Running lean or rich question?

[XS650D]

I have the bike running really good I feel.My only issue which may be normal is when in top gear cruising along at approx 4000 rpm with throttle barely cracked I get a very slight hesitation were it feels slightly jerky.
Is that a sign of rich or lean and if so what carb circuit should I be looking at ie: fuel/air screws, pilot jets or needle clip position.It will not do this while putting around the parking lot in 2nd gear with throttle barely cracked and will pull smoothly without hesitation or stumbling with approx 1/2 throttle roll-on.Full throttle is perfect. Could this be right were the Diaphragm just starts to lift ?

 

[TwoManyXS1Bs]

... when in top gear cruising along at approx 4000 rpm with throttle barely cracked I get a very slight hesitation were it feels slightly jerky.

Is that a sign of rich or lean

Coupla ways to figure that.

Apply a little choke when it does that, note any differences.

Put in new plugs, maybe alongside the road, to minimize running at speeds/conditions different from the test condition. Immediately enter that questionable cruise mode, and stay there awhile. Then, kill the engine, coast to a stop, do a plug read...

 

[Bob Kelly III]

Capatal thinking 2M ! perfect way to tell if it's lean or rich at that speed ! good show !
i'm betting too lean !
although I've played some with the cutaway/needle jet settings I have always come back to the stock position in the end
even with massive porting I've never had to change the needle jet.... main jet, Yes !!!
I have often wondered how they figured out HOW to get the right amount of air through the emulsifier tube at 3/4 throttle but they certainly did figure it out ! LOL
Makes me wonder if the designers had a micron counter of some sort to figure these things out ! HAHAHA!
..... you'ed certainly have to be better at math than I am, to do it mathematically ! LOL and it's not like a manufacturer to do it by trial and error method.....
....
Bob......

 

[XS650D]

Good point with the choke, but when I figure out if rich or lean at what fuel circuit would that be effecting , Pilot,fuel screw ? If I'm being exact
it happens at 4200 rpm at cruising speed about 60mph throttle maybe 1/8 to 1/4 cracked, take it up or down from that rpm and completely
smooth.

 

[grizld1]

Please believe me this time: there are very strong secondary effects involved in any component change, especially in older carburetors. I can tell you that your issue is not due to mixture screw setting or pilot jet selection. Beyond that, the crystal ball gets foggy. Remember that even a 1-step change in needle position can force a change in pilot jets. Good thinking re. slide lift; what are you using for air filters? Remember, minor defects that won't be felt at low loads can become troublesome at speed. Your issue is transitional, so if your ignition still employs the mechanical advance unit, you might take a close look at it.

 

[5twins]

4200 RPMs puts you in the mid to upper midrange or low main area. I think either setting could influence it. I have another bike that had pretty much the same symptoms and it turned out to be the main jet was one size too big. Running up and down through the RPMs was fine but rolling the throttle open after cruising in the midrange for a while would produce a slight hesitation or minor stumble. Your bike seems to be running well otherwise so your jetting is obviously close, maybe just not quite right yet.

I seem to recall discussing your jetting a while back. If I remember correctly, it was a bit richer than what I found works well in the '76-'77 carb set I have.

 

[XS650D]

Thx for reply,guys.I am running a 135 Main as it feels good and strong top end to redline, no hesitation stumble,it will pull hard and fast enough that
I have to let off throttle or it will shoot up to 8 grand before I can let off.Running Uni filters.Mikes muffs same as yours 5T.
Stock needle set in middle setting #3 ,tried dropping needle I clip to #2 but bottom/ mid felt flat, went back and power was back, pulls hard.Tried going to 30 pilot
but ended up at 27.5,feels cleaner crisper and pulls real hard off the line with no hesitation ect.Bike idles at 1000 rpm smooth with timing set with gun
to right side of F (On the line) Pamco ignition.Mechanical advance working good opens at approx 16/1700 rpm and full advance at approx 2900/3000.
Looking at at Advance with timing light and tach they both hit really close to 3000 rpm, so I thing I have that good.I have balance my carbs with a home made
syn tool, works great and air/fuel screws are adjusted for max idle or highest speed at 1200 rpm.Strange thing is when I check my plugs after a good ride one is
light grey, entire electrode and other is light brown both with same smoke ring level at bottom inside plug, both have a vey light ring of carbon around the entire
ring about the same on both plugs.I realize its hard to diagnose something like tis unless u know all the facts, so I hopefully got most on here.

 

[5twins]

Yes, I think you're a tad rich. I would try a 132.5 main, or lean the needle. I would do one change or the other, not both at the same time. I would probably try both (by themselves) to see which worked or which worked best. You can lean your needle by 1/2 step using a washer. That might be enough to cure your flat spot but not hurt the midrange performance very much.

 

[XS650D]

OK thx 5T. Will give the main a try first . PS: That timing setting you recommend works great! Smooths out the power band
and bike also runs smoother, till u get to approx 7 grand then it gets angry!

 

[5twins]

Yes, the main jet swap is the easiest to implement and it may be just what you need. Yes, setting the timing towards the retarded side of the idle timing range does work well, and it is more insurance against going over advanced. I ended up with mine like that to alleviate some minor pinging I was getting once the weather got really hot. The better running was just some "icing on the cake", an added bonus. But truth be told, and I know I've mentioned this before, we're actually not running the timing retarded, we're running it closer to the original timing spec these bikes used when they were first released. Yamaha changed the timing spec in early '71 to smooth out the idle and low speed running .....



When I changed mine, I did notice a drop in idle speed and it did get a bit more "lumpy", but I like the way is sounds, and a little tweak on the idle speed screw returned it to a stable, fast enough (no stalling) speed.

 

[XS650D]

Interesting stuff 5T. Question ,am missing the small washers behind the Advance fly weights, also was wondering if the cam side to side
play is important, should the play be tightened up or shimed up bit.I have had an issue lately with the Advance seal leaking.I installed 3 Mikes xs
seals and finally tried a Yamaha seal, still leaks from seal only, shaft end is smooth,im thinking that with the side to side play of
maybe 1/16" if I shimed the cam on one end it would keep the seal from possibly sliding of the end of the shaft as it really only on by maybe a 1/16" of cam shaft
showing.Im thinking shim it on the Advance side only.Whats your thoughts on this, may also help to keep the advance from bouncing around
when checking timing.

 

[5twins]

Do you mean the advance rod or the cam itself? The cam shouldn't move side to side. If it does, that indicates the bearings aren't pushed all the way in on both sides. This is a problem. If the cam is floating left and right, the cam chain sprocket is too. This could cause chain alignment problems and increased sprocket/chain wear. The "fix" would be to loosen the top cover and tap the bearings all the way in on both sides. They should be inset equally on each side. Check/measure the inset and compare side to side .....



Yes, the advance rod can have excess side to side play. Yes, shimming it out can help smooth out the timing. I shimmed mine by adding shims under the points cam (now the Pamco rotor).

 

[XS650D]

Sorry I meant the Advance rod has maybe a mil side to side play, not the cam.I played around with the spring tension on the advance
seal and I think its finally good after a approx 60 mile ride today, no leaks inside or at the back were the gasket is.
I found that I was missing the o-ring behind the advance but I did have it on the other side were points used to be.New o-ring from Yamaha
and all good.Thx for the advise. Also do I need washers behind the fly weights, mine are missing in action, will this cause any issues or premature wear
on the weights.I. am prob going with Electronic advance next season but regardless I can fit some small thin washers if its needed.

 

[5twins]

I don't know what would happen without the shim washers. Maybe as you said, premature wear. I just made sure I didn't lose them once I discovered they were there. I almost did lose them because they're hard to see or notice. If you're going to change to the E-advance, it probably isn't worth shimming the advance rod. It did help on mine but I still use the advance unit and probably will until it wears out. So far, the amount of advance it gives me is still well within spec, which does sort of amaze me, lol.

If you've ever had an original choke lever off a BS38, the advance shim washer is similar (but smaller) to the shim washer behind the lever. It's a very thin, blue steel wave washer. So, it shims some space but also applies a little pressure to take up the slop.

 

[XS650D]

OK thx, ill have a look at the hardware store.

 

[XS650D]

Thx,5T. Forgot to mention I dropped the main down 1 size to 132.5 and light hesitation or jerkyness at 3 to 4000 rpm is gone!
Top end still feels the same.Aside from everything running nice and smooth if I cruise around at 3000 rpm in a higher gear for 10miles
and pull plugs half the plug electrode is black so I'm probably still a bit rich, havent been able to find shims thin enough
for a 1/2 clip setting on the needle yet but thats probably something I need to do. 1 full clip setting leans it out to much and
bottom end feels flat.Its amazing how these very small changes effect to a fairly substantial change in power and smoothness.
These have to be the most finicky carbs I've ever worked on! Question does the main jet change effect the the lower end at all or just the
mid range upper and lower.U sure know your xs650,s,most off your recommendations are bang on!

 

[5twins]

Yes, one main jet size can make a real difference, especially when you're close. Also when close, it often takes a fair amount of miles to realize things still aren't quite right. One quick trip around the block won't show you the "glitches". You need a chance to put the bike through all it's paces, run it fast, run it slow, accelerate hard and easy, cruise at various RPMs for a while then do a throttle roll-on from there, etc.

The small change you made shouldn't reach and effect all the way down to the low end. A major change, like 3 or 4 sizes, might, but not just one like you did.

The Mikuni needle clip slots are 1mm apart so that means moving the needle clip one step changes the needle height by 1mm. These are the washers I use. They're just standard M3 washers and are about 1/2mm thick. I check the ones I'm going to use and if not a match I either grab another or lightly rub the thicker one on some sandpaper to match it to the thinner one .....

You are correct in thinking that by your plug color, the midrange is just a tad too rich. The needle setting has a big effect on plug color because you run in it most of the time. Like I said, the shims are just standard M3 washers but if you have trouble finding some, drop me a P.M., I'll send you some. I have more than I'll ever use, lol.

You say dropping the needle a whole step made the lower midrange too flat. Are you up one on the pilots too? I didn't think the midrange was bad on my set with the needle leaned a full step but that was with the 132.5 mains AND a 27.5 pilot.

 

[XS650D]

Yes,my current se-up is 27.5 Pilot, Needle Middle slot, Main 132.5 Fuel Screws 1.25 & 2.25 (set-up for fast idle on the lean side but with no
spitting on high throttle roll off) Currently runs best it ever has rite now! But I think there,s still room for improvement. It feels pretty amazing
when u make a change and notice an improvement. With every tweek its probably running closer to a totally stock bike thats well tuned.
Changing pipes, pods ect really screws up the smoothness that this bike is capable off.But I'm getting really close! Im sanding down a few washers to tests have Digital Calipers to measure.Thx for offer.