Cylinder 4 not firing - GPz 550

I'm trying to get a 1983 GPZ550 running and making slow progress - last night I got it to start for the first time, and it even idled for maybe 20 seconds.

This afternoon I put the gas tank on and tried to get it going. I could start it and rev it up a little to keep it going, but it clearly was pretty weak and I quickly realized that only cylinders 1 and 2 are firing; the exhaust tubes for 3 and 4 were stone cold.

I was trying to think of problems that would affect two adjacent cylinders, and all I could think of was two things:
1) the two carb atmospheric vents connecting 1 to 2, and 3 to 4. I made sure both tubes were venting freely and it made no difference, but that shouldn't matter at idle anyways because the needles don't raise at all (right?).
2) the emissions system valve on the right side of the front valve cover. From what I understand the two exhaust ports are connected here, but it seems unlikely that something could go wrong with that that would prevent the cylinders from firing.

Here are some initial tests I did:

-Pulled the spark plug out of #4 and cranked the engine with the plug grounded. The plug had strong, rapid sparks. I assumed for the moment that #3 would be the same, but I will double check that later to be sure. So that indicates that the ignition system is not the issue. Unless the timing is off - but if the #3 and #4 timing was off, so would #1 and #2 because they're on the same coil. So that wouldn't really make sense.

-Attached a tube to the #4 carb drain and checked the fuel level. there appeared to be the right amount of fuel in the bowl. I rapped all the float bowls with a screwdriver a few times to loosen up any rust that may be messing with the floats.

I'm not sure which direction to go from here. I feel like the cylinders might not be getting fuel because it doesn't smell especially like unburned fuel at the end of the #3/#4 exhaust pipe.

It is very likely that my fuel is contaminated with a few small rust particles. I cleaned it out as best I could but could still here a little silt before I put fuel in it. The carbs have just been taken apart, cleaned and put back together. New float valve needles have been installed and the float heights were bench tested before installing on the bike.

All input appreciated!! I'm sort of new to bikes and carbs, mostly just worked on EFI cars before. Thanks a lot,

Marshall

If not already done, would do compression test (hold throttle wide open during testing). And also check valve clearances.

Good Luck!

Okay, so I've done a little more investiagtion and I've got it down to one cylinder not firing, #4 - though I'm still a little skeptical of #3.

I think dirty fuel is the source of my problems because yesterday I spent a while draining the float bowls and trying to measure their fuel heights, then I filled the tank up with gas (it had been maybe 1/10 full before) and when I started the bike, the third cylinder was magically firing again. The #3 exhaust tube wasn't quite as hot as 1 and 2, but it was definitely running. #4 was still cold to the touch.

I couldn't do a compression test because I don't have an adapter that fits the spark plugs these engine use. What size and thread type should I look for if I try to find/make an adapter?

I did check the valve clearances. 7 of the valves were within the tolerance, but exhaust valave #3 was just barely too tight. my .127mm feeler could fit in the gap, but my .152mm feeler would only fit if I forced it - so the gap is probably about .14mm, just under the .15-.25 tollerance. Is it worth going through the trouble of adjusting that one valve clearance? The bike only has 14,000 miles on it and I'm not likely to ride it more than 1,500 miles in the next year, especially if it takes me forever to get it running - I'd like to put off adjusting the valve clearance if possible. Also, think a good ol' 2-ply cereal box gasket will do for the valve cover, or should I get some real gasket material from NAPA? Or should I order a pre-cut gasket?

Trying to measure the carb fuel levels had interesting results. I tried to do everything like the manual said, but it still seemed like a pretty subjective test. I found that you have to bleed the air out of the tube or it will throw the level off, and it seems impossible to completely get the air bubbles out. Here are my results, the distance of the bottom of the fuel meniscus above (+) or below (-) the float bowl mating surface:

Carb 1: +4.1 mm
Carb 2: - 7.5 mm
Carb 3: - 2.8 mm
Carb 4: - 7.5 mm

Obviously, something is wrong here because I know all of the floats were very close to the factory standard height before I installed them - there's no way they were off by up to 5 mm, so changing them now to suit these numbers would seem risky. I have the feeling these numbers are the result of either dirty fuel messing with float valves or me not understanding how to do the fuel level test properly. Advice?

I just want to ride my freakin' bike! Thanks for your help,

Marshall

if you had dirty fuel in there, you are going to need to pull the carbs again and clean them,, also install a inline fuel filter.
also have you installed new plugs in the bike

Yeah, I've installed new plugs and there's already a fuel filter that doesn't seem to be doing much (or the fuel's clean, but probably the former). I'd like to do a compression test before I pull the carbs, but I don't know what size adapter will fit the spark plug holes.

Thanks

I don't know if you already have done this. Make sure the ignition coils are hooked up correctly. On my 1981 KZ750 LTD, the positive and negative leads were switched around causing one cylinder not to fire. The spark plug visually had spark, but there was no combustion taking place in that cylinder. It took me a while to figure this out.

Carefully inspect the pilot jets . The holes are very small and are usually the first thing to clog from bad gas or debris . Paint brush cleaner (not paint thinner) cleans them quite well usually .

mjg15 wrote:

Carefully inspect the pilot jets . The holes are very small and are usually the first thing to clog from bad gas or debris . Paint brush cleaner (not paint thinner) cleans them quite well usually .

Ya, clean that pilot out, you might even hold it up to the light and see a little pinhole, but blow it out, that little pinhole opens up pretty big.
Also, when you clean the carbs again, and be very careful to keep the parts to the right carbs, take the floats and float valves out and spray the feeder hose. carb cleaner shoot spray out of the valve seats. I watched as a chunk of black stuff popped out of there.

Okay guys, thanks a lot for the advice. I'm dreading having to take the carbs/air cleaner off again but I'll do it if I determine it's definitely a fuel problem.

However, what I REALLY want to know right now is HOW you do a compression test on an engine with this size spark plugs??

Where can you buy an adapter to fit the holes that will attach to a "regular" compression test gauge?

Or do I actually have to buy a new metric compression tester?

The guys at the local garage didn't know what to do about it, as usual.

It's important to note that I have NEVER seen this bike run before, and even though it has only 14k miles on it, it could have been beat to hell before it came into my posession (hint #1: I got it for free). I just want to make sure it's worth messing with the carbs and ignition at all, because there's a chance I should be planning a rebuild as we speak.

Just stick your finger in the holes and spin it over to tell if it has compression. If you have small rust particles in the tank its the pilot jets. Problem is it will keep plugging them. The hole in them is so small a grain of flour will plug them. Even a wire off a wire brush is too big to go thru the hole. Just went thru the same thing with #4 cyl on my GPZ750

Good news! I managed to get the #4 carb off without taking the air cleaner or other carbs out, and I found that the pilot jet was clogged up. I managed to really clean it out well by rotating a bunch of times between carb cleaner, running a piece of sewing thread back and forth and around through the pinhole, and blowing compressed air through it. When I reinstalled the carb it still didn't seem to work, but then I got the bike hot and all of a sudden it was running pretty well on all 4 cylinders.

Now I have the bike running and have blasted it around a little, so I'm a lot more confident in the condition of the engine internals; there seemed to be plenty of power, although the clutch was slipping some.

Only problem is, the engine won't run at all with the choke, whether the motor is cold or hot. I'm sure the choke passage in at least one of the carbs is blocked; while cleaning the #4 carb, I was unable to squirt cleaning fluid through that long tube that pokes down to draw from the bottom of the float bowl. I assume that's what supplies extra fuel for the choke. I have no idea how I'm going to clean them out, because it's a long passage with a couple bends in it - not something I can use my 'flossing' technique with.

For now, I'll just warm the bike up by holding the throttle open without the choke.

Thanks for helping me out,

Marshall

Congrats and thanks for saving another 83.