Topic: 78 CB750 Slow jets

[TwoTired]

But if it stumbles when quickly opening the throttle cannot be cured with mixture screw then the #38 idle jets would be the next logical step.

I gotta disagree. The 77 and 78 PD carbs were mandated to idle lean to pass emission's PPM hydrocarbon test at the exhaust.  They were mandated so lean, that sudden throttle opening ALWAYS caused stumble as the mixture went way too lean with the sudden slide opening.  Later models went to CV type carbs, in part to alleviate this, but for these mechanical slide carbs, an accelerator pump was the only way to get the enrichment needed for snap throttle response.

If the accelerator pump is working at proper capacity, the pilot jets can be completely plugged and the engine will still advance vigorously when the throttle is twisted.  (Yes, I've tried it that way. 
It won't stay at idle without choke, but if you catch it with a throttle twist, it will pick up.  )

I've had it the other way, too.  Pilots jets clear, but the pump working weakly.  Idled great, but with throttle advance stumble.  In my case, the mouse ear holes on the diaphragm had swelled almost shut.  Drilled them to match the housing's holes size and wow, nearly got shot in the eye.

This bike has the stock F muffler, and a K&N filter in the stock air box.  Stock jetting and carb settings, of course.

Cheers,

[lucky]

But if it stumbles when quickly opening the throttle cannot be cured with mixture screw then the #38 idle jets would be the next logical step.

I gotta disagree. The 77 and 78 PD carbs were mandated to idle lean to pass emission's PPM hydrocarbon test at the exhaust.  They were mandated so lean, that sudden throttle opening ALWAYS caused stumble as the mixture went way too lean with the sudden slide opening.  Later models went to CV type carbs, in part to alleviate this, but for these mechanical slide carbs, an accelerator pump was the only way to get the enrichment needed for snap throttle response.

If the accelerator pump is working at proper capacity, the pilot jets can be completely plugged and the engine will still advance vigorously when the throttle is twisted.  (Yes, I've tried it that way. 
It won't stay at idle without choke, but if you catch it with a throttle twist, it will pick up.  )

I've had it the other way, too.  Pilots jets clear, but the pump working weakly.  Idled great, but with throttle advance stumble.  In my case, the mouse ear holes on the diaphragm had swelled almost shut.  Drilled them to match the housing's holes size and wow, nearly got shot in the eye.

This bike has the stock F muffler, and a K&N filter in the stock air box.  Stock jetting and carb settings, of course.

Cheers,

I agree with you on everything except there was no emissions testing except maybe for import purposes.

[TwoTired]

I agree with you on everything except there was no emissions testing except maybe for import purposes.

The EPA was expanding their purview toward motorcycles as part of the empire building/bureaucracy expansion.
Honda, at least, had to show them test data that the configurations sold in the USA met criteria set for import allowance.
I'm certain Honda did emissions testing, at least during development.  If for no other reason than insuring production samples would pass any "spot checks" made by the EPA later on.  Certainly they did testing to know for themselves what the machine put out the tail pipe.

As an agency, all the EPA had to do was issue a number to meet; x PPM hydrocarbons at idle.   And, then demand that the company applying for import approval supply all test data and test methodology used to create the test data.  Certainly, exhaust sniffer test equipment was readily available to verify manufacturer data fairly easily.  If it does not, then customs is notified to impound any further imports.   That usually gets a response out of the shipper/dealer network.  I'm sure Honda did what they needed to avoid that, and yielded to EPA pressure/authority.

[brewsky]

Well, you gotta give the Honda advertising people credit for trying to turn it into a positive!
Check out the "lean jetted" blurb circled on this brochure!

[bollingball]

Well, you gotta give the Honda advertising people credit for trying to turn it into a positive!
Check out the "lean jetted" blurb circled on this brochure!

brewsky I love that add. Got any more or some links to them? I wonder if it is a 77 or 78. Oh I see a 78K. Damn I would love to have a poster of that.

Ken

[brewsky]

Well, you gotta give the Honda advertising people credit for trying to turn it into a positive!
Check out the "lean jetted" blurb circled on this brochure!

brewsky I love that add. Got any more or some links to them? I wonder if it is a 77 or 78. Oh I see a 78K. Damn I would love to have a poster of that.

Ken

I found it lurking in the back pages of a digital 750K8/F3 Shop Manual.
May have gotten it from this site....don't remember....will try to find the source....if not, maybe I can figure out how to send it....it's 35,000 KB though.

[bollingball]

Well, you gotta give the Honda advertising people credit for trying to turn it into a positive!
Check out the "lean jetted" blurb circled on this brochure!

brewsky I love that add. Got any more or some links to them? I wonder if it is a 77 or 78. Oh I see a 78K. Damn I would love to have a poster of that.

Ken

I found it lurking in the back pages of a digital 750K8/F3 Shop Manual.
May have gotten it from this site....don't remember....will try to find the source....if not, maybe I can figure out how to send it....it's 35,000 KB though.

Thanks Don't worry about the size. If you find it you can pm it to me at the highest resolution you can or a link and I will do it. Maybe then I can blow it up with out it looking to bad
                                                   Thanks in advance
                                                            Ken

[lucky]

OK...back to the main thread focus.

CASO's BIKE:
Has figured his bike jetting out. Good result.
Set up:

A 1978 CB750K.
Bike is stock besides a 4-1 open header. No baffle no nothing. OPEN.
#40 slow jet
#120 main jet
Factory needle settings.
Mixture screws 1 turn out
Accelerator pump and nozzles working.
Clean fuel filter.

LUCKY' BIKE

A 1978 CB750K.  Looks like a good result went from #42 idle jet to a #40.
Set up:

Bike is stock besides a 4 into 2 into 1 with internal metal baffle.
Metal internal baffle from Cycle Shack.
**EDIT #42 It just worked better.
#120 main jet
Factory needle setting.non adjustable.
Mixture screws EDIT 3/4 turns out
Accelerator pump and nozzles working.
Clean fuel filter.


Road tested and it has a smooth transition of throttle.
Runs strong through the gears.



IMAGO's BIKE:
He is working on other issues than his jetting right now.
Same year and basic set up.

A 1977 CB750K.
Exhaust 4 into 1 MAC with MAC muffler.
#35 slow jet
#120 main jet
Factory needle settings. 1.5???
Mixture screws?
Accelerator pump and nozzles working?( working on it.)

From the video on another thread (How do these plugs look?) it shows
poor throttle response from idle. I predict he will be going to #40 jet also-LUCKY

[lucky]

Sorry ..I did not get to your comment right away.

Quote: ( FROM Blitzburgh207)

"I run pods and open 4-2-1, which I'm sure have a far greater effect on 1/4 - 3/4 throttle, but I was just saying I'm suprised he would have to increase his slows that much but did not have to raise his needles. - but I also see he's back to stock..."

IMAGO's bike is not stock ,it has a performance exhaust.
See his set up stats on this page above. Mac 4 into 1 with Mac muffler.

[lucky]

These three bikes I have been working to follow, with exacting standards should help all forum members with 1977-1978 CB750's jetting issues.

Three bikes all stock except the exhaust.
Each has slight variations.

I hope you appreciate our efforts-LUCKY

Three bike owners contributed to this jetting investigation.
IMAGO (almost finished)
CASO...Finished
LUCKY...Finished

[Tugboat]

Thanks y'all - this is good stuff.

[lucky]

Thanks y'all - this is good stuff.

Thanks TUGBOAT!!

If the search feature works on this site and a newbie reads this he will have struck gold.
Of course Google search would find this page. "CB750 idle jet"

[Blitzburgh207]

Sorry ..I did not get to your comment right away.

Quote: ( FROM Blitzburgh207)

"I run pods and open 4-2-1, which I'm sure have a far greater effect on 1/4 - 3/4 throttle, but I was just saying I'm suprised he would have to increase his slows that much but did not have to raise his needles. - but I also see he's back to stock..."

IMAGO's bike is not stock ,it has a performance exhaust.
See his set up stats on this page above. Mac 4 into 1 with Mac muffler.

Yeah I need to re-read what I write on here... I meant I see he's back to stock slow jet size, I know the exhaust is not stock.

Good luck all! Plug chops are your best friend! 

[Imago]

Success! Here's what I've done so far...

Stock bike, 1978 CB750F except for MAC 4 into 1 exhaust.

#35 slow jets and #105 main jets (which surprised me... I thought it would be larger).

I removed and cleaned the carbs again this morning paying particular attention to the "mickey mouse" ears on the accelerator pump diaphram. A small drill bit twisted through both ear holes seems to have done the trick. The one that passes gas was nearly swollen shut. Thanks "Two Tired" for that terrific advice.

I also ensured the tiny springs and balls held in place by tiny brass blocks were free, clear and passing gas only in the proper direction, (don't ask how I tested this). I didn't have to remove the tiny brass blocks, (thank god). They seemed like a bear to remove so once I assured myself they were working as designed, I left them alone.

I set all 4 mixture screws at exactly 1.5 turns from seated.

Rode to the local "Show and Shine" at the Snack Shack's summer opening, had some ice cream, (now you know why) and listened to a great old time band in the park. Lots of interest in these old bikes, some thought I was, or should be, in the show!

I'll post a short video in my other thread to show my progress...

[lucky]

What was the other thread title???

The Snack Shack event sounded like fun.
Tell us how the bike runs too.

[Imago]

What was the other thread title???

The Snack Shack event sounded like fun.
Tell us how the bike runs too.

Bike runs pretty well, but not perfectly yet. No bogging off the line but maybe a bit less acceleration or hard pulling after 3000 RPM than before, I need to ride it more to be sure. So far I'm pretty thankful for your advice, and others.

My other thread is: How Do These Plugs Look? New Movie - Much Better Throttle Response Off Idle!

[Kev Nemo]

just found this site if it helps for slow and mains jets

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750_1969-1971.html

http://www.motorcyclecarbs.com/Keihin_Pilot_Jet_N424-24_C92.cfm
peace

These will not work on 77-78 750. They do have good shim washers. Sirius is about the only place for the press ins.
Ken

They have the press-in slows on that site but they're $7.99 per jet- too rich for my blood (*rimshot)

[Kev Nemo]

Man- Sirius moved the back order date to the 29th = /
http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1109

Any other comparable sources?

[Tugboat]

^Bummer! I'm on that back-order list.

BUT, I found the original accelerator-pump-circuit hoses with the restrictors in a box last weekend!! Guess I saved them when I did the rebuild a few years ago. SQUIRT SQUIRT!

[Kev Nemo]

Awesome- I heard not having the restrictors will give you a headache trying to synch

[flybox1]

Awesome- I heard not having the restrictors will give you a headache trying to synch

No.  they just improve the function of your accel pump by decreasing the volume in the connector tubes between carbs. 
they make your squirters, squirt better 
once i got my brass tubing, it was maybe 10 minutes to make them, and 5 minutes to put them in.

[Tugboat]

@Flybox - do you notice a difference in throttle response, based on that one change?

[flybox1]

definitely!  its instant go.  Starts are less labored, too. my #4 squirter was kinda weak, so, when i bench tested before and after restrictors were installed, i had a noticeable increase in squirt.  force and volume. especially in #4, the farthest away from the pump diaphragm.

they are the little blue pills for your accel pump 

[Tugboat]

Hell yes!! OK, so when I get my #42's in I'll pop these on as well and hopefully, as they say, that will be that

Now to fix that head gasket leak......

[lucky]

Awesome- I heard not having the restrictors will give you a headache trying to synch

Where do these rumors start???

I ran my 1978 carbs for months not knowing about the restrictors until Brewsky told me about them. I installed them and noticed very little difference, but I am sure they sprayed a little better.

Also this RUMOR makes no sense because when you sync the carbs the engine is idling. The accelerator pump spray nozzles are not even working!


Nonsense rumors.