GS-650 Katana, Persistent Misfire

[Guest]

Hi Guys,

I'm at my wits end with this one.....

We have a 1981 650GX Katana. We bought it as a project, and it's nearly there. It's been stripped down and new piston rings, stem seals etc installed as it was a high miler and probably had a lot of rough treatment (it was a launch/press bike in it's early years).

It all started after rebuild. It started misfiring on 4, and swapping the LT/HT leads about between coils moved the misfire from cylinder to cylinder. A HT lead was suspected (as it gave me a nice jolt when I pulled the cap off), and as the coil was probably an original type with no interchangeable HT leads, we bought a new one.

Now, the misfire is on No.4 and we cannot make it move, no matter how we swap leads about.

This is what I have done to eliminate certain things:

Compression tested, all within a few psi of each other. A squirt of WD40 down the bores raised the compression by 25psi. So valves and rings are ruled out.

I've had the carbs off, and given them a thorough clean. There was some wild variations in the float heights, but they are all now set to within 0.5mm of what the manual says.

As far as vac leaks on the inlet stub are concerned, I've listened using a piece of rubber hose as a makeshift stethoscope and have squirted WD40 at them. The vac guages are plugged in, and all carbs are balanced.

I've swapped 1 & 4 spark plugs to rule a bad plug out, and no difference.

There is a mixture of WD40 and gas on the plug tip. I don't suspect over fuelling, as the downpipe on 4 is barely warm compared to the other 3 which are as hot as you'd expect them to be. If it was over fuelling and detonating, the downpipe would be just as hot as the other 3.... right?

The annoying thing, as that when it's running if I pull the plug cap off there is a good visible and audible spark.

Anybody else have any more suggestions? Surely if was a LT fault it would impact No. 1 as well wouldn't it? Although I have seen a video on youtube of a GS that is a pot down, and it is mooted as being a connector problem somewhere under the seat......

Any suggestions would be appreciated!

Thanks,

Jay

[tom203]

I'd rule out electrical since you switched 1 and 4 with no difference. So it might be carb related- not enough fuel passing thru #4 carb to make it consistently happy (pipe not hot) ,but sometimes just enough of a lean mixture to get misfiring. It's not easy to clean the low speed circuits in these carbs- how did you do it?
Of course, you got all valve clearances in spec. with all buckets moving nicely up/down.

[Tempted]

Too rich and the plug will foul, the pipe will never get hot. Too lean and may not be getting enough fuel to burn and the pipe won't get hot. What kind of air box is on it when you have it running? If I were to guess I'd say it was a carb issue if everything else is ruled out.

[G0FHM]

Hi Tom,

I've gotten to the point where I've ruled out electrical too.

I've checked the resistance of the primary and seconday coil windings as they are not a matched pair, and the primaries are at 5ohms as per the manual, and the secondaries although lower than the book figure, are 1.8Kohms apart, so that has potentially ruled out a mismatched load causing a problem.

Like you said, I think the problem may be in the fueling, or a leak in the vac guages - I'll pull the brass stub for 4 out and replace it with the grub screw and see what happens.

The misfire is across the entire rev range, from cold with choke open, to warm up to 4000rpm (I dare not push it any higher as it has new piston rings, and hasn't done any road work yet).

The carbs were cleaned ultrasonically using water, dishwashing liquid and vinegar as the medium (as suggested by a local mechanic) and blown through and left to dry for a couple of weeks. All the jets were removed and ultrasonically cleaned seperately (on a carb by carb basis, to keep everything together). And since then, a couple of cans of carb cleaner solvent has been squirted through the small pilot system bores.

If anything, I think the carb may be flooding as when I pulled the top off and removed the piston, there was a fair amount of gas in the throttlebody (enough to wet the end of my finger) and a little bit in the round tray under the diaphragm. The diaphragm seems good - I can push the piston up, and put my thumb over the hole at the back side of the carb and it will stay up and not move. The needle valves appear good, as I can fill the float bowls with gas to the point where they are closed, and blow down the delvivery tube, and it's a constant pressure. Do the same with the needle valves open, and you can hear bubbling and air coming from the vent pipes. I have the auxiliary fuel tank hanging from the shed roof and it has a clear delivery pipe. I can get a bubble in the tube, mark it and leave it overnight and it will still be in the same place in the morning, so nothing is leaking down.

The spark plug on 4 is getting wet with gas.

If it was over fuelling i'd expect to hear rough running, with a fair amount of noise due to the excess fuel detonating uncontrolledly. Unless of course, it's putting too much gas in and there isn't enough air to make it go bang at all.....

Right, I'm off back to the shed to cry quietly into my coffee! :-(

Thanks,

J

[G0FHM]

Hi Tempted,

It's the standard airbox, with new freshly oiled foam filter, not the paper type that screws on to all 4 carbs.

J

[tom203]

OK, #4 plug wet, signs of gas in area just below (?) vacuum diaphragm of #4 carb . Maybe the float needle seat o-ring is leaking, letting gas overfill.

[G0FHM]

Hi Tom,

I did replace all of the needle valve O rings with viton rings at the time I serviced them, as they were all very compressed, leaking and flooding.

I'll have another look at them - Maybe 4 has not quite seated properly and has come out of it's groove when I pushed the needle valve in.

I may invest in a new needle valve too, just in case.

J

[tom203]

While viton should work fine, I use plain old buna o-rings for carbs since heat isn't a problem. They swell nicely in presence of gas and seal tightly.

By the way a member on here deals with this carb stuff, have a look at his site sometime

http://www.cycleorings.com/

[G0FHM]

Tom,

I think I read the Mikuni BS tutorial from here actually.... Very helpful!

Just a quick thought - Would the rubber plugs in the pilot jet holes be likely to cause a problem? A couple aren't the best fit in the world, they stay where they are, but I'm wondering if gas/air bypassing those would cause a problem such as this?

J

[Nessism]

Pilot jet plugs aren't going to cause that problem unless they are missing.

Did you hold up the pilot jet to a light and make sure it's open?

Perform a valve adjust and vacuum carb sync?

Oh, and replace all carb O-rings and intake boot O-rings if you haven't done so already. Spraying crap on the parts is an unreliable method of finding vacuum leaks.

[tom203]

I doubt it - fuel still has to pass thru pilot jet ; lack of rubber plug apparently causes transition problems

[G0FHM]

Ok guys.... Make sense of this: :-)

I've popped out the intake air stub between the backside of the carb and the 4 way collector before the airbox.

Firing it up and revving gives 3 cylinders. If I put the end of my finger on the end of the piston, I can feel it pulsing up and down by a small amount, in time with the needle flick on the vac dial. So that appears to be working ok.

Here's the funny bit!

If I place my hand over the opening on the backside of the carb (not enough to completely seal it and prevent air from getting in), the engine will suddenly jump up to 4000rpm, and run cleanly. This is proved by a hot downpipe, and a hot sparkplug.

Also, I can feel the odd pop and splutter as the airflow going in is being momentarily interrupted.

Does that help give any answers?

Any ideas?

[tom203]

"If I place my hand over the opening on the backside of the carb (not enough to completely seal it and prevent air from getting in), the engine will suddenly jump up to 4000rpm, and run cleanly. This is proved by a hot downpipe, and a hot sparkplug."

Blocking intake causes engine vacuum to "reach past" throttle plate and access main jet/needle area- maybe it then scoops enough fuel here to take off. But why do you have a fouled #4 plug ?? more coffee needed

[G0FHM]

I'll pull the plug out and check it in a bit, after it has cooled down, to see what the state of it is after actually running.

It seems that when it is not firing, gas is still getting in.

I'm going to try swapping the diaphragm/piston/needle between #1 & #4 to see if that moves the problem. That will rule out diaphragm - But a visual inspection along with other tests, indicates it's ok.

Strange!

J

[tom203]

Don't forget to update us- your problem might happen to someone else, like me!