Topic: "Best" carbs for CB750

[fxef79]

OK... I completely understand using the word "best" is a touchstone for danger... but let me explain:

I'm curious what opinions are as to the "best" choice for a set of carbs for my CB750 trike.  I'm not necessarily in the market right now... I'll have to live with what I have for now which is a set of dual Mikuni VM34s on cycleX manifolds... but one day I'd like to consider simplifying my life... and so I'd like to know what my options are so that when I can be in the market later I'll know what to watch for.

By "Best", it should tick these boxes:

• Reliability.
• Availability of parts.
• Ease of setup/maintenance/tuning.
• Preferred to be a standard set of SOHC CB750 carbs, regardless of year, so that I can apply knowledge from this forum and other CB750 resources to maintain.  That's the fundamental issue I have with the current setup - nothing learned from all of you experts applies to my carb setup, so it's a big unknown.  I don't want to invent the solution myself... I'd rather use a recipe that's proven.
• Reasonable cost.  Frankly, preferably the least expensive option that remains reliable and functional without giving up too much quality.

What I do NOT need:

• "Best" power/performance.  This isn't a race bike.
• "Best" OEM match for the bike.  This isn't an OEM *anything* bike... it's a frankenstein engine with what appears to be K8 head on K2 cases, with unknown (as of yet) cylinders.  I'm not looking for "Honda correct".
• "Best" fit for an OEM bike.  It's already custom throttle cable, and not a stock frame or gas/oil tank, so I don't need something that needs to match up with those things.
  Expensive.  I just spit my coffee out a few minutes ago when I saw someone selling a $1000 rack of carbs.  Ain't no way.

This is a trike I expect my wife to be able to ride, and not have to "fiddle" with carb settings every time we want to ride.

I appreciate all opinions!  (even those who disagree)...

Oh... fair warning... I have no idea what some of the "OEM" references you guys might make mean, yet... I don't know the difference yet between a "PDxx" carb and a "whatever" carb.  I'll learn... but be gentle.

[Stev-o]

Hey fx...I find the "stock" Keihin carbs to be very reliable, inexpensive, fairly easy to set up and will offer years of trouble free use as long as you dont let ethanol fuel sit in them for months without starting the motor. You should be able to source a set for around $100, but at that price will need rebuilding.

Keihin "PD" carbs would have come on your K8 head, these offer a fast idle option for cold starts and can have a handlebar mounted manual choke, might be a nice feature for your partner.  However, seems some guys prefer the older "round top" carbs, everyone will have their own opinion on which is "best".

Thats a start, I'm sure others will chime in.

BTW - I'm sure you could sell your Mikuni carb setup, a lot of guys feel that is a higher performing option once set up properly. 

Good luck

PS. Got a pic of the bike?

[HondaMan]

I'm supposing that you will want the superior low-end torque arrangement, running a trike...?
In that regard, the early "round top" carbs have superior mixing and tuning abilities as compared to the later PD series, emission-controlled, carbs. Where the PD carbs can shine (comparatively) is in the upper 1/3 of the throttle range, as they mix a little more evenly up there than the round-top versions will. Interestingly enough, though, and as proven by the smaller CB400 and CB350-4, the upper RPM will mix even better in the roundtops than the PD carbs IF (and only if) the intake airbox system delivers very quiet air to the throats of those carbs, and you remove the little wasp-waist in the throats of those carbs. On the trike you may have more room to let this happen? On the roadrace bikes I worked with, this was vital to high-RPM throttle response, but was hard to do with the 750 because the battery was smack-dab in the way, with not much else of a place to move it. In the later SOHC4 bikes Honda moved the battery to where the oil tank is on the 750 so they could make their stellar-performing still-air intake systems. Those systems proved that still air beats velocity stacks or long, smooth intake runners, every time.

[fxef79]

I'm supposing that you will want the superior low-end torque arrangement, running a trike...?

You're an observant man!  Yes, that would be preferable!

In that regard, the early "round top" carbs have superior mixing and tuning abilities as compared to the later PD series, emission-controlled, carbs. Where the PD carbs can shine (comparatively) is in the upper 1/3 of the throttle range, as they mix a little more evenly up there than the round-top versions will. Interestingly enough, though, and as proven by the smaller CB400 and CB350-4, the upper RPM will mix even better in the roundtops than the PD carbs IF (and only if) the intake airbox system delivers very quiet air to the throats of those carbs, and you remove the little wasp-waist in the throats of those carbs. On the trike you may have more room to let this happen? On the roadrace bikes I worked with, this was vital to high-RPM throttle response, but was hard to do with the 750 because the battery was smack-dab in the way, with not much else of a place to move it. In the later SOHC4 bikes Honda moved the battery to where the oil tank is on the 750 so they could make their stellar-performing still-air intake systems. Those systems proved that still air beats velocity stacks or long, smooth intake runners, every time.

This frame is fairly roomy in that respect. See my next post with some pics.

Most likely downside would be lacking any of the standard Honda frame hardware for hanging the airbox.

I'm curious... does this also imply that one of the airboxes from these later SOHC CB400 or CB350 bikes could be used on an early CB750 carb rack ("Round tops"?) with positive effect?

Specifically in this case I don't know how much advantage I could take of this, though... as the implication of what you've said is the main benefit of doing so would be for higher end performance on round-top carbs... and high-RPM driving isn't a particular goal for a trike.  But it's definitely intriguing!

So sounds like your vote is for early model round-top carbs.

Forgive the next dumb question... having found a list of carb models and years, but not knowing how that translates in "round top" carbs:  What are the "round top" carbs?  Does that mean the B750A, 7A, 657A, 657B and 086A carbs?
Because it seems the '77 K carbs were "PD41A" and the '78 K carbs were "PD42B"?  and the '77 F carbs were PD41B and the '78 F carbs were PD42A  (from here: http://www.hondachopper.com/garage/carb_specs/carb_specs.html
  ) .  So if those are "PDxxx" then that means they aren't "round top"?

FWIW I bet you cover some of this in your book... I haven't made it there, yet!  I'm at the head inspection chapter right now.

[754]

Webers would be good if you can find or afford them. Or build a manifold and run a single Weber 2 throat, be good on a trike.
 They got big squirters that you can dial the amount and it really helps..

[fxef79]

Hey fx...I find the "stock" Keihin carbs to be very reliable, inexpensive, fairly easy to set up and will offer years of trouble free use as long as you dont let ethanol fuel sit in them for months without starting the motor. You should be able to source a set for around $100, but at that price will need rebuilding.

I'm not afraid of rebuilding carbs.  Doing it now on these Mikunis.

Keihin "PD" carbs would have come on your K8 head, these offer a fast idle option for cold starts and can have a handlebar mounted manual choke, might be a nice feature for your partner.  However, seems some guys prefer the older "round top" carbs, everyone will have their own opinion on which is "best".

Seems I read that the 77/78 PD carbs also had accelerator pump? I'm more used to carbs with pumps on them... in fact on these Mikunis I really find it hard not to twist the throttle while starting, which completely blocks the enrichener from working.... a habit I got from kickstarting my old Sportster, and has continued with my Shovelhead's Bendix with pump... grabbing a handful of throttle to get it started is kinda required.

Thats a start, I'm sure others will chime in.

BTW - I'm sure you could sell your Mikuni carb setup, a lot of guys feel that is a higher performing option once set up properly. 

Good luck

PS. Got a pic of the bike?

Thanks for the advice!

Here's some pix.

First is pre-cleaning state of one of the Mikuni bowls.  The rest are the bike and shots showing the room available for carbs and/or air box.

[fxef79]

Mikuni bowl nastiness.

[fxef79]

Couple side shots to show the room for carb racks,etc.

**edited to correct rotation of one of the pics.

[fxef79]

running/walkaround.

https://www.youtube.com/watch?v=ngFdpGGCI6M

[fxef79]

Webers would be good if you can find or afford them. Or build a manifold and run a single Weber 2 throat, be good on a trike.
 They got big squatters that you can dial the amount and it really helps..

Uh oh... that sounds dangerously like we're getting out of the "affordable" range. 

[754]

 I will admit, did not read your whole post before I replied.
 77 and 78  carbs have accelerator pump and in my experience I felt the difference, surprised nobody else mentioned it.
 It does however not have the hit like Webers do.. .. i think they may not pass emission tests.
 That said, I have found a few Webers sets cheap,  or at least reasonable. If you luck out. If you can get a manifold, you can always pick up used Webers.
 I always heard single carb Triumphs were sweet at low rpms around town and no synch problems. That is why I suggested if you could build a manifold,  and get a single Weber DCOE, it would be real easy to setup, and maybe feel pretty good.

[slikwilli420]

What's wrong with what you have now? Ive heard good reviews about the mikuni kits especially for just cruising around.

[fxef79]

What's wrong with what you have now? Ive heard good reviews about the mikuni kits especially for just cruising around.

Once I get them all cleaned up and rebuilt - maybe nothing.  Biggest issue right now is I know they're running very rich at idle, so that means I have to start the whole jetting process... starting with pilots, then needle and jet, and then mains...  and that means buying all kinds of sizes of jets, doing plug chops at, at least, 3 different throttle ranges, so a lot of road time and swapping jets around and trial and error.  There's not going to be any good "tried and true" recipe for this setup.

Whereas, if I had a stock CB750 carb rack, I feel like most of the "trial and error" I can bypass by using the collective knowledge found here on the SOHC4 forums. 

Maybe my reasoning is flawed... but that's why we ask questions!

[fxef79]

I will admit, did not read your whole post before I replied.
 77 and 78  carbs have accelerator pump and in my experience I felt the difference, surprised nobody else mentioned it.
 It does however not have the hit like Webers do.. .. i think they may not pass emission tests.
 That said, I have found a few Webers sets cheap,  or at least reasonable. If you luck out. If you can get a manifold, you can always pick up used Webers.
 I always heard single carb Triumphs were sweet at low rpms around town and no synch problems. That is why I suggested if you could build a manifold,  and get a single Weber DCOE, it would be real easy to setup, and maybe feel pretty good.

Love those pipes.

Not worried about emissions tests... Florida doesn't have 'em.  Thank goodness - no way my daughter's old pickup could ever pass!

[fxef79]

So far:
"later PD carbs because they could have remote choke set up, and an easy high-idle setting for cold starts"  (Oh... and accelerator pumps)
"earlier round top carbs because they might have better low-end for a trike" (and with a good airbox and some tweaks might also have good high-rpm mixing)
"Weber and make a manifold"
"keep what ya got"



Of the OEM CB750 carbs, is there a particular year/model that's considered the simplest to maintain reliable operation?

Oh... just realized I should make a point.  I do NOT have the equipment to vacuum sync carbs.  So the "keep what ya got" has a bit of merit, since it's jus t 2 carbs to bench sync but not be able to fully vacuum sync, rather than 4 carbs to be mismatched/out of sync.

[PeWe]

My CB750 K6 stock carbs have never needed any spare parts except for main jets.  (Used 100.000 km)
Air filter and rubber inlets on both sides of the carbs are service parts.

I bought a cheap repair kit from cruzinimage. I only used the top gasket and float bowl gaskets. I should have bought Honda stock. Cruzinimage top rubber gasket is a little bit too large.

Throttle pin rubber gasket replaced by cheap grommets from China
https://www.ebay.com/itm/201599266793
Enough for 25 sets!

Cleaned the carbs that had been clogging old fuel for 27 years.

I think K1-K6 use same carbs?

[70CB750]

Remember Lucky?  He was 100% sure the K0 carbs were the best ever.

[robvangulik]

Sure, he also set his ignition timing to the T mark...

[seanbarney41]

Sounds like it runs, revs, and idles on all 4 without much trouble...so you are already doing better than most newbs that come here for carb help.  I would keep what you have.

[Stev-o]

I would keep what you have.

+1.  I love Mikuni carbs, my Kawasakis have them

[fxef79]

Sounds like it runs, revs, and idles on all 4 without much trouble...so you are already doing better than most newbs that come here for carb help.  I would keep what you have.

Probably a wise statement I should heed... don’t fix it if it ain’t broke, right?

[2wheels]

Best sounding no muffler 750 i have heard.
I normally don't like the sound of open pipes on these engines.

[slikwilli420]

What's wrong with what you have now? Ive heard good reviews about the mikuni kits especially for just cruising around.

Once I get them all cleaned up and rebuilt - maybe nothing.  Biggest issue right now is I know they're running very rich at idle, so that means I have to start the whole jetting process... starting with pilots, then needle and jet, and then mains...  and that means buying all kinds of sizes of jets, doing plug chops at, at least, 3 different throttle ranges, so a lot of road time and swapping jets around and trial and error.  There's not going to be any good "tried and true" recipe for this setup.

Whereas, if I had a stock CB750 carb rack, I feel like most of the "trial and error" I can bypass by using the collective knowledge found here on the SOHC4 forums. 

Maybe my reasoning is flawed... but that's why we ask questions!

You are making the process far harder than it needs to be. Get a few main jet sizes smaller and see how it runs. All the plug chop business is totally unnecessary. Call Kenny and see what the starting settings should be and start there if not set that way already.

[fxef79]

You are making the process far harder than it needs to be. Get a few main jet sizes smaller and see how it runs. All the plug chop business is totally unnecessary. Call Kenny and see what the starting settings should be and start there if not set that way already.

I didn’t buy them from him. Two previous owners before me is when they were put in... and if I interpret the service work order I have in the records I was given correctly, they were something used the shop had on the shelf then. So I really hate to bug him... especially given his website statement NOT to call them for help if they weren’t bought from him. Even given that...  I did email and just ask for what they typically ship them setup as, and got that info.  I was stoked they responded to that - that was given to me as a pilot size, jet and needle size and clip setting, and a range of main jets, and even at that it’s just a suggested starting point.

Chalk it up to “fear of the unknown”. I like to start from a point of knowledge and then move forward. Just seems like the body of knowledge related to OEM carbs is much greater - gives me “warm fuzzies”.