1977 xs650 - rev's hanging up when i get off thottle

lol. you and me both. I'm getting pretty frustrated mainly because i've been dealing with one problem after another with getting this bike sorted. Im pretty close now but I'm running out of perseverance :banghead::banghead::banghead:
 
I'm definitely looking forward to that day! I took it for a quick burn yesterday and it was great. Despite the fact that it was like -3 outside and this stupid rpm issue it was still a good time
 
It sounds like you've got the advance unit sorted. The springs are probably fine. Now I think you need to take a closer look at your carbs and their state of tune. First off, what's your idle speed set at hot? It should be about 1200 RPMs, no more. If you have the idle set too high, like at 1500, you can be starting into the advance curve. As that happens, the engine speeds up, which advances the timing more, which speeds up the motor more, etc. It's a self-feeding problem which eventually has your motor racing without you touching the throttle.

You may also need to do some jet changing for those pipes. Your '77 carb set came with the smallest mains ever put in a 650, 122.5's. They're fine on an all stock machine but are quickly out-paced if you start modding. The bike may be happier with a slight increase, say 125's or 127.5's. There's also the fact that you reamed your original jets out with torch tip cleaners. Who knows what sizes they are now.
 
The idle speed is 1000-1100. I've considered getting into the jetting but my buddy who owned the bike for 8 years before me chopped the exhaust right when he first bought it and he never played with the jets or had any of these complaints that I'm dealing with. He used to ride it to work here so i know it WAS working ok haha. That being said it still could probably benifit from some bigger jets. I should have probably chose my words more carefully with the torch tips. I just poked them through to clear the orafice I didn't really hog them out but it could be possible that they've been opened up a bit? I should just bite the bullit and change them but I thought since they were working ok with this setup in the past I wouldn't change them until I got it running the way it was prior to it sitting before i bought it.

I did check the slides and diaphrams when I had the carbs apart and they seemed good. I could move the slides freely and when I pushed them up and covered the inlet passage with my thumb they didn't creep down at all so I figured they were good.
 
I think you are right to hold off changing jets until you've checked everything else thoroughly.
As you say it all worked fine once upon a time so if nothing has been replaced it should still work just as well now with the same jets.

I know its frustrating but I think the only answer is to systematically go through and check everything again meticulously, especially look and test for air leaks on the inlet side.
 
It seemed to improve when I lubed the advance assembly and cut one spring down. I'm wondering if I cut the second spring down if it would help? I was reading the links that peanut posted and one of the comments was "I've read in a couple of other places that if loading the engine against the brake brings the revs down and they stay down, it's the ignition advance. If the revs drop and then pick up again when you pull in the clutch, it's the carburation". I can force the RPM down with the brakes but when I pull the clutch in it revs right back up so based on that comment it seems that it is a carb issue? boo.
 
Ha! :D I read that too and filed it away somewhere....

I'm dreading starting my engine next week after a recent rebuild. I'm expecting a nightmare tuning the thing after all the problems you guys have had :(
 
Make sure your carbs are in sync. Out of sync carbs can cause some weird issues. If your buddy just putt-putted around on the bike, a slightly too small main may not have shown much detrimental effects. Something else to consider is how our gas formulations have changed. I don't think the gas today is as good as it was a few years back. I think it burns leaner now. So, the bike may have run rich enough years ago but today may be too lean with the new gas.

You've also made no mention of mix screw settings. The spec for your carb set is 1.5 turns out ± 1/2 turn. Make sure both carbs are set around that and the same as each other (or very close to the same). If all this is good then maybe you are sucking air somewhere. You said you tested (with spray) the manifolds but how about the throttle butterfly shaft seals? You can remove and inspect the condition of the one on the outside of the left carb without dis-assembling much (just remove the cable arm, washer, and spring). They should be very soft and flexible. They need to be because they work on vacuum.
 
I haven't checked or adjusted any of the carb settings so perhaps its time i look into those mixture screws. I'll see what they're set at now and i'll put them to 1.5 turns out each so at least I have a baseline. I'm trying to only adjust one thing at a time so I know if it made a difference or not. Based on these recent findings it does sound like my issue is carb related and not timing related so I think it's time to start playing with some carb settings. As i mentioned before I've had them apart and cleaned everything that I could but I've left the settings alone so far.

I haven't sync'd them yet but I did print off how to make a manometer so that is likely coming down the pipe pretty quickly here as well. I think I should probably check and adjust the cam chain tenson, valve lash and recheck timing and then i'll start in on these carbs. Although my symtoms seem to point directly at the carbs being the issue I keep reading to do the proceedure in this order...
 
Yes, you won't get good results carb tuning unless the other stuff is set correctly. If you happen to have the carbs off again, you can do what's called a "bench sync". That's simply looking at the butterfly plates and making sure they're both open the same amount. With a properly set idle, they will only be open a tiny crack. Hold the carb set up to a good light and look in the engine side at the butterflies. The light will show around the edges of the plates, mostly at the tops and bottoms, and should show about the same amount on each one. You can get the sync very close this way, right on with some practice. If the plate openings are mismatched, your carbs are out of sync. Fool with the sync screw on the linkage between the carbs to equalize the plate openings.
 
oh ok yeah I did eyeball the butterflies when I had the carbs off last time and they appeared to be centered and opening/closing the same amount. If they're off they're still very close at least to my untrained eyes.
 
peanut said:
I think you are right to hold off changing jets until you've checked everything else thoroughly.
As you say it all worked fine once upon a time so if nothing has been replaced it should still work just as well now with the same jets.

I know its frustrating but I think the only answer is to systematically go through and check everything again meticulously, especially look and test for air leaks on the inlet side.
Click to expand...


yeah I was hesitant to start messing about with the jets since it did seem to run correctly with this setup. Once i've gone through everything else i'll start looking at rejetting it. It's strage that the rev's dont hang every time. It seemed to be more when it warmed up as well. I'll go out today and fire it up again and see if i can get them to hang when it's cold. I wonder if somehow the enging temp is a factor? The choke didn't seem to make any difference to the rpm's when they were hanging up last night. i flipped the choke on/off several times in an attempt to get the rev's to drop and it didn't do anything so perhaps the choke isn't the issue. Also to reiterate the linkage and the cable aren't binding either. I reached down to the linkage and it was against the idle stop when the rev's were running away. Unless like previously mentioned the sliders are hanging up inside the carb but as far as linkage goes that seems fine.
 
Yes, on these old bikes, you need to check just about everything. Place your finger over the needle jet as it's mounted in the carb body and see if it wiggles at all in its hole. If it does, the o-ring's toast. It should be a rather snug slip fit into the carb body.
 
I modified the second spring on the advance unit to further increase the tension then I ran it for a few min and revved it to 5,000rpm several times and it returned to idle each time. Im not 100% convinced that it's fixed because it did the same thing to me last night and it eventually started hanging up at 5,000 again but it's possible that the springs just couldn't overcome the centrifugal force after having only modded 1 spring. Although that being said you would think that every time it revved up to 5k it would get stuck and that wasn't the case. I'll monkey with it a bit more after work and see if I can get it to act up again.

it seems as though the timing is off each time i've played with the advance unit to mod the springs. It would start last night after I modded the first one but it would die if I touched the throttle at all. I regapped the points and for whatever reason that got it going. I modded the second spring today and it would fire up but it seemed as though the timing or something was off. Would playing around with the advance springs cause the points/timing to change?
 
Fooling with the advance shouldn't alter the timing unless you originally set it with the advance stuck part way open. Then yes, now that it's returning fully the timing would be off. Best check that too.
 
Sync the carbs. Typically, one carb is just on the cusp of working or not working. When it decides to work, the revs go up. When it is not working, the idle is set by the other carb.
 

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