I have a set of 34mm VM Mikunis on my 81 street stracker that are working great, seem to have the jetting set (at least according to the plugs), but have a problem when riding with them acting like they're still partly open when I close the throttle - i.e. I don't get a lot of compression braking, and unless I pull the clutch in, it acts like the idle is set real high. Usually when I pull in the clutch or come to a stop, it idles down. Could it be the timing, since the slides and cables seem to run freely? Or is it some sort of emissions control in the ignition system, like a lot of manual shift cars seem to have? I am using regular gas, and saw some threads involving higher octane - not bsure this would even be related. Much thanks for any advice.....
Post by Don Murray on Sept 21, 2008 9:06:24 GMT -5
Sounds like a small air leak in the throttle butterfly shafts, or manifold connection. My 72 with 38 mikunis had worn throttle shafts and seals. I got rid of the hanging throttle by just replacing the outer shaft seals. Did the job without removing the carbs, but it would be easier if they were off. Don't know about doing that on an 81. That end of the shaft had worn more because that's where the 72 model throttle cables are connected. I found the leak by squirting some water around various places until the engine idle speed changed.
BTW, on the 38 Mikuni carbs, the throttle shafts themselves can be tough to remove and that's necessary if you replace both seals. The tiny butterfy screws are often frozen and have to be drilled out which could be a tense job. You can get new screws and seals from MikesXS.
Donmurray, there are no butterflies or throttle shafts in a VM-series carb. Claytonia, you probably don't have an ignition issue, and if you don't hear detonation your fuel is fine (I use and recommend high octane, but there are some very well-informed folks out there who run nothing but 87). Sometimes vacuum leaks at the carb mounts don't open up until the engine is warm; check with your aerosol of choice. These carbs are also very sensitive to mixture screw settings; a very small lean imbalance will elevate idle sharply at full warm-up.
Post by claytonia on Sept 22, 2008 19:41:39 GMT -5
Thanks for the quick responses! Yes, these are new VM Mikunis, so no mechanical problems that I see. BUT....I'm thinking its an air leak cause I installed my own vacuum nipples (actually brake bleeder valves) on the manifolds that came with the carbs, so I could sync them up, but can't say as I was real happy with the way they turned out (superglue probably not the best sealant!), and was thinking of spaying something on them while running to test. On the other hand, the run-on problem was there before I put the nipples in, but I don't think the backfiring was. I'll try the mixture screws (any suggestions? I followed the instructions that came w/the carbs), and air leak test and see what I get.
You can use blind plugs available from any auto parts store on the vacuum barbs. Don't know what instructions came with your carbs, but I recommend setting mix screws by running one cylinder at a time at the lowest idle the engine will hold, setting each side in turn to best idle. Be sure the engine is at operating temperature. You'll need to increase throttle a bit to maintain 1-cylinder operation; use a pull tie between grip and housing to hold your setting. With electronic ignition, don't kill a cylinder by pulling a plug cap; either shut down, stick a spare plug in one cap, and ground it on the engine, or as Pete suggests, pull the blind plug from one boot. Verify sync after setting mix screws.
Pull both carbs and tell us what you have for pilots, Mains, slide cut-outs, needle jets and needles plus mods that have been done to the engine ie; exhaust and air filters. Once we know how your carbs are set up we can help with your woes.
Sorry I took so long to check back - guess I thought that was the end of it. You all are great - thanks for all the suggestions! Please excuse the lengthy response....
Yes I have the Mikuni kit from "the guy on eBay". Will get back to you ASAP with the jets etc. This bike was a real basket case, only kept the bottom end, has a virtually new top end: max. overbore (680cc) cylinder and pistons from MikesXS, X-pipe exhaust, Shell cam, and a head that I got off eBay that was supposedly just rebuilt (it looked pretty clean). To my amazement, with all the new/mixed parts, it fired right up and has had no starting problems. It has pretty much had the run-on problem from day one (before I put the vacuum nipples in). The Compression (cold and hot) is 105 left/115 right - but with the carbs closed (didn't know they should be wide open). The plugs have always looked prertty good, but I did move the needles a notch richer because it was taking forever to warm up - that helped a lot, and the plugs still have a consistent tan color, so except for the run-on I've assumed it was jetted pretty close. It backfires a bit when I rev it standing still, and sometimes at constant moderate throttle.
It was running-on real bad today - I have upped the idle stops a fair amount so it will idle, without the chokes, sooner once warmed up - this does seem to aggravate the problem but its hard to say. It totally acts like the cable is sticking - it holds at around 2500-3000 pretty much all the time when its really warmed up, but the slides look like they're resting on the stops fine, and fiddling with the cable has no effect. I also tried pressing on the vacuum nipple covers, and wiggling them around some, but that doesn't seem to do much either. Even pulling the covers off doesn't make a huge difference (so maybe they really ARE leaking bad). Ok - that's probably TMI. I'll take a carb apart and check the jets this weekend.
I just pulled the carbs: main jet #180; needle #6DH4 (60H4?), clip in middle slot (#3); the airscrews i had set 9following the VM guide) to ~1.2 turn out. I really think its jetted fine - it pulls strong, runs cleanly throughout low to mod revs (haven't opened it up yet - still working omn getting it titled!), no pinging, no missing, no backfiring - just what acts like a stuck throttle. BUT.....I am going to have to replace one of the needles, and this may have been the problem all along (but i'm doubting that) - the slide slipped out of my hand and hit the floor, the needle was bent moderately when I picked it up - I'm guessing that just happened, but if it was already that way (from the last time I put the slide in, maybe it wasn't aligned?) then that may have been problem all along.....? It definitely had some level of run-on problem from the first time I rode it, just less. I'm tracking down a replacement needle today, and will try that along with sealing off the vacuum nipple holes, and see what happens.
Also checked compression w/throttles open - noe 115 both sides (cold), which I'm hoping is OK.
OK, hope you're still with me....i finally got the needle and sealed the holes in the manifolds pretty well, put the carbs back on, fired right up, but still the same problem. This all may be unrelated, but here goes....then, once it warmed up the right cylinder started cutting out and popping; when I put the choke on that side the revs went up pretty high, but with the left choke (only) it just went up a bit (air leak?). Then I noticed that the right fuel line was empty, while the left had some gas in it (tank is full), so I checked that the petcock was working OK, then pulled that carb apart - it didn't have much gas in it at all, blew every hole I could find (these are new carbs), put it back on and the gas flowed fine this time. But it still has the same symptoms described above.
Compression on both sides is 125PSI warm. Both plugs are coming out wet, which I haven't noticed before. So I now have a half-dead right cylinder, and as far as I know the original problem is still there. Any ideas?
Did you arrive at 1/2 turn out on the air screws by idle function, or did you just set 'em there and leave 'em? 1/2 turn is the inner limit of usable adjustment; if you're having to choke down the air that far to get optimum idle it's an indication of a lean pilot jet. What needle jets and pilot jets are in place? If you have generic Q-2 needle jets in there matching those generic 6DH4 needles, you'll never get clean operation.
Thanks for the advice again! I'm embarassed to say that this recent problem was caused by my wonderfully-designed :-) plugs for the holes in the manifolds - the right side fell out (or more likely got sucked in!) but it was just hot melt glue anyway - it was temporary, but I didn't expect THAT temporary. So I won't waste any more of your or my time until I replace both manifolds and just finally eliminate that possibility.
Re: jetting - I arrived at the 1/2 turn airscrew by following the Mikuni tuning suggestions. I figured it was the minimum. I will have to check the needle jets, but i'm replacing the manifolds before anything else. Like I said, it was running very cleanly, just had the annoying run-on problem when warm - which i'm now hoping was caused only by the manifolds leaking. Any suggestions on where to get them? Sportingforless.com has them for $35+shipping (pair), which i guess isn't all that bad.
The SFL boots are flimsy. I'd advise you to spend a little more and buy aluminum carb mounts from 650 Central or Shell Racing Specialties, and order some 6F9 needles, P-6 needle jets, and a copy of the Sudco Mikuni Tuning Manual, while you're at it.
r80rt: :)This is a splendid write up, THANK YOU!!!!
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buell88: Anybody have any idea who built the xs650 tracker used in the Draggin Jeans Ad on Pipeburn?
May 16, 2017 16:24:09 GMT -5
joshua: The regulator/rectifier unit I got from Mikes PMA has closer hole spacing than original. Are we supposed to mill the holes to fit?
Jun 7, 2017 19:25:58 GMT -5
joshua: My bad. I found the bracket they supplied to mount the rectifier/regulator... Uh oh. How is this meant to work? Which way is it meant to mount? Looks like I have to use some spacers or something. Good instructions would be nice.
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craig8johnson: I'm checking the charging system. I'm afraid I might have fried the last battery. With the solid state regulator (after market) what should it be charging at. The book says, I think, 15 volts. I'm hitting 17 when revved. Is that too high?
Jul 11, 2017 18:06:19 GMT -5
gggGary: Yes 17 volts is too high, about 14 volts is max, sounds like you don't have the right regulator or it's wired wrong.
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tt650: Trying to breathe life into a 1980 that's been stored (indoors) for 25 yrs. Air filters are dry rotted. Have cleaned carbs but she only runs briefly on choke then stops. Need jet setting info and other ideas that might help her idle and rev.
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trix13: Hey everyone! New to the forum and am fighting those gremlins!! New charging system, ignition switch, coil and fuse box. Resistance is higher than normal on the ignition pick up so I'm trying to find one. No such luck yet.
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trix13: Also, even though all of my connections are clean, I cannot seem to find out why , when at higher rpms, I hit the signals and the rpms pulse with the signals...hahah. same with the brake light. I appreciate all input!!
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