Weighing Fueling Options

[Syscrush]

Hey all. In the spirit of the amount of info I shared (and discussion generated) when figuring out stuff like brakes, suspension, cooling, electrical, etc. I think I'm gonna share my decision making process for sorting out fueling on my bike.

The bike has a +3mm overbore (near-stock compression), stock head, the correct 1979 cams, K&N filter in the OEM airbox (I'm open to going back to OEM if that will help the OEM carbs work best), and a 6-into-2-into-1 header with a baffled muffler.

Given the amount of back & forth I've dealt with in trying to get these carbs right, I strongly suspect that they're suffering from a mix of bad tune and problematic wear. It's been 5 years of 1 step forward, 1 step back.


I have been spending some time on the fence about how to proceed. I'm stuck between these broad choices:

1. Have the OEM carbs restored and set up by an obsessively meticulous CBX specialist. This is the simplest and most cost-effective approach and the one that gives me most confidence that the bike will look, sound, and work right. The concern I have is that after the rebuild there's still the matter of getting them final tuned on the bike, which has proven to be an ongoing challenge. And if they don't work right after the rebuild because of other factors on my bike (specifically the exhaust), or because I'm just not satisfied with the stock carbs even at their best, then what?

2. Aftermarket carbs - either the bolt-on CR setup from TIMS or a custom-built rack of Yoshimura MJN carbs. I'm not looking for more power, but these seem like the way to get everything fresh and new, with the potential for more power as a bonus. The drawbacks here are greater cost, and I don't like the looks or the sound as much - I also worry about the CR's being rattly and annoying for street riding, and about the MJNs not fitting (as nobody is making a CBX-specific application). Also, both are less common than OEM, so I worry about making it even harder to find relevant tuning expertise. I really do want to keep the stock airbox, which is presumably more difficult or impossible with these options. Lastly, it's not clear to me if either of those carb choices would make tuning any easier.

3. EFI conversion aka the nuclear option. I am tempted to go this way for 2 reasons - the first being that it can be tuned with a laptop and there's plenty of local expertise for doing that, and the second being that with a good closed-loop tune I could also add a catalytic convertor and leave less bad karma in my wake. The obvious cons are cost, looks, and complexity. I have not found an individual throttle body solution that's likely to work well, fit the CBX, and look good to me.

A bit more on EFI conversion options:

I have thought about gutting the stock carbs and building a plenum that looks like the stocker but with a single throttle body. The carbs would basically be just plumbing in that case. Injectors would have to be worked in somehow sneaky, which poses serious difficulties. I think that this idea is a non-starter.

I've also thought about trying to use the TB's from the Triumph Bonneville that are specifically designed to look like vintage carbs:



They look dandy, but I seriously doubt that 6 of them will fit into the space left by the OEM carbs.

Maybe a hybrid approach where the CBX carbs are gutted except for the throttle butterflies and those giant caps are hiding injectors?

The other class of options are all-custom solutions for throttle body, plenum & manifold, and airbox. Don't try to look stock, just try to look good.


I'm really torn on this, and open to ideas or suggestions.

[Syscrush]

Taimoshan Cycle Works made custom carbon fiber intake trumpets with 34mm throttle butterflies, injector bosses, and set them up in 2 groups of 3 with a u-joint in the middle so they could preserve the OEM style canted banks.

Some info here



I like that this is so tidy and sleek looking - nothing seems bodged together. My concerns are that I don't think that all trumpets should ideally be the same length (3-4 should be longest and 1-6 the shortest to compensate for the variance in intake runner length). Also, I would like to preserve the stock plenum and airbox if possible, and I don't think that a setup like this could be made to work. Still looks pretty rad, though, and an option that might appeal to others reading this.

Here is an intake from a twin-turbo bike that uses a single TB and has the kind of staggered runner length that I like:



That bike has an intercooler setup that feeds charge air into the 2 outboard sides and then flows from the top-middle of the intercooler, over the cam cover, to this forward-facing throttle body. Very cool, but not quite what I want. I also think that this specific shape would flow OK under boost, but it doesn't look great for an NA setup. Still, it shows promise - I can imagine the plenum part made to look like the stock plenum, and the TB opening into the filter box. I can also imagine it being a hellacious pain in the butt to get that whole rig onto or off of the bike.

Here is a similar setup that I saw in person at a local shop:
Throttle: (From some kind of Acura)


Plenum:


Runners & Injectors:


This is intentionally rough & ready to suit the overall character of the bike as well as to keep the conversion as simple as possible. I don't find it too hard to imagine finessing that plenum into something that would look at first glance like the stocker, and making the runners fit with the stock intake boots. It would be more tidy overall with an internal fuel pump, too. It would really look like something is missing without the carbs, though. Maybe if the whole thing was blacked out it just wouldn't draw the eye and remain stealthy.

[Syscrush]

To me, the Yosh carbs look amazing off the bike - I'm not sure how they'd look on the bike if they were used along with the stock airbox & plenum - or if they'd even work properly set up that way - it really looks like a lot of care went into those intake trumpets:



The concern with those carbs is that as far as I know, nobody is running them on a CBX. A custom builder I've been corresponding with who swears by these carbs and uses them on all of his bikes has suggested just going with the bolt-on CR roundslides instead as they're a known quantity.

They really are beautiful pieces, though.

[Syscrush]

But how good are the OEM carbs? I know that they have their fans here. If I can be talked into fixing the OEM carbs and using them for a few years, that would definitely make my life a lot simpler.

[Syscrush]

A custom builder I've been corresponding with who swears by these carbs and uses them on all of his bikes has suggested just going with the bolt-on CR roundslides instead as they're a known quantity.

I just got a clarification - he was not talking about the CR's, but rather this FCR setup with 2 canted banks of 3:



Hubba hubba! I haven't found anything on this site or CBX World about anyone actually installing and using these things. Looking a the wider net, this is the only evidence I could find of FCR's installed on a CBX. They look like older models with different intake trumpets, but they do have the V-shape, at least:

[Syscrush]

Another EFI option to consider is stock carbs with everything removed but the throttle valves, and then a shower injection setup like this in a modified version of the stock plenum:

https://youtu.be/QCKVirTcP4Y

[steve murdoch icoa #5322]

I have ZERO knowledge on the CRs, FCRs, EFI or Turbo setups but i think you would run into the same problems.
They will need to be set up for your particular bike and they could be even more finicky to deal with.
It sounds like your main concern/quibble with the oem carbs has been the set-up for your bike.
It makes most sense to me to send them to one of highly regarded carb rebuilders with all the pertinent info about your bike and i bet they come back VERY close to what you need. Easily half the cost i would think too.

[Mike Nixon]

The stock carbs can be and have countless times been made to work with all sorts of engine changes, even ones more involved than yours. Actually, your engine is stock. A big bore kit adds throttle response and torque. While on the other hand, "performance" carbs present many challenges and compromises. Not worth it in my opinion, especially if you don't see yourself riding at WFO all the time, and very few people do. However, rather than fuss with the icing on the cake, fix the cake. The very first thing to do is fix your engine. A 3mm big bore should yield considerably more than stock compression. At sea level some 190-200 psi. The likely deficit is valve issues, endemic to the CBX. I would get that fixed way before doing anything else. Then the stock carbs will amaze you. The more compression, the better they work. But even if you don't solve the loss of conpression, stock carbs done right will work so well you will never want anything else, and this is coming from soneone who has a bigger collection of CRs, FCRs, RS and other exotic carbs than anyone exceot probably Nigel Patrick. Stay with stock. But get them worked on right.

[Artie]

I can again attest to stock motor,stock air box,stock carbs,just as Honda designed it = perfect carburation
and also to Mike Nixons expertise

[Syscrush]

The stock carbs can be and have countless times been made to work with all sorts of engine changes, even ones more involved than yours. Actually, your engine is stock.

I agree with you on this.

While on the other hand, "performance" carbs present many challenges and compromises. Not worth it in my opinion, especially if you don't see yourself riding at WFO all the time, and very few people do.

This is very helpful, thanks. I spend very little time at WFO, which is a big part of why most of my decision making has focused on OEM or EFI.

However, rather than fuss with the icing on the cake, fix the cake. The very first thing to do is fix your engine. A 3mm big bore should yield considerably more than stock compression. At sea level some 190-200 psi. The likely deficit is valve issues, endemic to the CBX. I would get that fixed way before doing anything else.

I think I created an incorrect impression - when I said "near-stock compression", I was not referring to the numbers from a compression tester. I was referring to the description from TIMS that the pistons I have are "1147 BIG BORE (67. 50 MM) J&E STD COMPRESSION PISTON KIT". They also sell a "HI COMPRESSION" kit.

The engine made 105 hp (uncorrected) at the wheel after rebuild & break-in. Since then it has had a post break-in valve adjustment which helped throttle response, starting and rideability immensely, and it has had the pilot jets bumped up based on the sniffer results from the dyno. Throttle response at cruise in the middle of the rev range is not great, and fuel consumption is insanely awful (I'm hitting reserve at ~70 miles of steady-state highway cruising). Based on all of these factors, I am confident that the engine internals and valves are in good shape, and that the carbs are in need of serious TLC.

tock carbs done right will work so well you will never want anything else, and this is coming from soneone who has a bigger collection of CRs, FCRs, RS and other exotic carbs than anyone exceot probably Nigel Patrick. Stay with stock. But get them worked on right.

This means a lot. If the story from those who know is that FCRs etc. work better but are more expensive, I'd still be considering aftermarket carbs, but this helps me rule them out.

I do have one question for the "stock is best" crowd though:

I have seen repeated claims that certain not-uncommon mods result in intractable tuning problems - that certain 6-1 headers result in a midrange flat spot (I've heard this about DG specifically), or removing the airbox snorkel lid and/or using a K&N filter kills throttle response in part of the rev range. I don't doubt that an all-OEM setup (intake, carbs, and exhaust) will work exceptionally well - as it was developed with resources that the rest of us can't even dream of. But if certain seemingly minor mods cause problems with getting the carbs to work right, then how can I be confident that the problems I'm having aren't due to the OEM carbs' difficulties dealing with my exhaust? As noted above, I have the stock airbox with a K&N filter but am OK with going back to OEM filter if that will help. However, I am 100% committed to the exhaust on my bike for sentimental reasons.

If I have have the carbs sorted out by someone who is acknowledged for their ability to ensure they're "worked on right", but the exhaust won't play nice with even perfectly-restored carbs, then what's my next step?

[Mike Nixon]

Yes, the 6-1 DG is a poor choice.

[EMS]

I was not referring to the numbers from a compression tester. I was referring to the description from TIMS that the pistons I have are "1147 BIG BORE (67. 50 MM) J&E STD COMPRESSION PISTON KIT". They also sell a "HI COMPRESSION" kit.

Phil, I have said this on other occasions before, but some people who fly by the seats of their pants and have done so for the last 25 or 30 years with the CBX have accused me of being too technical about this.
For what it's worth: Many who sell oversize bore piston kits have little or no knowledge what these kits do to the compression ratio. Especially with the CBX kits. Many are just quoting some Far East sellers who claim their kits are standard compression, because their pistons look like the stock pistons. And that is actually the problem. If the pistons look like stock, i.e.: has a dome shape like the stock piston, an increase in bore WILL INCREASE the compression ratio. This is neither extreme science nor advanced combustion engine theory, it is plain physics and math.
The 67.5mm bore kit with "stock appearance pistons" will increase the compression ratio from a stock 9.3:1 to a 10.05:1.
I have communicated that to both Louis and Bill Brint (of Tim's) when they offered these kits on ebay, but they chose to ignore my comments.
The engine I have put together for my 79 has a 72mm bore which gives me a 1304cc. It would result in a compression ratio of 11.3 : 1, if I would not have used a flat top piston, which brought it back down to 9.8 : 1

[Syscrush]

Phil, I have said this on other occasions before, but some people who fly by the seats of their pants and have done so for the last 25 or 30 years with the CBX have accused me of being too technical about this.
For what it's worth: Many who sell oversize bore piston kits have little or no knowledge what these kits do to the compression ratio. Especially with the CBX kits. Many are just quoting some Far East sellers who claim their kits are standard compression, because their pistons look like the stock pistons. And that is actually the problem. If the pistons look like stock, i.e.: has a dome shape like the stock piston, an increase in bore WILL INCREASE the compression ratio. This is neither extreme science nor advanced combustion engine theory, it is plain physics and math.
The 67.5mm bore kit with "stock appearance pistons" will increase the compression ratio from a stock 9.3:1 to a 10.05:1.
I have communicated that to both Louis and Bill Brint (of Tim's) when they offered these kits on ebay, but they chose to ignore my comments.
The engine I have put together for my 79 has a 72mm bore which gives me a 1304cc. It would result in a compression ratio of 11.3 : 1, if I would not have used a flat top piston, which brought it back down to 9.8 : 1

Thanks for this, Mike. I used the term because it's the term that the seller uses, I do not claim it's actually stock compression - but I think it's good that this point gets made often so that it becomes common knowledge.

My pistons are JE and bought through TIMS.

[Syscrush]

After researching and weighing options, I've decided on the following plan:

1. Send the carbs to a CBX specialist/expert who I trust to get them into the best shape possible (or get an authoritative answer on if I need replacements).

2. Ride the good stock carbs for a while - hopefully they work well enough for me to get a few (i.e. at least 3) years of enjoyment out of them. If they give me problems, I'll probably fit a data logging WBO2 sensor with inputs for throttle position and vacuum sensors to get a picture of what's going on.

3. Eventually (hopefully not more than 5 years from now) convert to closed-loop EFI by gutting the carbs and fitting a shower injection system in the plenum - with the intake manifolds, rubbers, plenum/airbox, and air filter box modified as little as possible. If I can get it running well and maintaining the AFR in a tight range running closed-loop, I'll have a catalytic converter added to the exhaust.

[Syscrush]

I have an updated plan, due to 2 factors.

The first factor is the arrival of my second baby - I'm keeping the bike off the road for a couple of years because I learned with the first one that I'd get barely any opportunity to ride. I figure while it's off the road for a multiple-year hiatus, I might as well just take care of business and get the bike finally sorted out correctly.

The second factor is seeing the example of mlynch001 in this thread. I was close when I mentioned installing injectors in the caps of the stock carbs - he showed that it can be done the other way - by installing injectors in the float bowl area.

Going EFI will mean 2-5x the cost and 10x the duration as sending the OEM carbs to a CBX carb expert, but I'm choosing this path because it makes me more confident about the long-term viability of the bike, and because it opens up the possibility of adding a catalytic converter. To make the bike into something I can live with long-term, I need to get it to be something that the mechanics and tuners I have access to can cope with - and that means something that can be tuned with a laptop. My plan is to do research and planning until Nov-Dec of next year, and then start the real work. I would like to be able to ride it again in 2020, but expect that 2021 is a more likely outcome. We'll see.

Thanks to everyone who provided helpful advice and feedback here.