GS-650 Katana, Persistent Misfire

[G0FHM]

Hi Guys,

I did a little bit more playing last night, and when it's actually burning, the plug is clean and dry. A little black from a rich mixture, but no gas or oil fouling.

I ran out of motivation last night and that's as far as I went.

I'll have another coffee (or two) and go swap the pistons about. If that moves the problem to another cylinder, then that will indicate needle/piston/diaphragm. Not a cheap problem to fix! (not in the UK anyway!)

J

[G0FHM]

Well,

I've swapped the carb pistons/diagphragm/needles between #1 & #4.

Still no change!

I'm confused!

J

[Nessism]

Where are the pilot screws set?

Did you remove the pilot jets and hold them up to a light to make sure they are open?

Did you spray carb cleaner though the choke pick up tube and make sure it comes out the choke plunger passage?

Did you make sure the choke tube passage in the float bowl is open?

Did you adjust the engine valves (not likely the problem, but part of basic maintenance)?

Did you look though the Newbie Mistakes thread to make sure you are avoiding the common problems?

Did you go though the entire carb rebuild tutorial and replace all O-rings, including those on the intake boots?

Seems pretty obvious the problem is fuel related, since as you said there is spark (although you should verify the plug in the cylinder is firing). I suggest going back to basics and verifying each step in the process.

[hjfisk]

Yes I read the entire post twice and didn't see where you tried a different plug in cyl. #4. Even if they were new it wouldn't be the first 'new' plug that was junk. Some plugs just won't fire with compression against them. Seen it alot on snowmobiles

[Carter Turk]

Do you have the original ignitor?

[G0FHM]

Where are the pilot screws set?

2.5 turns, as stated as a starting point in the rebuild "howto"

Did you remove the pilot jets and hold them up to a light to make sure they are open?

Yes - And have blasted carb cleaner through them.

Did you spray carb cleaner though the choke pick up tube and make sure it comes out the choke plunger passage?

Yes

Did you make sure the choke tube passage in the float bowl is open?

Yes - Again, blasted with carb cleaner, and got a face full in the process!!

Did you adjust the engine valves (not likely the problem, but part of basic maintenance)?

All within tolerance!

Did you look though the Newbie Mistakes thread to make sure you are avoiding the common problems?

Yup!

Did you go though the entire carb rebuild tutorial and replace all O-rings, including those on the intake boots?

Yes!

Seems pretty obvious the problem is fuel related, since as you said there is spark (although you should verify the plug in the cylinder is firing). I suggest going back to basics and verifying each step in the process.

It's not the plug, I've swapped them between pots and it's still #4 that is inoperative. I've had brand new Bosch & Delco plugs fail before now - But i've never had an NGK let go on me... Yet!

The only conceivable thing left, is the seal between the intake stub and head. I've tested the inlet stub (don't ask me how!), and that doesn't leak.

I did fit new O rings, and the ring wasn't a very good fit within the groove - And I'm just wondering if it sprang out when I offered it up to the head, although on inspection it shows no signs of being pinched. I'll stick it in place with a smidge of silicon RTV when I put it back on, which will also help the seal.

If that is indeed the problem, then it hid itself very well. Normally a bit of rubber fuel hose stuck in the ear makes an excellent makeshift stethoscope, and I've successfully found leaks using that method many times over the years.

One thing I did find, was the revs were very slow to recover after being raised. Lubricating the advance mechanism (it was quite stiff) helped a little, but didn't totally stop the problem. That is consistent with a vacuum leak.

I'll put it all back together tomorrow and report back!

Thanks,

J

[tom203]

Carry on ! I'm still wondering why #4 took off when carb mouth was partially blocked, but I know there's an explanation.

[G0FHM]

It all points to a vac leak...

As somebody else mentioned earlier in the thread, if there is low vacuum, then it won't suck the gas through from the float bowl, by whichever route.

Putting a hand over the air intake will increase the vacuum, to the point that it will draw fuel through -Albeit much richer, due to a lower concentration of air.

I struggled to understand that explanation for a start, but the more I think about it, the more it makes sense!

I'll see what tomorrow brings, after I have put it back together!

J

[G0FHM]

Well, problem fixed!

I can only assume it was a vac leak that was undetactable.

I glued the O ring on the inlet stub in position with silicone, and put it back on.

Now have all 4 pots banging. Looking at the vac guages, there is now a higher reading on #4 - Not much higher, but probably it was enough to cause a problem.

Just glad it's sorted now.....

Thanks for bouncing ideas around guys!

Thanks,

J

[tom203]

Nice persistence ! and good warning to others!

[duaneage]

Silicone turns to jelly in the presence of gasoline. If you sealed the o ring on the intake boot with it the fix won't last. Unless I'm reading it wrong you need new intake boots and o rings.

Nice bike BTW, I would sell both mine for a shaft drive Katana.