Runaway Idle

Hi,
Please check this vid, lemme know what you think is causing this.
I have some theories, but its grinding me down


If its a leak, can you tell me how to figure which part is leaking?
If its lean, any guess as to how many sizes?

valves in spec: Intake 1- 4 (0.11, 0.14, 0.11, 0.14), Exhaust 1-4 (0.16, 0.19, 0.19, 0.18)
All carbs wet set @ 3mm
pilot screw @ 3 turns out.
Synched with a Morgan carbtune
Curent jets: Pilot 37.5 (N151.067), Main 115 (N100.604)*
Spark plugs look like:
*chopped airbox: http://xjbikes.com/forums/index.php?threads/airbox-chop-instead-of-pods.44782/#post-402031

 

Did you adjust your idle? The big screw pointing towards the air box between carbs 2&3.

Grab a small propane torch, turn it on (don't light it) and sweep it around the carbs, vacuum plugs, throttle seals, etc.
If there is a leak you will hear a change in the RPM's.

The CarbTune only sets the fuel/ air mixture. You need to do a running vacuum sync. (after setting the idle proper).

 

Cheers Stumplifter, I've trimmed the video to what I thought was relevant.
The original start of the video had me playing with the idle, bringing it down from fairly high to what you see.

I managed to get it to idle for a while this time, but the next time I started it, it went right up to 6K and all the idle twiddling in the world wouldnt stop it.
Besides which, I figure if it idled ok this time, then the setting is (was) prolly right.

My main concern with the propane method is that it will diagnose a leak, but I'll have no idea what part of the carb is leaking.
Is there a way I can test the carb while it's off the bike?

*You're thinking of the Colourtune. the Carbtune is a vacuum sync tool.

 

Right you are.

With the propane, if you just crack it open a wee bit it you won't be bathing the whole system with gas and it should get you close to where the leak is.

When was the last time the carbs were completely rebuilt?
Enrichment circuit isn't hanging open?

 

Sounds like a butterfly not shutting down, it will let just enough air in to lift the piston & start the whole thing racing.

 

Just a thought, cause I did this..once...did you make sure the springs were installed under the carb hats?
A vacuum leak, like a butteryfly a bit open, or shaft seal leak would make the bike run up right at the start. I don't think it would idle at all.
Possibly the plungers are sticking or the rubber has a hole or cracked...

 

Cheers fellas,
I'll try the propane trick after I double check the plungers/choke circuit
Carb cap springs are definitely in. I rebuilt the carbs October last year*, then they sat off the bike until july, where I have had them off and on again (wet setting and changing jets)
I've re-cleaned them a few times now.

you reminded me I have questions! - Clunk test
With the rubber diaphragm flipped up/disengaged I get a proper satisfying clunk
But when the carbs are fully built/diaphragm in place, they drop gently. is that right?
The spring is pretty weak yeah?

I have not visited the shaft seals because I chickened out of trying to dremel off the screw peening on the butterflys.
Then again, I haven't revisited the choke bits for a year..

The butterflys should be open just a crack tho, surely. it couldnt idle otherwise.

You got me convinced of the choke/enrichment circuit now. (stream of thought typing. long day)
Aight. Motivated. I'll let you know if I get a win.



*bike has not been on the road since august 2013. I've been.. modifying it*.


*I forgot you can see it on the video

 

are you certain you have the jets back in their proper locations? some of them can be reversed easily. just thinking out loud.

CN

 

Plugs look lean, lean, lean and lean.

Intake/exhaust mods? I see rejetting in your future. Welcome to the seventh circle of Hades.

The MIKUNI spec for floats is 2mm, +/-1mm. Which puts 3mm at the very low end of the range. That certainly won't help if you're fighting a lean condition.

You're dealing with Mikunis here. They are a bit more precise instrument than Hitachis, and do not take kindly to any sort of mis-adjustment or leaks, internal or external, fuel or air.

Mikuni jets cannot be easily reversed. However Mikunis have their own set of pitfalls: Float levels are absolutely critical. I wouldn't run them at 3mm. Get them as close to 2mm as humanly possible or if err you must, err UP.

Once Mikuni throttle shaft seals fail, they fail. And NO WAY would I take a running Dremel into the throat of a BS28. Get a dowel or appropriate diameter broom handle, and make a "plug." Then cut away slightly more than half, horizontally, about halfway and create a a half half-round/half round plug. Open the butterfly and insert your new wooden tool to hold it open, then use a small, sharp jewelers' file with a handle to carefully peel away the tails of the screws. (Most folks don't realize the pointy end of a file is supposed to be stuck into a handle.) If you slip you're more likely to jam the file into the wood than the carb. And one file scratch is nothing compared to what one little slip with the Dremel could do. I love my Dremel; but I wouldn't use it for that.

Were these carbs ever submerged in anything, and/or did the bike spend a considerable amount of time outside at some point in its sorry previous life? How many miles on it? I see throttle shaft seals looming big; however---

Mikunis can leak air IN in other places, like the enrichment plunger boots. The plungers themselves don't fit tightly enough in their bores to completely seal off the opening through which they dump fuel when open. When the plungers are closed (at a point below) those openings can "suck wind" past the enrichment plungers if the boots up top don't do their job to seal the bore and plunger. I cannot speak from experience as to whether or not Hitachis have this vulnerability; but I can tell you for sure Mikunis do. Carefully inspect the enrichment plunger boots; if they don't seal tightly to both the carb bodies and the plungers, you need to order some new ones from Len. Hardened/cracked enrichment plunger boots can drive you crazy chasing phantom vacuum leaks because you look right past them.

Mikunis don't like leaky carb-to-airbox boots, and it's also very easy to "pinch the lip" on #2 or #3 when installing the rack and not get them all fully seated in the intake manifolds.

Stepping back from the carbs themselves, are the rubber vacuum spigot caps new or old and hard? Proper clamps on them?

 

Cheers Fitz, mad detail as ever.
I cannot F&*$inkg believe I missed the float height being 2mm instead of 3mm
Every other XJ is 3mm.. except mine.
I cant tell you how much that feels like a cold stone in my stomach. mucking about with fuel for hours twiddling tiny bits of metal.
Sounds like I need to up my main jets again too. I don't mind the seventh hell of re-jetting, but wet-setting is miserable.

Cheers for the dremel warning. I'll do as described. I have been making myself sick artificially creating a vacuum and trying to block various parts of the carb to determine the source of the leak.
Translation: I've been sucking my carbs like a bong. yuk
#4 is baaad.

Carbs were never submerged, I scrubbed each piece individually with lots of carb-clean spray.
I don't know how sorry its previous life was, but I'm sure it was locked outside in the rain and beaten with a stick when it whimpered.
What can I say, they saw me coming. I was/am desperate to ride, or at least start the build I was dreaming of for the previous 6 months.
mileage: well.. the original clocks are buried deep in the parts-to-be-ebayed stack of shit. lets say "lots of miles"

rubber vacuum spigot caps. .. I think you mean the rock-hard little lumps perched on the carb-boot tubes.
However the bike is usually (and was in the video) plugged into the carbtune, so I can rule that out for now (but still need to remedy it.)

cheers all for the replies.
Any info on my clunk question?

 

Each diaphram slide should "clunk" with the carbs fully assembled.

Gary H.

 

this, did you polish the slide bores?

CN

 

They "clunk" with the hats off. Otherwise they should descend in "slo-mo."

The reason I asked about the sitting outside (especially on the SIDEstand) is that the #4 carb takes a real pounding from the weather in such case.

This squares with your findings in regard to #4, does it not?

***Fitz tip*** Find another RACK. The you can either rebuild that, or strip it for parts. But one way or another, you are probably going to need to do throttle shaft seals. Working without a spare rack is like working without a net.

 

Finally finished the operation, success, patient will recover.
Chacal is on the wrong continent for me, so my throttle seals came from NRP carbs.

Still need to wet set. Totally depressed at the thought

The new screws holding the butterfly have threadlocker.
I also have nylocks available but they do seem to fill up the available airspace.
Anyone had tuning/performance problems after putting nylocks on the butterfly screws?

 

I don't think I would use nylocks on the butterfly screws. The carbs wasn't designed with them. I would just use the threadlocker and be done with it. When you wet set, be sure to empty and refill the bowls at least 3 times between adjustments to make sure that nothing is "hanging" giving a false reading. The carbs reward the owner for perfection of adjustment and construction.

 

Ok, finally at 2mm
Funny enough, the 6mm tube im using has a 2mm meniscus. Handy.

Also, seems that at 2mm,when inverted the floats sit 20.5mm above the edge of the carb (with gasket removed)

Also, some of my carb leak issues appear to be caused by the float catching on the inner edge of the gasket.

Another note: I have 2 kinds of carb joiners in my rack. Straight metal tubes for petrol like everyone else and then, mysteriously, black plastic T sections higher in the carb as if for ventilation?
Image attachment attempt: http://exjayarrjey.tumblr.com/post/104781127157/i-dont-see-these-on-anyone-elses-carbs-wtf