engine trouble

devildrums

XS650 Enthusiast
Messages
95
Reaction score
53
Points
18
Location
Tampa, FL
Hey guys, I have a 77 D model and need a little assistance. I recently got it back together after about 4 months sitting in the garage. I have gone through the bike and cleaned the carbs, points, adjusted the cam chain and clutch, have new gas lines and filters and the bike fires up and idles nicely. The problem is when I take off, as the rpms reach above 3000, the engine starts sputtering, coughing, spitting, etc. It rides nice as long as I keep it under that. I had a 650 friend help me go over it and we just don't know what to look at next.
Thanks for your help,
Jimmy
 

Attachments

  • Fresh Bike 2019.jpg
    Fresh Bike 2019.jpg
    287.5 KB · Views: 202
Nice Yellow! Can you confirm the 77 ran better in the upper rpm's before sitting and carb cleaning? Just looking at your pic brings suspect to the small pleated cone air filters and the jetting. 3 thou rpm is in the transition zone from pilot to main depending on throttle. The rubber diaphrams are in good shape? Didn't get abused with carb cleaner? And another quick check being your bike sat awhile and the points were cleaned. The mechanical advance opposite side of the points, are they pivoting and functionally freely? .. more clues
-R
 
Nice Yellow! Can you confirm the 77 ran better in the upper rpm's before sitting and carb cleaning? Just looking at your pic brings suspect to the small pleated cone air filters and the jetting. 3 thou rpm is in the transition zone from pilot to main depending on throttle. The rubber diaphrams are in good shape? Didn't get abused with carb cleaner? And another quick check being your bike sat awhile and the points were cleaned. The mechanical advance opposite side of the points, are they pivoting and functionally freely? .. more clues
-R
Thanks Machine. Paint was just done by my friend Moe at moecolors.com and I have been a yellow fan all my life. It began running rough, which is when I cleaned the carbs, and then once the daily rain set in, I tore it apart for paint. As much as I can tell the rubbers looked fine in the carbs and I didn't drown it with cleaner so that should be okay.. We cleaned the points, but didn't really know how to check them for function, so that might be next. And I saw a few posts about those air filters, and since I had them on when I first bought it, I just replaced them with the same type. I did spray it with the filter spray, so it is possible I put it on too heavy.
 
Check you Diaphragms in the carbs. My engine ran like crap and I thought she just needed the carbs cleaned. Nope. A tiny hole in each diaphragm. I'm going to smear some RTV over the holes and down the road get in contact with JBM industries and order some of their diaphragms for the XS and the KZ project.
 
Last edited:
857, I installed a pair of the JBM diaphragms in my bs34 '80sg a few months ago. Instant smooth running engine. Old 'phragm in right carb had a very small hole. Smooth install, A+ recommend JBM.
 
Yes, those air filters gotta go. They're all wrong for several reasons - 1. They're physically too small. 2. They're the pleated element style that doesn't work well with our CV type carbs. 3. They're tapered. CV carbs prefer a straight filter. You'll never get the bike tuned right using those filters. Best pod for these CV type carbs is the long, straight, foam UNI pod, in your case for your BS38s, part #UP4200 .....

1GjnNlo.jpg
 
Last edited:
Oh yeah I agree such a nice bike to throw cheap air filters on. Oh and since it’s a two into one you need to find the sweet spot for tuning the carbs. Speaking from experience the needle seems to need to be dropped, because it runs richer in the mid range.
 
Last edited:
Sorry its been so long, got into season at work and been hustling. Changed the filters, exhaust gaskets, and carb settings and got it going better. New thing is more of a pain. I have had the last few rides where it just randomly shuts off as I am gearing down. Restarts fine, then may or may not cut off. One key example was I cranked it 4 times and it just idled down and shut off. 5th time kept running and idling fine and got me back home ok. Any idea what this intermittent problem is? Usually if it keeps happening every time it is easier to locate the problem. Thanks.
 
Have you rejetted the carbs? Did you check the float heights? Are the slide diaphragms in good condition? You can lift the slide with the pod filter off and make sure it makes a woosh noise going up and falls slow when it drops. What exactly have you adjusted on your carbs? If you are still running stock jets then that is your issue. Make sure your vacuum lines aren’t old too.
 
Last edited:
Have you rejetted the carbs? Did you check the float heights? Are the slide diaphragms in good condition? You can lift the slide with the pod filter off and make sure it makes a woosh noise going up and falls slow when it drops. What exactly have you adjusted on your carbs? If you are still running stock jets then that is your issue. Make sure your vacuum lines aren’t old too.
No, hadn't gotten as far as any of those things yet, this just started happening recently and I have been seeing posts about that. Had it down for about 5 months and just wanted to ride it a little. Guess its time to tear it all down again. Thanks.
 
You should probably go at least one size up on the pilot and two sizes on the main jet. There is a helpful guide in the tech section.
Oh, duh, did change to the foam filters as everyone says, and was wondering if I have to run it more rich or lean to account for the air flow difference. I think it is lean, but wanted another opinion.
 
dd, of course fuel is important, but so is ignition, and breaker point ignition needs regular TLC. There's been plenty of good how-to written here on adjusting and maintaining the points and advance governor (AKA "automatic timing unit"), and a few searches and a scroll through the Tech menu will turn up plenty. If you don't have a xenon strobe timing light, consider buying one.
 
dd, of course fuel is important, but so is ignition, and breaker point ignition needs regular TLC. There's been plenty of good how-to written here on adjusting and maintaining the points and advance governor (AKA "automatic timing unit"), and a few searches and a scroll through the Tech menu will turn up plenty. If you don't have a xenon strobe timing light, consider buying one.
But would those things make it cut off after it is running? That is my biggest problem right now. It fires up fine, but then decides to run several different ways and then sometimes bogging down and cutting off, all while sitting in one spot. Very frustrating.
 
Yes those things and many others will cause your issue. I am working on a buddies xs750 right now and it has the same issues as yours. First I fixed his timing which got the bike running but now I have to fix his carburetors and also rejet for his pod filters. I have had issues with the ignition system, electrical system, and fuel tank vent system all that cause similars issues. The fact is that if you add modifications to these type of carbs you need to rejet or they don’t run right through all throttle positions. It may idle fine but that is only a small part of the throttle circuit. If your carbs run lean in the mid range the bike will die because it isn’t getting fuel. If you have bs38s you can raise the needle, but I still bet money you need pilot and main jets altered. Check out niche-cycle store on eBay they carry cheap close to oem jets (https://www.ebay.com/str/nichecyclesupply).
 
Last edited:
Yes those things and many others will cause your issue. I am working on a buddies xs750 right now and it has the same issues as yours. First I fixed his timing which got the bike running but now I have to fix his carburetors and also rejet for his pod filters. I have had issues with the ignition system, electrical system, and fuel tank vent system all that cause similars issues. The fact is that if you add modifications to these type of carbs you need to rejet or they don’t run right through all throttle positions. It may idle fine but that is only a small part of the throttle circuit. If your carbs run lean in the mid range the bike will die because it isn’t getting fuel. If you have bs38s you can raise the needle, but I still bet money you need pilot and main jets altered. Check out niche-cycle store on eBay they carry cheap close to oem jets (https://www.ebay.com/str/nichecyclesupply).
Thanks you for the help, will check that site.
 
Back
Top