Have a case of the carb tuning blues

MadRat 88

MadRat 88

XS650 Enthusiast
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This is my first post on the forum, I've been an info stalker up until now. I have an 81' xs
650 chop, stock elec. ignition and I'm running open pipes. I have adjusted the valves, re-built the carbs, adjust cam timing, vented the tank and keep coming up with the same problem, I can't get over 60mph. I have one up on the pilot jet, and have tried main sizes 137.5 up to 150. I can find a comfy spot with all sizes by adjusting idle screws between 2.75 and 3.75 turns out but keep choking as soon as get into the high rpm's. I just went back to 147.5 mains and I'm at 3 turns out. Idles good but LH cylindar keeps popping and backfires on decel. Plugs are gapped at .030 and bike still runs good low and mid with a stutter as soon as I hit the high rpm's. Also I've switched back and forth on air filters, Mikes XS and standard cones.

Any suggestions or avenues to pursue will be greatly appreciated!
 
What do the plugs look like? Stock BS 34's? Tried using a thin washer under the clip to raise the needle?
 
Have you checked your diaphragms for holes? Small holes will only allow partial slide lift and the bike will only go so fast. Could be the cheap pods too.
 
Stock 34's, plugs are pretty dark but not where they look totally burnt. I haven't put a washer under the needle and haven't checked for holes in the diaphragms. Thanks for the direction I'll see what's inside.
 
What air fliters? The cheap ebay pods cover the air jet. I had this problem. Try running it with no pods and see what happens
 
I run 145 mains, 45 pilots, 3 turns out on the mix, good pods, open exhaust, and I'm at sea level. I would do as 5twins suggests and check those diaphragms. Also sync the carbs (causes backfires on decel).
Also, and this is a major also, cut exhaust sucks!!!!!!! It kills performance. I ride everyday, about 50 miles a day minimum on a hardtail. I have cut exhaust and my bike lags due to it. If I put good XS megaphones on it, it will zoom. That's just my 2 cents.
 
I took apart the left side and found some small spots, one of them I can see what looks like a gold mesh, but I can't see light through it. Not sure if this qualifies as a "hole". I guess that may be enough to throw the performance off.
 
On the air filter side of the BS34's you will see 4 holes with a 5th, oblong one on top. With one finger, lift the round slide/ diaphragm up, cover the oblong hole securely, and release the slide. It should fall, very slowly or not at all. This means the diaphragms have a good seal. If the slide just drops then there is a hole or bad seal. You can do this with the carbs installed, just take the pod filter off to access the ports. Make sue the carb is completely together if you do this on the bench. Parts missing can cause a false reading. Just looking for a hole doesn't work too well.
 
Thanks Little Bill, I'll give that a shot and see what happens. When I got these carbs the slide was stuck bad enough where I couldn't move it. Weeks of spraying and soaking in carb cleaner finally got them to break loose. I'll spray and blow'em out again before I put them back together and see what happens...
 
Be very careful when using harsh cleaners around rubber parts. It is best to strip the carbs of all rubber parts before cleaning. Carb cleaner will deteriorate diaphragms, o-rings, shaft seals, and the float valve tip. Most backfires are caused by a lean condition, which in turn, is caused by air leaking past a seal. Even the rubber boot around the enricher/choke will cause a leak, driving you batty searching for it. Those seals need to be pliable, tight, and new. Reusing an old o-ring or seal is not advised. Just think about what carb cleaner does to the oils in your hands. Now imagine what is does to nice, soft rubber.
 
I tested the diaphragm, they both drop a bit then slow up to a crawl on the drop, hard to totally close the opening with my thumb but seem OK with this test. The air jets? located on the lower left side of the 4 hole pattern, are covered with the small cheap air filters but are not with the Mikes XS pods. Will changing these jets make any kind of difference good or bad?
 
Oh ya! They need to be clear and unobstructed. The 34's have a removable air jet, bottom left, which is actually a small main jet. They are the same thread size, just different head diameter. That's why the cheap-o pods suck.
 
I have never messed with changing them. I've been able to adjust with just the main, pilots, or mix screw. If you are at a very high elevation I could maybe see it, but stock should be 132.5. They do not match the main jet.
Read www.amckayltd.com/carbguide.pdf until your eyes bleed. It is the carb bible. It is a must read.
 
Every bike is different. There is no "set" recipe for modifications. However, I suggest a 142.5 or 145 main, 42.5 or 45 pilot, 132.5 air jet, and start at 3 turns out on the mix. Now that is what I find works good, as a starting point, for pods and cut exhaust. You will always need to test, sync the carbs, use the "dead cylinder" method for the mix setting and read the spark plugs. It takes time, but you ar dealing with a 30 year old bike with 2 carbs.
 
Stock air jet size is 135 so yours is correct. You seem to have tried all manner of jet sizes without success so if you plan on keeping those carbs, I would be inclined to replace the diaphragms.
 

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