rebuilt carbs and now it wont idle down

I just got done completely tearing down a set of 750 maxim carbs for my XJ700. I know 750 carbs on a 700 isn't correct but that's what I had laying around. I figured these would work fine since it's only 50cc's smaller. And these bikes run lean from the factory. I was riding the bike last summer with these carbs and wound up putting 4000 miles on it and the whole time I had inconsistent idle. So I decided I would tear them apart this winter and give them a complete cleaning. And I installed new intake boot gaskets. I broke the rack apart and cleaned all the carbs in chem dip and then my ultrasonic cleaner. I put in new jets, new fuel needle and seat, new air needle and o-ring, polished vacuum slide and carb throat. Bench synced the carbs and reset the float heights. I put them back on the bike today and it fired right up but would not idle down. The bike sits there running 8000 rpm until it runs out of gas. I checked the vacuum slides and they move free. Does anyone have any ideas where I should start trouble shooting? Any ideas why it won't idle down at all? help please I am pulling my hair out and I'm almost bald already.

 

You didn't mention replacing throttle shaft seals, if they were left in while the carbs were dipped they may be toast. I would start by checking the outer ones since you can get to them without pulling the carbs again.

 

I did not check those. I will look at them tomorrow and see what they look like.

 

The Throttles are open!
8000 rpm's ...
That's WOT's.

Check a few things:
The Throttle Linkage Connection stuck under the Head.
A misplaced Manifold Clamp holding the Linkage open.

Return Springs not loaded.
Something keeping the Throttles open.

Oh, yea ... one more thing:
Don't let that Mill spin at 8-Grand without a load on it.
The Engine's 30 years old.

If you let that Plant rev too long at 8-G's, ...
You are at risk for throwing a rod.
Then, ... that's all she wrote.

 

Its doing that with the throttle closed as far as i know. I did get it to drop down to 6000rpm's a few times and I twisted the throttles and it reved up so I know the butterfly valves are closed. Its almost like there is gas just running into the intake unrestricted by the carbs. I filled up the float bowls with gas on my bench before I put them in the bike and there was no gas seepage. The gas stayed in the feeder tank I have. I rigged up a little wheel in the line so I can see when gas is flowing. None was so I am pretty sure the fuel needles are closing off the flow of fuel.

 

Maybe way too simple but are you sure your idle knob isn't twisted way in-Also if your floats were flooding it I don't think it would even run-you are getting proper atomization (or close to it) or it would have other problems. Someway, somehow it's behaving as if the throttle is opened.

Agree with Rick about letting it run that high off load - anything above 6k is dangerous to run w/o a load on the crank, even for a short time.

 

How far open were the butterflies when you bench synced? They only need to be just open past the first of the two low speed bleeder holes.

 

Air leak!

Replace your shaft seals and check your air jets for correct
placement.

 

I used a couple of allen wrenches. One holding the number 3 open slightly and another to adjust the other butterfly valves so they just touched. I know its not a perfect system but it gets me close enough to get the process started. I have a YICS blanking tool installed right now so I am ready to do the final sync. I have a mercury manometer. I am also going to use my colortune to set the air screw once I get it to idle down.

 

How small were these wrenches?

The throttle blades should drag on a strip of 3x5 index card when bench synched. I'm going to guess you have them open a good deal more than that and possibly way out of balance which is causing your problems.

 

I used 3mm allan wrenches holding the number 3 butterfly valve open and used another one to adjust the other 3. They all look like they are touching in the same place. I used a magnifying glass to adjust them so I could really see exactly where they touched. Could a leaking enrichment circuit be passing fuel through? Isn't there a seal at the end of those plungers?

 

Raw fuel won't burn. It needs the air. You're getting air somewhere and on a CV carb once you get air it lifts the slide and that adds the fuel.

A 3mm opening has your throttles open way to far for an idle. An index card is .009" or 0.22 millimeters thick.

Even a paper clip is only 1mm.

 

Ok I didn't understand what you were getting at. I put the allan wrench under the butterfly valve to hold it upen against the spring pressure to close the valve. I put another one under the other butterfly valves on the other carbs and adjusted them to touch the wrench. I then took the tools out and the butterfly valves close all the way. There is no space at the top and bottom of the valve when they are closed. In other words they touch the throat of the carb at top and bottom.

 

Somebody put on here already, but when I bench sync I open the throttles to where I can only see two of the three air metering holes at the top of the butterfly

 

Check the simple things first...

Idle adjustment, Idle adjustment, Idle adjustment!

Whilst it's running, slowly back-off the idle adjuster (knurled knob) anti-clockwise.

It's very sensitive and reaction from the motor seems slightly delayed so don't go too far all at once.

Let us know what happens...

 

To rev that high, ... the Butterfly's have got to be open.
It takes Main Jet Supply to rev that fast.

Go back to Bench Sync.
Use a 1/4" Wide Strip of 3X5 Card -or- make a Feeler Gauge out of a Strip of 35mm Film you trim to 1/4" Wide.

You have to Begin Tuning with the Throttles Closed.
Make the Bike IDLE -- Adjusting the Pilot Screws.

IF, ... the Engine "Runs-away" on you while you are Adjusting Pilot Mixtures...
Conduct an examination for Air Leaks.

Air leaks, ... ALONE ... won't generate 8,000 rpm's.
The Throttles need to be near wide-open to get 8-Grand on the Tach.

 

Ok I will check that the next time I go work on it. I was so hopeful that with this cleaning would solve my problems. Concerning the butterfly shaft seals if they are still soft and pliable how do I tell if they are still usable? Is it difficult to replace them?

 

As the idle adjuster is wound in it will incrementally open the throttles/butterflies, surely?

It does the same job as the throttle cable, but from the other end, doesn't it?

 

Yes. But there should be a Spring on the Idle Rod that effectively limits how far the Throttles can be opened by the Idle Rod.

If the Mill is still revving over 3500 rpm's:

Start from scratch.
Bench Sync the Throttle Butterfly's closed.
Start with 3
Use 3X5 Card or a Strip of 35mm Negative.
Mechanically sync the Throttles --- Closed!

Work on getting the Plant to IDLE.
If you dont have a ColorTune -- Do it "By Ear"
Search: By ear.

Once you get the Engine Idling without Main Jet Fuel you can commence Fine Tuning.

 

I still think this is the first thing to check. you can get thousands of revs by turning in the idle adjuster.

In view of the fact that the carbs have been rebuilt, they will most likely be far more efficient and be overcoming any previous adjustment of the idle to compensate for old, badly adjusted carbs.

Plus, I bet he didn't count the number of turns there were on the adjuster before removing it and taking the carbs to bits.

I'm still confident that this is a major part of the problem, though willing to look daft if ultimately it's not.

Nige

 

Just as an experiment I turned my idle adjuster knob all the way in (idle up) on my Mikuni BS28 equipped XJ550. Normal operation (idle set at 1200rpms) it will start from cold (air temp 50F) with 1/2 enrichment in about 2 seconds and run at 2500-3000rpms within 10seconds, then after 1 minute at 1200 rpms no enrichment.

Idle adjuster knob all the way in or high:
-From cold with 1/2 enrichment: started in 1 second-ran up to 6500-7000rpms.

-From cold with full enrichment: started in 4 seconds-ran up to 7000rpms then begin to sputter and ran erratically.

-From cold no enrichment: started after 3 seconds-ran up to 3500-4000rpms.

-From hot no enrichment: started after 1 second-ran up to 5000rpms. After throttle blip it actually went higher briefly then v e r y slowly came down to 4000.

One thing clearly different with the idle adjustment set too high is throttle response-set correctly it is very crisp returns immediately, set too high it lags and returns very slowly, set too low the bike will die after a blip.

I would do the bench synch over to be sure. You really shouldn't be able to get those maximum revolutions with just the idle adjuster set to max. Air could be finding a way in as well as previously mentioned.

 

IDLE ... is the Engine running on the Idle Function of the Carb; alone.

Idle is alone and of itself.

Once you add something to IDLE; you are no longer Idling.
Fast Idle w/ *Choke*

Once the rpms reach the limit that Pilot Mixture Jets can supply to make power; if the rpms are increased the Engine is OFF IDLE.

You added something to IDLE.
More Fuel
More Air
Both.

The challenge, of Fine Tuning, is to get the Engine to IDLE and TRANSITION from IDLE -to- OFF IDLE without creating an Imbalance of Fuel and Air that will cause the Engine to:
Hesitate ~ Wait for Fuel
Bogg out ~ Have too much fuel.

Regulating the IDLE is the Key element to Fine Tuning.

 

Yes Rickomatic, but you know what I mean - and what Mercuryman has demonstrated - is that you can adjust the adjuster so far that instead of acting as an IDLE adjuster, it becomes a THROTTLE.

 

semantics..ok so I've edited my prior post to eliminate all the improperly included "idle" terms, and just to be clear I only included enrichment in the experiment to show how it could contribute-as a real world comparison for our posterity.

 

When all is said and done:

When you release the Throttle the Engine should IMMEDIATELY return to the Idle you have set and tuned too.

It shouldn't take a moment or two to settle down.

 

Quite right - where you have set it to.
If you've set it to 8000rpm that is where it will settle down at.

 

If the Idle Rod will open the Throttles to let the Engine run at 8-Grand:

Something isn't right.

The Idle Adjustment Rod's Tension Spring should bind and prevent the Mill from being over-revved.

 

Hey Rick, all I was doing was making a suggestion which might help the bloke.

I think he's run away by now.



PS. If you were to come to my garage I could start up my bike and then get it to rev at 8000 in the blink of an eye by turning the adjuster - but then, there's no spring fitted.

Anyway, I think we've exhausted this one - have a Merry Christmas.

 

There is only so much we can do rendering assistance in these forums.

Not being able to look for problems that might be simple to find if we were aside the bike oft times complicates matters because we stress what the common causes are.

Overload becomes a problem sometimes.

Generally, the right info reaches the guy and he gets on track.

 

In two pages of responses I didn't see anyone suggest the most common cause of this symptom--

It's possible the throttle quadrant is snagged on the back of the motor and not being allowed to return fully; the first thing to do is to carefully inspect the mechanical function of the throttle cable and linkages to see if it operates freely and returns fully to rest. (With the motor off.)

There's another possibility as well: Since you "broke the rack" you may have induced a bind in the mechanism when reassembling it if the carbs didn't end up straight and true. It doesn't take much in the way of misalignment to cause this. You should be able to detect this with a careful bench sync.

To answer another of your questions that wasn't addressed: replacing throttle shaft seals isn't super-difficult but it is tedious and requires not only breaking the rack, but removing each individual throttle shaft from each carb. Lots of fussy, precise work.

 

No i have not run yet.... i had to go to work and i am on yhe road now. I wont be able to get back to the bike until next week. Its been very interesting reading though. Thanks for all the info guys.

 

In my first Post I offered the two most common reasons for Carbs causing the Plant to run-away.

The Throttle Linkage Hook-up gets jammed under the Head.
A misaligned Manifold Clamp fouling the linkage.

If the Engine is making 8000 rpm's ...
The THROTTLES are OPENED and the Engine is running on Main Jets.

Remove the Carbs.
Secure the Hook-up for the Linkage to the *Choke* Rod to keep it from dangling.
Back-out the Idle Adj Rod to insure Butterflys will CLOSE.
Close the Throttles.
Lube the Shafts with Marvel Oil by standing the Rack on-end and placing a few drops of MMO on the Shaft Openings near the Flys.
Invert Rack: Repeat.
Assure Linkage is not binding by "Exercising" Linkage.

Lube Sync Screws.
Lube Idle Rod Threads

Bench Sync after assurance linkage is operating correctly.

Bench Sync:
Use a Feeler that will let the Bench Sync be done with the Minimum opening of Throttles
35mm Negative Strips
3X5 Card Slices
Resume' Paper

Mechanically Bench Sync the Carbs.
Use Idle Rod
Set Clearance of 3 to a Tight Drag on the Feeler
Match 1, 2 & 4

Inspect Lifting Forks of *Choke* Plungers.
Inspect Top Hats of Enrichment Plungers

Correct any misalignment causing the Plunger to not be seated.
Plungers with BENT Top-Hat Stems must be removed, heated, straightened, cooled to assure the Valve seats when closed.

Mount using CORRECT Manifold Clamps.
If the Carbs were mounted using incorrect clamps, ... Inspect Manifolds for Hose Clamp Incisions made be edge of incorrect clamp.

Good Luck.

(Also: Seal the Manifolds to the Head while the Carbs are off. Tuck some RTV into the Seams of where the Manifold meets the Head.)

 

Today I finally got around to pulling the carbs and braking them down. Gladly all the butterfly valve screws came out and I was able to get all the shafts out. 5 of the 8 seals were hard and some were even degraded to the point they looked like they were made of tar. So today I ordered 8 new seals and I am looking forward to putting these back together. I can't believe that these could have made it not idle down but we will see when I get them back on my bike. Thanks to all for your input, I really appreciate it. I will keep you posted. I have my manometer and colortune ready for the big day.

 

Cool. Now, while you're waiting, get your valve clearances in spec; they need to be if you hope to get a decent sync.

 

Hey,

Sorry to resurrect a dead thread if your problem has already been solved, but I had the same exact problem today and it turned out to be the throttle cable not allowing the linkage to return all the way down on the idle mixture screw. I popped the throttle cable off and turned the idle screw all the way out, then in until it moved the butterfly valves open just a touch. It started right away and was at about 1000 RPM. I am glad I tried all these fixes before pulling the carbs out again!

 

I got the new butterfly valve shaft seals in and put the carbs back on the bike and then it developed a starter problem. Now that the starter has been rebuilt hopefully my problems are done. I still have not had the bike running yet since I rebuilt the starter but hopefully it will be back up and running soon. Ill keep everyone posted.

 

I am having the same problem. I used a large paper clip. And did not count the turns out on the adjusters. So I had to set to 2 and 1 half 360 turns out, by the book. I will reset to paper thin this time.

 

You will save a great deal of time if you BENCH SYNC the Throttle Plates with the THINNEST Medium possible.

The Key to Getting the Bike running right is setting IDLE.
REAL Idle.

If the Throttle Plates are OPEN the width of a Paper Clip, ... You have trouble setting the Mixture Screws to get the Bike to IDLE close to 1050 and below.

Start-off with the LEAST amount of AIR Gap you can accurately set too.
35mm Film
3X5 Card

With the Throttle Plates CLOSED, ... COLORTUNE to Bunsen Burner Blue
(Not to where you 1st achieve BLUE, ... but ... to the RICH side of the Blue Flame.)

The Bike should be able to IDLE at that setting without stalling until it runs out of gas.

From there, ... Vacuum Sync the Carbs.
Done right, ... the Bike should IDLE like a Sewing Machine.

Once you arrive at the point where you Vac Synced and have a good Idle,..
The only thing left to do is add just enough SUPPLEMENTAL RICHNESS at the Mixture Screws to get you out-of-the-hole.

 

ignore this

 

and this

 

It's been a while since I last posted on this thread but I finally got the starter issue fixed and the bike will start again. In the mean time I checked the vales and they are in tolerance. I also took the carbs apart again and put new butterfly valve shaft seas in. I put the carbs back on and the thing now runs at 8000 rpm every time it starts and it wont idle down at all. I do not know where to go from here. I also must tell you guys that the set of carbs I have on the bike are from a xj750 and not the stock carbs for an xj700. They were too far gone to rebuild but I did use the butterfly valve shafts in the 750 carbs so they are spaced the same as the stock carbs. The idle adjuster screw is turned all the way out and it is not even touching. So I am at a loss as what to do except try and find another set of carbs and rebuild them. But really shouldn't the 750 carbs work just fine since they are for an engine just a little bigger and the bike did run well with these carbs in the past. All I was trying to do was make it idle better and all I've done by sealing the carbs back up is make it worse. So any suggestions where to go from here would be helpful. If I can't figure is out the bike will be for sale here soon.

 

Triple Post! Awesome.

Did you bench sync the new carbs after you change the seals?

The jets maybe wrong for the 700 vs 750.

 

Yes I did re-bench sync the carbs after I got done with replacing the seals. I don't know why it posted it 3 time. Is there a way I can delete the other 2 post?

 

Don't know about delete but you can click the edit button and at least remove the text from two.

 

To make 8-Grand, ... the Throttles have to be almost WIDE OPEN.

You, ...
Might have ...

Hooked the linkages to the wrong cable.
Have the Linkages stuck-open because the Cable attachment is stuck under the Head.
Have the Manifold Clamps on upside-down ... not letting the Linkages close.
Have the Throttle Cable bound ... routed improperly ... control misplaced ... broken cable ... linkages stuck open.
Using the wrong Hose Clamps.

Unhook Cable
Press on Sync Screws with Pencil eraser forcing linkage shut.

Don't run the bike at 8,000 rpms.
You could throw a rod.
Scrap that Engine real quick.

 

Don't run the bike at 8,000 rpms with no load.

(You can run it well past 8000 once it's driving the bike down the road.)

Otherwise, the above is exactly what I'd look at. I've done the snagging the linkage thing myself.

 

i will be pulling the carbs when I get home this coming Monday so well see if I have something snagged on the cable or what's going on.

 

I just went back and re-read this whole thread.

You have "Frankenstein" carbs. You talked about using the carbs from a 750 but using the throttle shafts from the 700's carbs, etc.

The throttle jet (slide) NEEDLES are different between the Hitachi-equipped 700's and 750's (Y-20 vs a Y-13.)

Most of the jets are different, some greatly so:

The 750 Maxim uses a 120 main jet, the 700 a 107.

The 750 Maxim uses a #40 pilot fuel jet, the 700 a 36.5.

Main air jet: #80 on the 750; the 700 calls for a 70.

PILOT Air: 750: 225; the 700: 210.

I don't think they're going to work anywhere near right unless you swap over the jets and needles too.

 

I did not change any of the internal parts. just the shafts. Its weird that the bike did run really well last summer it just would not idle consistently. Which I figured was because of the seals being bad. Now they are new. These bikes are jetted lean from Yamaha anyway to meet the epa regulations. So I figured I could make them work. If not I will just have to find some xj700 carbs and rebuild them and get them installed. They are very hard to find and even harder to find usable ones.