You might consider this item: Randakk’s GL1000 Carb Rebuild Video
Honda GL1000 Carbs Restored by Randakk Customer Jouko Tomminen of Lahti, Finland
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 on 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:
• Jet needles
• Shape of CV slide (“cutout”)
• Bushing material in carb caps
• Size of idle by-pass port ports
• Throttle linkage
• Choke linkage
• Synchronization adjuster screws / brackets
• Carb stays
• Main fuel jet emulsion tube
• Secondary main fuel jet emulsion tube
• Fast idle mechanism
• Throttle bell crank / spring
• Carb-to-plenum locating dowels (not present on 755A carbs)
• Intake runner bore diameter
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 were 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:
I get a few email inquiries about putting GL1100 carbs on a GL1000. Even though I’ve thoroughly evaluated this swap myself, I have been crucified online by a few for daring to criticize this swap. Typically, my views are distorted or misrepresented. Frankly, I don’t care what you decide. Henceforth, I will make no further public comment about this topic beyond this Tech Tip. This 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 a further pointless debate. If misinformation creeps into online discussions, that’s not my problem and I don’t feel any compunction to correct or dispute anyone over this. KEEP ME OUT OF THIS PLEASE!
Feel free to consider this information, but do me a favor and please don’t post this link into any of these silly “debates.”
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).
• GL1100 carbs have an accelerator pump designed to provide improved “off idle” response …a good feature.
• But, GL1000 carb “off idle” performance is easily improved via this inexpensive mod detailed here
• Due to the smaller main venturis, GL1100 carbs on early GL1000 models will reduce peak power output (simple physics).
• GL1100 carb jetting is not optimized for either early or late GL1000s.
• GL1100 carbs have an accelerator pump (1) and 4 air cutoff valves (4 vs. 1). This makes overhauls considerably more expensive. This also introduces additional points of failure as well. That’s why Honda eliminated the accelerator pump and reverted to a single air cutoff valve on the GL1200 carb design. Honda was responding to the high volume of complaints they received from dealers and service techs about the unnecessary complexity of GL1100 carbs.
• GL1000 and GL1100 intake runners are sized differently (internally). Mismatching the intake runners from one model to the other will result in an unnecessary “step” at the cylinder head opening. This is bad for peak performance.
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:
• Expense and complexity of rebuild – you will need 4 aircut-off kits + an accelerator pump kit
• Mismatched jetting…esp. On ’75-’77 models
• Reduction in top end performance.
• Reduction in bike’s collector value
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.
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.
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 the 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
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