mikuni carbs

hi,up until a week ago i had never heard of mikuni carbs or the xj900 even though i am sure i have seen one over the years,i am not a biker and never have been so what i,m about to say is based on logic ,my logic ha ha.last week my neighbour asked me to look at his bike xj900 f he had a problem with the carbs,so i took them off bike and stripped them down to clean and polish i noticed all 4 diaphrams were perforated and ordered new ones,got these about a week later from the states,this gave me time to clean all of the jets and polish interally and external the carbs,i cut off the old diaphrams and sealing rings to fit the new ones--no problem they fit perfect,so put carbs back on bike but still would,nt start as no fuel was getting to engine--took carbs back off and used a vacuum cleaner and a bit of hardish foam for a seal on the carb barrel,each slider lifted and the foam bit was soaked with fuel so it seemed that i done the job with the carbs mmm so back to the engine this time fitting new o rings to the carb boots--still no start,just a massive backfire from exhaust strangley after i take my finger off the crank button--also no iol light on dash---checked oil and found at least a litre in the airbox ooops not good--took about 4 litres out of the sump and checked level--still full--took out another litre looking good i can see the level in the window--would this have stopped the vacuum to the carbs--still wont start, any ideas welcome

 

It fires when you RELEASE the button?

Flat battery. Or one that's not up to the task.

This is common; our screwey ignition systems won't work without the right amount of current to bring everything "up" as it were; and it's VERY possible (happened to me too, more than once) to have enough 'oomph' left in your battery to very smartly spin the engine BUT have nothing left to fire it. If it's not the battery, your starter is sucking all the juice. It fires when you release the button because you "unload" the system.

Oh and if you've soaked the plugs you'll need to replace them once you figure out the problem.

Mikunis look pretty much like this inside, although this is a BS28: http://xjbikes.com/Forums/viewtopic/t=31061.html

 

Those Carbs probably need a Vacuum Piston Bore re-finishing.
Click on the "Clunk Test" link below my signature.

FYI
Aftermarket Diaphragm Rubbers probably didn't come with a Location Orientation Tab. Something to check.

A 900 is a beast to start COLD.
The Battery needs to be strong.
Cold-start Enrichment Circuit: Bowl Metering Jets, Bowl Wall Well, Siphon Tube, Enrichment Circuit Passages and Plungers need to be CLEAN, Open and well-functioning.

Test Petcock for Leak-by. Overhaul as needed.
Replace Float Valves and Seats with New Kits.
Search for Float Height Setting procedures.

Install "Insurance" Fuel Shut-off Valve.
(Hdwe Store ~ Briggs & Stratton Fuel Valve)

 

I have that 900 beast. As long as my battery is at its best (3 yrs old so far). She fires even in the coldest of weather. Once you get it to run it in my opinion is to as least fire it up or ride every few days. That way the day you want to take it out it fires with no issues. 60K miles & still a beast.

 

I had this same issue until I ditched the $35 Walmart batteries (1/year) and opted for a better quality $125 gel battery and tender. Now I am on year 2 and not a problem. Minus her need to go through one or 2 starter solenoids per year, but that will have to be a new thread.

Don't go cheap, use a tender.

 

hi,Ed crunchbucket here---today i stripped down the starter motor on the xj 900f {I know what the F stands for ha ha}and yes it was the dirtiest motor i have ever seen,one continuous lump of crud around the armature,so after an hour with the dremmel,cutting paste and T cut it now looks like a mirror---did this help the bike to start---NO---back to the drawing board

 

Every time it's happened to me it's been a battery on its way out.

 

hi,ED crunchbucket again--does anyone know how long the springs should be in mikuni bs36 carbs as today i found out that my nieghbour has streched them--oooops now i will have to cut some off for the original tension---yamaha xj900 f thanks

 

DO NOT cut the springs off to "restore" the original tension; that will make them stiffer.

If the bike only fires the ignition when you let go of the button, the issue is not in the carbs. At least, THAT issue isn't.

 

ok i must admit i have taking a liking to the xj900 ,it just has something about it---when i was cleaning all of the crud off with my dremmel drum sander i could smell the 80s,brought back memorys of my beloved cortina MK3 GXL,yes i,m an old git ha ha

 

yamaha xj900f---can anyone tell me---does the oil light sense pressure or is it an oil level light and why its on only when cranking the engine--cant say what happens when the engine starts as i have,nt managed to start it yet---thanks dudes

 

The Oil Light is a Oil LEVEL Light.
It comes on at Start to show the Bulb is working.

In cooler temperatures it is not uncommon for the Oil Light to come-on.
The Engine Oil "Scavenges" back down to the Sump slower, than when the Plant is running at a higher temperatures when it warm outside.

Adding a few ounces of Oil will help with the false level warning.
When the Bike is warmed up; place the Bike on the Center Stand ... level.
Observe the Oil Level in the Oil Level Sight Glass.
Add more Oil to the Plant.
SLOWLY drip additional Oil into the Plant until the Oil Level in the Sight Glass rises to where the Level is just about filling the Glass.

 

EDIT-:think my eyes are playing up, kept seeing multi-quotes earlier :?

Backfire when letting off the button, if the starter checks out good (armature not knackered, resistances OK, brushes +10mm) then my money'd be on the battery, literally...

 

hi,Ed crunchbucket here---today while i hade the mikunis off to do a clunk test--works great--i noticed that if you screw the pilot jet as far down as possible it protrudes in the carb venturi,backing it off 2 and a half turns i looked closely with my magnifying headset it then became flush with the venturi wall---so i backed it off another 2 and a half turns to make the hole suposedly where the air should enter---good or bad idea-?-as i would have never known this with the carbs in situ

 

I don't know why the emphasis continues to be on the carbs when there's a starting circuit issue, myself.

 

I second that. The battery is probably flat as Fitz pointed out earlier. It is not the first time I read this kind of story.

 

Agree 100%, to reiterate my last post on this thread...

 

Back in the day, I used to love it when my fellow MG, Austin, Triumph (car and bike) BSA and Norton owners would F* with their carbs and F* with their carbs and F* with their carbs when good ol' Joseph Lucas was the culprit all along. (Those of you who have ever seen the insides of a Lucas magneto understand. Or a Lucas mechanically-regulated electric fuel pump. The British drink warm beer because they have Lucas refrigerators.)

Now that at least the three of us agree, let's go grab a (cold, hopefully) beer.

 

Or indeed a Lucas distributor.... at least they don't rely on paper for insulation though :roll:

Lucas was still miles better than the patriotic alternatives of Miller or Wipac (ever stripped a Wico-Pacy mag?)

My beer resides in a Bosch fridge

 

hi,Ed crunchbucket here---today i have wired a car battery direct to the starter motor and left the bike battery in to power the ignition--this will stop the drain on the spark igniter and soon as both batterys are fully charged i will test this theory and post back

 

---all 4 diaphrams were perforated and the sliders were sticking plus a ton of crud inside and out,the choke did,nt work as all of the grub screws on the choke bar were loose--all diaphrams replaced carbs cleaned synced and floats adjusted to exact specs plus a clunk test done on the sliders--all good---the emphasis is now on the ignition system and i have 2 car batterys 1 for the starter alone and 1 for the igniter system---i hope this cleared up the word --PROBABLY--for you

 

anyone know if the oil level switch is a also a safety cut out switch like the side stand has a cut out switch which i have already soldered a jump wire onto as the switch was knackered and constant open circuit no matter what position the side stand was in---like most of the wiring on this bike all i keep finding is corroded multi points in the wiring harness---so i cleaned evey wire and connection--long live the dremmel ha ha

 

---mmm the jet engine?who would have thought of that?mmm the hovercraft and the bagless hoover---the english--well thats my three

 

Nobody likes a smart alec lol

At the end of the day, American or not these guys are giving their own time freely to try and help other people get their bike running, if these people were a bit clearer about where in the process they were & what they had or had not already tried or done then I'm sure said people would get more relevant advice.

As far as I at least was aware, we were done with carbs & were now diagnosing a non-start with backfire when letting off the button. Of course if the wiring was all corroded that's not going to have helped but I dunno, whats even happening with it now? :?

 

hi dude--right now i have 2 car batteries 1 for the starter and 1 for the igniter will test this tomorrow---i am done with the carbs they work fine--i tested them with a vacuum cleaner--but not a bagless one ha ha

 

The oil level switch is just for a warning light and does not interrupt the starting circuit in any way.

However, the bike IS equipped with a safety circuit system consisting of a sidestand switch, sidestand relay, neutral switch, clutch lever switch and safety cutout relay that prevents you from starting the bike in gear unless the clutch is pulled in or AT ALL if it thinks the sidestand is down.

If you suspect an issue with any of the above, simply unplugging the safety cutout relay will effectively bypass the entire system.

And for the record, the Supermarine Spitfire (or its racing predecessors) the classic Edward Turner designed upright parallel twin that begat the classic Britbikes like Triumph, Norton and BSA and of course, the jet engine get my votes.

We'll pretend we never saw the TR7 or the De Havilland Comet.

 

And let us not forget also, that our beloved Rover V8 is really a Buick (hence the B designation of it's first Rover application in the P5B)

If the bike's turning over on the starter OK I guess the next port of call would be to use a spare spark plug attached to one plug lead at a time to verify a spark when cranking. (Don't leave any plug lead off while cranking though as it could hurt the coils, just pull one at a time & replace it before doing the next).

The 2 car batteries is a pretty good idea IMO as it eliminates the possibility of a dead starter "pulling down" the voltage while cranking.

Oh and Fitz, don't forget Henry Maudslay too. Without him we'd all be holding our bikes together with dovetail joints & cotter pins 8O

 

yep you got me there dude,if you read my first post i said i know nothing about bikes,the bike is my neighbours and its the first time i have ever worked on a bike,,as for the comet it had square windows and the cracks formed in the corners,notice all planes have round windows,a good lesson,i dont think many liked the tr7 or the wedge as we used to call it although i have seen a couple around here,the rover engines were crap until honda started putting their engines into them,i should know ,i got one.i have bypassed the side stand which i only found by accident and today i will use the 2 car batteries to try and start the engine,1 for the starter motor and 1 for the ignition system--i have sparks on all 4 plugs the timing checks out ok and the valves are moving and in the correct order,i,m not using the fuel tank i have a bottle filled with fuel,new fuel like a drip feed and a clear fuel filter so i can see the fuel going in,i have put new o rings on the carb boots so i really am at a loss if this engine wont start today

 

Sounds like you got most bases covered. A petrol engine only needs 4 things to run, Fuel, Air, Compression & Spark so If she don't go with spark, fuel, air & fast cranking I'd be checking compression & expecting low readings.

EDIT-: Just a thought, you have spark outside the engine but fouled plugs can appear fine & break down under compression.
Have you fitted NEW spark plugs? Use non-resistor type & brand new, not cleaned old ones. They seem to foul easier than ever these days & have caught me out a couple of times few years back before I made fitting new ones a routine operation on any engine I work on...

 

your right i too would expect little compression,as its not my bike i dont know how much my neighbour would be willing to spend,I.E new plugs compression tester so far nobody has loaned me a battery or even a car to jump start it so it just sits outside my caravan,motivation is whats needed here and i cant keep chasing people to wipe their arseholes---thanks for the info dude,let you know what happens

 

Well for how long it'd take to test it out I'd have lent you the use of a couple of batteries if it didn't mean a 700 odd mile round trip, someone must have 5 minutes to spare it's not like you're asking to borrow their wife huh 8O

He should at least budget for new plugs, if he's willing to take your time he can at least spring for them lol

 

i am with you on that one---all i need is a good battery--i live on a caravan site so i am going around asking for old lesuire batteries,i have one on charge now but it takes so long to charge,the last 2 died in seconds,if i still had my car i would have used jump leads but i had to scrap it to pay the rent--long live jobseekers allowance ha ha the plugs look quite new but i did clean them and i,ve had all 4 out whilst cranking it goes from a pissy yellow colour to a blueish tint---not happy with that--so must be a current drain hence the 2 battery test--if it charges---let you know

 

yamaha xj900f does anyone know if the HT leads unscrew from the coils,they look like they are fixed in with resin i dont want to damage them by turning,my neighbour says he has a spare set of coils but now i have them he has cut the leads off half way up,why would someone do that???

 

The Plug Wires don't come with Plug Caps on them.
They screw off.

Search: "Coil Surgery"

 

What rick said ^

Maybe he has two 2/3 coils & that's why the wires seem too short? (the 1/4 coil has to have longer wires to reach obviously).

AFAIK the wires are sealed into the coils, but you can get inline joiners to splice new leads on, not sure where you'd get them over here but chacal does stock them & new copper cored wires.

That said, these bikes don't have a strong spark like you'd expect to see from a CDI system or even a points & oil coil setup so it may be fine. Best way to check is to measure the resistances of the coils.

For the 900F the primary is 2.7 Ohm +/-10% and secondary is 13.2K +/-20% both at room temp (20c).

Another test Haynes gives is the minimum spark arcing gap of 6mm (remember Gunson flashtest? ) but the resistance tests are best

 

FINALLY the xj 900 lives after much tweaking with plugs and air screws plus 2 car batteries she can now start on her own---there is a slight misfire but my neighbour is getting new plugs tomorrow--so 3 weeks into this challenge he takes it to the service station and fills the tank with DIESEL----WTF ha ha ha ha ha ha ha ha

 

well it looks like i,m done here,thanks to you all for your advice,sure i will miss this site,long live the xj--------Ed crunchbucket

 

No helping some people :roll: lol

 

thats cool rick,yes the caps do unscrew---but thats not my problem---the HT leads have been cut?no idea why,so do the HT leads unscrew from the coils---i now see that they dont and are sealed in with some kind of ceramic resin,so on this bike it,s not possible to change the leads for new ones as you would have to purchase 2 new coils as well, and they look expensive

 

thanks for the measurements here are my readings on the spare coils---primary 4.4 ohms secondary 11.70 k ohms------ next coil--primary 0.9 ohms and secondary 3.75 k ohms---not much point trying to use these i guess

 

If the Plug Wires are too short:

Extend their lengths using Spark Plug Wire "Splice" attachments.

http://www.ngksparkplugs.com/products/p ... licers.asp

Do the so-called "Coil Surgery"

http://xjbikes.com/Forums/viewtopic/t=1 ... rgery.html

 

The first spare coil may be a 750 unit (secondary is closer to their spec)

Second one looks pretty knackered though yeah.

Did you get a chance to see what the ones on the bike read?

 

OEM Coils and Wires are at their Service Life and beyond.
Like Fuse Panels, ... they should be Upgraded.

DYNA Coils and High Performance Wires and Caps is a Performance Upgrade.
ACCEL Coils do the trick; but are a bit of a hassle to install.

Click on XJ4Ever.
Len has Mounting Kits for DYNA's

 

Len's not in Hampshire though

 

Dyna is closer; not sure if they have a dealer in Hampshire though. DynaTek UK You'll want the DC 1-1 if memory serves. I hate to steer business away from Len, but import duties for just coils would probably be a budget breaker.

 

Same here, but chances are in this case there's no business anyhow, the guy was diagnosing & fixing the bike for a cheapskate who wouldn't even lend the guy a decent battery to test with lol