Rpm's hover, but not finding air leak.

hey xs folks.
first of all i'm a mega novice, but i did just finish a top end rebuild of my 77 xs. got the jugs bored out, and got a new 1 over piston and ring sets. got the motor back in the frame, and she started right up. but after a few minutes the rpms "hover" after i let off the throttle. i know this is a classic air leak on the intake boots, but i've sprayed carb cleaner and wd40 all over the manifolds before and after the carbs, and they're is no change whatsoever in the rpms. after a little bit longer the rpms won't go down at all unless i put on the choke for a second. i've cleaned the carbs twice now and have noticed nothing new. no stumbles while rolling thru the throttle at all. the plugs are show slight signs of being lean, but not crazy gray/white. some oil toward the outside with the electrode tip a little gray. the mixture screws are 2 rotations out. the same that they were before the rebuild. any ideas? really appreciate any help.

There are other causes for idle hover, including the need to sync your carbs and an advancer hang up.

I used to have this problem. I set the tension on the timing chain then set the valves then syncd the carbs and it went away

thanks for the quick response. i'll check those things and go from there.

As others have already mentioned, its important to have both carbs sychronized.

As part of that, you want to be sure that both carb butterflys will close fully. When they close fully, the butterfly will fully cover the 3 bypass holes in the top of the carb throat.

When I have my carbs removed from the bike and sitting on the bench, this is what I do.
Cut a piece of paper about 1/2" wide by 3" long, to use as a feeler gauge. Use the throttle speed screw to adjust the left butterfly for a small amount of drag on the piece of paper. Then use the sync screw to adjust the right butterfly for the same amount of drag. If you can't achieve that, then the butterflys may not be centred in the carb bores, which would require you to loosen the butterfly screws so that the butterflys can centre themselves in the crab bores.

allright folks,
well i adjusted the cam chain. was a little on the loose side. adjusted the valves. intakes were a little loose (barely though). now they're right where they need to be. i used the paper in the carb throat against the the butterfly valves like described above. seems right on to me. also the bike is not new to me. i've ridden it constantly for 5 years. i did a top end rebuild because my cam chain guide (the one toward the front) fell apart and black bits were all over in my oil. i got the jugs measured and they were just at the end of tolerance. so i got 1 over pistons and rings. got the jugs bored out accordingly. intake boots were replaced not all that long ago, plus i'm just not finding an air leak. today i also raised the needle setting one slot. i was feeling good, and hoped i was gonna finally have a well working machine. she starts right up, but after even less time than before the rpms wanna run away from me. have to put the choke on for a second to bring it down. so what's next people. advance unit? replace the springs? also i haven't checked the points. i did mark the backing plate to the points housing, and replaced it exactly where it was before.
i guess my hang up is; the only thing difference (as far as i can tell (ie. no left over bolts or nuts when i put it back together)) is the new gaskets, and 1 over pistons and rings. i know it's user error or something. really want to finish the job on my own instead of selling something in order to bring it to a shop. any help would be rad. thanks

the very first step on my to-do list would be set those points properly - what could be causing your problem is an overadvanced state of ignition. set the point gap using clean (oil free) feeler gauges, then use a test light set the inital timing then use a timing light to fine tune with the engine running.

if the bike is basically stock (aside from the one over pistons) put the needles back to the original clip.

also you need to sync your carbs properly, the paper method only works for a rough sync and to make sure the butterflies are centered. use a home made manometer or buy a set of carb sync gauges (i got a set of motion pro gauges that work great).

When you assembled the advance rod, did you grease the bushings it turns in?

tie the advance unit in place so it can't open and see if that chages things. Easy to get to and do.

thanks for all the advice. finally went out and bought a timing light, and will get the points next. i'll make sure to sync the carbs next. it's taken a long time to do this rebuild, and i really just want to make sure the bike is running top notch. no shortcuts, i'm realizing fast. also, i did grease the advance unti rod during assembly. i'll keep you all informed as i find things out. thanks.

checked my points and lo and behold...they were all out of wack. got them right where they need to be and i'm not noticing any hover! still want to sync the carbs. does anyone have a picture or diagram of a homemade meter. i'm a real visual learner. any help would be greatly appreciated. thanks.

Here's a link with instructions and a pic. Very simple to make .....

http://www.powerchutes.com/manometer.asp

I'm having the same problem with intermittent high RPMs. ( '77 xs650 stock ) I've set the valve clearance, set float levels, and synced carbs. In that order. Still no fix.

So I'm checking points/timing. But when I pulled points cover noticed the points cam has worn spot and crap coming off ( see picture ) and the advance on the other side has rust coming off. That's normal?

No, that's not normal, lol. You need to spend some quality time with the old girl and do some much needed maintenance. The felt pads rubbing the points cam are there to hold grease and lube the cam as it spins. The grease on yours is all dried out and chunking off. That's the crap you found. The advance unit should be clean and oiled, not all rusty like yours. You need to service it and the advance rod that runs through the cam. The advance rod is probably dry and hanging in the advanced position. That will cause the hanging idle.

Clean and lightly grease the advance weight pivot posts. Clean and oil the springs. Lightly wipe the whole unit down with oil so it shines. In the future, a small shot of foaming chain lube on the posts and springs will keep them lubed.

The outer bushing at each end of the cam that the advance rod turns in has grooves cut into it's I.D. These are to hold a reservoir of grease and keep the rod lubed. I just serviced mine recently and re-greased the bushings. I had done it initially 5 or 6 years ago. There was still some grease left in there but I took the opportunity to refill the bushing grooves. I'll get some pics up shortly.

Yeah I guess it does look bad.

I should take everything off of the points and advance sides in order to withdraw and lube advance rod?

Here's my cam end bushing after 5 or 6 years of service, some grease left in the grooves but not full anymore .....



..... and after re-filling .....



The factory recommends some sort of lube containing moly. I use VW CV joint grease because it has a very high moly content.

Easiest way to service the rod is to remove the little disc from the advance side, the one that hooks into the weights, and then withdraw the rod from the points side. The points cam will remain attached to the rod. Clean any old grease out of the bushing grooves (you probably won't find any) and fill with fresh stuff. Lightly smear the entire rod with grease to protect it from rusting. You don't have to remove the advance unit or points and points plate. This won't disturb your timing or points settings at all.

Just be sure to align the little disc properly upon re-assembly or you'll throw your timing off 180°. There is a locating pin on the rod for the disc. There is also one for the points cam. They must both point in the same direction .....

Thanks for the help. I've managed to lube advance rod, cam, advance weights. Got rid of a good amount of crap. Set the points gap while I was at it between .3mm and .4 mm. RPM still climbs high for some reason. Happens when I blip the throttle.

One thing I noticed was that points cam seems to have worn spots ( notice darker area on cam in photo ). It's not completely smooth. Would this cause my RPM problem?

lafayette said:

Thanks for the help. I've managed to lube advance rod, cam, advance weights. Got rid of a good amount of crap. Set the points gap while I was at it between .3mm and .4 mm. RPM still climbs high for some reason. Happens when I blip the throttle.

One thing I noticed was that points cam seems to have worn spots ( notice darker area on cam in photo ). It's not completely smooth. Would this cause my RPM problem?

Click to expand...

I recently had my idle hanging when I would blip the throttle and changing gears. I installed new pilot jets that are 2 sizes larger than stock (#32.5). After installing the new PJ's, I adjusted the mixture screws, and found the sweet spot where the rpm no longer hangs. I was unable to buy the first size larger (#30) than my stock PJ's, so I tried the 2 sizes larger and its working for me. I have to drive more kms to see if this will be the best way to go, and I will likely try 1 size smaller PJ (#30).

Your carb may be too lean, so a larger PJ may help you.

no one mentioned throttle shaft seals yet eh...

spray carb cleaner where the butterfly shaft pivots in the carb body....guarantee it stalls the bike out.

I wont even run a set of carbs now without doing those seals. every set i have had has been bad.