Randall Washington, aka "Randakk"
President and Chief Executive Rider
Chapel Hill, NC USA
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Return to main Tech Tips Index Early vs. Late Carb Mixing and Matching / GL1100 Carbs on GL1000? / GL1000 Carb SpecsYou might consider this item: Randakk's GL1000 Carb Rebuild Video GL1000 Carb Interchange Issues I get lots of questions about mixing and matching GL1000 carbs. My general advice is to avoid doing this. It can get you into real trouble. When buying new or salvage parts, always try to match the correct carb code for the carbs on your bike. In addition, you must verify that the carb rack on your bike is correct for the engine actually installed in your bike. Unless you know the entire history of a bike, you can't rule out that a previous owner made an ill-advised carb and/or engine swap. Believe me, this happens all the time. I cringe every time I see an EBAY ad that claims "GL1000 carbs from 197X GL1000...fits all '75-'79 GL1000s." They will "fit" and bolt up fine, but unless they are carefully matched to the actual cams in your engine, they will not deliver optimal results. You can chase your tail forever trying to resolve the tuning challenges you create by running mismatched carbs with a given GL1000 engine. The reduction in main venturi bore diameter from 32 mm to 31 mm in 1978 is one of the main reasons you should never mix early engines with late carbs (or vice versa). Carburetion for the later engines is significantly different to match the "softer" cams used in these engines. See the tables below for complete GL1000 Carb Specs. See also: Emulsion Tube Variations See also: GL1000 Carb Code Location See also: GL1000 CV Slide / Cap Variations GL1000 Carb Codes - US Market"Early" GL1000 Carb Series
"Late" GL1000 Carb Series
Carb Interchange Rules: 1. Never mix carbs from early series to an engine in the later series. 2. Never mix carbs from late series to an engine in the early series. 3. Never mix carbs within one rack that have different codes. Why? Externally, all GL1000 look about the same and there are quite a few interchangeable parts. However, there are quite a few significant changes from year-to-year involving such things as:
Then, Honda made really big changes to the GL1000 engine in '78 vs. the earlier models. The main change was very different, "softer" cams designed to improve low and mid range running at the expense of top end performance. GL1000 Camshaft Specifications (per page 5-2 of Official Honda Workshop Manual)
Lift values did not change. Unfortunately, the new cam delivered only minor benefits to low and midrange running but it really killed the top end. The new cams necessitated a major revision in carburetion specs as well. The most important change was a 1 mm reduction in main venturi size which has a big impact on flow characteristics. Initial timing advance (and hence the corresponding reference mark on the flywheel) and dynamic ignition advance was changed on these later engines as well. For these reasons, the jetting in the 769A and 771A carbs is quite different from the earlier specs. Running GL1100 Carbs on a GL1000? Warning: Controversial Material! Update: January 29, 2008:
Putting GL1100 carbs on GL1000s is oft-repeated advice from some. Based on my experience, I personally advise against it. I consider it so-so advice for '78-'79 GL1000s...worse advice for '75-'77 GL1000s. This is because the early GL1000s have hotter cams and larger main carb venturis than later GL1000s and GL1100s (discussed above). Other points:
You may be tempted to trash your GL1000 carbs and install GL1100 carbs. These will certainly run, but in my experience not as well as the correct GL1000 carbs. Most people who invest in this swap report significant performance "improvements." Typically, they are comparing their results with dirty, poorly maintained and malfunctioning original carbs. The accelerator pump advantage is offset by:
I have no real axe to grind since I sell rebuild kits for both the GL1000 and GL1100 (and GL1200 as well). Folks are free to make their own choice. The GL1100 kits are naturally more expensive since there are 4 air cutoff valves + an accelerator pump kit. I don't believe I am as smart as the Honda engineers who calibrated these setups. That's why I'm a strong advocate of the OEM setup You are free to decide for yourself. Real World? I recently put a freshly rebuilt set of GL1100 carbs (rebuilt to a very high standard by me) on one of my '75 GL1000s as a test. It ran fine. It had an OK idle, good off-idle performance, and a decent midrange If you are perceptive and have a finely calibrated butt, the accelerator pump operation is noticed as a positive. If you had never ridden this bike with the "correct" carbs, you might think the performance was great. Not me. I consider throttle response a top priority on any bike I ride. The GL1100 setup was acceptable, but not an improvement. There was no perceptible increase in midrange torque as advocates of this swap insist. Worse - it was sluggish at mid-to-higher rpms compared to the correct and perfect GL1000 carbs I removed (they were the carbs I rebuilt for my GL1000 Carb Rebuild Video). No "theory" is necessary to understand the limitation on max performance of decreased main venturi size! I could not get the GL1100 carbs off fast enough! Why settle for OK, when you can have stellar performance? Why give back performance Honda has already bestowed on these bikes? I understand that some folks don't care about max. performance. That's fine, but I do. Note: Both of the carb sets (GL1000 and GL1100) in this test had my proprietary mods as well as the "normal" mods discussed in these Tech Tips as well as the performance mod discussed in the video, so it was "apple-to-apples." Installing GL1100 carbs on a GL1000 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. Final Detail If you make this swap, be aware that GL1100 carbs have a port for the vacuum advance mechanism found on GL1100 engines. These are not present on GL1000s, so you will need to cap this vacuum port.
GL1000 Carb Specs - US Market"Early" GL1000 Carb Series
"Late" GL1000 Carb Series
Note 1: To remedy the infamous "off idle glitch" common to all "early" GL1000s, make the mod detailed here (not necessary for "late" GL1000s). Note 2: Beware of the "Air Jet Mix-up" problem in the Clymer workshop manual for the GL1000. Details here. Note 3: The reduction in main venturi bore diameter from 32 mm to 31 mm in 1978 is one of the main reasons you should never mix early engines with late carbs (or vice versa). Carburetion for the later engines is significantly different to match the "softer" cams used in these engines. Note 4: To my knowledge, this is the only place you will find OEM CV Needle ID numbers published. Keep in mind that each aftermarket carb kit supplier uses their own needle identification scheme which will not match these numbers. Very important - aftermarket needles are often not matched well to the OEM specs. For this reason, I recommend reusing the original needles whenever possible. Note 5 : See also: Emulsion Tube Variations 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 |