xj550 engine runs high speed after carb cleaning

OK now I 've seen everything.

1981 xj550 Max with only 2,900 original miles.

Engine starts on Choke then climbs to 4,500 - 5,000 rpms.
Will not idle at oem bench synch settings (not even close).

Cable is new, not binding, throttle opens and closes normally. Fuel line is new with a new in-line clear filter. Petcock flows 100% in ON or PRIME.

HISTORY:
Bike was running OK when I got it but #4 Carb was flooding fuel, from dirty inlet valve.
Petcock was bad, rebuilt it & tests fine for fuel flow. 11/26 Replaced carb holders. No vacuum leaks.

11/28/08
Just re-checked carbs & floats (set at 21.5mm) and oem bench synched at .7 mm open on butterflys.
Soaked all bowls and put guitar wire thru enrichment tubes & jets - flashlight shows them clean & clear.
All carb jets and tubes are open, clean and clear. Sprayed fluid & air goes thru them perfectly.
Pilot - Idle air screws, bleed holes are open, idle screws are set out at 2.5 turns counterclockwise. Applied anti-seize to threads.

Carefully inspected carbs for dirt, obstructions, with no visible changes, floats and valves work perfectly.
Carbs pass fluid-in-bowls test and clunk test.

PO had put plastic caps on the two inter-carb plastic Ts between left and right carb banks just above and behind the tops of the bowls. Per Rick's instructions I re-opened these Ts to atmosphere.

ANYONE have any ideas why this engine now runs as if it was WOT?
No other changes, additions or deletions have been made other than opening the caps. SHOULD THE CAPS ON the Ts BE CLOSED? SEE PIC

 

Ok, I know the Seca and Max differ slightly but they both have Mikuni's. I just went out in the garage and looked at the carbs on both my 550 Secas and took a good close look at my spare rack of carbs. In all 3 cases those ports on the carbs are blocked from the factory and do not have the interconnections nor the fittings you are talking about. I HAVE seen them before on other carb racks, always with LONG (vent?) hoses atttached. Since the bike seemed to run halfway OK before everything you did, and since there seems to be no harm done by having those ports blocked (they're not open on any of my 3 racks of original Mikunis) why not RE-BLOCK them as a troubleshooting step? If it runs better that way, leave it and we research why they're there in the first place on some racks of Mikunis and not others.

 

bigfitz

The confirmation of the various 550 racks & appearance helps. Never saw this vent port difference on 550s before. I'll block them again to get the carbs back to their "as before" running ok state.

thanks again

 

Weird. I've seen those hoses on racks up on eBay but not on my bikes, or my spare rack. Those spots on the carbs are just "blanked" I always wondered what went there. I looked in the factory book too, and they're not shown, just the actual fuel pipes between the carbs. They don't show up in ANY year of the 550's in Yamaha's parts fiche; but they're there on the 1984 and '85 FJ600's. I KNOW that Yamaha's parts fiche aren't always 100% accurate (compared to the actual production bikes) but it makes me wonder exactly when those extra pipes appeared, their actual purpose, and whether they are supposed to connect to anything or simply be pointed at the ground. I say block 'em but good and see what happens.

 

Check the linkage.

In order to support revving that high there has to be AIR and Fuel entering the Engine.

Stuck Linkage
Butterflys Open
Major Air Leak
Stuck Diaphragms

 

If those ports are open when they shouldn't be all the carbs are sucking extra air BIG TIME. Rick, can you shed any light on the fact that they seem to be "blanked" in all 550 reference materials I can locate as well as on both of my original 550 Secas and my spare rack of carbs? Are they just "vent" or "drain" ports, or ??? Remember, the 550's almost exclusively have Mikunis.

 

Your situation may be different with those extra holes you are talking about, but I had the exact same mind boggling situation a few weeks back that i successfully fixed. My problem was that one (or more) float pins was stuck open. This was letting in extra fuel and it was making the engine race at 6k on full choke an 4k without. In my case however, adjusting the butterflies to closed would kill the engine or bring rpms pretty low and then just the slightest blip on the throttle or adjust the idle screw jumped the rpms back to 4-5k. I took the carbs off and went through them again, polishing the inside of brass fittings of the float pin assembly and the edges of float pins. I then measured the fuel level (on the bench) with a plastic tube. and one of them was still flooding . upon examination the "stop tang" or the little bent edge that stops against the post was not right. so everything would look fine when the carb was upside down, but when you turned it over, the floats went too far down and pin was crooked and when the fuel pushed it up, the pin wouldnt close. I once i pushed the tang out, it worked just fine. My engine runs just fine now. May be this will help you - I hope your drain plugs are not frozen. But if they are use heat - works everytime!

 

Oh i forgot, Since I was too cheap to spend 50 bucks on new float pin assemblies, I replaced the existing worn out o-rings with nitrile o-rings from freight harbor. I figured what the hey, an o-ring is an o-ring - as long as there is flexible non porous material pressed against a flat metal surface, it will provide a leak free seal. nitrile resists fuel. so there. if these fail, i will buy new assemblies. Yea i can buy new ones, but then i would have gone and bought a newer bike too- whatever.

 

If I remember correctly the tubes are drain tubes added to later Mikuni's to keep fuel out of the crankcase if the float valves and petcock malfunction. Most common reason for a high idle is a lean mixture caused by an air leak. This can occur at the carb boots or intakes. A low float level can also cause it. If these check out then it may be just out of sync carbs. I have tuned some bikes that would idle right down once the mixture and sync were dialed in. Some may not if the static or bench is off. This is from the butterflies being too far open with the idle screw bottomed out. The carbs have to be pulled to correct this. The easy way to check is to just push down on one of the throttle set screws and see if the idle drops. These are the set screws on the throttle rod linkage, the rod that rotates when the throttle cable pulls it up.
The best way to set fuel levels is with a bench setup using clear tubing to measure the fuel in the float bowls. Check my gallery for a typical setup.

 

Start the bike and let it run.
Once it's warmed-up a bit test it for the Air Leaks.

Shoot some Carb Cleaner om the following areas:
>Manifold connections to the Head.
>Manifold connections to the Carbs
>Hose Clamps at Carb Manifolds
>Carbs Throttle Shaft Pivots

If there's an Air Leak; the RPM's will spike-up when the Carb Cleaner gets sprayed-on.

Disconnect the Throttle Cable from the Throttle Grip.
Check to see that it isn't a Tight Cable or maladjusted cable.

Push Down on the Sync Screws to check that the Linkage isn't binding.
If the RPM's come-down when you push down on the Linkage EITHER the Linkage is hanging-up in a Tight Throttle Shaft Seal, The Throttle Shaft Seal is Leaking Air -or- The Sync Screw is holding the Butterfly Open from being set-up incorrectly.

Also check that the Carb Linkage Pull isn't stuck up under the Head, in the front of the Carbs.
That happens a lot on Maxim's.
I wouldn't rule it out without checking it on the Mikuni's.

 

Bike finally started after plugging the drain hose T Caps with silicone:

Petcock was set on PRIME ... bike finally started on Full Choke.
Throttle butterflys are Bench Synched e.g. set @ 1mm.
Pilot - Idle Screws set at 3.5 turns OUT.

SAT. morning.
Engine ran fitfully at 1,800 rpm for 4 - 5 mins., then # 3 & 4 plugs fouled.
Cleaned the spark-plugs and restarted. Engine finally began to settle down at 1,200 rpm, then rpms climbed back up to 2,500 ... wtf!? I said a few choice words ... then looked down.

Now fuel was overflowing from both #3 & #4 carb bells. Duh! I'd left the petcock set on PRIME = not good. Reached a flash / stopping point.

So its time for new spark plugs, will re-start the engine tomorrow to see if it can run steady and go from there work with the rack butterflys & then idle mix screws - just enough to permit a steady and clean idle state. Hopefully the floats, set at 21.5 mm, are not too high (open) for this particular engine + carb combo.

11:58 am final note
With the old spark plugs and idle screws being perhaps too far out (idle rich) the bike is now hard starting - "I'm almost, but not quite". Will be dialing in the idle mixture, step by step.

Speculations:
It might turn out the '81 xj550 Maxim carbs need to have those 2 drain tubes intact in order to run well at "equilibrium". Owing to these blocked drain tube vents this bike is touchier than I've experienced about initial carb settings. Possibly I'll have go back to a Max 550 Airbox with hoses etc. Could a stock airbox actually run better than PO's pods? Has a stock exhaust system.

 

That's a most definite YES.

Pods can be set up to run well, but it takes lots of effort and trail and error.

IMO, it's not worth the work involved to get pods set up correctly for the minimal, if any, gain in performance.

 

Getting the bike to run well with Pods other than the stock Airbox and Boots is quite the time and labor intensive process.
A few who have gone that way are satisfied with the performance they are able to find.
I think many others get to a point of tuning their bikes with Pods that "they can live with." Not the best tune-up they can get; but enough to ride and have fun.

I think Pods should be categorized as more of a Cosmetic Mod than a Performance Mod. In the hearts of those who did the Mod; it was a "Look" they were going for, more than adding ponies.

That your Bike is a 550 and you are running Mikuni's should make it a bit easier to find an arrangement of Jets to bring the bike to a good running state.

Although, getting there is like baking a cake from scratch. You need a good recipe, with all the needed ingredients. That, we don't have. Just a few numbers others have tried.

 

Rick - You were correct about those carb vents (aka T splitters) having to be open to atmosphere on 550 Maxims.
Gasoline on the floor has a way of proving that.

James, an experienced rebuilder with a Carb exchange service on eBay says;

"If the xj550 vents are blocked the carbs will flood."

Two areas that need to be restored, fixed.

1. Its probably time for new throttle shaft seals.
2. Hello stock AIRBOX and Goodbye to PO's el cheapo PODS.

Trial & error tuning has a way of chewing up big chunks of time.
Ah The joys of the season! A bit cool to be riding much anyway.

 

Mind you, I'm NOT doubting this, but I don't understand why none of the Mikunis on both my 550 Secas ('81 and '83) have those vent tubes and don't have a problem. Is it because carbs that HAVE the vent tubes have other ports open inside them that the "ventless" ones don't or is it the other way around?

Good idea, remember you will have to pull the motor. I see stock 550 airboxes on eBay all the time, finding one won't be a problem. Make sure the one you end up buying doesn't have splitting seams anywhere, that's what's happening to the stock airbox on my '81. I have a replacement, it's that "pull the motor" thing... holding off the inevitable with black adhesive RTV until I have another reason to yank it.

 

hey guys - wonders never cease!

Buy motorcycle, buy funnel, pour money in ... it works!

OPENED THE VENT CAPS ... as Rick recommended
Backed off rack screws so throttle shaft was "dead" e.g almost closed.
On a hunch cranked idle screws out another 1 turn.
Hit the starter button and ...
Engine started after a few more stabs and ran fast until warmed up.
Then it settled into a nice e v e n p u r r

Idles smooth as velvet ... albeit at 1200 rpm. Could use a real carb synch.

Rode it around the block. Whoa! This thing's got awesome power for a little ole stocker 550! Not as much grunt as either of my 1100s ... still very satisfying. Soft springs notwithstanding it handles worlds better.

MC + $$$ + XJ Forum = BIKE RUNS GREAT

A duffer 'tuner' is reborn. Thanks ... Great day.

 

Pods are bad because they are not restrictive enough, right? Could you block a portion of the filter elements on each pod to get similar results? You could even do the math to find out how much air can/does flow through the stock airbox and use that to guide your modifications.

Bad idea?

 

The street Yamahas I've had liked the stock airbox restriction better than Pods. Plus it seems the factory did airflow studies; notably on the XS1100s that proved they would run harder and faster using internal runners inside the airbox. (from an old magazine test article.) The XS11s could wail using the stock airbox, doing 11.79 in the quarter, while weighing in at over 500 lbs.

Pods work in WOT applications e.g. like on my old quasi-desert racer XT500 where maximum air flow was a necessity.

Restricting the flow in pods may work and permit easier carb service. One more use for glue and silicone sealant.

Out here ATV and buggy riders sometimes wrap extra filter gauze around their UNI-Filter pods but this may be for dirt insurance. I'll give it a go ... while waiting for the stock airbox to arrive.

 

Excellent. That is good news indeed. I still want to know why some (a lot?) of the Mikunis on XJ550's HAVE NO vent tubes (the ports are cast shut) but work fine? Vented internally somehow?