I think it's POSESSED!!!

[Ironhead]

I just picked up a PILE of xs 650's and a truck load of parts, I'm going to build my son a hardtail bobber over the winter. Here is the thing, one bike is a 1975 XS 650 B,(447) it has been converted to a 'super moto' style bike and is in really good shape, after getting it running I can not get the idle below 4000rpm!!! I rebuilt both carbs (VM36mm mikuni's) split cable set up ( 1 into 2), all the cables-hoses-intakes are in great shape. When I adjust the carbs with the idle adjuster screw, all the way in is at about 6000rpm- and when I adjust each one the same number of turns out(and I've gone all the way out) it will not go below 4000rpm Could this be a timing issue??? The bike runs fantastic accept for this one thing, I have even disconnected the cables totally, to see if they were hanging up or maybe too short, but same story Any ideas would be greatly appreciated.

 

[5twins]

Check the ATU to see if it's stuck at full advance. It's also possible the slides may be swapped side to side in the carbs (or in backwards maybe) but I think it would race to redline then.

 

[Ironhead]

Thanks, I'm gonna go try that right now.*fingers crossed* I don't think it's the slides I did them each separately, one at a time, but that doesn't mean the previously owner didn't...

 

[Travis]

Take a picture of the slides in the carbs (as viewed from the air filter side) so we can see how low they go.

 

[Ironhead]

O.K. Travis, but that'll have to wait til tomorrow, but will have them up A.S.A.P. (no camera right now).

 

[jayel]

throttle plates (butterflies) holding open?

 

[Ironhead]

There are no butterflies in these Carbs, I do however have 6 sets of the stock type carbs, I am thinking of going back to them if I can't sort this out. I'll post some pics sometime today of the carbs that are on it now. I know my way around S&S carbs and Sportster motors pretty well-but these things are a whole new animal...Thanks again.

 

[Ironhead]

Here is a pic of the carb so you can see the slide.( the other is the same) It started and idled this A.M. and I took it for a quick ride, but when I got back, when it was warmed up sufficiently she started reving to the sky again...

 

[5twins]

The slide looks OK. If they were installed wrong and sat too high, the bike would rev high immediately upon start-up, not only after hot. I would check for air leaks and also check the operation of the advance. If the advance is sticking, that will cause the RPMs to stay high. You may need to service the ATU and the advance rod (clean and lube). Most of these bikes haven't had the advance rod lubed since they left the factory.

 

[Ironhead]

Thanks for the info, that was actually my next step. Swapped out the carbs for a set of the originals and just doesn't run right (popping and backfiring) so back to the other carbs and servicing the ATU...

 

[Sundie]

<thinking> Isn't it impossible for full-slide carbs to get into a 4k race condition? I mean, no way enough fuel is coming through if you don't roll the throttle and raise the slides...

 

[Travis]

What do you have jetting? Those carbs look funny to me for some reason..

 

[5twins]

I've never seen a long air screw w/ knurled end like that and I've never seen anyone plug the air jet completely with a screw before. Usually, for use on the 650, you remove the air jet and run with the hole wide open.

 

[JasperLP]

I agree with 5twins, where does it get its air? I don't see the idle screw on the side of that carb. Got any more pictures?

 

[Ironhead]

The air screw is just an addition that I installed same as the other just different in looks (originals installed in one of the pics) as I said, I installed the original carbs and really don't like them. The guy I bought all this stuff from installed these carbs and only rode the bike for about 110 miles. I got it running and it worked great for a short 2 mile ride, but the revs just kept building , and when I got back she was pegged at 4000-6000 rpm. Getting kind of worried I have 3 more motors to go through to see which one goes in the first bobber for my son.At this rate I should just plan on one for my grand son, haha...Thanks for all the help guys.

 

[JasperLP]

Ironhead,
I see, you have two left hand carbs. Makes it kinda hard to reach that one idle screw. I also don't recognize that black plug behind the fuel inlet, my vm34s don't have that, and they don't have that brass screw in the intake. Go to the tech section under carburetors, good info there.

 

[5twins]

I don't think there's a right hand version of the 36, only 32s and 34s.

http://www.carbparts.com/mikuni/vm__round_slide_standard_specifi.htm

 

[tx650]

Check for blockage in tank cap

 

[Ironhead]

Yeah,Jasper it makes it difficult to get at that thing for sure. I think you are right 5twins everything I've found for info always show the left (it's basically a dirt bike carb). I also think tx650, you might be on to something there, the other day (when it was running) after I shut her down I could hear some odd sound, I traced it down to the gas cap popped it open, and it stopped!!! So back to the drawing board today Vm's going back on clean and service ATU (ATR) timing check and clean (switch) gas cap- tank. gonna remove the air screw in the carbs see how that works. Thanks for all the great info guys, like I said before I'm new to the XS's I'm used to pushrods and a single carb. It is most definitely a learning curve.

 

[5twins]

That screw blocking the air jet may be part of the P.O.s attempt to tune the carbs. You need to make a list of the jetting that's in the carbs now (including the needle) and compare it to an established baseline. Some good baselines for the VMs used on a 650 are given here .....

http://xs650temp.proboards.com/index.cgi?board=carbfaq&action=display&thread=730

Running with the air jet removed is part of the established baselines. If you still have some or all of the "off the shelf" dirt bike jetting in there, removing that blocking screw may make things worse.