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Hi fellas,
I just finished rebuilding my 79 xs650 special and it runs great except for one issue. When I am on the freeway it runs great but I when I stop from the freeway it either dies or when I rev it, it take a while for the rpms to come down. It will start right back up, but same issue with the slow rpm drop. Seems like lean mixture. However, when I am riding at 40+ and pull the clutch and rev it they come down fast. What seems to be happening is that when the bike is stopped the mixture is lean and when moving it is fine. I think it is due to my air filters creating a low pressure zone at the intake at speed, and when stopped, it is the pressure is normal. A venturi action at the intake. Does this seem likely? Want to know before I buy new filters. Will I have the same problem with pods? Thanks a bunch!
with my background in fluid dynamics and heat flow theory dealing with reactors, yes i know not quite the same thing but fluid is fluid, what could be happening because of the shape of the filter its fine upon first start cause the air has been trained by the starting of the engine how to flow intially. But when your going higher in the revs with load the air flow retains the same path just greater velocities. When you come off the freeway the air flow is signifigantly decreased causing turbulence back in the filter area causing less than suitable airflow and giving you a lean condition. I've never seen a filter like that work perfectly on a bike before. I think you would have much better succes with an operfilter design but you would have to retune the bike more than likely.
As a side note this is all theoretical and I'm only going off of what the filtershape and design looks like to me.
Yeah, I agree. I ran the bike a little bit with no filter and it ran for crap. At idle the rpms would be real high then come way down, and speed didn't seem to be a factor. So pods it is. Any objections? Thanks
They look like emgo filters, not the greatest things in the world. I personally would go with unis or kn's. But honestly for 10 bucks i would try them out and see if they work better than what you have. if they do then that was a cheap mod to fix a problem. If they still give trouble, oh well it was just 10 bucks.
I've got those same Emgo style filters on my 750 and they work just fine. You get a little air gasp if you crack the throttle from idle, but at highway speeds they work just fine, and coming down to a stop, it doesn't kill your airflow. The nice thing is, if they get dirty, wash them with a water hose, hit them with some degreaser, rinse 'em out real good and let em sit in the garage overnight and they're good to go.
Yeah, K&Ns are pricey, thinking mikesxs for around 40. I was just thinking that what if I turned the filters so that they were near the center of the bike and out of the turbulent air? Might help.
well it only happens as your slowing down right, if thats the case i think shape is more the problem than direction. when you run at speed they run fine (more turbulent air) but when you slow down they give you problem (less turbulent air). I think thats what you meant correct me if I'm wrong.
Yeah, it didn't work and apparently I didn't tighten down the clamps enough on one of them so it looks like I am getting new filters. Probably from mikesxs.
well i definately hopes that it helps you. I imagine the lose of surface area for suction also creates a bit of a problem also, i think with new open element filters it'll run alot better when slowing down because it still has plenty of surface area to draw air from.
hey no problem man, its been a while since i got to flex those brain muscles. I don't work in that field anymore, decided to get my Math education degree and then went to work at geico in the process where i do auto damage adjusting. Get the flex the mechanical and structural side, just not fluid flow and heat transfer theory anymore.
I could be wrong about this and if I am wrong please correct me.
If you ask me, it sounds like a vacuum leak at the base of the carb flanges. I can't imagine that any air filter would have enough of an impact on the air flow to cause the revs not to come down. When you're riding around with your engine under load and your revs up, that causes the amount of manifold vacuum to drop. If you have a very small leak there might not be enough vacuum at speed to draw additional air into the engine. At idle you have the most amount of vacuum thereby exacerbating the problem.
Thats true but it would also cause one of the cylinders to be on a lean condition which could be read by the spark plugs and also lean sounding when accelerating. It depends though on how bad that leak was.
Yeah that was my first thought, I checked for a leak but didn't find one. The Rpms did come down just slowly which is a sign of a lean mixture. I think the problem was a mixture of incomplete tuning plus crappy air filters. Bit more tuning plus good air filters should do it.
Crazy question, but if you are on the highway and pull the clutch and rev it it's fine, so have you tried to do the same on the off ramp or as you slow down?
be safe when you try this, but instead of using the motor to aid deceleration, just pull in the clutch and keep blipping the throttle and use only brakes to slow the bike. Do the revs keep rising and falling normally or do they start to stick and take a long time to come down as the bike slows down?
If it were speed related, the revs would stay up before you move off and all the time when stationary and I'm not hearing you say that.
If it revs up ad down ok, then it's probably related to the slowing down ie to the higher vacuum when you shut the throttle and allow the engine to slow the bike. It's unlikely to be airflow related, but those filters and emgos are not very good and teh bike needs to be jetted to work with a non stock airbox arrangement.
Pete an I have talked briefly about this and agree that the road he was on because of the lack of original parts for the 77-78 is somewhat odd but not uncommon . He found out nearly the same way I did that there are drastic differences in bleed and non bleed pilots . I found later that although the K&N's might have looked better ( remember my frame is red also) the UNI is actually a better deal all around . What we didn't get into the last time I abused you was I used a somewhat larger UNI than you now see on the red bike and hid a velocity stack in it . Read Pete's final for the 650 before the 800 went in . That is so close to what I came up with I have only one caveat . Be very careful drilling the "lift" holes in the bottom of the slide . I feel that 1/8" is actually a bit large even with the original parts .
There is an advanced course but it will have to wait for accreditation or hell to freeze over . I was told the next time I get personal time to actually work on my own crap would be when hell freezes over ...
I'm real close to getting the chassis updated and serviced and see no reason not to stick the 650 mule motor in it so it can be ridden . I'll be starting with
Main jet 142.5
Pilot jet 40 (type 22/210)
502 Needle (this is similar to the 77-78 Canadian UK needle ) clip- middle
Now keep in mind that the larger UNI filter was in place and a short velocity stack was hidden inside the filter ( same hose clamp attached them both . I can't measure atm but I recall the slide lift hole was 7/64 or about .109 " . The main air bleed was also opened slightly and showed enough improvement that I left it alone ( got chicken) . I don't recall the drill size used only that a little bit allowed a consistently , slightly off idle , under load response . I was also using a fairly decent exhaust system at the time and was still using points .
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