How to Cure "Off-Idle" Flat Spot

Allow me a little soapbox time. Early GL1000s are routinely criticized for having poor carburetion. The criticism has been repeated so often by so many that it has become accepted as fact. Here is my contrarian view: the early GL1000s have great carburetion set-ups! My personal '76 GL1000 show bike has flawless, near-linear carburetion…rheostat-like throttle response, no flat spots, starts easily, warms up fast enough and has a decent idle. My carbs are completely stock except for a few minor tweaks I make on all the carbs I service. I've owned and ridden many bikes. I believe the stock set-up on a GL1000, properly maintained and tuned delivers results as good as you can reasonably expect on a 4 cylinder, 4 carb bike of this vintage with this level of performance. In fact, GL1000 carburetion is easy to sort compared to some of the demonic setups I've encountered (my Weber-equipped Ducati 750 Nuevo Sport wins the prize as the worst I've ever personally grappled with).

The single carb conversion units which are available for GL1000s appear to be well engineered. However, my view is that this is an unnecessary "improvement." I'm convinced that if anyone bothered to do a dyno test, the stock set-up would produce more peak power and torque with flatter curves. Automotive-type carbs ( e.g., Holley) aren't normally expected to perform over the wide rpm range associated with motorcycle engines. Typically, they also don't tolerate much departure from steady vertical operation. Aggressive sustained cornering can become a problem with these carbs. A well-tuned, stock GL1000 will easily pull to red-line in 4th gear (being over-geared, fifth is another matter). I can't personally verify, but some claim single carb conversions "run out of steam" on top end. Some other points to keep in mind:

1975-1977 GL1000s are blessed with a Superbike-class engine. Although riders tend to think of them as "torquey" since there's plenty of low-end grunt and a heavy flywheel effect, it's actually a rather "peaky" engine. In fact, the torque peak is up around 6500 rpms in typical Honda fashion for performance engines from this era. Also, the cam specs (duration, lift and overlap) are fairly radical. Part of the issue with the GL1000 idle is the slightly "lumpy" cam that causes semi-irregular cylinder filling/firing at low speeds. Not as bad as a AA fuel dragster but very noticeable to a good ear. It just seems to be a fact that flat "boxer" engines like this (also Porsche, Subaru, VW, BMW, etc.) generally don't sound sweet at idle. It's unreasonable to expect a peaky engine, with a semi-radical cam to have a perfect idle. GL1000s will tolerate a great deal of low speed "lugging" before they complain…usually by fouling plugs. Personally, I make a conscious effort not to spend much time below 3,000 rpms.

GL1000s also have rather long intake runners. This is a good idea to increase torque especially given the cam timing. But, the penalty is longer warm up times. Once you understand the design intention (superbike performance), you have to reconcile this with the inspired and semi-illogical combination with a touring geometry frame and running gear. Personally, I love this quirky stealth combination, but your expectations have to be reasonable.

If your carbs are clean (I mean REALLY), all your vacuum hoses are good, your air-cut off valve fresh, choke linkage free, butterfly pivots not worn out, floats set and operating correctly, vacuum pistons free, air and fuel filters fresh and you've synchronized AFTER all other tune-up issues are perfect, you should have excellent results. The first point bears amplification. Cleaning a set of GL1000 carbs is straightforward, but tedious and time consuming (takes me about 6 hours total). Most dealer personnel don't have the aptitude or patience required to do a proper job. I've cleaned many carbs that were supposedly just cleaned by someone else and found lots of grunge in obvious places. You don't necessarily have to split the carbs from the plenum, but everything else has to come apart. For example, if you don't remove ALL the jets and emulsion tubes, you're wasting your time. Also, don't go to all the work of disassembly and cleaning without using new o-rings and blanking plugs when you reassemble. The action of the vacuum slides must be perfect. Generally, this means you must not only clean the slides and bores, but also CAREFULLY polish with crocus cloth (don't damage the locating bore pins!).

As mentioned previously, Robert Overby wrote a very interesting article titled "Improving Your GL1000's Performance" which appeared in Wing World in the June, 1995 issue.

In addition to the ignition quirks, he addressed another common problem with these bikes: the "off-idle" flat spot on '75-'77 models. These bikes have slightly different carb jetting, but they are all rather lean on the idle circuit. This makes them require excess choking (which wouldn't be so bad if the fast idle ramp was curved better). Worse, this typically results in a flat spot just above idle that most riders overcome by revving the engine slightly and slipping the clutch to move from rest. Once you've become accustomed to this compensation, you don't tend to notice the minor annoyance.

However, there is an easy fix. Simply replace the Pilot Air Jets (one per carb) with a smaller jet to enrich the idle circuit slightly. '75 and '77 models use stock Keihin Pilot Air Jets - size #110. All '76 models use a size #115. Robert recommends a replacement jet - size #98. The Honda part number for this replacement jet is 99164-601-0980. The good news is that these can easily be replaced without carb removal. They are mounted in the brass elbow on the carb end of the 4 short vacuum hoses coming from the carb plenum. The bad news is that these replacement jets were discontinued by Honda in 1996!

Here are 3 solutions:

1. Cheap, but tedious: Remove the stock pilot air jets, solder the orifice closed and re-drill an opening using a size #62 micro drill bit which is approx. 0.038" (for reference, a stock #110 jet is about a #57 micro drill size = 0.043" …the reduction in jet size is only about 11.6%). Replace the altered jets and you're done. Micro drill bits are available from a variety of sources that sell Dremel accessories. One good source for micro bits is Widget Supply: http://www.widgetsupply.com/

2. If you've recently bought carb rebuild kits, you probably have some leftover #60 secondary air jets. These are from the same jet "family" and can be drilled out with a size #62 micro drill per above.

3. Easy, but costs about $25: Secure 4 replacement jets from SUDCO with Keihin Part #99101-124-100 (SUDCO part # is 99101-ZF5-1000). These have the right size orifice for correction (size #100), but they aren't sized to screw into the elbow. Instead, here's what you do. Remove the stock jets (you won't be using them!). Then insert the jets from SUDCO into the end of the vacuum tube nearest the elbow. Push it in exactly 5/8" using a drift or piece of dowel. Make sure the slotted end is oriented away from the elbow. Re-attach the vacuum tube and you're done. By the way, I recommend you replace these vacuum hoses while you're at it. The correct size vacuum tubing is 5.3 mm id (good substitute is 7/32" id). While you're messing with vacuum hoses, you might as well replace the hose to the air cut-off valve. This hose is smaller...3.5 mm id (good substitute is 5/32" id). This mod REQUIRES that you keep the clamps to secure the vacuum hoses. Here's a link to SUDCO: http://www.sudco.com/

All 3 methods work great!

Note: This mod is for '75-'77 GL1000s...not required or necessary for later bikes.

Additional Note dated 5/22/03:

Recently, I discovered a small difference here. In '76, Honda slightly enlarged the 3 idle by-pass ports (located under the round aluminum "puck"). In '77, Honda reverted back to the original size for these by-pass ports. This explains why stock '75 and '77 carbs had #110 idle air jets while '76 carbs had #115 idle jets (more air was needed to balance the additional fuel passing through the slightly larger by-pass openings).

I've discovered that Robert Overby's recommendations (detailed above) regarding changing to #098 idle air jets is perfect for '75 and '77 bikes. However, on '76 bikes it's a bit more correction than optimal.

The solution for '76 carbs is to use #105 idle air jets. Basically, you can make these following method #1 or #2 above, but use a #59 micro drill bit instead of a #62 (I know it's confusing, but micro drills get bigger as the micro drill number gets smaller). This is the set-up I run in my own '76 show bike.

By the way, I live at about 350 feet above sea level. If you ride extensively at high altitude (above 6000 feet) I would forego this "off-idle" mod.

Return to main Tech Tips Index