Rev and return to idle problem

Hi everybody , I hope someone can help with a problem I have with my XJ 550 Maxim H 1981...I have removed the carbs and replaced the float needles and rubber o rings and jets , cleaned out 3 times with carb cleaner, replaced the air intake boots, manifold boots and all clamps, vacuum pipe to the petcock, replaced the throttle cable and adjusted to linkage on the carbs so snaps bak real good.....My problem is when revving the revs respond exactly as you would expect but as you twist the throttle grip back just as fast the revs come down slowly....any ideas what this could be .......thanks any help would be much appreciated....Col

 

hmm, kinda sounds familiar - check this out for ideas and ponderings:
http://www.xjbikes.com/forums/index...at-subside-after-3-seconds.47814/#post-425286

My free time has been very limited - messed around a little bit last weekend, still have issues, pulled the rack off again and planning on breaking it apart cleaning up the (new in May) throttle shaft seals (and anything else that looks possibly gunked up); and doing a thorough inspection of all the springs in the linkage.

Good Luck and re-post if you find an answer!

 

Thanks Stumplifter, loads of food for thought ....a lot of it was way above my head but I really like the bench test of using the break / clutch to see what the revs do after pulling in , if they go back up would indicate a vacuum leak.....I am not sure this will work for me because I only have the issue of the revs coming down slowly.......if my memory serves me right I have already done this and nothing changed......I feel more inclined to edge towards a binding issue...ie diaphragm slides binding and not closing correctly...my dilemma is do i replace the whole diaphragm or just the rubber .....the cost is so restrictive just to eliminate that as the problem.....
Thanks ever so much for you help.....will let you know if I get the answer and remedy to the problem.....Col from the UK

 

Have you checked your valves or synced the carbs? Did you break the rack?

 

Hi, no not checked the valves yet , do you mean the shim clearances, only visually synced the carbs and the butterflies look to be in exactly the same positions as each other, and no I have not taken the rack apart if thats what you mean...certainly not dropped it .....lol
Col from the UK

 

Well I would start with "breaking the rack" taking everything apart yes. Get those carbs zestfully clean. Replace all rubber parts. Throttle shaft seal and fuel supply o rings. Do a good bench sync! But get those valve shims checked out. If any are tight take care of them. Then do a sync. I'm sure I'm missing something... And you can search everything on this wedsite.

Good luck!!!

 

Hanging idle indicates an out of sync condition and incorrect float levels can also contribute.

You can't properly sync the carbs until the valves are in spec.

 

Eliminate future headaches. +1 What they ^ said.

Gary H.

 

Thanks for the input guys, I have been watching quite a few vids on u tube and all the bikes seam to rev the same as mine so I am wondering if I have a problem at all or is it that I am expecting something that will just not happen. When I rev the bike the revs do come down, just not as quick as what I was expecting them to.....Should the revs come down very quick or a gradual decline....I could do with a bench mark to look at to see what I am aiming for....

 

Spec the valves , then check all the carbs again.....I have checked the float levels 3 times now and cannot find any adjustment I could make to make them better they are all exactly the same hight...The butterflies are the same although I have not synced them with a measurement... I have checked the floats are not damaged and taking in fuel.....So I am off rip the carbs out again
Col

 

http://www.xj4ever.com/setting fuel levels.pdf

Have you checked this out yet?

 

Also take a look at the cheat sheet on the last page of the pdf.

 

Hi Yes , that is a fantastic how to thanks.....I have replaced all needles and seats with new o rings...made sure needles where exactly same size, all seamed bang on....however I have not checked the hight of fuel level with a clear tube as all screws are chewed and I cannot get them out....will this mean I would be better off getting a screw extraction kit..
Col

 

The idle is hanging. The only way to truly synch your bike is to do a "running" synch. The valves must be in spec in order to do a proper running synch. The answer(s) you seek have been given. Hope this helps.

Gary H.

 

What shim kit do you get for the 82 xj 650, I can't find one on ebay?

 

No need for a kit. Shims are sold individually. You swap shims to get the clearance you need for a given valve then order the ones you need. Send Chacal a e-mail, tell him what you need and your bike info. Hogfiddles also runs a shim pool. Send him a e-mail (he will explain how it works). Hope this helps.

Gary H.

 

Once the bike is properly tuned, the motor should SNAP to throttle and return immediately to idle as you would expect. There are a LOT of vids on YouTube of improperly running bikes whose owners seem to think they're fine. A hanging idle is quite common, as it's the primary symptom of a motor that isn't fully in tune. But no, it shouldn't do that.

 

From my experience, if the rpms hang up, the fuel mix is lean. Check for vacuum leaks.

Conversely, if rpms dips down below idle and recovers to a stable idle, typically means a rich mixture.

However, any troubleshooting without valve clearances in spec, is all a moot point.

 

Well, I'm going to throw some fuel onto the fire--

Yes, you CAN sync your bike with valves out of spec ...... But the syncing will be affected by, and relative to, the out-of -spec valves. There ARE enough variables to keep the bike running rather well. But OF COURSE it won't run nearly as well as when everything is in spec.

Don't tell me a bike can't run when a valve clearance is out out of spec. If that were the case, then it would be much easier for us-- just ride the bike til it won't run, then do the shim swap. But, that's not how it works. How do we know if a shim clearance is out? By measuring at regular intervals so if a clearance is out of spec, we can get it back in to maintain OPTIMUM performance.

End of rant

Sounds to me like there's a vacuum leak somewhere yet, or the mixture is a bit lean

 

ONLY MAYBE.

However, if say, 6 of the eight valves are at zero clearance plus the carbs haven't been properly serviced yet IT AIN'T GONNA RUN.

And once the carbs are all sparkly clean, float levels all wet-set and bench sync'ed, if the bike still has a handful of tight valves it probably won't want to start. And will be all but impossible to tune.

There is a reason that valve clearances are the very first maintenance procedure in the service manual.

 

service manual, factory service manual, for dealers, follow the money. if i take a bike to a dealer and say it's running a little rough.......adjust the valves first $$$. no matter what, it's always adjust the valves. i might have a tank full of water....adjust the valves first. this insures that nobody gets out cheap. if you say tune it up, that's adjust the valves. if you say make it run, not so much.
if it ran ok when you parked it, it will run again without touching the valves
i'am done

 

wet setting the floats after rebuild is very important. use a left hand twist drill bit to remove the old screws ( al little heat helps them break loose) and replace them with the hex head ones from len.

CN

 

Well, yeah......if all or almost all of the valves are at zero clearance, of course---but from what I could tell from the discussion, we weren't talking about almost every valve being SO far out of spec that there's absolutely no clearance left. I just find it amusing that sometimes the comment along the lines of "if you have a valve out of spec, your bike can't run" comes along. There's more to it than that. And with that, I simply mean that even when a valve is out of spec, there can still be plenty of clearance that will still allow for fuel to enter, compress, combust, create power, etc..... Of course, it wont be optimum, but with the variables that are inherent in the bikes, there is enough give-and-take that will allow for some lattitude.

and I'll throw some other fuel on the fire, too, while I'm at it.

I have NEVER done a wet-set on the floats. I carefully set them and that's it. I haven't had a single bike that has had any running issues from having done it this way. I'm not saying it's the right way, but I'm not saying it's the only way, either............. But I will also always suggest that it be done.

Just like with fuseboxes......I fully and wholeheartedly think the blade-fuse upgrades are awesome. AND........of course, having said that-----every bike that I have on the road is still running on the original factory fusebox.

dave

 

I'm running (rebuilt) original fuseboxes too. And also recommend the upgrade to anyone so inclined.

You missed my point on valve clearances. Of course, one or two or even six valves slightly out of spec won't keep a motor from running. When they get tight enough they affect valve timing and that cylinder starts to lose compression and will then affect how the motor runs. Completely ignore an XJ long enough and the day WILL eventually come when it won't make enough compression to run due to all of the valves being tight and half of them dead tight.

My point was IF YOU HAVEN'T EVEN CHECKED THEM then you have absolutely no idea what you're dealing with; you might very well have a motor that was neglected until it would no longer run. If you get your hands on an old bike with no first-hand knowledge of its history the first prudent step is a compression test. Due to the design of these motors, a few way-too-tight valves can seriously skew said compression test. So if you're serious about the recommissioning, step one is to get the valves in spec and step two is to do compression tests. Then and only then do you know for sure what you're dealing with. To try to adjust or tune carbs on a bike with unknown valve clearances is a complete waste of time. Sure, it might run. Or not. But if you're truly serious about getting it running right, then that's where you start.

Wet-setting is more precise than dry-setting can ever hope to be. Please keep suggesting that it should be done. It's also a very good way to catch other float related inconsistencies or leaky valve seat seals, fuel rail issues, etc.

 

and THAT is exactly my point