Cylinders 3 and 4 not firing

Hi, my XJ750 has been dormant for a couple of years now with multiple issues... the first of these was the carby vacum diaphragms (now replaced).

The second issue which I can't identify now is that two of the cylinders aren't firing, cylinders three and four. both the coils are working as cylinders one and two are on different coils... Any thoughts please?

 

Swap #3 and 4 spark plugs with 1 and 2. If you find that 1and 2 are not firing, then there is internal damage to the plugs and require replacing. However, if there is obvious damage to plugs 3 and 4, go ahead and replace them.

 

the plugs have been replaced. swapped the plugs and leads and still had the same cylinders firing...

 

tried reversing all the leads from 1234 to 4321 and cylinders 1 and 2 still the only ones firing

edit - forgot to mention compression tests are ok

 

Since the coils, plugs, boots, and wires seem to be ok (I would still OHM them out), then the problem would point toward carb related issues.

Have you:
Cleaned the carbs?
Wet-set fuel levels?
Bench synched?

 

the carbs have been stripped to individual single parts and cleaned, put back together, wet set the levels and synched...


edit: I actually thought it was the carbys myself so they've been stripped down and put back together 3 times now, but nothing has changed

 

Dave when you did your carbs did you pull apart the pilot screw assembly and clean it's passages? Have you tried adjusting the mixture on those cold cylinders. If you can't get any response from turning the screws (idling up or starting to die) then you've found the problem.

If you know you have good compression, good spark, then the only thing left is the air/fuel mixture.

You could try attaching a fuel line to the vacuum ports of 3&4 and squirt in some gas/starting fluid and see if the pipe gets hot-if so air/fuel is missing or wrong somewhere.

 

Hey Mercury - yes I did pull apart the pilot screw assembly and clean it along with everything else, position was noted before removal and upon reassembly was restored to original position (manual says 3 1/4 turns is standard, all 4 were set at 3 1/2 turns)

I haven't actually tried messing with the mixture adjustment yet - that was the last resort I haven't dared play with yet as all 4 of them are set the same!I was hoping that wasn't it, but as you say, compression and spark are good...

I hadn't heard of the fuel line trick, I'll have to try that trick thanks!

 

okay. Just tried a couple of things. I attached the fuel line to number 4 and squirted in some starting fluid, started the bike and number four fired up instantly for a few seconds. mmm. fuel not getting to number four at the very least.

quickly checked the fuel level in bowl number four. fuel is really low - on the bench it was on spec.

to me this points to something being dirty somewhere around the mixture screw I think - I'm now going to strip the carbys and try and clean them again. Being new years day I dont think I'll be able to find anywhere open that sells carby cleaner...

Did I read somewhere that carby cleaner is bad for motorcycle carbys anyway?

 

Carby cleaner isn't bad for carbs, just the rubber seals.

Spray carb cleaner is ok, but if you are going to do a carb dip, you want to pull all rubber seals, i.e. throttle shaft seals, out before you do.

 

Gee I hope not, or a whole bunch of us are in trouble...

A mixture screw problem has absolutely nothing to do with a low fuel level in the float bowl. Completely unrelated.

How did you set the floats? They need to be "wet-set" individually using fuel and clear tubing; with the rack level in all planes.

 

okay, before I pulled the carbys, I rechecked the levels (clear tubing and fuel still connected) - they're all back to where they should be (as much as I can tell given they're now at a slight angle). Needle clogged or sticking?
I think I'm gonna do another strip and clean of the carbys as the only thing that I know for certain now is that fuel just isn't getting to the cylinders somehow, given that starter squirted into the vacum ports makes it run properly...

Good to know about the carb cleaner- as I say, new years day here already so I won't be able to buy any probably...

I agree that mixture screw problem shouldn't be a problem, but if there's a lump of junk sitting in the pipes anywhere the only place I might not have seen may be around there as there's so many twists and turns there. (maybe)
The fuel tank on the bike isn't an XJ tank as it doesn't sit right (rubs against frame in one spot) and doesn't match pics of this model bike in it's original spec. it's also had a lot of rust in it which I've had bronze brazed up. it might be that a couple of lumps of rust have made their way through somewhere that I haven't picked up on - hopefully I can find something this clean (4th carby stripdown now). I must not be being anal enough!

 

started off stripping down carbys 3 and 4. came across something strange - in the pans of each carby there's a hole drilled down to nearly the bottom with a brass plug/jet, which looks like it draws fuel up into either the choke or idle system. My father in law made the observation that they were blocked, and given that they'd gone to the trouble to drill and plug them it was strange that they were blocked... I then removed the pans from carby one and two and sure enough, they weren't blocked on those two...

(carby's are HSC32's in case of any confusions)



hmmm. next step attempt to unblock these things...

 

That is where the Enrichment Circuit draws fuel from when you open the "choke".

Brass tube on the bottom of the carb goes into the hole on carb bowl edge (follow green line)

When you slide the choke lever, the cable pulls up on the Enrichment Plunger, and vacuum from the motor draws extra fuel up the brass tube.

These need to be open for your Enrichment Circuit to work correctly. It is best to fill the bowl about half way with carb cleaner and let it sit.

You should be able to spray carb cleaner into the hole on the inside and have it shoot out the other hole.

CAUTION::::: it will SHOOT out, powerfully, a good 8-10 foot jet stream:::::CAUTION

So make sure it's pointing AWAY from humans and pets.

But it would have nothing to do with your carb bowl not getting fuel. I would suspect something else is clogged also.

Could also be valve seats, fuel rail, etc.

Time to do a meticulous cleaning, me thinks.

 

The carbs look very clean, but there are a lot of tiny passages that could have "gunk" in them, and they all need to be spotless for the carbs to work properly.

 

yes that HAS to be clear. there is some further info on that at XJ4EVER, I think in the "information overload"?
In any case, Chacal sells a super tiny drill bit that works perfect (use it as a file) then slightly taper the tube of a can of carb cleaner to "fit" the jet and spray untill it shoots out like a high pressure nozzle

 

okay, managed to get one of the bowls clear, but the other one is being stubborn. ARGH. ended up with carb cleaner in my eyes (no sense in my head, can you smell the neurons burning?) - water water water water and call paramedic friend for advice lol

anyway, one of them is still blocked, pressure isn't shifting it. going to soak it for a bit and see what happens.

also continuing to check all other holes and nooks and crannies for other blockages. man these things have a lot of nooks and crannies!!

 

you might try boiling that bowl. here is somthing that has worked for me on various items simmilar:
1. boil in lemon juice for 5 minutes the let sit in hot lemon juice till it cools to the point you can pick it up with your hands
2. repeat process with white vinegar (go straight from the lemon juice to the vinegar)
3. boil in water then immediatly after get clearing it and try the re spray

also an oriface cleaning tool set may help?

 

if that don't work take the unused lemon juice and add some grapefruit juice, orange juice and vodka
mix well and have a drink while you have a happy new year

 

another very odd thing I found. I have a haynes manual for the bike, and it contains this photo (rotated for comparison with the next pic)



when compared with a genuine yamaha manual



I think I may have been putting the air jets in back to front... going to try putting the air jets in according to the yamaha manual rather than the haynes manual. the mechanic who put this bike back together last has a lot to answer for!

 

haynes is wrong, they also are wrong about the intake valve specs.
go with the factory manual

 

skw, yes, they're definitely wrong, I was just leafing a bit further through the manual and one of the other diagrams contradicts the earlier one. Oh joy!

 

Um...I wouldn't exactly call that person a "mechanic".


The Haynes and factory manuals both have errors. It's best to use a combination of both, then use this forum as a backup when the manuals contradict themselves

 

quick question, the haynes manual says fuel level should be 3 mm below the bowl edge. I only got the yamaha manual recently and it's saying 1 mm below the bowl edge. (I only recently got the yamaha manual) I'd previously adjusted the fuel level to 3 mm as per the haynes manual, I'm guessing the haynes manual is probably wrong. Can someone please confirm the 1 mm figure?

edit: carbys are the HSC32's

 

I use the the bowl screws. With the carbs level for and aft, side to side I wet set to the bowl screws. Just seems easier, hold on and I'll masure that...........

 

HSC32 Fuel height is 3mm +/- 1mm

So the height can be set anywhere from 2mm to 4mm, but getting as close to 3mm as you can is best.

 

yep thats what I got. I never "measured" it till just now, I always went by Ricks green/yellow/red bowl picture thing.

 

And make sure you use fuel to set the heights.

Other fluids don't have the same properties, so petrol will measure differently than, say, windsheild washer solvent.

 

Everytime there's a Thread on Float Heights, the issue of what to use to set Heights is bantered about.

I have always used Windshield Washer Fluid.
Because SAFETY in a shop connected to my home required it.
Those who recommend GAS ONLY seem to believe there is such a great difference in Specific Gravity Washer Fluid will induce a great error in calibrations.

On the LEFT is WASHER FLUID, ...
On the RIGHT is High Test GAS.
The Red "Float" is the Cap from a Sharpie.

 

so in theory if I used the washer flluid and set to the lowest end of spec,
I shoud be around mid or higher end of the spec when using fuel? Or is it not even enough to try and figure? I have an OCD thing going now

 

Oh just use fuel, and be careful.