Randall Washington, aka "Randakk"
President and Chief Executive Rider
Chapel Hill, NC USA
|
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. Feel free to consider this information, but do me a favor and please don't post this link into any of these silly "debates."
Pictured above are a few of the single carb conversions I've evaluated (Ford, Solex and Weber carbs shown left-to-right)
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 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:
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:
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...
Another early Wing "guru" states:
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 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. The only company that I'm aware of with a current offering is: Here is my review of LD's Ol' Wings Single Carb Conversion. 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:
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 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. 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 siamesed, synchronous carb is 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 a 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 significant reduction in the main venturi size on these carbs. Likewise, getting the most power at midrange and above required much larger main venturies 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 well designed manifold (like the heated C.C. Products unit) yielded the best results. That setup has a very "sweet" personality that I like. It's a bit down on power in the upper rpm ranges compared to OEM carbs, but otherwise quite pleasant. On the negative side, this was a very expensive manifold to manufacture and almost none can be found today at any price. Plus, new Weber carbs are not cheap either. 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 forum: Back Yard Built Goldwing's Bar & Grille Return to main Tech Tips Index Intellectual Property Notice: All material appearing in this website is the property of Randall Washington, and is protected under United States and international copyright laws. The photographs, text and other content may not be copied, reproduced, distributed, stored, or manipulated in any manner without the express written permission of Randall Washington. For more information on the use of material from this website, click here |
