GL1000 Engine pieces parts
Posted: Mon May 04, 2009 12:43 pm
GL1000 engine differences
We have had many discussions recently regarding the interchangeability of parts on the 1000 engines. Mixing and matching of 1000 parts happens all the time. Playing with the mix of engine parts is not as simple as “will it bolt up?” Odds are yes it will bolt up and yes it will probably run but that is not the whole story.
The 75 was produced with highest horsepower and peak performance of all the 1000’s and was rated at 84 HP. Each year a few things changed but the dramatic changes happened between the 1977 and 1978 model years. The most common place for these changes were the Carburetors. The changes were driven by the times as Emissions controls were becoming commonplace and by the development of the GL1000 as a touring machine. The Torque curve was flattened to provide more mid-range torque at the expense of some peak HP. Some reports had the 78/79 series as low as 79 or 80 HP.
Carburetion changed every year, the biggest change was from the 77 to the 78 models with the reduction of the venturi size by 1 mm and more changes in the jetting profiles. The CV slides also changed several times with the most dramatic of those changes happening between the 75 and 76 Model years. The 75 slides were shaped at the bottom so when they were raised the entire carb throat was open. The Carb linkage also changed between 75 and 76.
---------------1975---------1976---------1977---------1978---------1978EC---------1979
Venturi bore
Diameter----32mm-------32mm-------32mm-------31mm--------31mm----------31mm
Main Fuel Jets
Pri-------------65-----------62-----------62------------60-----------60--------------60
Sec-----------125----------120----------120-----------120----------120-------------120
Slow Fuel Jet-35-----------35------------35------------35-----------35--------------35
Main Air Jets
Pri------------120----------120-----------120-----------120----------140-------------140
Sec------------60-----------60-------------60------------60-----------60--------------60
Slow Air Jet--110----------115-----------110-----------120----------130-------------130
The Cams, Valve timing, ignition timing, and exhaust system also all changed between the 77 and 78 model years.
The Camshaft peak intake lift was reduced from 37.07- 37.23 in 1977 to 36.72-36.88 in 1978
This reflects the Valve timing changes driven by the reshaping of the cams.
Camshafts-------------1975 - 1977-----------1978 – 1979
Intake opens:----------5 Degrees BTDC-------5 Degrees BTDC
Intake closes:----------50 Degrees ABDC------35 Degrees ABDC
Exhaust opens:---------50 Degrees BBDC------40 Degrees BBDC
Exhaust closes:---------5 Degrees ATDC-------5 Degrees ATDC
The initial Ignition Timing point changed from 5 BTDC in the 75-77 models to 10 degrees BTDC on the 78/79 models. Also the timing advance curve was redesigned by the use of a different timing advance unit mounted behind the points plate. This resulted in the peak advance of 32 degrees being reduced to 27 degrees on the 78/79 model.
The exhaust was completely redesigned in 78 to a 3 piece muffler unit from the single piece Horseshoe. The lack of exhaust noise of the earlier models was a complaint for many owners. The redesign in 78 increased the noise and changed flow characteristics to further the progression toward the touring engine profile and away from the sport/performance model of the earlier years.
Those are the changes that were performance related. Numerous other small changes were made during the various production runs to address other issues. The cases were slightly modified starting in 78 in an effort to mask certain engine noises. And the rear cover was modified in 78 also, removing the kick-starter option.
Everything above is factual to the best of my knowledge. This next part is my advice/opinion/observations.
Each year we get hit with questions like “will my 79 run better with 77 carbs” or “can I improve the performance of my 78 by running 75 heads” or “will my 76 be faster if I switch to straight pipes” or “a guy told me…….” etc etc etc etc etc etc……
If you are trying to get a bike running and all you have are 77 carbs to put on a 79 go for it. Will it run? Yes it will, will it run correctly? No it will not, will it be adequate? Well that is entirely up to you. If you ruin a head and need to replace it then replace it with the same series i.e. 75-77 or 78/79. Do not buy the first one you see on Ebay just because you can’t wait any longer. If you want to bolt on 75 parts to a 78/79 to improve performance then you need to do the whole thing. Carbs, Timing Index, Timing Advance, and Cams. Doing anything less will likely result in a loss of performance vice any supposed gain.
The Engineers that designed these bikes had test equipment we can only dream of, and data built on years of tests and decades of educated experience. The intricacies of ignition timing, fuel air mix, valve timing, combustion chamber flow dynamics, exhaust scavenging all have to be balanced. Bottom line is that I do not doubt that performance improvements can be made to the stock setup, but the odds of getting the right combination by bolting up pieces parts from another model are miniscule. Unless you really really really know what you are doing if you want to get maximum performance from your GL1000 keep it stock and in good tune.
We have had many discussions recently regarding the interchangeability of parts on the 1000 engines. Mixing and matching of 1000 parts happens all the time. Playing with the mix of engine parts is not as simple as “will it bolt up?” Odds are yes it will bolt up and yes it will probably run but that is not the whole story.
The 75 was produced with highest horsepower and peak performance of all the 1000’s and was rated at 84 HP. Each year a few things changed but the dramatic changes happened between the 1977 and 1978 model years. The most common place for these changes were the Carburetors. The changes were driven by the times as Emissions controls were becoming commonplace and by the development of the GL1000 as a touring machine. The Torque curve was flattened to provide more mid-range torque at the expense of some peak HP. Some reports had the 78/79 series as low as 79 or 80 HP.
Carburetion changed every year, the biggest change was from the 77 to the 78 models with the reduction of the venturi size by 1 mm and more changes in the jetting profiles. The CV slides also changed several times with the most dramatic of those changes happening between the 75 and 76 Model years. The 75 slides were shaped at the bottom so when they were raised the entire carb throat was open. The Carb linkage also changed between 75 and 76.
---------------1975---------1976---------1977---------1978---------1978EC---------1979
Venturi bore
Diameter----32mm-------32mm-------32mm-------31mm--------31mm----------31mm
Main Fuel Jets
Pri-------------65-----------62-----------62------------60-----------60--------------60
Sec-----------125----------120----------120-----------120----------120-------------120
Slow Fuel Jet-35-----------35------------35------------35-----------35--------------35
Main Air Jets
Pri------------120----------120-----------120-----------120----------140-------------140
Sec------------60-----------60-------------60------------60-----------60--------------60
Slow Air Jet--110----------115-----------110-----------120----------130-------------130
The Cams, Valve timing, ignition timing, and exhaust system also all changed between the 77 and 78 model years.
The Camshaft peak intake lift was reduced from 37.07- 37.23 in 1977 to 36.72-36.88 in 1978
This reflects the Valve timing changes driven by the reshaping of the cams.
Camshafts-------------1975 - 1977-----------1978 – 1979
Intake opens:----------5 Degrees BTDC-------5 Degrees BTDC
Intake closes:----------50 Degrees ABDC------35 Degrees ABDC
Exhaust opens:---------50 Degrees BBDC------40 Degrees BBDC
Exhaust closes:---------5 Degrees ATDC-------5 Degrees ATDC
The initial Ignition Timing point changed from 5 BTDC in the 75-77 models to 10 degrees BTDC on the 78/79 models. Also the timing advance curve was redesigned by the use of a different timing advance unit mounted behind the points plate. This resulted in the peak advance of 32 degrees being reduced to 27 degrees on the 78/79 model.
The exhaust was completely redesigned in 78 to a 3 piece muffler unit from the single piece Horseshoe. The lack of exhaust noise of the earlier models was a complaint for many owners. The redesign in 78 increased the noise and changed flow characteristics to further the progression toward the touring engine profile and away from the sport/performance model of the earlier years.
Those are the changes that were performance related. Numerous other small changes were made during the various production runs to address other issues. The cases were slightly modified starting in 78 in an effort to mask certain engine noises. And the rear cover was modified in 78 also, removing the kick-starter option.
Everything above is factual to the best of my knowledge. This next part is my advice/opinion/observations.
Each year we get hit with questions like “will my 79 run better with 77 carbs” or “can I improve the performance of my 78 by running 75 heads” or “will my 76 be faster if I switch to straight pipes” or “a guy told me…….” etc etc etc etc etc etc……
If you are trying to get a bike running and all you have are 77 carbs to put on a 79 go for it. Will it run? Yes it will, will it run correctly? No it will not, will it be adequate? Well that is entirely up to you. If you ruin a head and need to replace it then replace it with the same series i.e. 75-77 or 78/79. Do not buy the first one you see on Ebay just because you can’t wait any longer. If you want to bolt on 75 parts to a 78/79 to improve performance then you need to do the whole thing. Carbs, Timing Index, Timing Advance, and Cams. Doing anything less will likely result in a loss of performance vice any supposed gain.
The Engineers that designed these bikes had test equipment we can only dream of, and data built on years of tests and decades of educated experience. The intricacies of ignition timing, fuel air mix, valve timing, combustion chamber flow dynamics, exhaust scavenging all have to be balanced. Bottom line is that I do not doubt that performance improvements can be made to the stock setup, but the odds of getting the right combination by bolting up pieces parts from another model are miniscule. Unless you really really really know what you are doing if you want to get maximum performance from your GL1000 keep it stock and in good tune.
