A ponderous flat spot

[richard]

I have been riding my all stock 1980 XS650 for about six months now. When I first got the bike I ended up cleaning the carburetors three times before I got them sorted out. It had been sitting a while when I bought it. I run a single inline fuel filter and a manual petcock, but other than that it is all stock. I also only run Chevron premium gasoline in it. Lately I have noticed a slight stumble at 3100-3300 rpm at slight load. This just became noticeable with the 90+ degree weather. The strange thing is I can accelerate through it, and with that it smooths right out with higher rpm. Also it's only noticeable in 4th and 5th gear. I figure because the engine has no load on it in lower gears. At first I thought it was a batch of bad gas, but now I think it may be fuel delivery. Timing and advance has all been checked. The ignition is all stock with no centrifugal advance, just the usual magnetic trigger on the rotor. Warmer ambient temp seems to be one factor so I think the mains could be too lean or the alcohol in the gas may also be a factor. Right now it just kind of a nuisance, but little things like this bug me, so I'll have to sort it out eventually. I've been reading the Carb guide again to see if I'm missing something. My question is has anybody else run into this problem recently? and will one size up on the mains be any help? BS34 so I'm stuck with the needles. I'm going to change the fuel filter tomorrow, but it is only three months old. Strange?

I bought this bike because I was riding my 71 way too much. My intent was buy a special and make lots of changes to it, but I liked it so much just the way it is I think now I'm going to just leave it all stock.

[grizld1]

Richard, tuning carbs is always a trial and error job, and working at a distance makes the guesswork even more uncertain, so take the following with the whole shakerfull of salt.

In the rpm range where your flat spot is happening, the mains have minimal effect. Component-and-range charts are misleading when it comes to vacuum carburetors: you can whack the butterflies wide open at low rpm and get no lift on the slides because there's not enough vacuum developed to lift them, and this effect is especially strong in old-style vacuum carbs with heavy round slides. In your carburetors, the low rpm mix is most heavily affected by the PJ's and the needle position. As to the stock needles in your carbs: you can shim them to richen the mix, you just can't drop them to lean it out. Radio Shack sells a pack of assorted small washers, and you'll find some in it that will give you what's effectively a half-step change (0.20"). There are very heavy interactions between needle position and PJ selection, and since the PJ's in early BS34's tend to the lean side anyway, the ones you have in place will probably be fine. You'll need to reset the mixture screws.

One word to the wise: if your low rpm flat spot in 4th and 5th is occurring only with the motorcycle under the very heavy load shown in your photo, the motor may just be telling you that it would be happier if you'd shift a little later where there's more power on tap.

[richard]

Thanks Grizld1, that is very helpful.

The low RPM range seems to be fine. 3100-3300 it just seems to need something. As I'm sure you know having lived here we have a lot of switchbacks in the Sierras. That is where my XS really excels, I can roll into a switchback in 2nd gear and it pulls like a tractor coming out of it. Even loaded down like in the pic. I guess I just want 80 to run as well as my 1971, but that took me a lot of fine tuning, plus the 71 is just breathes differently. Shimming the needles on the 80 is not all that time consuming so I may try that. I can mic each washer, and I really have nothing to lose except to change them back. For a while California was experimenting with more alcohol in the gas. I even saw an MPG drop in the pickup and the Prius, so I switched to banded gasoline and they all returned to normal. I hate alcohol in the gas, but I'm kind of stuck with it here. I'll have to tinker I guess. By the way no matter what year people will stare at a XS650 loaded down like a pack mule. The bike seems to handle it just fine. By the way my 1980 will run on 87 octane, if you can't get premium at your local gravity pump.

[richard]

FIXED :-)

I figured out what it is. I have been making a stand-off for the saddle bags for a couple of days, and finished yesterday, so I parked the bike in its usual spot, and this morning I change the fuel filter. After that I opened the petcock and noticed the float bowl on the right side over flowing for the first time. So a tapped on the right side float bowl with my pocket knife and it seated. Turns out there was a piece of grit in the right side float needle seat. I clear it and all is well. No more flat spot. The grit was restricting the flow to the right side just enough to starve it at 3100 rpm. I say it was grit, but it looked like some crap of some kind. Like a varnish colored piece of something that wouldn't breakup between my fingers. Anyway cleared and it runs good now. Runs out clean from bottom rpm to top.

[grizld1]

Congrats, Richard! Sounds like it might be time to upgrade that fuel filter. As far as operating range goes, I regard anything under about 3800 as low rpm; but if you can get that machine to pull for you at 3200 loaded with gear on mountain roads, you're a helluva tuner!

BTW I think you have me confused with somebody else; I never lived out that way, and my only experience of the Rockies was a run from Champaign, IL to Boulder, CO and back in the summer of '68. The board handle refers to a graying head, not the Rocky Mtn. bear!

Always thought that gray G-model had one of Yamaha's classier looks.

[richard]

Yeah I'm still trying to find a way to put a larger automotive filter on it. If the flat spot comes back I may just have to clean them all again. It does pull well. In the switchbacks the limitations of traffic keeps the rpms just a little too high for 1st and a little low for 2nd. It will roll out of a ten MPH hairpin in second very well when all my other friends are forced to slip the clutch. In those situations much below 2000 rpm it start to complain. The rest of the time I'm usually between 3-4k climbing or going down the road. Where I live is at 340 feet, in thirty minutes I could be at every bit of 6000 feet elevation. It's nice when you want to get out of the heat.

I may have gotten you confused. There was a guy I talked to here that had a Norton for a while living near Topanga before he moved east.

[richard]

One last question:
BS34 Float needle screens. Should I remove them like Mikes suggests, or not? Opinions?

[grizld1]

If you've installed an effective inline fuel filter and cleared the float valve screens well, it really shouldn't matter whether the screens are left in place or not; anything that gets past a decent fuel filter should be much too small to block them.

[richard]

Yeah that's what I thought. I didn't know if they represented a restriction issue or not. That's why mikes suggestion to pull them made me wonder. Some of these places above the snow line here with seasonal gas supply make an inline filter a must.

Thanks for you answer. I appreciate it.

[grizld1]

Identical screens are present in the float valves of Mikuni RS series racing carbs, which are used on motors with much higher fuel demand than our old twins. The only time they'll present a flow issue is when debris blocks them!

[richard]

OK thanks

[headcase]

Re: automotive filter: I have the decent pod air filters on my old nag, so the factory airboxes aren't in the way, assuming yours is bone stock.(?)I can't do a photo at the moment, so I'll try a verbal description of the Rube Goldberg setup that seems to work great on mine. I feed the 2 lines out of the petcocks to a tee off to one side of the frame tube by the air filters, one line out of the tee to a longish small diameter plastic auto filter (which one is a mystery, whatever will fit should do the job).The filter is zip-tied...yeah I know...to that center frame tube, and the now-single fuel line feeds into the top of the filter. Out the bottom comes a single line to a second tee, which then feeds one line to each carb. It can get cluttered if you don't spend some time routing the stuff right though. I can get shots next time I have the covers off if needed.
Nice bike, boss!

[richard]

Thanks, I left the silver bike is all stock because I figures to ride it year round. Really I bought the bike to make a bunch of modifications, but I like it so much the way it is I just decided to clean it up and ride it. I carry several of those inline 90 degree filters in my tool pouch now. The gas at the pumps in the mountains are the same as seasonal gas everywhere. It could have anything in it.

Room is kind of tight for the inline fuel filter, but I'm thinking of getting an bigger 3.9 tank to give it more range. That tank would probably have two petcocks which would give me more options for in line filters. I want to ride to Alaska, but I haven't figured out how to do it on an XS650 that wouldn't be really painful. I have three bikes and no room for another. I could do a lot to an XS650 for half the price of a BMW GS or v-strom and I wonder if a rephrase and some other changes would be enough, and would last. Ether that or I would have to sell the 71 to make some room. I don't even like to think about that. If I could just hang my motorcycles from the ceiling like bicycles? There is a Transalp for sale not far from here for $2500 I'd like to have that in my garage too. After this picture was taken the gas pumps and the lodge burned down in the rough fire a week later. I'm glad I got to go back one last time before it was gone.

[headcase]

And as to the subject line of this post....I have a rather big flat spot I ponder occasionally too, if I can get the bathroom mirror angled just right to see the top of my cranium.

[richard]

LOL