Topic: CB350F Jetting Help for Mid RPM Stumble

[Mikerts_Garage]

I know what most of you are thinking right now "here we go again, another SOHC jetting thread". But here me out for a minute because I will show you I have done my requisite research. Hopefully this thread can be referenced to some of the other people who seemingly have this problem.

First, a description of the running condition and the problem,

Bike starts up no problem cold(34 degrees F) with choke. Idle is rock solid at 1200 rpm when warm. Off idle performance is exactly what you'd expect from a small displacement 4 cylinder with round slide carbs. Bike pulls hard through the mid range and very happily all the way to redline(new main jets really woke this bike up above 7k). Cruising at any RPM is very smooth, this bike is such a sewing machine! Now for the problem, when cruising at a constant speed everything is great, until you get on the throttle to accelerate. Upon opening the throttle the bike responds by breaking up slightly for about ¼ second and then clearing up and acting normal. I can only describe this behavior as "choking on your own spit" as that's what the bike feels like it's doing. It exhibits this behavior anywhere from 3k-7k rpm, under that I'm at a stop light, and over that the bike is always happy. I also don't find it does it when accelerating from a stop and rowing through all the gears, only when cruising or decelerating and getting back on the throttle(very annoying coming out of corners).

Second, some background on the current condition of the bike:

It has ~8k original miles.

Lives in SLC, UT, ~ 4300ft elevation.

Stock 4-4 exhausts were shot when I bought it, I welded them back together where the mufflers meet the headers. If I had to guess, Exhausts 1 and 4 probably flow slightly more than 2 and 3 due to being more rusted out.

I replaced the rubber intake boots going to the engine as well as the O-rings that go with them. I also replaced the rubber boots going from the air box to the carbs.

Stock air box, filter, and snorkel all in good condition.

PO clearly had the carbs cleaned/ vapor blasted recently, they are immaculately clean.

Carb body 1 was replaced at some point with a number 3 carb body from another bike. To achieve this they swapped the throttle pivot cap onto the outside hole and epoxied the gas port feed through. They also used a cut off wheel to get the slide to work , but I ordered the correct slide and replaced it.

Brass looks original, but it's hard to know for sure, at least the idle jet is stamped Kehin.

Main jets have been sized down to #70's to correct for altitude.

Floats set at 22mm not the standard 21mm, this was done using calipers, not the clear tube method.

All vent hoses and drainage lines are in place.

Point, condensers, and ignition coils are all oem (I'm pretty sure they are original).

Points gap was set to 0.014", ignition timing was set with a timing light, and ignition advance showed to be fully working.

Fresh NGK D8EA plugs.

All spark plug wires trimmed back 1cm and boots rescrewed in with care to make good contact with the copper.

Valves all set to proper spec about 50 miles ago, timing chain adjusted as per the manual.

Carbs synced using blue chinese carb sync, I got the cylinders all within 50 mmhg of each other, all of them were sitting at roughly 200 mmhg vacuum when synced, I don't want to dial it in further until I have the current issue figured out.

Idle set screws at 1&¼ out(highest idle), very happy starting hot.

Engine has never blown blue smoke as far as I know. I have not checked the compression yet though

Thirdly, an account of settings tried so far.

1. I replaced the stock main jets(75) for 70's and set the slide needle position to second closest to the needle end(pos #2?). Before the needle was at the stock position third from the needle end(pos #3?). This really helped high rpm performance, I'm assuming due to the main jet, but the stumbling problem was worse than ever.

2. I tried moving the slide needle position to fourth closest to the needle end(pos #4?). This was the best performance I've seen yet, much less stumbling than stock and my first configuration. but still present.

3. I tried the furthest, leanest needle position, fifth from the needle end(pos #5?). This made the bike very unhappy all over, rough cruising, bad acceleration, hard to get moving, ect.

4. I tried using a small washer to set the needles at a position half way between #4 and #3. This has been working okay, but maybe not as good at position number 4.

As a side not I re-synced the carbs after every adjustment as disturbing the throttle linkage to get the needles in and our always threw it out of whack slightly.

Fourthly, a checklist of things from other forum posts and speculation. I have tried to pull from the threads that best match my issue:

http://forums.sohc4.net/index.php/topic,187566.msg2175240.html#msg2175240
Suggestion: set float height using clear tube method.
Thoughts: I can do this, but I'm not sure how to measure the gas in the tube. Do you measure the height of the gas against the bottom flange of the carb body? Kind of like you would with the bottom of the floats?

Doesn't seem like this issue got resolved.

http://forums.sohc4.net/index.php/topic,182703.msg2114706.html#msg2114706
Suggestion: Make sure carb vent hoses are intact and running to a place with calm air.
Thoughts: Hoses look original, but to be in good shape. I have them running just behind the engine terminating slightly above the swing arm pivot. I think this is good enough?

Suggestion: make emulsifier holes slightly bigger to adjust for modern gas.
Thoughts: I could try doing this with some guitar string, I'm hesitant though because I feel like my problem might be too rich not to lean.

Suggestion: Try lower octane gas.
Thoughts: I would like to try this as I'm curious, however i just filled up with 91 a few days ago so it might take me a second.

Doesn't seem like this issue got resolved.

http://forums.sohc4.net/index.php/topic,189761.50.html
Suggestion: Switch to Denso 4099 Spark Plug for hotter spark.
Thoughts: Is this worth it? I've always stuck with NGK.

Doesn't seem like this issue got resolved.

Finally, speculation:

I'm inclined to think the next thing to try is adjusting the float height a bit higher. I've had decent success doing this on my CB550. However, the floats are already "statically" set at 22mm. I have a hunch that floats get less "floaty" with age, and needle springs get a little less springy, but I don't have any real evidence to back that up.

How important do y'all find it to do the "clear tube" method? How would I go about it?

How high of float height would be too high? I don't want to lean this thing out at high rpms.

I've also heard tales of people re-shaping slide needles or even slides themselves to fix fueling issues, but that sounds terrifying and I've always had good luck with stock needle shapes.

Please, If you have any other Ideas, or if you've encountered this yourself, please let me know.

Thank you for reading this very long post.

[Mikerts_Garage]

Quick update after commuting to work on the current set up.

I tried turning the fuel tap off while riding; after about a minute the behavior did seem to improve. But just after the bike was about to die.

I also tried pulling over and taking off the air box cover(Man does this bike make some intake howl!). This also seemed to improve behavior, but the problem doesn't totally go away.

[Don R]

 Just a thought, my gl1000 hates high octane gas, especially 91 octane Shell.  It spits back and bogs, If I use half of it and top off with no alcohol 87 octane and it gets happy again.
 Last year, I stored it with half low lead 110 aviation gas and half no alky 87 just because the AV gas doesn't spoil, it liked that mix pretty well too.
 

[Mikerts_Garage]

Just a thought, my gl1000 hates high octane gas, especially 91 octane Shell.  It spits back and bogs, If I use half of it and top off with no alcohol 87 octane and it gets happy again.
 Last year, I stored it with half low lead 110 aviation gas and half no alky 87 just because the AV gas doesn't spoil, it liked that mix pretty well too.

This seems like the easiest thing to try for the time being. The lowest octane ethanol free around me is 88, so I might try some 85 in green slime flavor and see what happens.

Also I love "no kill motorcycle shop" sometimes I feel that way when the bike is more rust than steel lol.

[carnivorous chicken]

I am a fan of the clear tube method, and although someone here once said it doesn't work for 350Fs, it certainly does. I've done it a few times.

You've seen the thread on it so you know the drill (no pun intended) -- extra drain screw, drilled out, small brass tube JB welded, and clear tube. But to answer your question, when you turn the gas on the carb with the clear tube will fill and fill the tube (the end of which you're now holding vertically next to the carb). When the carb fills and the float valve closes, you will see where the fuel level is for the carb -- theoretically just below where the float bowl seals.

Replacing that #1/#3 carb body with a #1 might help as well as it seems there's been a bit of fudging with that. If you've got actual holes in the exhaust still despite the welding that's another issue that's not gonna help.

Others may have a different opinion, but I'm always good with NGK plugs and if you have the stock brass I wouldn't mess with it.

[Mikerts_Garage]

I am a fan of the clear tube method, and although someone here once said it doesn't work for 350Fs, it certainly does. I've done it a few times.

You've seen the thread on it so you know the drill (no pun intended) -- extra drain screw, drilled out, small brass tube JB welded, and clear tube. But to answer your question, when you turn the gas on the carb with the clear tube will fill and fill the tube (the end of which you're now holding vertically next to the carb). When the carb fills and the float valve closes, you will see where the fuel level is for the carb -- theoretically just below where the float bowl seals.

Replacing that #1/#3 carb body with a #1 might help as well as it seems there's been a bit of fudging with that. If you've got actual holes in the exhaust still despite the welding that's another issue that's not gonna help.

Others may have a different opinion, but I'm always good with NGK plugs and if you have the stock brass I wouldn't mess with it.

Thanks for the advice, It turns out one of the fittings for my carb sync actually fits the thread of the bowl drain screw(how convenient!). Should I be aiming for a specific fuel level? right now when I measure the level it comes up to ~8mm below the bottom flange of the carb body(top of carb bowl where, or where the two meet to be specific). Or is it more important to just ensure all the levels for all the carbs are the same? I haven't played with it much yet, just had the thought that my carb sync fitting might work and it did.

I'm also a bit skeptical on the carb body situation, my guess is they replaced it because the other 3 are badly pitted and number one was probably completely shot. So far it hasn't given me any issues besides that stupid slide situation. It syncs up just like the others, and the adjuster is in mostly the same spot. That being said, if someone has a #1 carb body laying around they'd want to send me, I wouldn't be apposed.

I try not to wander away from stock brass if I don't have to. I'm certain the idle jets and needles are genuine, the main jets are from sigma 6 jet(I've had good experiences with them in the past). The emulation tubes on the other hand I'm not certain about.

[Mark1976]

   Start with the basics, get a good rack, the mix and match thing going on doesn't help (barring that, it is what it is).
   Make sure you've got the right pilot/main sizes (35 pilot 75 main) and preferably keihin brass, with good snug fitting o-rings. Ya need a good starting point, those are the original sizes.
   Make sure you have oem needles, using the stock clip location, 3rd from the top/middle?. The aftermarket needles aren't even good for sewing with, junk...
   Do set the floats to 21and clear tube on all 4 carbs and get the level even across the 4 of them. That 8mm number you reference should work, I like it a smidge higher...
   Reset the pilot screws to what the manual recommends, 7/8-3/8, your at over one turn out (lean) and at 70 on the main (lean) with what maybe a lower than recommended float levels (lean).
  Get it good and warmed up and resync them.
   Take it for a ride, report back, what does it do cold, warm, hot...
   Pull a plug, pull all of them if ya want, what do they look like...
From what I can tell regardless of your altitude, its running lean, that mid throttle stumble is a general indication your needles aren't correct, but you have so many non standard variables, mains, needle hght (needles), float hght, its difficult to tell. Go back to stock, clear tube it, resync it and see what you have, go from there
A good reference would be hondaman (mark) pretty sure he lives at a bit of altitude...
Just a suggestion... your bike...
   

[HondaMan]

The symptom you are describing suggests slightly low-float-bowl issues. The CB500/4 is sensitive to this same thing, and on those bikes it shows up in spades when the float valves were changed, especially with the new ones being the "modern" versions with the stiffer-than-OEM springs. This then causes the float bowls to run low, even with the same [mm] setting number. In the end of that [prolonged] troubleshooting, I found that setting the floats 1mm DEEPER than OEM with the stiffer springs in the valves then made the bike run much better, without the stumble from throttle roll-on. This has proven to cure many of the CB750 roundtop carbs with new float valves, too. Could yours also have the stiffer aftermarket float-valve springs? I ask because the OEM versions have such soft springs that one wonders if they were supposed to be THAT soft, and all of the replacement valves I've used had stiffer ones. So, the float bowl got set deeper to prevent hesitation in the throttle. It's a close call in the 350F/400F carbs, though,as there isn't a lot of vertical space before the fuel is sitting right against the float bowl gasket. Then they will always weep gas [very] slowly around the bowl's perimeter - if they do run better for it.

What happens is: the engine can run OK with slightly low float bowls as a 'lean burn' engine, but it settles into a cadence that is consistent with the lean-burn combustion. Then when the throttle is advanced, the needle is essentially in the "wrong spot of the taper" for that airflow, and the engine gets disturbed in its cadence from an increased airflow that is also short of fuel. It stumbles while it recovers from the too-lean change: carbs without a method of adding fuel at throttle increases (like accelerator pumps for lean-burn setups) will usually stumble as they re-settle into the lean-burn cadence. This cadence is why lean-burn setups always feel "weaker" than the older arrangements that basically ran on the rich side a little bit, like the CB750K0-K6 & F0/1 engines.

The CB650 SOHC4 (1779 and 1980 version with those miserable lean-burn carbs) does exactly the same thing, and for the same reasons. It has an air bypass valve too, to stop decel popping, but when it leaks it runs like a much-worse version of what you are describing.

[Mikerts_Garage]

   Start with the basics, get a good rack, the mix and match thing going on doesn't help (barring that, it is what it is).
   Make sure you've got the right pilot/main sizes (35 pilot 75 main) and preferably keihin brass, with good snug fitting o-rings. Ya need a good starting point, those are the original sizes.
   Make sure you have oem needles, using the stock clip location, 3rd from the top/middle?. The aftermarket needles aren't even good for sewing with, junk...
   Do set the floats to 21and clear tube on all 4 carbs and get the level even across the 4 of them. That 8mm number you reference should work, I like it a smidge higher...
   Reset the pilot screws to what the manual recommends, 7/8-3/8, your at over one turn out (lean) and at 70 on the main (lean) with what maybe a lower than recommended float levels (lean).
  Get it good and warmed up and resync them.
   Take it for a ride, report back, what does it do cold, warm, hot...
   Pull a plug, pull all of them if ya want, what do they look like...
From what I can tell regardless of your altitude, its running lean, that mid throttle stumble is a general indication your needles aren't correct, but you have so many non standard variables, mains, needle hght (needles), float hght, its difficult to tell. Go back to stock, clear tube it, resync it and see what you have, go from there
A good reference would be hondaman (mark) pretty sure he lives at a bit of altitude...
Just a suggestion... your bike...
 

Thanks Mark! Starting from stock is always my first move as well. When I got the bike i set it to completely stock settings with new plugs. It always exhibited the stumbling, and got worse when warmed up which made me think rich. When checking the plugs after 500 miles they were very black, gas mileage was terrible as well, only about 30mpg.

Thank you for confirming the 8mm number, that helps a ton, makes me not feel totally crazy 

I can't seem to find the recommended idle air screw settings in the factory manual, I can look in the clymer manual tomorrow. 1 and 1/4 is just fastest idle plus 1/4 turn in.

I will definitely report on plug color after a decent ride.

[scottly]

Main jets have been sized down to #70's to correct for altitude.

Replace the stock main jets!

[Mikerts_Garage]

The symptom you are describing suggests slightly low-float-bowl issues. The CB500/4 is sensitive to this same thing, and on those bikes it shows up in spades when the float valves were changed, especially with the new ones being the "modern" versions with the stiffer-than-OEM springs. This then causes the float bowls to run low, even with the same [mm] setting number. In the end of that [prolonged] troubleshooting, I found that setting the floats 1mm DEEPER than OEM with the stiffer springs in the valves then made the bike run much better, without the stumble from throttle roll-on. This has proven to cure many of the CB750 roundtop carbs with new float valves, too. Could yours also have the stiffer aftermarket float-valve springs? I ask because the OEM versions have such soft springs that one wonders if they were supposed to be THAT soft, and all of the replacement valves I've used had stiffer ones. So, the float bowl got set deeper to prevent hesitation in the throttle. It's a close call in the 350F/400F carbs, though,as there isn't a lot of vertical space before the fuel is sitting right against the float bowl gasket. Then they will always weep gas [very] slowly around the bowl's perimeter - if they do run better for it.

What happens is: the engine can run OK with slightly low float bowls as a 'lean burn' engine, but it settles into a cadence that is consistent with the lean-burn combustion. Then when the throttle is advanced, the needle is essentially in the "wrong spot of the taper" for that airflow, and the engine gets disturbed in its cadence from an increased airflow that is also short of fuel. It stumbles while it recovers from the too-lean change: carbs without a method of adding fuel at throttle increases (like accelerator pumps for lean-burn setups) will usually stumble as they re-settle into the lean-burn cadence. This cadence is why lean-burn setups always feel "weaker" than the older arrangements that basically ran on the rich side a little bit, like the CB750K0-K6 & F0/1 engines.

The CB650 SOHC4 (1779 and 1980 version with those miserable lean-burn carbs) does exactly the same thing, and for the same reasons. It has an air bypass valve too, to stop decel popping, but when it leaks it runs like a much-worse version of what you are describing.

This is a great bit of advice, and exactly what makes this forum so great!

If i had to venture a guess, these carbs have the stiffer float needles than stock. definitely not as floppy as I've seen in other Hondas of the period.

I think I will go through and set the floats to 20mm (1mm lower than stock, 2mm lower than it is right now) and double check that the levels are equal using the clear tube method(I will report back what the fluid level is for future reference).

I will also change the slide needles back to the middle position, not the half step leaner they are now.

Also great facts on the CB650! I've had a few carbs with air bypass valves(xl350, CM400, CX500), they always turn to rubber mush after a while.

Fingers crossed I don't get any bowl leakage or overflowing 

[Deltarider]

First: have everything standard and use OEM parts only. I see no need to deviate from how Honda originally delivered the bike.

[...] until you get on the throttle to accelerate. Upon opening the throttle the bike responds by breaking up slightly for about ¼ second and then clearing up and acting normal. [...]

Back in the day the advice was: for a sporty acceleration, first shift down one or two gears.

Idle set screws at 1&¼ out(highest idle), very happy starting hot.

Although aimiing for a highest idle is common practice, it is wrong for our bikes which lack acceleration jets. We have warned in this forum more than often. I see my slow jets as some sort of auxillary jets that happen to double as idle jets. It was a Honda mechanic who informed me that the slow jets also play a role in driveability whilst accelerating. BTW, Honda advised 7/8 of a turn for your model, you have 1&¼... With these airscrews ¹/₈ of a turn can already make a big difference. I can make my CB500 pur like a kitten with the airscrews turned out 2¹/₂ turn out, but the acceleration is then gone. Realise that our carbs are very basic as there were no emission laws back then. Also many don't know the difference in concept between our oldfashioned airscrews and the IMS type adjusters later found on PD carbs. I am quite disappointed that even 'experts' in this forum can have it wrong and that many here, who I know, know better, nonetheless are reluctant to correct them. I consider that an unhealthy situation for this forum and I witness it with sadness...
My advice on checking fuellevels is: keep it simple. First diagnose if something is really the matter. I never had to readjust floats on any CB500/550. So read this: http://forums.sohc4.net/index.php/topic,193811.msg2264903.html#msg2264903 The method is simple and foolproof.
Are you sure the little O-rings are OK and seal the main jets well?

[Herr Supersport]

Your approach is perfect, and all the important points have already been mentioned.
Just a small addition from my experience, particularly regarding the fuel level:

I’ve meticulously set the float/fuel level to 4mm below the seam using the clear tube method for years, always had stumbles and black spark plugs. Smaller or larger jets didn’t make any noticeable difference.

After installing new floats and valves [Honda OEM parts]:
Out-of-the-box 21mm float height: Runs well, but is hard to start after sitting for a long time.
19mm float height: stumbles and black spark plugs.
20mm float height – Hondaman recommendation: Optimal. The engine responds smoothly and spontaneously to throttle, spark plugs are clean and a nice chocolate brown color.
This corresponds to a fuel level of approximately 9mm below the seam and a float bowl volume of 35cc.

One more thought: advancing the ignition by plus 2° might be worth a try.

Yes, the carbs on these small bikes are very sensitive. I’ve never had to adjust float levels on any of my other motorcycles.

[Deltarider]

[...]
One more thought: advancing the ignition by plus 2° might be worth a try.

Yes, the carbs on these small bikes are very sensitive. I’ve never had to adjust float levels on any of my other motorcycles.

Grützi, Her Supersport. I agree a 100%. I run my CB500 with 2° extra advance which results in a more eager respons.
Like you I never had to readjust floats.
For years this float adjusting madness has spread like an epidemic in this forum. The culprit is probably the Clymer manual that presents it as a maintenance thing, which it is not. Itchy hands do the rest and the next owner most likely will have to readjust.
Because there is a considerable risk of damaging the delicate parts involved, I have developed a safe, foolproof method for diagnosis.
You probably run your bike at a higher altitude than us in the low countries at the sea.
Tchüss.

[Herr Supersport]

 
Dutch mountains may be flat - I live at 300m, elevation on my rides is about 900m max.
Cheers!

[Deltarider]

Holland has one 'mountain' only - a shocking 322,4 meters high - which we share with the neighbouring countries: Belgium and Germany.

[Mikerts_Garage]

First: have everything standard and use OEM parts only. I see no need to deviate from how Honda originally delivered the bike.

[...] until you get on the throttle to accelerate. Upon opening the throttle the bike responds by breaking up slightly for about ¼ second and then clearing up and acting normal. [...]

Back in the day the advice was: for a sporty acceleration, first shift down one or two gears.

Idle set screws at 1&¼ out(highest idle), very happy starting hot.

Although aimiing for a highest idle is common practice, it is wrong for our bikes which lack acceleration jets. We have warned in this forum more than often. I see my slow jets as some sort of auxillary jets that happen to double as idle jets. It was a Honda mechanic who informed me that the slow jets also play a role in driveability whilst accelerating. BTW, Honda advised 7/8 of a turn for your model, you have 1&¼... With these airscrews ¹/₈ of a turn can already make a big difference. I can make my CB500 pur like a kitten with the airscrews turned out 2¹/₂ turn out, but the acceleration is then gone. Realise that our carbs are very basic as there were no emission laws back then. Also many don't know the difference in concept between our oldfashioned airscrews and the IMS type adjusters later found on PD carbs. I am quite disappointed that even 'experts' in this forum can have it wrong and that many here, who I know, know better, nonetheless are reluctant to correct them. I consider that an unhealthy situation for this forum and I witness it with sadness...
My advice on checking fuellevels is: keep it simple. First diagnose if something is really the matter. I never had to readjust floats on any CB500/550. So read this: http://forums.sohc4.net/index.php/topic,193811.msg2264903.html#msg2264903 The method is simple and foolproof.
Are you sure the little O-rings are OK and seal the main jets well?

I stick with stock brass whenever possible. With stock jetting, this bike was running very rich, jet black plugs, terrible fuel economy ect. I'm not confident #70 main jets will be the right call, but on my CB550F I had great success moving from #98 to #90. Most of what I ride is canyons going up to 10,000ft elevation. I will check the color of the plugs and make the call in a few weeks.

Down shifting on these little 4's is essential for sure, but it can be hard when blipping the throttle to rev match causes the engine to fall flat on it's face lol.

Good advice on the idle settings. I've only ever heard the "turn them out to fastest idle, then turn back in 1/4 turn." I will set the screws accordingly next time I'm out. I'm also unfamiliar with the differences between " oldfashioned airscrews and the IMS type adjusters later found on PD carbs".

Thank you for the specs on how much gas should be in each gas bowl, I feel like I have so many options to really dial in the floats now!

O-rings on the main jets are brand new, I've run into that problem before on my 550 as well.

[Mikerts_Garage]

Your approach is perfect, and all the important points have already been mentioned.
Just a small addition from my experience, particularly regarding the fuel level:

I’ve meticulously set the float/fuel level to 4mm below the seam using the clear tube method for years, always had stumbles and black spark plugs. Smaller or larger jets didn’t make any noticeable difference.

After installing new floats and valves [Honda OEM parts]:
Out-of-the-box 21mm float height: Runs well, but is hard to start after sitting for a long time.
19mm float height: stumbles and black spark plugs.
20mm float height – Hondaman recommendation: Optimal. The engine responds smoothly and spontaneously to throttle, spark plugs are clean and a nice chocolate brown color.
This corresponds to a fuel level of approximately 9mm below the seam and a float bowl volume of 35cc.

One more thought: advancing the ignition by plus 2° might be worth a try.

Yes, the carbs on these small bikes are very sensitive. I’ve never had to adjust float levels on any of my other motorcycles.

I can not thank you enough for this hyper specific fuel and float information. I will be singing your praises when I only have to take on the carbs and futz around with them one more time!(hopefully)

Good advice on the ignition timing. I've had good luck with a little more advance on my 71' CT90, I've never thought to try it with other bikes.

[carnivorous chicken]

That being said, if someone has a #1 carb body laying around they'd want to send me, I wouldn't be apposed.

I've got a couple 350F carb bodies but not sure if there's a #1, and... I wont' be back in AZ until April or May, where they're sitting. but if you still need one then holler.

[Kevin]

I have nothing to add other than to say that I sure wish this forum had a "Like" button.
So much great information and so well written.
Thank you to everyone who took the time to reply and to Mikerts_garage for the well written question and follow-ups.

I will be watching to see what the final resolution is.
Best of luck!


Sent from my SM-S908U1 using Tapatalk

[Deltarider]

Here is the best video:

From 16:00 on the verifying of fuel levels is demonstrated by Marco. His floats were at 21mm, so NO NEED to adjust. If you prefer the clear tube check (I don't) fuel level should be 3-5 mm below the flange. The drained quantity of each CB350F/400F bowl should be 46-48 cc. Video is in German but the images speak for themselves.
courtesy Marco

[Mikerts_Garage]

That being said, if someone has a #1 carb body laying around they'd want to send me, I wouldn't be apposed.

I've got a couple 350F carb bodies but not sure if there's a #1, and... I wont' be back in AZ until April or May, where they're sitting. but if you still need one then holler.

I will probably take you up on this come April or May. Until then, I think the bodged together carb is working okay. I had another(worse) CB350F just before this one that I sold. I regret not taking the carb #1 off that before selling!

I have nothing to add other than to say that I sure wish this forum had a "Like" button.
So much great information and so well written.
Thank you to everyone who took the time to reply and to Mikerts_garage for the well written question and follow-ups.

I will be watching to see what the final resolution is.
Best of luck!


Sent from my SM-S908U1 using Tapatalk

I appreciate this sentiment, I think it's important we all remember that these posts not only help the OP(me), but also hundreds or thousands of people in the future who experience similar problems. I was sort of bummed when I found the previous posts about this issue unresolved, I will do my absolute best to get to the bottom of it

Here is the best video:

From 16:00 on the verifying of fuel levels is demonstrated by Marco. His floats were at 21mm, so NO NEED to adjust. If you prefer the clear tube check (I don't) fuel level should be 3-5 mm below the flange. The drained quantity of each CB350F/400F bowl should be 46-48 cc. Video is in German but the images speak for themselves.
courtesy Marco

Great video! Definitely way more gas in those carbs than what I saw in mine. Since I will have the carbs out on the bench anyway, I will do both the clear tube method and the bowl capacity method to be doubly sure.


Sadly it dumped a good bit of snow here this weekend thwarting my plans to dig into this bike(my shop is more of a shack ). College starts up for me this next week; progress will have to wait until next weekend if the weather permits.

Thanks again y'all for all the advice!

[HondaMan]

When all is said and done and working right, you'll be an expert as concerns the early SOHC4 type carbs (before the PD emission-control versions appeared). The 350F is THE pickiest, trickiest, snotty-to-the-owner carbs I've ever seen on  any Honda!

The other things on this particular bike to "sort out" once the carbs are close-to-right, and which might confuse one as to the cause of the symptoms, is the springs in the spark advancer: if the engine has more than 8k miles on it, they have softened too much from heat-annealing. When this is added to our modern gasolines that burn FAR slower than the ones, it makes these smallbore bikes tend to "hang" in high idle during deceleration maneuvers and in city riding on hot days. The "fix" is to increase the tension on one (or both) of the spark advancer springs. It will act just like the carbs are overly rich following decel, but it's not the carbs. It's the advancer.

I find this on 100% of the 350F and 400F (not to mention their larger brothers).

[Mikerts_Garage]

When all is said and done and working right, you'll be an expert as concerns the early SOHC4 type carbs (before the PD emission-control versions appeared). The 350F is THE pickiest, trickiest, snotty-to-the-owner carbs I've ever seen on  any Honda!

The other things on this particular bike to "sort out" once the carbs are close-to-right, and which might confuse one as to the cause of the symptoms, is the springs in the spark advancer: if the engine has more than 8k miles on it, they have softened too much from heat-annealing. When this is added to our modern gasolines that burn FAR slower than the ones, it makes these smallbore bikes tend to "hang" in high idle during deceleration maneuvers and in city riding on hot days. The "fix" is to increase the tension on one (or both) of the spark advancer springs. It will act just like the carbs are overly rich following decel, but it's not the carbs. It's the advancer.

I find this on 100% of the 350F and 400F (not to mention their larger brothers).

There's not really anything going for this bike when it comes to ease of tuning lol. Just about anything would make it less fussy: less cylinders, more displacement, electronic ignition, shim in bucket valves, and of course different carbs. But isn't it fun when they play hard to get? 

Is there a good place to buy the correct tension springs? I assume having the wrong spring constant either way ruin the advance curve. Seeing as i just rolled over 8k a month ago i better get on that!

[HondaMan]

When all is said and done and working right, you'll be an expert as concerns the early SOHC4 type carbs (before the PD emission-control versions appeared). The 350F is THE pickiest, trickiest, snotty-to-the-owner carbs I've ever seen on  any Honda!

The other things on this particular bike to "sort out" once the carbs are close-to-right, and which might confuse one as to the cause of the symptoms, is the springs in the spark advancer: if the engine has more than 8k miles on it, they have softened too much from heat-annealing. When this is added to our modern gasolines that burn FAR slower than the ones, it makes these smallbore bikes tend to "hang" in high idle during deceleration maneuvers and in city riding on hot days. The "fix" is to increase the tension on one (or both) of the spark advancer springs. It will act just like the carbs are overly rich following decel, but it's not the carbs. It's the advancer.

I find this on 100% of the 350F and 400F (not to mention their larger brothers).

There's not really anything going for this bike when it comes to ease of tuning lol. Just about anything would make it less fussy: less cylinders, more displacement, electronic ignition, shim in bucket valves, and of course different carbs. But isn't it fun when they play hard to get? 

Is there a good place to buy the correct tension springs? I assume having the wrong spring constant either way ruin the advance curve. Seeing as i just rolled over 8k a month ago i better get on that!

Every now and then I see an OEM spark advancer (-333- partnumber) that is NOS. That's the only way to get the OEM springs now. But, cutting back 1/2 turn at a time (and then reassembling, re-riding, re-doing it all again...) is how I approach them - and their bigger brothers.

One thing that helps speed it along: I punch-mark 3 places around the edge of the points plate and its 3 support bosses on the engine so it is quicker to reinstall and realign. If you're lucky enough to have an OEM TEC or Hitachi plate, they fit nice and tight, too.