:59:01 PM »

So I enlarged the top four emulsifier holes in the 45 pilot jets and it still hangs up before idling down. The video is from when it runs best. When the air screws are open 1 turn it hangs at 2500RPM for way longer before idling down. But with the air screws only open a quarter turn it doesn't idle that well once it finally does settle into the idle..
Everything is clean inside these carbs. http://cloud.tapatalk.com/s/5aad8062b2eeb/20180317_131920.mp4

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seanbarney41 · « **Reply #19 on:** March 17, 2018, 02:22:46 PM »

that's a weird one...does look like you got the idle speed set way too low. Try about 1100 rpm, not 600

Johnny340 · « **Reply #20 on:** March 17, 2018, 02:23:54 PM »

Quote from: seanbarney41 on March 17, 2018, 02:22:46 PM

that's a weird one...does look like you got the idle speed set way too low. Try about 1100 rpm, not 600

If I set it that low then it hangs up at 2500 even longer... Sometimes indefinitely.

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seanbarney41 · « **Reply #21 on:** March 17, 2018, 02:34:38 PM »

have you tried to ride it? how does it perform?

seanbarney41 · « **Reply #22 on:** March 17, 2018, 02:39:12 PM »

Quote from: Johnny340 on March 17, 2018, 02:23:54 PM

Quote from: seanbarney41 on March 17, 2018, 02:22:46 PM
that's a weird one...does look like you got the idle speed set way too low. Try about 1100 rpm, not 600
If I set it that low then it hangs up at 2500 even longer... Sometimes indefinitely.

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ok, not understanding...from your video it looks like your idle speed is set at 600 rpm, it should idle at about 1100-1300

Johnny340 · « **Reply #23 on:** March 17, 2018, 02:52:50 PM »

Quote from: seanbarney41 on March 17, 2018, 02:39:12 PM

Quote from: Johnny340 on March 17, 2018, 02:23:54 PM
Quote from: seanbarney41 on March 17, 2018, 02:22:46 PM
that's a weird one...does look like you got the idle speed set way too low. Try about 1100 rpm, not 600
If I set it that low then it hangs up at 2500 even longer... Sometimes indefinitely.

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ok, not understanding...from your video it looks like your idle speed is set at 600 rpm, it should idle at about 1100-1300

If I set the idle speed to 1100 or so, then blip the throttle it revs up high but hangs even longer at 2400-3000RPM.

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seanbarney41 · « **Reply #24 on:** March 17, 2018, 03:00:33 PM »

I would say you need bigger idle jet, but is anything bigger available?
Any chance you got confused on the clip position and put it all the way at the top(leaner) instead of all the way at the bottom(richer)?
Something is not right here
Have you had a look at the plugs?

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Author Topic: Jetting (Read 3329 times)