Wharfcreek
Near 50 Yrs of Experience
My thoughts are that this subject should become a separate category, as it contains aspects of the Carb section, Ignition, Electrical, Engine Mechanical, and even the exhaust section. So, that thought in mind, I go forward:
I am fortunate enough to have have owned a good many XS650s in past years, and currently have 4 still title in my name, though 2 are in the possession of each of my 2 sons. But, this post is about an '81 that I'm going through right now. And, now that it's 'running' and 'drivable'.....I have to address the issue of 'fine-tuning' the engine to get it up to optimum performance level.
This process starts with certain 'mechanical' and 'electrical' aspects being confirmed as correct. In the case of my '81, as it's an electronic ignition bike, I've made sure those components are now working correctly. Mine originally had a bad Ignition Module....but I'm currently using a 'spare' that my son has for his '81.....and have acquired a replacement for my bike that I'm waiting to arrive. But, I know the one I have is good......and the bike is now running smoothly.....up to about 5K RPMs. I should also mention that this bike has had it's valves properly adjusted, and it's compression is confirmed as well balanced and at proper levels. Final note: It also has an added 2-into-1 exhaust....and the 'stock' BS34 carbs have been replaced with older (and as some would say: 'Better') 38 mm units from an old '79 'Standard' I had years ago and changed over to some Mikuni Flat-Slide units. AND....the stock air box has been removed and I now have 'pod' air cleaners on each carb.
So....why this post: Well......as previously mentioned, I have a performance problem at RPM above 5K. Were this a 'points' bike, I might be suspect of those. But, as a EI bike......and one with a known good Ign. Module AND a new coil, AND new plugs......I'm thinking: CARBS!! Also, I've run into this in the past, and found it to be a carb problem where some basic 'tuning' solved things nicely. Also as previously mentioned, this bike has a 2-into-1 exhaust system and individual 'pod' air filters, so....more 'air' generally creates a need for more 'fuel'. So.....a better breathing 'extractor' type exhaust, less air restraint on the 'intake' side.....and this stacks up to be a 'lean' miss over 5K. Pulling a spark plug and seeing a damn near clean, white insulator more or less 'confirms' that lean diagnosis as being true. So.....how does one correct this???
Note that the problem is above 5K, so what does that mean? Well.....it means it's idling pretty smoothly....and 'under' 5K....it seems to perform pretty well. That being the case, one can assume that the idle jet and idle circuit is not in need of attention. And...that the requirement for more fuel is most critical at higher RPM.....meaning the main jet is probably too small. BUT...what about slide needle position? Well.....good question!! Here's my thought on it....and where perhaps some further entries on this thread may make it a potential 'sticky' for 'tuners of the future. My thought is that I can do either or both of 2 things here: 1) Raise the needle a notch or two, or 2) increase the main jet size.
Typically the 'needle/main' relationship shows up as a problem in 'off-idle' and mid rpm ranges.....or so I've experienced. This is really a function of throttle position. When running on 'part-throttle'....you use the relationship between the main and needle ...where as at WOT (wide open throttle) you're running basically on the main jet. With more air on the intake, and better breathing out the exhaust, I'm thinking I need a bigger main jet. I may be able improve my situation some by raising the needle......but I think I'll still have some remaining symptoms that won't go away until that main jet is opened up some.
So..........I now conclude this post with hopes that some experienced 'tuners' will add their comments and provide the benefit of their knowledge and expertise. And, some additional posts may make this worthy of long term redemption for 'future tuners'!
With much appreciation............
WC
I am fortunate enough to have have owned a good many XS650s in past years, and currently have 4 still title in my name, though 2 are in the possession of each of my 2 sons. But, this post is about an '81 that I'm going through right now. And, now that it's 'running' and 'drivable'.....I have to address the issue of 'fine-tuning' the engine to get it up to optimum performance level.
This process starts with certain 'mechanical' and 'electrical' aspects being confirmed as correct. In the case of my '81, as it's an electronic ignition bike, I've made sure those components are now working correctly. Mine originally had a bad Ignition Module....but I'm currently using a 'spare' that my son has for his '81.....and have acquired a replacement for my bike that I'm waiting to arrive. But, I know the one I have is good......and the bike is now running smoothly.....up to about 5K RPMs. I should also mention that this bike has had it's valves properly adjusted, and it's compression is confirmed as well balanced and at proper levels. Final note: It also has an added 2-into-1 exhaust....and the 'stock' BS34 carbs have been replaced with older (and as some would say: 'Better') 38 mm units from an old '79 'Standard' I had years ago and changed over to some Mikuni Flat-Slide units. AND....the stock air box has been removed and I now have 'pod' air cleaners on each carb.
So....why this post: Well......as previously mentioned, I have a performance problem at RPM above 5K. Were this a 'points' bike, I might be suspect of those. But, as a EI bike......and one with a known good Ign. Module AND a new coil, AND new plugs......I'm thinking: CARBS!! Also, I've run into this in the past, and found it to be a carb problem where some basic 'tuning' solved things nicely. Also as previously mentioned, this bike has a 2-into-1 exhaust system and individual 'pod' air filters, so....more 'air' generally creates a need for more 'fuel'. So.....a better breathing 'extractor' type exhaust, less air restraint on the 'intake' side.....and this stacks up to be a 'lean' miss over 5K. Pulling a spark plug and seeing a damn near clean, white insulator more or less 'confirms' that lean diagnosis as being true. So.....how does one correct this???
Note that the problem is above 5K, so what does that mean? Well.....it means it's idling pretty smoothly....and 'under' 5K....it seems to perform pretty well. That being the case, one can assume that the idle jet and idle circuit is not in need of attention. And...that the requirement for more fuel is most critical at higher RPM.....meaning the main jet is probably too small. BUT...what about slide needle position? Well.....good question!! Here's my thought on it....and where perhaps some further entries on this thread may make it a potential 'sticky' for 'tuners of the future. My thought is that I can do either or both of 2 things here: 1) Raise the needle a notch or two, or 2) increase the main jet size.
Typically the 'needle/main' relationship shows up as a problem in 'off-idle' and mid rpm ranges.....or so I've experienced. This is really a function of throttle position. When running on 'part-throttle'....you use the relationship between the main and needle ...where as at WOT (wide open throttle) you're running basically on the main jet. With more air on the intake, and better breathing out the exhaust, I'm thinking I need a bigger main jet. I may be able improve my situation some by raising the needle......but I think I'll still have some remaining symptoms that won't go away until that main jet is opened up some.
So..........I now conclude this post with hopes that some experienced 'tuners' will add their comments and provide the benefit of their knowledge and expertise. And, some additional posts may make this worthy of long term redemption for 'future tuners'!
With much appreciation............
WC
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