Tuning an XS650 engine for maximum performance

[Wharfcreek]

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

 

[scabber]

Have you cleaned the carbs and checked the float levels. A larger main jet should help the 79 carbs had 135 mains try a 140. The 2into 1 usually needs a jet change.

 

[NONclow]

'Uni' podsthumbsup, or K&N, Mikes XS perf, etc???thumbdown

 

[Wharfcreek]

Scabber...... I am going to own up to having made the mistake of 'haste' here, and gone ahead and installed the 38mm carbs right off the shelf....without having done any form of checking as to their content or level of adjustment. Likewise, when I pulled of the original 34's, I didn't check them either. So....spent last night correcting that error.

Turns out that the 34's I pulled off were jetted with mains at 137.5, and a pilot jet at 42.5. Also, the float levels in the 34's were set to low it was a wonder it ran at all. The bad news is that my 38's had 122.5 mains, and 27.5 pilot jets. Needles on the 38's are set one up from the bottom.....or what some would call the 4th step down. So, the low speed running mixture was actually pretty rich from the already high needle....which explains why I got pretty good performance at under 5000 RPM. But....that 122.5 appears to clearly be a limiting factor on the high end.

On the bright side, I had one more set of 38's in the bag.......which contained the same 27.5 pilot jets, but had the 135 mains that you refer to. So, out with the 122.5's, and 'in' with the 135s. While I was at it I corrected the float levels in the 34's, which involved some repair on one float pivot. Like SO many things I've found wrong with this bike.......someone had slightly 'bent' the pivot pin on one of the 34's......but I got it straightened out, polished up, and working smoothly.

I'm still a bit unsure of one aspect of all this.....that being the question of the relationship between the jet size and carb size. If the 34's used a 137.5 main and ran properly with that jet size.....does that mean that the 38's should also use the same jet size.......or is there no relationship at all? Honestly, my inclination is that there probably is a 'similarity'....but not a 'direct' relationship. And, other variables....like adjustable needle height on the 38's and NOT on the 34's (without doing some changing around).....that may come into play....as may emulsion tube, and even basic changes in overall design. But, clearly, the 122.5 was not big enough......so hopefully the 135 will improve the situation if not resolve it altogether. I may still have to get some larger pilot jets.....but, the bike was idling pretty smoothly.......so, maybe just go to a 30 or 32.5?

On the pilot jet issue....... I believe there is some interchangeability between the 34's and the 38s.....yes/no? That 42.5 seems HUGE to me......particularly with my bike idling pretty well on the 27.5. But, maybe someone can tell me if the two different carbs DO take the same pilot jets?

Final note to NONclow: Guess you're not a fan of those pods from Mike's? I have to admit that the pods I have are NOT from an English-speaking nation. I bought a bunch of them years ago.....used them on other bikes....and never had a problem with them. But, I've also had some FAR superior type units......and if I had a set of those now I'd quickly make the change. I may make a change down the road....but for now....these will have to do!!

Thanks for the comments scabber and NONclow! Further comments welcome!

WC

 

[NONclow]

That sounds OK.... for now. I'd also inspect and pay close attention to the top portion of the mounting area of the pod to make sure it doesn't hinder, block partially, or dissrupt the airflow into the vacuum slide slot in the bellmouth of each carb. They can be trimmed to work, but these CV carbs tend to open the slide more thoroughly at high R's with a tubular tract that retains the intake velocity.

 

[Brian902]

You may want to visit this post for some excellent info;

http://www.xs650.com/forum/showthread.php?t=43

 

[scabber]

Have you read the carb guide. Yamaha changed the BS 38 carbs several times over the years the 122.5 indicates the carb set is 76 or 77 carbs. The set with the 135 main came in the 78 or 79. Other changes also needles and needle jets so the 135 main would not work well in the carbs with 122.5 main. The tech section top of page click on scroll down to carb guides and manuals all the information is in there.

 

[jd750ace]

Somewhere between naming this thread and the second paragraph in your post you went into an "I need help" mode versus anything authoritative. I'm confused. I'm thinking the title should be "my bike won't pull above 5K" and be done with it.

Posted via Mobile

 

[5twins]

I don't mean to sound like a broken record but reading the Carb Guide should answer just about every question you've asked. That being said, I will touch on a couple points .....

First, you must absolutely I.D. the year of the carb set you're working with. While they all may look similar from the outside, there were many internal changes over the model run. Re-jetting is based on what was stock in the year of the carb set, not for the year of the bike. Red flags go up for me when you mention finding 122.5 mains in the one set of BS38s you have and 135s in the other. As mentioned, that could indicate a '76-'77 set (with the 122.5 mains) and a '78-'79 set (the 135s). Those two sets are very, very different when it comes to jetting specs. They also have different slide assemblies and needles that differ in length by 5mm.

Two types of pilots were used in the 650 carbs over the years, the VM22/210 and the BS30/96. Externally, they look the same and will physically swap, but internally they are quite different which effects the way they flow. The carb sets are designed to use one type or the other and you must use the correct type for that carb set.

The BS30/96 type flow the already made mix of fuel and air. That's why they are sized so large, in the 40s. The VM22/210 type flow just fuel, the air being added after the jet. That's why they are sized smaller in the 20s.

The K&N pleated element style pods just don't work well on a CV carb. The pleats create turbulence in the air flow, especially around the outer edges where the air first enters through the pleats. The vacuum inlet port for the slide is in this outer edge area so this messes with the slide lift. It is most pronounced in the upper midrange-lower main area of operation, say 4 to 5 or 5.5K RPMs. This is where the slide is going through its initial and lower part of lifting. Instead of doing so smoothly, it "flutters".

Most 2-1 exhausts are not designed for ideal performance but rather just to bolt up properly and fit on the bike. Header lengths are often wrong along with collector diameters (where the "2" become "1"). Because of this, they can be difficult to tune for. Some have had some luck with sleeving the pipe down with inserts at the collector.

 

[650performance]

While you freshened the module, plugs, etc. you never said that you ensured it is timed properly. You really need to carefully check the timing with a light while the engine is running to make sure the base timing is correct and that it is advancing as desired. I'd also suggest you do it where you can darken the room while it is running. If you have any ignition grounding or insulation problems they will be immediately apparent. The problem you are describing - white plugs, no ooomph over 5,000 - may be that simple.

 

[Wharfcreek]

OK.....finally back into this again after a few days away from it. First, "THANKS!!" to all who have posted!! LOTS of good info, and I really appreciate it. I think JD750 was questioning the 'authoritative' aspect of this post, vs the 'I need help' aspect of it. And, to some degree I acquiesce to both. Some things I'm relatively sure of, others.....not so much. And, I'd be an idiot if I claimed to be an expert on any of this........so......what I offer as commentary based on my experience......I still put out there for 'correction' if I'm wrong.

650Perf.....you mention 'timing'.....but as far as I know, this bike has no accommodation for that. If I'm wrong....please correct me. But, I was under the impression that the 'crank sensor' as mounted in the stator......and the pick up as mounted in the field coils.....that was all a 'fixed' set-up. And, that all the timing 'advance' curves were part of what was programmed into the ignition module. So, I openly admit that I've done nothing with that aspect of things....as I didn't think I could.

Back to the carbs: Let me begin by saying that the pods I've got are 'crap'....with a capital 'C'!! I guess they work well for some purposes.....but with them installed the engine continues to break up at 5K RPM +. With them OFF....the engine runs REALLY well!! In fact, when the bike hits 5K in 2nd gear, it pulls the clutch loose. Needless to say.....3rd, 4th, and 5th are equally limited and substantially worse as you go up the gearbox. I guess with that header on it there's some kind of tuning curve sensitivity that figures in at about 5K.....as that's where it breaks up with the pods on.....and that's where the ending HP and Torque really kicks in with the pods off. So.....while there's still clearly some further tuning to do.......the 'pods' I've got....have GOT TO GO~!! Now the question is: With what do I replace them? I think NonClow mentioned UniPods. I'll have to look in to those.....and if anyone knows a source for them, please advise me.

As to the rest of this.......I REALLY appreciate the advice! I'm incline to think I'm very close here....with now a 'clutch' issue to deal with. I guess one thing always leads to another. But, I certainly don't mind buying a new clutch for the bike....particularly knowing that it's needed because the engine is putting out the kind of power I believed it should have. Now at least I know I'm heading in the right direction. I suppose I could drop the main jet size from the 135 to something less and see if that would work better with the pods I have.....but that means I'd be 'accommodating' the air flow restriction....which having a header on the other end is kind of hypocritical. Perhaps going to a 2-into-2 system would be a better choice????

More comments welcome..........

Many thanks!! Tom D.

 

[5twins]

Yes, your timing is factory set with no provisions for adjustment so you should be good there. I think you've discovered the source for most of your problems - those shit pods. Yes, get UNIs. For your BS34s, you will want the UP 4229s or UP4229STs (dual layer). Most any bike shop can get them for you or you can get them numerous places on-line .....

http://www.amazon.com/Uni-Filter-UP...ref=lh_ni_t?ie=UTF8&psc=1&smid=A1TFBK3C7LO58P

http://www.amazon.com/Uni-2-Stage-S...413919174&sr=1-2&keywords=uni+filter+up4229st

Jetting for "pipes and pods" has been pretty well documented for these bikes. Those kind of mods aren't that major but do require some carb tuning on these bikes. Your BS34s were the last and leanest carbs used on the 650 (E.P.A. mandates) so do need some tweaks, more so than some of the earlier, richer from the factory sets. The usual routine is one up on the pilots and one to three up on the mains. Now, if you had only one mod like just the pods or just the pipes, one size up on the mains might do. But you have pods AND pipes, so figure at least two sizes up for the mains. And then, you should probably try 3 sizes up just to insure the 2 size increase is best.

 

[Wharfcreek]

5twins, many thanks for the comments....though I think I should re-state that the bike no longer has the 34s on it.....but now has 38s. Your comments in post #9 are most helpful.....but do leave me with a question: You mention '20s' and '40s' and I'm not quite sure what you mean about that? Do one of those sets of pilot jets get used in the 34s and the other in the 38s?

I've measured the outside diameter of the two carbs in the 'air intake' side....and it looks like the OD of the 38s is actually a bit smaller at 52mm than the 34s....which measure 54mm. I'll check on the Uni-Pod avilability on those amazon sites......and check around and see what I can come up with. I would not have guessed that those 'Chinese' pods would have had that adverse an affect on performance. Like I said....used 'em before without this kind of result. Won't EVER use them again!!

For the record....... I know I'm cutting some corners here and there. But, it was never my intention to 'renew' this motorcycle......just get it running as best I could and then ride it some. In reality......there's a pretty substantial list of things the bike needs; ie rear shocks, front fork tubes, tire, a 'proper' battery, not to mention that I'm still without the 'ears' I ordered to mount a head light on. Plus I'm also sans turn signals at the moment. So...this is a 'work-in-progress' and will likely stay that way for a while yet. And, to say it's 'cosmetically challenged' would be hugely generous!! But....it's a 'budget' project......and my intention is to use as much of what I've got around here as I can....which included the 'upgrade' of the carbs. In retrospect, I'm not sure that the 38s are really all that superior....although they evidently do have some distinct advantages in terms of being both a bit more 'tunable'.....and flowing more air (or some some 'experts' have told me).

Anyway...... clearly my 'shit' pods are just that!! And, replacing them is my next order of business. I'm also going to need to address the clutch.....so if anyone has any recommendations of which way to go with that, I'll appreciate any advice there as well. Hopefully when I'm done...while I won't have a 'pretty' bike....I will have a reliable and well-performing XS that I can be both proud of and enjoy on the road!!

TSD