Single Carb Conversions?

Warning: Controversial Material!

Pictured are a few of the single carb conversions I’ve evaluated (Ford, Solex and Weber carbs shown left-to-right)

I get a few inquiries about single carb conversions. This Tech Tip is my comprehensive answer.

Even though I enjoy tinkering with single carb rigs myself, I have been crucified online by a few for daring to criticize these conversions. Unfortunately, my views are often distorted or misrepresented.

One of the common criticisms is that my views on this subject are economically motivated since I sell carb kits to service the OEM carbs. Believe what you wish, but that is simply not true. I’m driven by a simple guiding principle to give honest, reasoned advice when asked.

I generally make no additional public comments about this topic beyond this Tech Tip. However, I will “call out” individuals as necessary who make false assertions, lift comments out of context or distort my viewpoints.

This Tech Tip covers my entire view on the subject. I’m not asking for agreement. Decide for yourself. Feel free to disagree, but I will not be drawn into pointless debates. If misinformation creeps into online discussions (and there is plenty!), I don’t feel any compunction to correct or dispute anyone over this.

This topic tends to generate “wishful thinking” in otherwise genuinely sincere folks. Sometimes this clouds their judgment and objectivity.

Feel free to consider this information, but do me a favor and please don’t post this link or any portion of this content into any “single carb conversion” forum threads.”

Likewise, quoting me “out of context” is a fool’s mission that might lead to regret. Ditto for “out of context” and misleading publication of private communications on this subject.

My views are here. As of this writing, there are outrageous misrepresentations published about my views. Discount anything you see attributed to me elsewhere.

A few more single carb rigs from my collection

Likely the best commercial single carb attempt ever: Weber carb + coolant-heated plenum by C.C. Products

My view in a nutshell: I strongly recommend that you retain your original 4 carb setup.

I’ve probably installed and tested more single carb setups than anyone. I have evaluated commercially produced conversions such as the ones by Cycle Innovations and C.C. Products as well as various “one-off” creations including my own. More evaluations are planned. Which makes my crucifixion by some on this issue all the more amazing.

You may be tempted to trash your carbs and go to a single carb conversion. These work well enough, but I don’t personally recommend them in most cases. People who invest in these setups sometimes report significant performance “improvements.” Typically, they are comparing their results with dirty, poorly maintained and malfunctioning original carbs so the comparison is skewed a bit.

Some folks love the apparent simplicity and easy maintenance of a single carb. I absolutely understand that appeal. If you like your single carb conversion, ride and enjoy!

If you are debating the merit of a single carb conversion for yourself, consider this. The theoretical simplicity advantage of a single carb is offset by:

• Conversion expense
• Reduction in top end, peak performance
• Reduction in bike’s collector value
• Auto-type carbs (i.e., Holley) aren’t normally expected to perform over the wide rpm range required of performance motorcycle engines
• Due to the “packaging” and space issues involved in designing these manifolds, these setups are plagued by inconsistent performance across the 4 cylinders. A very common complaint is that cylinder #X is chronically rich (or lean)
• Due to the long intake tracks, these rigs are prone to carb icing in cold damp weather (less than 45 degrees F). In some climates, this may be a serious matter.
• Large mass of air fuel mixture in the runners on single carb conversions is sluggish to move creating poor throttle response
• Long intake runners require richer jetting that otherwise needed which reduces fuel mileage
• Inconsistent, irregular idle is a common issue
• Early ‘Wing cams produce minimal vacuum at idle…not the robust vacuum expected by automobile-type carbs. This compounds the idle issue.
• Some single carb conversions have actually suggested and required frame “surgery” to achieve necessary clearances. The need to hack away part of the frame is an obvious “show stopper.”

Many single carb conversions that I have tested exhibit lots of intake noise….either accidentally or by design given the air cleaner assemblies supplied. On the other hand, the air intake assembly supplied by Honda on the OEM setups is highly engineered to be remarkably quiet. Some folks interpret all the extra “commotion” generated by their single carb conversion as increased power. I do not make that mistake. Noise does not equal power!

Also, many owners resort to this drastic solution out of frustration. It certainly doesn’t address the root problem which in many cases is often something else such as:

• Poor compression
• Ignition timing “off”
• Dirty fuel tank
• OEM carbs need a thorough overhaul
• Etc.

As these landmark bikes become even more collectible, discriminating buyers are going to pay top dollar for only the most original specimens. Originality is important!

Other Expert Opinions:

I’m not alone in my views. A very well-regarded GoldWing expert says this…

“After careful examination a few years ago at CI’s request, I told them that their Innovation was a lack luster performer and a fire hazard.”

GWWRA Sr. Technical Editor

Another early Wing “guru” states:

“I believe that your performance and mileage will suffer severely if you convert to single carb.”

Former GWWRA Workbench Columnist

“Real World” Feedback:

This owner report is typical of the feedback I see…

“I ran a single carb on my GL1100 for over a year. Cycle Innovations was the manufacturer. They are out of business.

The bike started fine, but had a definite loss of power and top end no matter how it was adjusted.

With the stock carbs, the bike would top out about 115mph, with the single about 90mph. Not that I normally ride that fast, but I wa,s curious as to top end performance. The biggest problem though was that the bike would hardly run at outside temperatures below 45 degrees. The single carb setup would ice up and not run.

Finally, I spent $200 to have my stock carbs rebuilt and was much happier. My $500 conversion trial was definitely not a winner. If you live in a warmer climate and don’t mind a loss on top end, the single carb conversion that I ran wouldn’t be too bad. As others have found out, the stock carbs are the best.”

Forum poster on NGW

Details:

The part most people miss is that ANY single carb conversion on an early Wing will necessarily increase the length and volume of the intake tract by a very large amount. In practical terms, the intake runners on a single carb conversion will need to be about TWICE the length of the OEM setup. This is not a good development in terms of peak performance nor torque in this particular application.

On the OEM setup, just the plenum is centralized. The carbs are outboard (semi-sidedraft) with fairly short intakes in typical practice for high performance motorcycle engines.

Plus, the long runners on single carb conversions introduce problems such as unequal mixtures across cylinders, fuel condensation, carb icing as well as introduce the need to create (and regulate ) heat to control carb icing and improve atomization. The large mass of air fuel mixture in the runners on single carb conversions is sluggish to move creating poor throttle response. Worse, jetting has to be richened beyond ideal calibration to compensate for the long runners. Fuel economy necessarily suffers.

The most analogous situation to compare is racing applications with VW-powered vehicles. You will find various dual and single carb setups used successfully depending on the target objective. Max peak performance vehicles tend to favor outboard, multi-carb setups with short runners. Off-roaders seeking low maintenance, simplified throttle linkage and better fuel control (closer to vehicle center of mass) will tend toward single carb setups with long intakes. But, even these long intakes on performance VWs are far from horizontal as you tend to find on single carb Wings. Both approaches can work for their intended task, but the high performance option is always toward multiple, outboard carbs.

VW engines are air-cooled and naturally run hotter than early Wing engines. Yet, the single carb performance VWs all need carb heat to perform.

My contention that ALL the other engineering variables built into the early Wing engine design clearly favor a short runner design. Compensations (if you want to call them that) have already been built into the equation to generate more than adequate torque.

There is no discernible reason (other than stubbornness?) that I can conjure up to seek more torque…especially at the expense of overall performance. But since I happen to like carbs, why would I want less of them?

Single carb conversions can be made to work. There are plenty on the road. The maintenance “advantage” of only one carb is appealing to some. I understand that appeal. I’ve done considerable experimentation and built several single carb conversions myself…more are underway. Some of my single carb rigs ran well. Others had more obvious limitations. Many folks would probably find them acceptable to live with. But, in all cases, the performance clearly fell short when compared to the OEM carbs.

Most, unfortunately, the various marketers of single carb conversions have intentionally created the erroneous impression that they offered a performance upgrade. But, don’t let anyone convince you that single carb conversions are a “high performance” option. They are simply not due to the physics involved. The OEM setup will outperform any single carb conversion yet to be devised.

Some purveyors of these setups never accounted for the various ignition issue differences across the early GoldWings they were trying to support. The carbs selected (and most other single carbs as well) prefer much more static advance than the OEM Keihins. The folks marketing these systems might well have considered providing with their kits (for GL1000 at least) the later 431 style advancer set at around 10 degrees BTDC. Reworking the advance curve would have been a good idea as well. As it was, the install instructions provided with some of these kits advised cranking in LOTS of static advance for a good idle (trial and error just shy of “kickback!”) This resulted in way too much dynamic advance…especially on the early bikes with “371” advancers. This partly explains some of the “poor power” and detonation complaints. I’ve received reports of kickback being so bad, that starter shafts broke under the stress of rapid reverse rotation!

Many companies that have tried marketing single carb conversions for early Wings are now out of business. Most were run by earnest and hard-working individuals who tried very hard to deliver good value for their customers. A few were nefarious charlatans.

Single Carb Plenum Design Details?

As mentioned above, carb icing is a problem that plagues single carb conversions. Many strategies have been tried to overcome this. Unfortunately, many of these strategies employ the addition of “heat” with poor (or no) control. The result is the unintended consequence of vapor lock in warm weather.

Setting aside all the previously discussed limitations, I recommend that you avoid any single carb conversion that includes a “rigid” one-piece plenum that spans the entire width of the engine and eliminates the OEM intake runners (or equivalent).

Some form of flexible isolation is recommended to protect the intake from the normal thermal expansion/contraction cycles of the engine in use. The OEM intakes are excellent for this purpose. There are several good reasons why Honda engineers included durable rubber couplings in their intake runner design. Without them (or some other form of isolation), a rigid plenum is at some risk for eventual failure via stress fractures.

If you don’t believe me, check timing belt tension on a “cold” engine then again with the engine fully warmed up. The results in tension increase are pretty amazing and undeniable proof for “growth” of a warm engine. (By the way, this is why it is considered a best practice by fastidious mechanics to set cam belt final tension with a warm engine.)

With careful engineering and manufacturing, it is certainly possible to match the thermal expansion/contraction rates of a boxer engine to the corresponding expansion/contraction rates of a rigid intake. But, it is a daunting proposition generally beyond the merits of the effort. Many OEM manufacturers don’t bother to try. I agree with their reasoning. “Back-yard engineers” are much less likely to get this “matching” calibrated correctly.

Even the GL1500 intake is an “isolated” design. Contrary to some claims, this is clearly not a 1-piece rigid design. As shown below, it is a multi-piece design in standard engineering practice. In this case, the Honda engineers moved the flexible isolation to an inboard position underneath the centralized dual carbs as shown below:

Red Arrow Points to Isolation Point on GL1500 Manifold Pair. I’m pretty checked on the practical definition of the word “rigid.” When assembled into a “unit” with associated companion hardware, the components above do not approach “rigid.”

Carb-to-cylinder head isolation is actually a prevalent aspect of Honda (and other manufacturer’s) design considerations on a wide array of engines. Here’s another example: the isolation provided for the Honda CBX which is an inline-6 layout:

 

Honda CBX “Carb Holders” – necessary for carb-to-cylinder head isolation

All engines “grow” when warm. This fact of physics has to be recognized and accommodated by engineers who design anything that bolts to an engine.

What about Subaru, Porsche, Alfa Romeo or Other “Boxer” Engine Design Practice?

I happen to own two modern Subaru automobiles as of this writing. I love my Subarus! Subaru makes terrific vehicles. And yes – I have looked under those hoods. 

One of Randakk’s Subarus

Both of my Subarus are fuel-injected and have one-piece “rigid” plenums. This means the fine engineers at Subaru have done their homework on matching thermal expansion / contraction issues. They clearly have the resources to handle that. Again, “back-yard engineers” are less likely to get this “matching” calibrated correctly. It’s worth the effort for Subaru, because a one-piece intake (made in numbers) can save manufacturing and labor costs for a mass production item.

But, the best aftermarket intake makers for modern Subarus take this approach:

Superior Subaru Intake Manifold – “3-Piece” Design with Isolation

According to Forsa, this provides “improved durability.” I agree. By the way, adapting a creation based on that gorgeous Subaru intake above to a GL1000 would not be that difficult and would allow the use of a high-performance Mikuni HSR flat slide carb or similar. That would look really trick. Someone should try that!

Likewise, below is a typical intake setup as found on modern Porsches. Porsche is a brand renowned for high performance and superior engineering. Note the isolation.

Typical Porsche Intake Manifold – with Isolation

Even the Italians understand the desirability of intake isolation on a boxer engine:

Alfa Romeo Intake Manifold – with Isolation

I spent some time as a flyer and I’ve looked under the cowling of many airplanes. Boxer engines are quite common in general aviation. Below are typical examples of the intake setups found on such engines. Engineering decisions in aviation are literally “life-and-limb” issues. Again – note the isolation. The facts speak for themselves on the issue of a rigid plenum.

Typical Aviation Intakes – with Isolation (Lycoming on left, Continental on right)

Surpassing Honda’s Engineering?

If you install a single carb conversion, please don’t contact me (as many have) to help make your bike run properly. You will have already ignored my best advice!

The reason that the OEM carbs are considered by some to be sophisticated and complex is that Honda was trying to optimize performance over a wide range of operational modes:
• Cold start
• Idle
• Off-idle
• Cruise
• Deceleration
• Acceleration
• Max performance

Simplicity and performance are somewhat mutually exclusive in this engineering challenge. Trust me, if Honda could have gotten away with a single carb in this application without giving up any performance, they would have most certainly done so.

Ram Tuning?

Some proponents of single carb conversions point to the advantages of so-called “ram tuning” (popularized by Chrysler performance engines from the 50’s and 60’s). Ram tuning (technically resonant manifold tuning) is a very complex topic that involves an understanding of wave theory and harmonics. Ram tuning attempts to take advantage of the inertia contained within a moving column of air/fuel mixture as it comes to a stop against the closed intake valve. By adjusting the length of the intake runners, this energy can be used to improve cylinder filling when the valve opens. This is not a “free lunch” as the beneficial effect is limited to a rather narrow rpm range. In fact, performance penalties at non-optimized rpm ranges can more than offset the gains achieved.

Simplistically, the idea is that long intake runners increase torque. Ram tuning is a valid tuning concept for building torque and in certain limited applications horsepower as well.

But, there is a maximum torque value any engine can achieve. Extending the runners beyond that limit does not add any more torque and actually begins to reduce both torque and horsepower. Even if you don’t exceed the theoretical limit, longer intake runners generally reduce overall horsepower potential except for very narrow ranges. These narrow “blips” can be harvested by clever drag racer types, but they have limited applicability for street driven motorcycles. My strong belief is that Honda optimized both torque and horsepower with the OEM design. In practical terms, single carb conversions compromise both.

What about GL1500 Carbs?

Sometimes I’ll hear the argument that the later Honda GL1500 used quite successfully a centrally located “single carburetor.” If that “advance” was good enough for Honda, then surely would not a single carb retrofitted to an earlier ‘Wing likewise be an improvement? The short answer is no.

It’s true that Honda fitted a rather odd, synchronous “2 barrel” carb on the GL1500 that mounted transversely to a plenum in a central location above the engine. Fine point of clarification: in reality, the GL1500 setup is actually two separate individual carbs….not a 2 barrel. Look here for Rebuild Kits for  GL1500 carbs and other vintage Hondas.

Some comments about the GL1500 rig relevant to this “debate.”

1. By the time Honda developed the GL1500, the GoldWing had evolved entirely into a touring machine with minimal sporting pretensions and bodywork that completely covered up the interesting mechanical bits. Some might say that it had evolved toward “appliance” status.

2. The GL1500 sold in very large numbers. Those sales volumes (and long model life) allowed Honda to amortize huge R&D expense to develop a very sophisticated cast plenum. The cost savings vs. 6 individual carburetors made this investment worthwhile on such a high volume, medium performance touring machine. If you examine a GL1500 plenum, you will marvel at the very elegant and complex shape Honda was able to craft. It has very elaborate curved tubes that approach art. There are no “corners” anywhere to be found. There are amazing transitions everywhere you look. Honda even altered the tube diameters on various cylinders to compensate for the necessary differences in runner lengths and volumes. These are examples of amazing engineering capabilities. However, they are all compromises driven by economics from a performance point of view. It would be next to impossible to create this shape in a simple workshop.

3. Extensive measures were obviously employed to optimize flow in the GL1500 plenum. Heating was incorporated to prevent icing and other measures were taken to mitigate the effects of the long horizontal distances the plenum had to span. One example: the two carbs are mounted transversely across the frame). This simple measure reduces the distance the air / fuel path has to navigate by a small but important amount.

4. The Valkyrie was a later variant of the GL1500 model. The Valkyrie was targeted at performance and sporting demographic. The engine had the same displacement, but it had different tuning specs designed to increase output. Honda equipped the Valkyrie with six separate 28mm carburetors. I rest my case!

5. I’ve actually experimented with a single transversely mounted synchronous 2 barrel carb on a GL1000. I did extensive testing using a single Weber IDF44 mounted on a “one-off” plenum. One of the brilliant aspects of Weber carbs such as these is that you can easily change the main venturi size. Here’s what I found: Getting a decent idle required a significant reduction in the main venturi size on these carbs. Likewise, getting the most power at midrange and above required much larger main ventures that ruined the idle quality. Neither strategy was very satisfactory and no synchronous 2 barrel carb configuration performed as well as a progressive 2 barrel (like the Weber 32/36 DGV). Practically speaking, there are no commercially available, tunable synchronous 2 barrel down-draft CV carbs available with similar packaging values like the ones on a GL1500. I suppose you could attempt to re-sleeve the venturies and rejet a GL1500 “carb” on a facsimile of the GL1500 plenum. That would be a very heroic effort that would yield lackluster performance. Again reference the Valkyrie to understand why!

“Best” Single Carb Approach?

I’m a big fan of Weber carbs. Of all the rigs I’ve personally tested, a progressive 2 barrel Weber on a reasonably well designed manifold yielded the best results. Such a setup has a “sweet” personality that I find agreeable. It’s a bit down on power in the upper rpm ranges compared to OEM carbs, but otherwise fairly pleasant. On the negative side, new Weber carbs are expensive.

Commercial Viability?

As noted above, my critics sometimes accuse me of “bias” linked to my commercial interests. My view: they are mistaken and misinformed. As a fairly successful business person, I am constantly looking for commercially viable business opportunities that mesh well with my unique business model. Since the idea of single carb conversions is so alluring to some, I do get suggestions from time-to-time to offer such a product.

I have in fact looked quite seriously at manufacturing my own single carb conversion offering. I have access to the resources and manufacturing capabilities to do this if I desired. But, I always reach the same conclusion:

1. No single carb design evaluated, contemplated or possible would be equal to the OEM carbs from a performance point of view. Again, this is simple physics which cannot be denied.

2. The market is very “thin.” For example, about 100,000 GL1000s were made by Honda. Even if a few thousand single carb conversions could be made and sold, it would have no meaningful effect on my core business.

3. Ignoring the performance penalty, a good single carb design would require that the plenum be made in fairly large numbers to justify the tooling necessary to elevate the effort to reasonable, “commercial” engineering standards. This means high start-up costs for any serious effort.

4. Any reasonable effort that I would put my name and reputation on would necessarily require a new carburetor built to a high spec. That would eliminate all of the “economy” available via the salvage carbs favored by some. Junkyard carbs (any brand) are notorious for hidden problems.

5. Add the cost of a well-designed and manufactured plenum to the cost of a new Weber 2-barrel carb and you will quickly see the economic reality that I see: the vast majority of customers will save money and have a better final result via a proper rebuild of the OEM carbs.

6. The customer service exposure and workload from the inevitably large numbers of disappointed customers would be very daunting.

7. I consider (and dismiss) a fair number of business ideas that fall short of initial promise when examined critically on the issue of economies of scale. Single carb conversions are a perfect example.

8. On a quite regular basis, new single carb conversion “vendors” emerge from back-yard workshops amid great fanfare, hype and enthusiasm that sometimes approaches euphoria. Predictably, they “run out of steam” and cease operation as they discover for themselves the realities of physics and economics detailed above. I’ve noticed that they don’t have nearly as much to say as they retreat.

I’m sure more new vendors will continue to try their hand at this. I hope they take what I’ve said to heart and consider all the commercial failures that have preceded them. Otherwise, I hope they have very deep pockets and great customer service skills!

Conclusion:

Installing a single carb conversion does not make you a “bad” person and I won’t lose any sleep if you make this swap. Just don’t let anyone convince you that it is a performance “upgrade.” It simply is not.

If you like to tinker (I do), then creating a single carb ‘Wing might be a satisfying project. Just don’t expect to win any drag race, fuel mileage or dyno competitions! Single carb conversions are often discussed on this great Unique Carburetion forum at NGW Club (free registration required for full access):

https://www.ngwclub.com/forum/viewforum.php?f=47

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