Slow return to idle, sometimes it won't return

I have an '85 XJ700X with 16K on the clock. I recently bought it, the tank was rusted and it was running terrible. I coated the tank with the POR-15 system, tore down and cleaned the carbs thoroughly, replaced all the o-rings and float bowl gaskets & set the floats to spec.

I took it out for a ride today, it has decent mid - high range power, is sluggish off idle and low range and is slow to return to idle or won't return at all. I did the propane vacum leak test but no sign of a leak showed up. The idle is rough.

When it does return to idle (1050) it will hang around 2500 for a bit then drop right back to 1050 quickly....the rest of the time it stays up at 2500 or so.

When I hook up the carbtune the bike won't run, although it did run for a short while once and I managed to get it pretty close on the sync.....within a couple lines I believe.

Anybody have any idea what's causing this?

 

A few things I can think of:
1)Throttle cable sticking
2)Vacuum leaks, check again and try some starting fluid instead of propane.
3)Idle mixture to rich, if your screws are out much more than 2 1/2 turns that might be too much.
4)carbs way out of sync
5)float levels to low or high
Could be one or a combination of these things.

 

If you are clever enough, ... you can Refinish and Polish the Diaphragm Piston Bores without removing the Carbs.

Pull the Tank
Remove the Hats on the Carbs.
Remove the Springs and Diaphragm Pistons
Scrub the Bores with ScotchBrite Pads
First, with GRAY (Medium)
2nd, with WHITE (Extra Fine)

Get the Bores as Shining and Bright as possible.
Clean.

(If you want to go the Extra Mile, ... do the Bores with a DREMEL Tool with the Flex-Wand Attachment, ... and POLISH the Bores until they are Mirrors.)

 

Thanks Rick...I paid close attention to the slides & bores when rebuilding the carbs....all (4) were shiny like new and passed the 'drop' test with flying colors.

It acts like a vacum leak but propane flooding didn't indicate one.

 

mlew...Thanks for the info

1) The throttle snaps back 'with authority' don't believe it's stuck
2) I'll try this today
3) I checked each one before removing for the rebuild...one is out 3 3/4 (I wondered about that) is 2 1/2 out considered a good ballpark starting point? (I don't have a colortune)
4) Think they're pretty close but it wouldn't run with the petcock vacum line removed (even tho it was in the 'prime' position) I'll plumb in a gas bottle bottle today and re-check
5) Before re-setting the float levels each one was showing a wet level of about 6mm - 8mm....pretty sure no one has been in these carbs so that could be a factory setting. Went with the 3mm spec but that seems really high....do you think I should re-set to 6mm - 8mm?

Thanks for the help guys, this is probly something pretty simple because sometimes it acts normal and returns to 1050 rpm like it should....also, just a touch of the starter button and she's running. Just seems the warmer it gets the more the idle will hang.

 

+1 on checking for vacuum leaks with carb spray.

Sounds like its finding some air to burn that rich idle screw mixture somewhere.

Its also odd that when you hook up the carbtune it won't run

 

Sometimes the reason a Bike will run a bit fast is that Fuel is sneaking-in.

You might have "Flattened-out" Pilot Mixture Screw O-rings.
One or two might be "Split"

The Vacuum Line to the Petcock might be squirting-in Fuel from a Perforated Membrane in the Vacuum-Op'd Valve of the Petcock.

One Carb "Slightly" out of Sync with the others; opened a bit more letting Main Jet Fuel cause the Engine to Hunt.

 

Going to set the A/F's at 2 1/2 out shortly.

It did run one time with the carbtune hooked up and I got them sync'd pretty close, but am going out now to set up a temp fuel supply and remove the tank....see what it does then.

 

All new o-rings on mixture screws as part of my rebuild. Incidentally, I have several new o-rings for the mixture screws if anybody needs any just let me know (I re-build V Max carbs and they take the same size o-rings)

Going out now to eliminate the tank, petcock et-als, do a re-sync....I'll post the results.

Thanks for the help guys!

 

Well...it started with just a touch of the starter button...stalled quickly and needed a little choke, but started right back up and after a brief warm-up sat idling at 1050~1100.

When I disconnected the petcock vacum line there was a tiny bit of gas at the tank end of the line....think a petcock rebuild is in order.

While it was warming up it would return to idle fairly quickly but when it was up to temp it would hang around 2K for a few seconds and then drop back.

The sync is right on altho low....only at 14~15 on the carbtune. Is that normal?

I haven't gone to town yet for starter fluid but did the propane thing again with negative results. Just for verification I removed the #4 carbtune hose and shot some propasne at the spigot, the idle immediately jumped up....I'm doubtful there's any vacum leak.

It will take WOT after a brief hesitation...if revved to 2K first then whacked it takes it well, but still hunts for idle.

The remote fuel source did make a difference...didn't solve the idle hunting issue but helped.

Oh, I reset the A/F screws at 2 1/2 each

I'll re-check my float levels after it cools, I set them on the bench with the carbs level and they were well within the ±1mm

kind of at a loss here.......

 

I hate to bring this up, but have you checked the valve clearances yet?

 

AGH!!!! I was hoping that wouldn't come up!

I'll probly be drummed out of the XJ Forum for this but....I replaced a leaky valve cover gasket last week.....looked at all those little valve stems and decided to tackle the valve lash issue during the winter. I wondered if that decision might bite a big chunk off my butt at some point!

In my defense....with only 16K on the clock I assumed (yeah, bad word!) they were probly ok.

Could clearances out of spec cause this problem?

 

Indirectly. A couple of too-tight valves can throw your sync off, and get in the way of getting a "true" sync.

I know the the "X" motor has longer intervals, but the initial check is no less important than on the aircooled motors. Further, because the "X" IS more difficult to adjust, the likelihood of being neglected is even greater.

I would at least CHECK them asap, so that way you know if that's what's throwing things off.

This is also why the mantra is "valves first." A couple of out of spec valves can drive you buggy trying to do any serious carb tuning.

 

I noticed yesterday my choke was sticking open a little. I tapped it with a screw driver and it got unstuck.. Something to look at as well.

 

OK, I'll bone up on the procedure and check them.

Checked the wet levels again....all (4) are within spec. Boy, #2 & #4 are a PITA with the airbox on....ey?

Started it up and it ran nice, returned to idle, and was fairly responsive off idle but the more it warmed up the more it began to hunt for idle.

 

As part of the rebuild I removed, cleaned and lubed the starter plungers, all (4) are working smoothly with free play in the connecting rod when they're closed.

Thanks for the idea tho.

 

Pulled the plugs this am, they are a paper bag brown....not bad really, but the center 2 plugs #2 & #3 look like they haven't been changed since I sold it back in the late 80's!!

The crush washer for #2 was bent and laying in the plug well somewhere, don't even know if it was on the plug or not.

Anyhow, backed the A/F's out to 4 turns, straightened & installed the crush washer, let warm up till the idle went up to 2K or so, turned the tick-over adjust so it was at 1K± and sync'd it.

Picked up some vacuum, it's about 16 on the carbtune scale and isn't running bad at all....especially considering those nasty plugs! Gonna run up to the auto-parts for some new plugs and see if I can't fine tune her some. Definitely on the right track tho....thanks guys for the helping hands!

 

The valves won't go away by pretending they don't exist; you still need to at least check your clearances. No special tool other than a metric feeler gauge; no more disassembly than the valve cover.

CHECK them before it's too late.

 

The Mikuni Carbs on that Bike aren't very forgiving.
They need to really be "Attention-to-details" clean to perform right.

Especially, the Cold Start "Enrichment Circuit".

 

I hear ya bigfitz....valve lash is next on the schedule.

 

They are Rick, if I can't do a job right, I won't bother to do it at all.

Must of worked cause she rewarded me today with 100 miles of trouble free smiles! :lol:

Takes throttle off idle well, smooth through the entire power range, snappy & responsive, plugs looked great.....she's running just like she used to 20 some years ago.

 

I don't mean to hijack but what as it that fixed it?

I have a 1981 Seca 550 and I am having the same problem just higher rpm, I have checked valves, 2 a little tight, replaced throttle seals, replaced intake boots, super cleaned the carbs (not my first time at the rodeo), checked throttle and choke free play, synched the carbs. I've got nothing :evil: .

Mine is just like yours, fine when it starts then as it warms up it sticks worse. Do the mikuni carbs for the 550 have a pilot air screw? Any insight would be greatly appreciated.

 

No hijack Tswami. My problem was a combination of idle set too high, A/F adjustment and sync. Being cold-blooded like it is, the high idle didn't show up for a few minutes kinda confusing the issue.

I don't know about the Seca carbs but being Mikuni's the A/F screws are probably right behind the top hats, same as on the 700X carbs.

 

Are the are screws on the top of the carbs? I've never seen a carb with them on top but that is the only place I can see that they would be.

I guess if that is it the caps are still on so no one has messed with them and they need to be cleaned as well as adjusted.

Did you check your valves yet

 

On the 700X (33mm Mikuni's) they are on the top right behind the top covers....I'm guessing your carbs will be the same.

No, no valve check yet....I still have 10K miles to go per Yamaha maintenance schedule and with limited riding time left up here in the northeast they can wait till winter. It probly won't see another 500-750 miles before snow.

 

Ha, I understand that. I'm trying to get this thing ready for fall riding and its coming quick. My 2 tight valves are going to have to wait.

 

Here is a MIKUNI Carb with the Pilot Mixture Screw "Anti-Tamper Plug" still in place.

Left:
Factory Plug in Pilot Screw Orifice.

Right:
Drilling the Plug.
The PLUG (Red area) is only 2mm thick.
The PILOT MIX SCREW (Yellow) is just below the Plug and must not be damaged.

 

Rick, Thank you kind sir.

Are the diaphrams on these things kind of thick and stiff? I know bad ones and these seem just to be made this way. I was thinking they weren't returning easily enough but maybe the thick feel is just normal

 

I shoot some Armoral into a Shot Glass and soak Q-Tips in it, ... then -- wipe-down both sides of a Diaphragm Rubber every time I'm in there for cleaning.

I used to use TEAC Rubber Cleaner.
TEAC Rubber Cleaner became Unobtaineum.

 

Tswami, did your slides pass the 'clunk' test, do they move freely in the bore?

 

BAD IDEA. You'll never get the carbs sync'ed. Plus, if they're exhaust valves, you take a good chance of burning one.

Your high/hanging idle is the result of an out of sync condition that you won't be able to correct if your valves are out of spec; the more you fiddle with "adjusting" the carbs the worse it will get until you DO IT RIGHT.

You can take the advice, or find out the hard way. Of course, if you'd like to ride and enjoy the bike, you'll need to DO IT RIGHT.

Or you're gonna be at it until you do. The book says "valves first" for a reason.

 

Using somewhat left-handed reasoning then, if the carbs sync well...and the bike runs good, could one feel comfortable that the valves are within spec?

Especially if there is still 10K miles remaining before the first Yamaha recommended valve check interval?

 

Using somewhat left-handed reasoning then, if the carbs sync well...and the bike runs good, could one feel comfortable that the valves are within spec?

Especially if there is still 10K miles remaining before the first Yamaha recommended valve check interval?[/quote

I think the point is that Fitz and Rick are trying to explain in short is that most people never had their valves looked at back in the day when these bikes were new. Therefore everyone who are trying to sort out any running issues or trying to adjust carbs should take the time to follow the steps the factory laid out. Check and set the valves first! before making any other adjustments. Becides it only takes less than an hour to remove the valve cover and check the clearances anyway.

 

I think the point is that Fitz and Rick are trying to explain in short is that most people never had their valves looked at back in the day when these bikes were new. Therefore everyone who are trying to sort out any running issues or trying to adjust carbs should take the time to follow the steps the factory laid out. Check and set the valves first! before making any other adjustments. Becides it only takes less than an hour to remove the valve cover and check the clearances anyway.[/quote]

Don't take me wrong, I understand what Fitz & Rick are saying, and agree with them 100%....and I'm definitely NOT saying that a successful sync will eliminate the need for a valve lash check....certain trouble will follow there!!

Just that, with other factors present, could sync be used as ONE indication of valve tolerance condition?

 

I don't beleive that you could use that as a indication. You could still have valves that are slightly tight and get it to sync. If you want to go with it thats your decision.
I however know where my valves are because I once had to pull a head and find out how much it costs for replacement valves and a valve job. It was not a pretty sight to say the least

 

I'm not making any decision, I'm just asking a question.

So, it could sync with a slightly out of spec valve.....but a slightly out of spec valve won't cook a motor, right?

 

Yes I beleive you can get it to sync and I'm saying not a good idea to gauge your engine by that.

If your valves are too tight in any case here's what happens. The valve needs to sit on the seat for certain length of time to cool down. If that duration is shortened it can lead to valve failure sooner than later. Valves fail and burn due to not cooling properly. Over head cam engines that do not use hydraulic valve buckets tend to burn valves because no one pays attention to them untill its too late.

As far as cooking the motor don't think so, but cooking the valves over time will for sure make work for the machine shop.

 

Totally agree....it is an extremely poor idea to use sync as the sole indication of engine health! Almost certain destruction will follow!

 

If by "not cook a motor" you mean not ruin the cylinders/pistons, true; however a VALVE JOB on a 20-valve motor might qualify as cookage, wouldn't it?

CHECK THE VALVES. FIX TIGHT ONES ASAP.

Unless you really want to go through pulling the head, replacing valves, lapping the rest, installing new valve stem seals, etc.

I guess it depends on your idea of a "cooked motor."

This advice is even more critical on an AIR COOLED MOTOR.

 

Absolutely busted, bent valves qualify as 'cookage'! Hell, anything that forces you to the machine shop qualifies as cookage in my book.

AND, most certainly out of spec valve clearances should be fixed....I hope nobody thinks I'm poo-pooing the importance of periodic valve lash checks, cause I sure ain't!!

 

I know that; but the prevailing idea seems to be that it's "ok" to put it off until last, when in fact it's the first step in carb tuning.

Simple fact of the matter is, putting off adjusting a tight valve, especially an exhaust valve, is asking for a burnt valve; ESPECIALLY in an aircooled motor.