Review: LD's Ol' Wings Single Carb Conversion

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Warning: Controversial Material!

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Single Carb Conversion by L.D. Welch (shown with Randakk's High Performance Intake Clamps)

Note: I purchased this unit in September, 2009 and tested it on the road in October, 2009.

The kit was installed on a 1975 GL1000 with perfect compression and 100% of all tune-up issues completely checked and adjusted "to spec." The test bike is the "video" bike used to create my GL1000 carb video. I mention that because the stock carbs I removed work absolutely perfectly...of that I am certain. The only mods to the bike are a Dyna/S electronic ignition, Dyna coils and Dyna wires. New spark plugs were installed for this testing.

On the day before I began testing this single carb conversion, I ran the stock carbs on the road and did some baseline performance testing: observed top speed + timed "roll-on" accelerations (the weather was 60 degrees Fahrenheit with low humidity).

Initial Observations:

1. The construction, "fit and finish" and selection of components for this kit is very good. The materials and welds are quite excellent.

2. Surprisingly, the kit arrived in a large box packed very poorly. There was no bubble wrap or other protective material whatsoever. The plenum, chrome air cleaner assembly and lid were all loose and rattling around in the box. Worse, a portion of the carb's electric choke was actually protruding through the side of the box. For the nearly $500 purchase price (with shipping and tax), you might reasonably expect the unit to be packed carefully to avoid damage. Fortunately, there was no apparent damage to any of the kit components in this case.

Note: As of this writing (November 3, 2009) the price for this setup has increased to $498.00 + shipping and tax.

Shipped without protective packing materials. The carb's electric choke was actually poking through the box when it arrived.

3. The website supporting this product promises an "Instruction Book," but only one page of instructions was provided. A reference on the one page provided is made to a second page which was not included either.

Single page of instructions provided

4. This kit is marketed to all: "…Gold Wing motorcycles of the 1000 cc and 1100 cc class of engines built between 1975-1983." No jetting / timing / tuning / installation advice is provided. My understanding is that this kit was developed using the GL1100 platform. No distinction made for differences that might be needed for GL1000 owners...especially the early GL1000s ('75-'77) with hotter cams.

5. No instructions were supplied for connecting the 12V wire for the electric choke. A wire with crimped connectors was provided. The connectors were not correct for the GL1000 wiring harness on the test bike.

6. No instructions were supplied for connecting the required ground wire for the electric choke. A wire with crimped connectors was provided. The connectors were not correct for the GL1000 wiring harness on the test bike.

7. The plenum miters were not jigged and welded accurately. There is a large difference in the angles of the left vs. right side. Result: the main top "tube" (where the carb mounts) is not parallel relative to the engine as it should be: Right side angle = 14 degrees. Left side angle = 6 degrees. The measured difference is 8 degrees. This unnecessarily tilts the entire carb assembly by a significant amount when installed. At first, I thought this might be a flaw unique to my kit, but a picture on the website supporting this product appears to show the same flaw.

Accuracy of plenum jigging needs attention

8. There are no OEM-style machined ridges on intake plenum runners to mate to the OEM intakes.

9. The heavy-duty intake clamps supplied with the kit are poorly matched to the task. They are serious "over kill" and very difficult to install. As big and stiff as they are, they are actually nearly undersized (on the diameter range) and they do not apply forces evenly. The supplied instructions indicate that these clamps should be installed "extra tight." Instead, I ended up substituting aircraft-style clamps that fit correctly, are easier to install and apply clamping forces much more evenly without distorting the rubber on the intakes. I used these clamps instead.

10. The throttle bracket is well made and the method used to connect the "pull" cable is simple and works well. There is no provision to use the OEM "pull" cable.

11. The supplied fuel hose was about 3/4" shorter than needed. One end was cut at a sharp, irregular angle. I substituted a slighter longer section of fuel hose from my own supplies.

12. No mention is made of the open vacuum port on the carb that must be capped for use on a GL1000.

13. The new Solex carb needed some minor setup "attention" before it was put into service. For example, I had to resolve a binding issue with the choke butterfly. Ditto for burr removal on the float pivot pin. There were a few other minor aggravations with the new carb.

Business Issues

1. I would characterize vendor communications on my transaction as "disappointing." The vendor was slow to respond to some questions. Some answers were not particularly useful and some questions were never answered.

2. The vendor charged sales tax even though I don't live in Tennessee and I also have a valid sales tax exemption. There was no response to my questions about this.

As Tested on 1975 GL1000 (shown with Randakk's High Performance Intake Clamps)

Performance?

This conversion is marketed as a "high performance" conversion according to the vendor's website, but…

1. The design is clearly biased toward low speed performance. It produced easy starts (cold or hot) with a reasonably quick, stabilized idle (better than most single carb conversions but no better than stock carbs). This is not surprising given the small plenum volume and small carb venturi.

2. Steady state operation at most speeds is sluggish but OK. Decelerations are handled without issue. There were no detectable backfires on overrun.

3. This is a non-sophisticated carb (Solex 30 PICT-1) with limited tuning flexibility, but I can't find much fault with the supplied jetting. Plug checks looked fine. However, this Solex was designed for small economy cars and is not particularly well suited for the demands of a high performance, 1 liter engine capable of 80+hp at the crank and 9,000+ rpms. The very high intake tract velocities generated by this particular combination are no doubt overwhelming the main jet's ability to deliver adequate fuel to support the power potential. There is a "saturation point" beyond which the main jet can't deliver any more fuel even as engine speed (and fuel demand) continue to rise. Likewise, the small main venturi is also "strangling" the engine of needed air at higher rpms. This is not a "jetting" problem, but a basic design limitation of the plenum design and the carb selected.

4. The plenum is sturdy and well constructed (aside from errors with the plenum miters mentioned above). This finish is excellent. The plenum is also compact...a generally good feature for "packaging" issues and to support a reliable idle. Beyond that, the plenum design doesn't have many desirable attributes. It imposes numerous fairly "abrupt" corners for the air/fuel charge to navigate that are less than ideal. The small volume of the central cross section of the plenum means that individual cylinders have to "compete" for the limited availability of air/fuel charge at higher rpms. I also question the usefulness of the small upright section of the plenum. In my view, this just adds another restriction and turn to the equation. Finally, due to cylinder offset and simplistic design - the flow path for cylinders #2 and #3 is more convoluted than for the other two cylinders. My judgment is that this plenum would fair poorly in any rigorous "flow" testing analysis. Plenums for single carb conversions necessarily require many decisions and compromises to fit the available real estate. The choices made with this plenum are fairly typical of such compromises.

5. There is very strong evidence of undercarburetion. The main venturi of the Solex carb chosen is only 24mm! Compare to the OEM setup which provides 4 carbs of 31mm each (for early GL1000)! Basic physics: this is a fairly restrictive detail that limits overall performance.

6. Acceleration transitions are problematic. This is true even at relatively low speeds. Off-idle transition is poor…especially if the throttle is opened abruptly. Accelerations from various speeds and various loads produced noticeable hesitations. Some of these throttle response issues are what I would call a full-out "bog." At first, I suspected a defective accelerator pump, but it was working properly. The accelerator pump is a useful feature. However, it is not perfectly calibrated to the task. The Solex is not as adjustable in this regard as typical Weber carbs. I did try the inboard cotter hole on the accelerator pump rod (the carb came with the cotter in the outboard hole) for more "shot" but this did not provide improvement. The vendor suggested that the discharge side of the pump (orifice in the end of the copper tube) needs more tuning to overcome this issue.

7. The electric choke works well (after some setup attention).

8. The idle easily stabilized at 1000 rpms, but there is strong evidence that carb's progression circuit is partially activated at idle as the idle mixture screw (aka "volume control screw" on a Solex) has very limited effect. This is not desirable and limits low speed tuning possibilities. By the way, the "volume control screw" on a Solex controls idle AIR not FUEL. So turning it "in" richens the mixture. This detail is backwards from the OEM Keihin carbs. The kit's lack of installation documentation on this single point is guaranteed to get some folks into trouble!

On the Road Trials

Note: the weather was 60-65 degrees Fahrenheit with low humidity on all days of testing.

1. Measured 50-70 mph roll-on times in high gear were 3.1 seconds slower (5 runs averaged) compared to properly functioning stock carbs tested on the same bike a few days earlier. That's a significant performance reduction. It's actually a significant safety issue as it limits the avoidance envelope available via acceleration. Note: For the conversion unit, I didn't start the stopwatch until after the acceleration "hesitation" was over. The roll-on times for the conversion unit would have been considerably longer if I had kept in "hesitation" time. I tried 60-80 mph roll-ons, but the performance of the conversion was too sluggish to bother with those comparisons.

2. The conversion unit struggled mightily to achieve a top speed of only 100 mph (indicated) on a straight and level stretch of road (no significant wind). Acceleration above 60 mph was quite sluggish. The single carb conversion was not able to pull to redline in fourth gear (a good running GL1000 is always able to do that). The same bike running its stock carbs accelerated briskly to 120 mph+ (indicated) without fuss in a considerably shorter stretch of the same road a few days earlier.

3. No carb icing was observed, but the plenum remained "cool to the touch" even when the engine was fully warmed up. That suggests the possibility of carb icing in some operational modes.

4. The power loss is very noticeable above 6,000 rpms.

5. In my experience, top gear "roll-on" times are highly predictive of dynamometer testing results (and less stressful on the equipment). These simple tests suggest a horsepower penalty in the neighborhood of at least 15-20% compared to properly functioning stock carbs.

Conclusions

This is a reasonably serviceable product. Once installed, it should be a reliable setup that should be fairly easy to maintain.

Contrary to claims, it is NOT a performance upgrade however. It falls short of the overall performance of the OEM carbs in virtually every respect. It also provides less performance than every other single carb conversion I have ever tested...notably those with 2 barrel, progressive carbs.

The performance penalty is a stiff price to pay for a good idle and quick starts.

This is not a "plug and play" setup. Considerable tuning refinement remains to be done. There are some relatively easy to fix issues that the vendor could address and correct that would improve performance somewhat. Other limitations are more problematic.

For example, carbs such as this prefer quite a bit of static advance. If I were interested (and I'm not!) I would substitute a later GL1000 "431" advancer and advance the static timing quite a bit. I would also recurve and restrict total dynamic advance. This would provide several benefits. I actually experimented with "over-advanced" timing settings. Low speed performance improved slightly, but pinging resulted under load. The early GL1000 "371" advancer provides too much dynamic advance for this strategy to work well.

I would also put some "by-pass" opening in the throttle butterfly to allow its position to be more "closed" at idle. This old hot rodder's trick would deactivate the progression circuit at idle and return sensitivity to the idle mixture screw ("volume control screw"). This would also help off-idle throttle response. This change would probably uncover the fact that idle jet size is not optimized. Tinkering with the air corrector jet would probably be worthwhile.

If I were really set on running a single Solex carb (and I'm not!), I'm not convinced that this is the best Solex carb to chose. The Solex 30 PICT-1 is an excellent carb, but there are other Solex carbs that would probably outperform it in this application. For example, there are similar Solex carbs with somewhat larger main venturies. Some have "power valves" and other desirable features.

Even with the tuning refinements I allude to above, there is no fix for the significant overall performance penalty this rig imposes. My testing reconfirms what I've learned about the built-in limitations of single carb conversions.

Due to various technical details engineered into these bikes by Honda, this conversion will probably work better with a GL1100. Likewise, a later GL1000 ('78-'79) would be a bit more tolerant of this conversion. I can flatly say that early GL1000s with the hotter cams ('75-'77) will not find this conversion very agreeable.

Respectfully, I'm convinced that many folks who tout performance claims for single carb conversions may never have experienced the exceptional performance stock carbs routinely deliver. The OEM carbs are back on this bike now and the stellar performance is restored.

This single carb conversion is available here:

L.D. Welch

LD's OL' Wings

Corryton, TN

website: http://ldwingnut.com/default.aspx

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